Draft version August 6, 2018
Preprint typeset using LATEX style emulateapj v. 4/12/04
MID-INFRARED FINE STRUCTURE LINE RATIOS IN ACTIVE GALACTIC NUCLEI OBSERVED WITH
SPITZER IRS: EVIDENCE FOR EXTINCTION BY THE TORUS
R. P. Dudik
, J. C. Weingartner
, S. Satyapal
, J. Fischer
, C. C. Dudley
, & B. O’Halloran
Draft version August 6, 2018
ABSTRACT
We present the first systematic investigation of the [NeV] (14µm/24µm) and [SIII] (18µm/33µm)
infrared line flux ratios, traditionally used to estimate the density of the ionized gas, in a sample
of 41 Type 1 and Type 2 active galactic nuclei (AGNs) observed with the Infrared Spectrograph on
board Spitzer. The majority of galaxies with both [NeV] lines detected have observed [NeV] line flux
ratios consistent with or below the theoretical low density limit, based on calculations using currently
available collision strengths and ignoring absorption and stimulated emission. We find that Type 2
AGNs have lower line flux ratios than Type 1 AGNs and that all of the galaxies with line flux ratios
below the low density limit are Type 2 AGNs. We argue that differential infrared extinction to the
[NeV] emitting region due to dust in the obscuring torus is responsible for the ratios below the low
density limit and we suggest that the ratio may be a tracer of the inclination angle of the torus to our
line of sight. Because the temperature of the gas, the amount of extinction, and the effect of absorption
and stimulated emission on the line ratios are all unknown, we are not able to determine the electron
densities associated with the [NeV] line flux ratios for the objects in our sample. We also find that the
[SIII] emission from the galaxies in our sample is extended and originates primarily in star forming
regions. Since the emission from low-ionization species is extended, any analysis using line flux ratios
from such species obtained from slits of different sizes is invalid for most nearby galaxies.
Subject headings: Galaxies: Active— Galaxies: Starbursts— X-rays: Galaxies — Infrared: Galaxies
1. INTRODUCTION
Mid-infrared (mid-IR) emission-line spectroscopy of
active galactic nuclei (AGNs) is used to investigate the
physical conditions of the dust-enshrouded gas that is
in close proximity to the active nucleus. In particular,
many spectral lines are emitted in the so-called narrow-
line region (NLR) of these objects which typically ex-
tends between tens to at most a thousand parsecs from
the nucleus (Capetti, et al. 1995, 1997, Schmitt & Kin-
ney 1996, Falcke et al. 1998; Ferruit et al. 1999, Schmitt
et al. 2003).
The NLRs of AGNs have been studied extensively us-
ing optical spectroscopic observations. However, there
have been very few systematic studies of the NLR using
infrared spectroscopic observations. Infrared (IR) fine-
structure emission lines have a number of special char-
acteristics that have been regarded as distinct advan-
tages, particularly in determining the electron density of
the ionized gas very close to the central AGN. Infrared
spectroscopic observations allow access to fine-structure
lines from ions with higher ionization potentials than the
most widely used optical diagnostic lines. This is impor-
tant in many AGNs, where a significant fraction of the
line emission from lower ionization species can originate
in gas ionized by star forming regions. In addition, it
is generally assumed that the density-sensitive infrared
line ratios originate in gas with temperatures around 104
K and are less dependent on electron temperature vari-
ations, enabling a more straightforward determination
of the electron density in the ionized gas. Finally, it
1 George Mason University, Department of Physics & Astron-
omy, MS 3F3, 4400 University Drive, Fairfax, VA 22030
2 Naval Research Laboratory, Remote Sensing Division, 4555
Overlook Ave SW, Washington DC, 20375
has long been assumed that the IR diagnostic line ratios
are insensitive to reddening corrections–a serious impedi-
ment to optical and ultraviolet observations, particularly
in the NLRs of AGNs, where the dust composition and
spatial distribution are highly uncertain. For these rea-
sons, IR spectroscopic observations, especially since the
era of the Infrared Space Observatory (ISO), have pro-
vided us with some of the most reliable tools for studying
the NLRs in AGNs. However, while there are clear ad-
vantages of mid-IR fine-structure diagnostics in studying
the physical state of the ionized gas, very little work has
been done to investigate their robustness in determin-
ing the gas densities of the NLRs in a large sample of
AGNs. The Spitzer Space Telescope Infrared Spectrome-
ter (IRS), with its extraordinary sensitivity and spectral
resolution, offers the opportunity to examine for the first
time the physical state of NLR gas in a large sample of
AGNs.
The focus of most previous comparative studies of the
infrared fine-structure lines in AGNs has been on the
excitation state of the ionized gas, in an effort to de-
termine the existence and energetic importance of po-
tentially buried AGNs and to constrain their ionizing
radiation fields (Genzel et al. 1998, Lutz et al. 1999,
Alexander & Sternberg 1999, Sturm et al. 2002, Satya-
pal, Sambruna, & Dudik 2004, Spinoglio et al. 2005).
Remarkably, very little work has been done in the in-
frared on studying the line flux ratios traditionally used
to probe the NLR gas densities in a significant number of
AGNs. We present in this paper the first systematic in-
frared spectroscopic study of the line flux ratios of [NeV]
and [SIII] in order to 1) test the robustness of these line
ratios as density diagnostics and 2) if possible, to probe
the densities of the NLR gas in a large sample of AGNs.
http://arxiv.org/abs/0704.0547v2
2. THE SAMPLE
We searched the Spitzer archive for galaxies with an ac-
tive nucleus and both high- and low-resolution Infrared
Spectrometer (IRS; Houck et al. 2004) observations cur-
rently available. Only those galaxies with indisputable
optical, X-ray, or radio signatures of active nuclei (such
as broad Hα or X-ray or radio point sources) were in-
cluded in our sample. The sample includes three AGN
subclasses: Seyferts, LINERs, and Quasars. The galax-
ies in this sample span a wide range of distances (4 to
400 Mpc; median = 21 Mpc), Hubble types, bolomet-
ric luminosities (log (LBOL) ∼ 40 to 46, median = 43),
and Eddington Ratios (log(L/LEdd) ∼ -6.5 to 0.3; me-
dian= -2.5). The entire sample consists of 41 galaxies.
The basic properties of the sample are given in Table
1. The black hole masses listed in Table 1 were derived
using resolved stellar kinematics, if available, reverber-
ation mapping, or by applying the correlation between
optical bulge luminosity and central black hole mass de-
termined in nearby galaxies only when the host galaxy
was clearly resolved. Bolometric luminosities listed in
Table 1 were calculated from the X-ray luminosities for
most objects. For Seyferts, the relationship LBOL = 10 ×
LX was adopted (Elvis 1994). For LINERs
1 we assumed
LBOL = 34× LX , as derived from the spectral energy dis-
tribution of a sample of nearby LINERs from Ho (1999)
(see also Dudik et al. 2005 and Satyapal et al. 2005).
The bolometric luminosities and black hole masses for
quasars and radio galaxies were taken from Woo & Urry
(2002) and Marchesini, Celotti, & Ferrarese (2004), re-
spectively. A detailed discussion of our methodology and
justification of assumptions for determining black hole
masses and bolometric luminosities for the various AGN
classes represented in Table 1 can be found in Satyapal et
al. (2005) and Dudik et al. (2005). Table 1 also lists the
AGN type (1 or 2) for the galaxies in our sample based
on the presence or absence of broad (full width at half
max (FWHM) exceeding 1000 km s−1) Balmer emission
lines in the optical spectrum. We emphasize that the
selection basis for the objects in our sample was on the
availability of high resolution IRS Spitzer observations.
The sample should therefore not be viewed as complete
in any sense.
3. DATA ANALYSIS AND RESULTS
We extracted archival spectral data obtained us-
ing the short-wavelength, low-resolution module (SL2,
3.6”×57”, λ = 5.2-7.7µm) and both the short-
wavelength, high-resolution (SH, 4.7”×11.3”, λ = 9.9-
19.6µm) and long-wavelength, high-resolution (LH,
11.1”×22.3”, λ = 18.7-37.2µm) modules of IRS.
The data presented here were preprocessed by the
IRS pipeline (version 13.0) at the Spitzer Science Center
(SSC) prior to download. Preprocessing includes ramp
fitting, dark-sky subtraction, droop correction, linear-
ity correction, flat-fielding, and flux calibration2. The
Spitzer data were further processed using the SMART v.
5.5.7 analysis package (Higdon et al. 2004). The slit for
1 We include all galaxies that are classified as LINERs using
either the Heckman (1980) or Veilleux & Osterbrock (1987) diag-
nostic diagrams.
2 See Spitzer Observers Manual, Chapter 7,
http://ssc.spitzer.caltech.edu/documents/som/irs60.pdf
Table 1: Properties of the Sample
Galaxy Distance Hubble log log log AGN
Name (Mpc) Type (MBH) (LX ) (L/LEdd) Type
(1) (2) (3) (4) (5) (6) (7)
Seyferts
NGC4151 13 SABab 7.13a 42.7b -1.53 1r
NGC1365 19 SBb 7.64b 41.3d -3.42 2s
NGC1097 15 SBb · · · 40.7e · · · · · ·
NGC7469 65 SABa 6.84a 44.3a 0.34 1t
NGC4945 4 SBcd 7.35b 42.5f -1.97 2u
Circinus 4 SAb 7.72b 42.1g -2.74 2v
Mrk 231 169 SAc 7.24c 42.2h -2.16 · · ·
Mrk3 54 S0 8.65a 43.5a -2.21 2w
Cen A 3 S0 7.24b 41.8i -2.54 2x
Mrk463 201 Merger · · · 43.0j · · · 2y
NGC 4826 8 SAab 6.76b · · · · · · · · ·
NGC 4725 16 SABab 7.40b · · · · · · 2r
1 ZW 1 245 Sa · · · 43.9k · · · · · ·
NGC 5033 19 SAc 7.39b 41.4l -3.13 1r
NGC1566 20 SABbc 6.92a 43.5a -0.57 1t
NGC 2841 9 SAb 8.21a 42.7a -2.64 · · ·
NGC 7213 24 SA0 7.99a 43.30a -1.79 · · ·
LINERs
NGC4579 17 SABb 7.85b 41.0b -3.47 · · ·
NGC3031 4 SAab 7.79b 40.2b -4.16 · · ·
NGC6240 98 Merger 9.15b 44.2b -1.52 2z
NGC5194 8 SAbc 6.90b 41.0b -2.43 2r
MRK266NE 112 Merger · · · 40.9b · · · 2t
NGC7552 21 SBab 6.99b · · · · · · · · ·
NGC 4552 17 · · · 8.57b 39.6b -5.52 · · ·
NGC 3079 15 SBc 7.58b 40.1m -4.05 · · ·
NGC 1614 64 SBc 6.94b · · · · · · · · ·
NGC 3628 10 SAb 7.86b 39.9n -4.58 · · ·
NGC 2623 74 Pec 6.83b · · · · · · 2aa
IRAS23128-5919 178 Merger · · · 41.0b · · · 2bb
MRK273 151 Merger 7.74b 44.0o -0.31 2t
IRAS20551-4250 171 Merger 7.52c 40.9b -3.23 · · ·
NGC3627 10 SABb 7.16b 39.4p -4.33 2r
UGC05101 158 S · · · 40.9b · · · 1cc
NGC4125 18 E6 8.50b 38.6b -6.47 · · ·
NGC 4594 10 SAa 9.04b 40.1q -5.47 · · ·
Quasars
PG 1351+640 353 · · · 8.48a 44.5a -1.08 · · ·
PG 1211+143 324 · · · 7.49a 44.8a 0.22 1t
PG 1119+120 201 · · · · · · · · · · · · 1y
PG 2130+099 252 Sa 7.74a 44.47a -0.37 1y
PG 0804+761 400 · · · 8.24a 44.93a -0.41 1dd
PG 1501+106 146 E · · · · · · · · · 1y
Columns Explanation: Col(1):Common Source Names; Col(2): Dis-
tance (for H0= 75 km s
−1Mpc−1); Col(3): Morphological Class;
Col(4): Mass of central black hole in solar masses; Col(5): Log of
the hard X-ray luminosity (2-10keV) in erg s−1. Col(6): log of the
Eddington Ratio. (* = We include all galaxies that are classified as
LINERs using either the Heckman (1980) or Veilleux & Osterbrock
(1987) diagnostic diagrams. Col(6): AGN type based on the presence
or absence of broad Balmer emission lines.) References:aWoo & Urry
2002, b Satyapal et al. 2005, c Tacconi et al. 2002, d Risaliti et al.
2005, e Terashima et al. 2002, f Guainazzi et al. 2000, g Smith &
Wilson 2001, h Gallagher et al. 2002, i Evans et al. 2004, j Iman-
ishi & Terashima et al. 2004, kGallo et al. 2004 , l Terashima et al.
1999, m Cappi et al. 2006, n Roberts, Schurch, & Warwick 2001, o
Balestra et al. 2005, pGeorgantopoulos et al 2002, q Dudik et al. 2005,
r Ho et al. 1997, s Storchi-Bergmann, Mulchaey, & Wilson, 1992, t
Veron-Cetty & Veron 2003, u Marconi et al. 2000, vOliva et al. 1994,
w Khachikian & Weedman 1974, x Veron-Cetty & Veron 1986y Dahari
& De Robertis 1988, z Andreasian, Khachikian, & Ye, 1987, aa Laine
et al. 2003, bb Duc, Mirabel, & Maza 1997, cc Sanders et al. 1988, dd
Thompson 1992.
http://ssc.spitzer.caltech.edu/documents/som/irs60.pdf
the SH and LH modules is too small for background sub-
traction to take place and separate SH or LH background
observations do not exist for any of the galaxies in this
sample. For the SL2 module, background subtraction
was done using either a designated background file when
available or the interactive source extraction option. In
the case of the latter, the exact position of the slit on
the host galaxy was first checked using Leopard, the data
archive access tool available from the SSC. The source
was then carefully defined according to the boundary of
the slit and the edge of the host galaxy. The background
was defined at the edge of the slit, where no other obvious
source was present. In some cases, the slit was enveloped
in the host galaxy and background subtraction could not
take place. For both high and low resolution spectra, the
ends of each order were manually cut from the rest of the
spectrum.
The 41 observations presented in this work are archived
from various programs, including the SINGS Legacy Pro-
gram, and therefore contain both mapping and staring
observations. All of the staring observations were cen-
tered on the nucleus of the galaxy. The SH, LH, SL2
staring observations include data from two slit positions
overlapping by one third of a slit. In order to isolate
the nuclear region in the mapping observations so that
we might compare them to the staring observations, we
extracted only those 3 overlapping slit positions coin-
ciding with either radio or 2MASS nuclear coordinates.
Because the slits in both the mapping and staring obser-
vations occupy distinctly different regions of the sky, the
slits cannot be averaged unless the emission originates
from a compact source that is contained entirely in each
slit. Therefore the procedure for flux extraction was the
following: 1) If the fluxes measured from the two slits
differed by no more than the calibration error of the in-
strument, then the fluxes were averaged; otherwise, the
slit with the highest measured line flux was chosen. 2) If
an emission line was detected in one slit, but not in the
other, then the detection was selected. This is true for
all of the high and low resolution staring and mapping
observations.
In Tables 2 and 3 we list the line fluxes and statistical
errors from the SH and LH observations for the [NeV]
14.3µm and 24.3µm lines, the [SIII] 18.1µm and 33.5µm
lines, as well as the 6.2µm PAH emission feature. For
all galaxies with previously published fluxes, we list in
Tables 2 and 3 the published flux values. Our values dif-
fer by no more than a factor of 1.9, much less in most
cases, from the Weedman et al. (2005) or Armus et al.
(2004, 2006) published values. These differences can be
attributed to differences in the pipeline used for prepro-
cessing. In all cases detections were defined when the
line flux was at least 3σ. For the absolute photometric
flux uncertainty we conservatively adopt 15%, based on
the assessed values given by the Spitzer Science Center
(SSC) over the lifetime of the mission.3 This error is cal-
culated from multiple observations of various standard
stars throughout the Spitzer mission by the SSC. The
dominant component of the total error arises from the
3 See Spitzer Observers Manual, Chapter 7,
(http://ssc.spitzer.caltech.edu/documents/som/som7.1.irs.pdf
and IRS Data Handbook (http://ssc.spitzer.caltech.edu/irs/dh/dh20v2.pdf,
Chapter 7.2
uncertainty at mid-IR wavelengths in the stellar models
used in calibration and is systematic rather than Gaus-
sian in nature. We note that the spectral resolution of
the SH and LH modules of IRS (λ / ∆λ ∼ 600) is in-
sufficient to resolve the velocity structure for most of the
lines. There are a few galaxies which do show slightly
broadened [NeV] line profiles (FWHM ∼ 200 - 1200 km
s−1). These results will be discussed in a future paper.
Abundance-independent density estimates can readily
be obtained using infrared fine-structure transitions from
like ions in the same ionization state with different crit-
ical densities. The density diagnostics available in the
IRS spectra of our objects are: [NeV] 14.32µm, 24.32
µm (ncrit ∼ 4.9 × 10
4 cm−3, and 2.7 × 104 cm−3, where
ncrit = Aul/γul, with Aul the Einstein A coefficient and
γul the rate coefficient for collisional de-excitation from
the upper to the lower level), [NeIII] 15.55µm, 36.04 µm
(ncrit ∼ 3 × 10
5 cm−3, and 5 × 104 cm−3, Giveon et al.
2002), and [SIII]18.71µm, 33.48 µm (ncrit ∼ 1.5 × 10
cm−3, and 4.1 × 103 cm−3). The results are very insen-
sitive to the shape of the ionizing continuum. Since the
[NeIII] 36µm line was either not detected or was outside
the wavelength range of the LH module in virtually all
galaxies, we omit any analysis of the [NeIII] line ratio
from this work.
4. THE [NEV] LINE FLUX RATIOS
In Figure 1 we plot the calculated 14µm/24µm line lu-
minosity ratio as a function of electron density ne for gas
temperatures T = 104K, 105K, and 106K. We include
only the five levels of the ground 2s22p2 configuration and
neglect absorption and stimulated emission. The results
are nearly identical if only the lowest three levels of the
ground term are included. We adopt collision strengths
from Griffin & Badnell (2000) and radiative transition
probabilities from Galavis, Mendoza, & Zeippen (1997).
Fig. 1.— [NeV] 14µm/24µm line flux ratio versus electron den-
sity, ne, for gas temperatures T = 10
4 K, 105 K, 106 K
In Table 2, we list the observed [NeV] line flux ratios
http://ssc.spitzer.caltech.edu/documents/som/som7.1.irs.pdf
and their associated calibration uncertainties. In calcu-
lating the upper and lower limits on the ratios, RMAX
and RMIN , shown in Table 2, we did not propagate the
errors in quadrature as would be appropriate for statis-
tical uncertainties, but propagated them as follows:
RMAX =
F [NeV]14 + 0.15(F [NeV]14)
F [NeV]24 − 0.15(F [NeV]24)
RMIN =
F [NeV]14 − 0.15(F [NeV]14)
F [NeV]24 + 0.15(F [NeV]24)
We note that this is conservative, since some components
of the calibration errors should cancel in the ratio. Both
line fluxes were measured for 19 galaxies. In what fol-
lows we compare the line flux ratios measured in all but
one, MKN 266, for reasons that are discussed in detail
in Section 5.2. Of these 18 AGNs, 13 have ratios that
are consistent with the low density limit to within the
uncertainties, while only 2, both Type 1, have ratios sig-
nificantly above it. The remaining 3, all Type 2, have
ratios significantly below the low-density limit. Inter-
estingly, we note that a similar range of ratios was also
measured with the ISO SWS (Sturm et al. 2002, Alexan-
der et al. 1999). There are several possible explanations
for this finding. The observed, unphysically low ratios
could result from artifacts introduced by variations in
the slit sizes from which the line fluxes are obtained,
from calibration uncertainties, or from substantial mid-
IR extinction. Alternatively, perhaps important physical
processes were neglected in calculating the theoretical ra-
tios. In addition, errors in the collisional rate coefficients
for the [NeV] transitions associated with the mid-infrared
lines may be important. We explore these scenarios in
the following sections.
Observational Effects: Because the IRS LH slit is
larger than the SH slit, if the [NeV] emission is extended,
or multiple AGNs are present, the 14/24 µm line ratio
will be artificially reduced. However, since the ionization
potential of [NeV] is ∼ 97 eV, we expect that the [NeV]-
emitting gas is ionized by the AGN radiation field only
and is concentrated very close to the central source. Vir-
tually all of the [NeV] fluxes presented in this work were
obtained from IRS staring observations. Thus it is im-
possible to determine whether the emission is extended
using Spitzer observations alone. However, a number of
galaxies have been observed at 14 and 24 µm by ISO. In
Table 2 we list in addition to our Spitzer [NeV] fluxes, all
available [NeV] fluxes from ISO. The ISO aperture at 14
and 24 µm (14”×27”) is much larger than either the SH
or LH slits. In Figure 2 we plot the ratio of the [NeV] flux
measured by ISO to that measured by Spitzer for both
the 14 and 24 µm lines. The ranges of the [NeV] line
flux ratios are consistent with the instrument uncertain-
ties and are similar for all galaxies in the sample. Only
the 14µm ratio for Mrk 266 falls outside of the expected
range. This strongly suggests that the [NeV] emission is
indeed compact and originates in the NLR and that the
ratios are not affected by aperture variations, except for
Mrk 266 which is discussed in detail in Section 5.2.
If the data were affected by aperture variations we
would expect to see an overall systematic increase of
the 14µm/24µm line ratio with distance(See Figure 3).
The Spearman rank correlation coefficient (rS, Kendell
& Stuart 1976) corresponding to this plot is -0.069 (with
 [NeV] 14 micron Ratio
0.0 0.5 1.0 1.5 2.0 2.5
[NeV]
(ISO) / F
[NeV]
(Spitzer)
Mrk 266
[NeV] 24 micron Ratio
Fig. 2.— Ratio of the ISO to Spitzer [NeV] 14µm and 24µm
fluxes for those galaxies with overlapping observations. The range
indicated with arrows is that corresponding to the absolute flux
calibration for ISO (20%) and Spitzer (15%). Within the calibra-
tion uncertainties of the instrument, the [NeV] fluxes are virtually
the same for all of the galaxies except Mrk 266 (See Section 6.2).
This strongly suggests that the [NeV] emission is compact and orig-
inates in the NLR. We note that Sturm et al. 2002 find that the
[NeV] 24µm detection for NGC 7469 is questionable. The ISO to
Spitzer ratio for this galaxy (0.43) is the lowest shown here.
a probability of chance correlation of 0.78), where a co-
efficient of 1 or -1 indicates a strong correlation and a
coefficient of 0 indicates no correlation. Thus we find
that there is no correlation between the [NeV] ratio and
distance in our sample. However this does not completely
rule out aperture effects, if the size of the [NeV] emitting
region increases with the bolometric luminosity of the
AGN and the sample displays a significant trend in bolo-
metric luminosity with distance. In this case, a correla-
tion between the [NeV] ratio and distance would not be
apparent since aperture variations would affect all galax-
ies in the same way, regardless of distance. However this
scenario is unlikely since the size of the [NeV]-emitting
region would have to increase proportionately with dis-
tance in order to remain extended beyond the slit for all
galaxies. Nevertheless, we checked for this possibility,
both by examining the [NeV] ratio vs. bolometric lu-
minosity and by plotting the ratio vs. distance, binning
the galaxies according to their bolometric luminosity. We
find neither to be correlated over 5 orders of magnitude
in LBOL. Thus, in the case of the [NeV] line flux ratio,
we find no indication that ratios below the low density
limit are artifacts of aperture effects.
We point out that the [NeV] 24µm IRS line fluxes in the
small overlapping sample plotted in Figure 2 are system-
atically higher than the corresponding ISO-SWS fluxes,
despite the smaller IRS slit. This indicates that one or
both of the instruments is affected by systematic errors
more severe than are indicated by the calibration un-
certainty estimates. The SWS band 3D that includes
the [NeV] 24µm line was characterized by strong fring-
ing effects that when combined with the narrow range
of the line scan mode introduced sometimes large un-
0.5 1.0 1.5 2.0 2.5
Mrk 266
 = 0.069
log(Distance, Mpc)
 Seyferts
 Liners 
 Quasars 
Fig. 3.— The [NeV] 14µm/24µm ratio as a function of distance.
Open symbols signify Type 1 AGNs, Filled symbols signify Type 2
AGNs. The error bars shown here mark the calibration uncertain-
ties on the line ratio. If the ratio were indeed affected by aperture
variations we would expect a systematic increase of the ratio with
distance. As can be seen here, this is not the case, and we find no
indication that the low ratio is attribuable to aperture effects.
certainties in the baseline fitting, and therefore the line
flux measurement accuracy. In contrast, the baseline fit-
ting over the entire Spitzer IRS SH and LH full spectra
can be much more accurate. Moreover pointing accu-
racy and stability are an order of magnitude improved
over that obtained by ISO. We therefore assume in the
sections that follow that the adopted conservative Spitzer
IRS calibration uncertainties are accurate characteriza-
tions of the IRS measurements. Importantly, regardless
of which instrument is used, [NeV] ratios consistent with
the low density limit have been observed in a number of
sources with both ISO (e.g. Sturm et al. 2002 NGC 1365,
NGC 7582, NGC4151, NGC 5506; Alexander et al. 1999,
NGC 4151) and Spitzer (Weedman et al. 2005, Haas et
al. 2005, and this work).
Extinction: We consider the possibility that mid-IR
differential extinction toward the [NeV]-emitting regions
is responsible for the low [NeV] line ratios. Adopting the
low-density limit (LDL) for the intrinsic value of the ratio
([NeV]14µm/24µm ∼0.83 for ne≤200 cm
−3) for galaxies
with ratios below the LDL, the observed line ratio gives a
lower limit to the extinction, for a given MIR extinction
curve. We examined the visual extinctions correspond-
ing to the mid-IR differential extinction derived using
three separate extinction curves: 1) the Draine (1989)
extinction curve amended by the more recent ISO SWS
extinction curve toward the Galactic center for 2.5-10µm
(Lutz et al. 1996), 2) the Chiar & Tielens (2006) ex-
tinction curve for the Galactic Center using 2.38-40µm
ISO SWS observations of a bright IR source in the Quin-
tuplet cluster (GCS3-I) 3) the Chiar & Tielens (2006)
extinction curve for the local ISM using 2.38-40µm ISO
SWS observations of a WC-type Wolf-Rayet (WR) star
(WR98a). The Draine (1989) and Lutz et al. (1996)
extinction curve yields AV ∼ 3 to 99 mag (See Table
2). However, these values result from an extinction law
that is unexplored beyond 10µm. The Chiar & Tie-
lens (2006) Galactic center extinction curve cannot ex-
plain the observed [NeV] ratios since the extinction at
24µm is greater than the extinction at 14µm, so we do
not discuss it further. The visual extinction resulting
from their local ISM extinction curve is unrealistically
high (AV median=500mag). The calculated extinction ob-
tained using the Draine (1989), Lutz et al. (1996), and
Chiar & Tielens (2006) local ISM extinction curves are
given in Table 2.
The AV derived from the two extinction curves de-
scribed above are extremely high in many cases. Even
if the extinction is calculated from the upper limit on
the ratio to the LDL for the three galaxies whose upper
limits are below the LDL, the corresponding visual ex-
tinction is still very high (for the Draine 1989 and Lutz
1996 extinction curve AV = 21, 26, and 30 mag for these
three galaxies; for the Chiar and Tielens extinction curve
AV = 260, 330, and 370 mag). However, we caution the
reader that the actual value for extinction is highly un-
certain. Indeed very little is known about the 8-40µm
extinction curve in AGNs. Specifically, the 10 and 18
µm silicate features in this band are the source of in-
consistency. Even within the AGN class, extinction may
vary dramatically from 8-40µm because of variations in
the silicate features due to differences in grain size, poros-
ity, shape, composition, abundance, and location in each
galaxy. Hao et al. (2005) show that in five AGNs (4
of which are in our sample), both silicate features vary
considerably in strength and width. Sturm et al. (2005)
also show that the standard ISM silicate models do not
accurately fit NGC 3998, a LINER with silicate emis-
sion. Sturm et al. (2005) suggest that increased grain
size and possibly the presence of crystalline silicates such
as clino-pyroxenes may improve the fit, but that clearly
circumnuclear dust in AGNs has very different proper-
ties than dust in the Galactic ISM (see also Maiolino et
al. 2001a, 2001b, but Weingartner & Murray 2002 for
an alternative view). Chiar & Tielens (2006) even show
that the GC observations and the local ISM observations
within the Galaxy deviate from each other most dramat-
ically in the wavelength region between the two silicate
absorption features. In their observations, this is the re-
gion between ∼ 12-15µm -directly overlapping with the
14µm values in which we are interested. Because of ir-
regularity of the silicate features in the mid-IR, it is very
difficult to interpret the true extinction there. Moreover,
in addition to the uncertainty in the extinction law, the
geometry of the obscuring material is unknown and can
vary substantially from galaxy to galaxy. The most that
can be said here for the galaxies with ratios below the
LDL is that if extinction is responsible for the low ratios,
then the extinction must be less at 24µm than at 14µm.
Physical Processes: It is possible that important
physical processes have been neglected in calculating the
[NeV] line luminosity ratio as a function of electron den-
sity shown in Figure 1. We consider three physical pro-
cesses that may affect the line ratios:
(1) A source of gas heating in addition to photoioniza-
tion (e.g., shocks, turbulence) that may yield gas temper-
atures substantially higher than 104K. As can be seen
in Figure 1, higher gas temperatures do not yield sig-
nificantly lower line ratios in the low-density limit, but
could explain the generally low values of the ratios that
lie above the LDL.
(2) Pumping from the ground term to the first excited
term, e.g., by O III resonance lines. The specific energy
density required for this to significantly affect the line
ratio exceeds 10−14 erg cm−3 Hz−1, which is implausibly
large by orders of magnitude.
(3) Absorption and stimulated emission within the
ground term, which could be important if, e.g., a large
quantity of warm dust yielding copious 24µm continuum
emission is located close to the [NeV]-emitting region.
Figures 4a through 4e show the line ratio as a func-
tion of the specific energy density at 24µm, uν(24µm).
We display results for electron density ne = 10
2, 103,
104, 105, and 106 cm−3; gas temperature T = 104, 105,
and 106K; and ratio of the specific energy density at 14
and 24µm, uν(14µm)/uν(24µm) = 0.4, 1.0, and 1.8 (val-
ues were chosen to reproduce the observed range of the
14µm/24µm continuum flux ratios; see Section 5).
For the moment, assume that the NeV is located suf-
ficiently far from the source of the 14 and 24µm contin-
uum emission to treat the source as a point. If hot dust
within or near the inner edge of the torus is responsible
for this emission, then this assumption requires that the
distance to the NeV, rNe, be large compared with the
dust sublimation radius, rsub ∼ 1 pcL
bol, 46 (Ferland et
al. 2002); Lbol, 46 is the bolometric luminosity in units of
1046 erg s−1. In this case, we can obtain a simple estimate
of uν(24µm) at the location of the [NeV]-emitting region
from the observed specific flux Fν(24µm), the distance
to the galaxy D, and rNe:
uν(24µm) ∼
Fν(24µm)
. (3)
With rNe = 100 pc, uν(24µm) estimated in this way
ranges from ≈ 10−24 erg cm−3 Hz−1 to somewhat less
than 10−20 erg cm−3 Hz−1 for the galaxies in our sam-
ple. These can be compared to the results of Hönig
et al. (2006), who modeled the infrared emission from
clumpy tori. They presented plots of Fν at a distance
of 10Mpc from an AGN with bolometric luminosity
Lbol = 4 × 10
45 erg s−1. Extrapolating to a distance
of 100 pc, we find uν(24µm) as large as a few times
10−21 erg cm−3 Hz−1, close to the estimate for the most
luminous AGN in our sample.
From Figures 4a through 4e, we see that the infrared
continuum can only reduce the line ratio significantly at
rNe ≈ 100 pc if T & 10
5K when ne ∼ 10
2 cm−3 and
T & 106K when ne ∼ 10
3 cm−3. However, the NeV, as
a high-ionization species, may lie closer to the central
source than does the bulk of the narrow line region. If
rNe ≈ 10 pc, then uν(24µm) increases by a factor ∼ 100.
In this case, the observed low line ratios can be explained
by this mechanism with T ∼ 104K, if ne ∼ 10
2 cm−3.
Higher values of electron density would require higher
gas temperatures.
In Section 5.1, we suggest that the [NeV]-emitting re-
gion may lie within the torus. In this case, absorp-
tion and stimulated emission within the ground term
are probably important. For the high-luminosity objects,
these may even dominate over collisional excitation and
de-excitation. At these central locations, gas tempera-
tures T ∼ 106 K may be natural (Ferland et al. 2002).
Relatively high densities may also be expected, in which
case the infrared continuum may not appreciably depress
the line ratio (see Figure 4d).
Adopting the Mathews & Ferland (1987) spectrum and
T ≈ 106K, the ionization parameter U ≡ nγ/ne ∼ 10
in order for a substantial fraction of the Ne to be NeV;
nγ is the number density of H-ionizing photons. For
this spectrum, nγ ≈ 1.7×10
3Lbol, 46 r
Ne, 100 cm
−3, where
rNe, 100 = rNe/100 pc. If rNe = 1pc, then either (1)
ne ∼ 10
10Lbol,46 cm
−3 or (2) the nuclear continuum is
filtered through a far-UV/X-ray-absorbing medium be-
fore reaching the [NeV]-emitting region.
If absorption and stimulated emission are indeed rele-
vant processes in [NeV] line production, we might expect
a relationship between the [NeV] line flux ratio and the
24 µm continuum luminosity that is consistent with one
of the curves shown in Figures 4a through 4e. In Figure
5 we plot this relationship for the [NeV] emitting galax-
ies in our sample. As can be seen in Figure 5, we find
no relationship between the [NeV] line flux ratio and the
24µm continuum luminosity for our sample of galaxies.
The Spearman rank correlation coefficient for this plot
is -0.01 (probability of chance correlation = 0.95), indi-
cating no correlation. As a result, as can be seen from
Figures 4a and 4b, stimulated emission and absorption
at low densities can be ruled out as possible scenarios be-
cause the scatter plot shown in Figure 5 does not follow
the model predictions. We note that the location of the
[NeV]-emitting region relative to the source of the 24µm
continuum emission is uniform among the galaxies in the
sample. Variations in the location might obscure any
correlation in these plots. Figures 4c and 4d reveal that,
for some values of ne, T, and uν(14µm)/uν(24µm), the
line ratio is very insensitive to the value of uν(24µm). In
these cases, the line ratio remains above ∼ 0.8. Thus, al-
though absorption and stimulated emission may be con-
tributing processes to [NeV] production, another mecha-
nism is required to explain the low (<0.8) [NeV] line flux
ratios in our sample.
Computed Quantities: Finally, it is possible that
there is significant error in the adopted collisional rate
coefficients. The accuracy of collisional strengths of in-
frared atomic transitions has been a longstanding ques-
tion. We adopt the collisional rate coefficients from the
state of the art IRON project (Hummer et al. 1993)
which produced the most up-to-date and accurate colli-
sion strengths for a large database of atomic transitions.
While these calculations have been questioned based on
recent ISO observations of nebulae (Clegg et al. 1987,
Oliva et al. 1996, Rubin et al. 2002, Rubin 2004), it is
likely that the discrepancies between the observational
and theoretical values can be explained by inaccuracies
in the fluxes employed (van Hoof et al. 2000). Uncer-
tainties in the collisional rate coefficients for the [NeV]
transitions are unlikely to exceed 30% (van Hoof et al.
2000). It is therefore unlikely that the low critical den-
sities implied by our data can be attributed to uncer-
tainties in the theoretical values of the [NeV] collisional
strengths.
5. EXTINCTION EFFECTS OF THE TORUS AND AGN
UNIFICATION
Although low electron densities, high gas tempera-
tures, and/or high infrared radiation densities may play
Fig. 4.— The [NeV] line ratio as a function of the specific energy density at 24µm, uν(24µm), for temperatures, T = 10
4, 105, 106 K,
for 14µm/24µm continuum ratios of 0.4, 1.0, and 1.8, and finally for electron densites, ne = 10
2, 103, 104, 105, 106 cm−3.
Table 2: NeV Line Fluxes and Derived Extinction
Galaxy [NeV] [NeV] [NeV] [NeV] [NeV] AV AV AV AV
Source 14.32 14.32 24.32 24.32 Ratio Ratio to Ratio to Ratio to Ratio to
SH ISO LH ISO LDL (D&L) HDL (D&L) LDL (C&T) HDL (C&T)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Seyferts
NGC4151 7.77a 5.50c 6.77a 5.60c 1.15
+0.41
−0.30
· · · 126 · · · 1686
NGC1365 2.20±0.06 2.50c 5.36±0.06 3.90c 0.41
+0.14
−0.11
45 189 570 2519
NGC1097 <0.05 · · · <0.18 · · · · · · · · · · · · · · · · · ·
NGC7469 1.16a <1.50c 1.47a 0.63c∗ 0.79
+0.28
−0.21
3 149 41 1990
NGC4945 0.28±0.03 <0.50d <0.75 · · · >0.38 · · · · · · · · · · · ·
Circinus 23.94±0.61 31.70c 24.00±3.90 21.80c 1.00
+0.35
−0.26
· · · 135 · · · 1799
Mrk 231 <0.44a <1.50e <0.69a · · · · · · · · · · · · · · · · · ·
Mrk3 6.45a 4.60c 6.75a 3.40c 0.96
+0.34
−0.25
· · · 138 · · · 1835
Cen A 2.32a 2.70c 2.99a 2.00c 0.77
+0.27
−0.20
4 150 56 2005
Mrk463 1.83b 1.40c 2.04b · · · 0.90
+0.32
−0.23
· · · 141 · · · 1886
NGC 4826 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
NGC 4725 <0.09 · · · 0.09±0.03 · · · <1.04 · · · · · · · · · · · ·
1 ZW 1 <0.11a 0.27c <0.10a · · · · · · · · · · · · · · · · · ·
NGC 5033 0.07±0.02 · · · 0.11±0.02 · · · 0.65+0.23
−0.17
16 161 198 2146
NGC1566 0.16±0.05 · · · 0.22±0.04 · · · 0.74+0.26
−0.19
7 153 92 2041
NGC 2841 <0.04 · · · <0.03 · · · · · · · · · · · · · · · · · ·
NGC 7213 <0.04 · · · <0.09 · · · · · · · · · · · · · · · · · ·
LINERs
NGC4579 <0.06 · · · <0.03 · · · · · · · · · · · · · · · · · ·
NGC3031 <0.06 · · · <0.04 · · · · · · · · · · · · · · · · · ·
NGC6240 0.51b <1.00e <0.39b · · · <1.31 · · · · · · · · · · · ·
NGC5194 0.41±0.04 <0.20c 0.39±0.09 · · · 1.06
+0.37
−0.28
· · · 131 · · · 1751
MRK266∗∗ 0.21±0.02 0.50f 1.19±0.06 · · · 0.18
+0.06
−0.05
100 240 1254 3203
NGC7552 <0.11 · · · <0.83 · · · · · · · · · · · · · · · · · ·
NGC 4552 <0.06 · · · <0.07 · · · · · · · · · · · · · · · · · ·
NGC 3079 <0.07a · · · <0.14a · · · · · · · · · · · · · · · · · ·
NGC 1614 <0.28 · · · <1.49 · · · · · · · · · · · · · · · · · ·
NGC 3628 <0.06 · · · <0.34 · · · · · · · · · · · · · · · · · ·
NGC 2623 0.30±0.04 · · · 0.47±0.07 · · · 0.63
+0.22
−0.14
17 163 218 2167
IRAS23128· · · 0.22±0.02 <0.40e 0.34±0.10 · · · 0.65
+0.23
−0.22
16 161 203 2152
MRK273 1.06±0.05 0.82e 2.74±0.19 · · · 0.39
+0.14
−0.10
49 192 617 2565
IRAS20551· · · <0.06 <0.25e <0.25 · · · · · · · · · · · · · · · · · ·
NGC3627 0.08±0.01 · · · 0.19±0.05 · · · 0.45+0.16
−0.12
40 184 504 2453
UGC05101 0.52b <1.50e 0.49b · · · 1.06+0.37
−0.28
· · · 131 · · · 1750
NGC4125 <0.03 · · · <0.07 · · · · · · · · · · · · · · · · · ·
NGC 4594 <0.03 · · · <0.04 · · · · · · · · · · · · · · · · · ·
Quasars
PG1351· · · <0.04 · · · <0.07 · · · · · · · · · · · · · · · · · ·
PG1211· · · 0.04±0.007 · · · <0.04 · · · >1.01 · · · · · · · · · · · ·
PG1119· · · 0.30±0.06 · · · 0.22±0.02 · · · 1.39
+0.49
−0.36
· · · 115 · · · 1531
PG2130· · · 0.42±0.03 · · · 0.42±0.05 · · · 1.00
+0.35
−0.26
· · · 135 · · · 1798
PG0804· · · <0.06 · · · <0.07 · · · · · · · · · · · · · · · · · ·
PG1501· · · 0.78±0.02 · · · 0.83±0.02 · · · 0.94
+0.33
−0.25
· · · 138 · · · 1846
Columns Explanation: Col(1):Common Source Names; Col(2): 14.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from
Spitzer; Col(3): 14.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(4): 24.31 µm [NeV] line flux and statistical
error in units of 10−20 W cm−2 from Spitzer; Col(5): 24.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(6):
[NeV] Line Ratio used in plots and calculations; Col(7): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL,
calculated using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µm
(Lutz et al. 1996); Col(8): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density limit (HDL), calculated
using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µm (Lutz
et al. 1996), Col(9): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL, calculated using the Chiar & Tielens
(2006) extinction curve for the local ISM, Col(10): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density
limit (HDL), calculated using the Chiar & Tielens (2006) extinction curve for the local ISM.∗ Sturm et al. 2002 find that the [NeV] 24µm detection
for NGC 7469 is a questionable one, ∗∗ As discussed in detail in Section 6.2, Mrk 266 is the only galaxy in our sample where we find that aperture
variation may affect the observed [NeV] line flux ratio. For this reason it has been excluded from relevent plots and calculations. References for
Table 2:
a Weedman et al. 2005, b Armus et al. 2004 & 2006, c Sturm et al. 2002, d Verma et al. 2003, e Genzel et al. 1998, f Prieto & Viegas
Table 3: SIII Line Fluxes and Derived Extinction
Galaxy [SIII] [SIII] [SIII] [SIII] [SIII] Av Av Av Av PAH6.2
Source 18.71 18.71 33.48 33.48 Ratio Ratio to Ratio to Ratio to Ratio to SL2
SH ISO LH ISO LDL (D&L) HDL (D&L) LDL (C&T) HDL (C&T)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Seyferts
NGC4151 7.50a 5.40c 6.57a 8.10c 1.14
+0.40
−0.30
· · · · · · · · · · · · 168.1
NGC1365 5.73±0.05 13.50c 27.20±0.38 36.10c 0.21
+0.07
−0.05
· · · · · · · · · · · · 132.3
NGC1097 2.18±0.02 · · · 11.40±0.23 · · · 0.19
+0.07
−0.05
· · · · · · · · · · · · 151.7
NGC7469 7.70a 9.20c 9.80a 10.40c 0.79
+0.28
−0.20
· · · · · · · · · · · · 415.0
NGC4945 3.18±0.03 6.30d 38.70±1.80 51.40d 0.08
+0.03
−0.02
· · · · · · · · · · · · 671.7
Circinus 19.10±0.70 35.20c 56.30±3.31 93.20c 0.37
+0.13
−0.10
· · · · · · · · · · · · 1018.4
Mrk 231 <0.47a <3.00e <2.30a <3.00e · · · · · · · · · · · · · · · 175.6
Mrk3 5.55a · · · 5.25a · · · 1.06
+0.37
−0.28
· · · 83 · · · 72 18.4
Cen A 4.54a 6.40c 14.80a 22.30c 0.31
+0.11
−0.08
· · · · · · · · · · · · 220.8
Mrk463 1.50b <0.80c 1.35b 1.20c 1.11
+0.39
−0.29
· · · 81 · · · 70 55.4
NGC 4826 3.39±0.03f · · · 4.61±0.08f · · · 0.74
+0.26
−0.19
· · · 96 · · · 82 66.2
NGC 4725 0.02±0.02f · · · 0.11±0.02f · · · 0.23
+0.08
−0.06
23 135 20 116 3.7
1 ZW 1 <0.11a <0.50c <0.18a <1.00c · · · · · · · · · · · · · · · 22.2
NGC 5033 0.85±0.11 · · · 2.42±0.10 · · · 0.35
+0.12
−0.09
· · · · · · · · · · · · 33.9
NGC1566 0.55±0.05f · · · 0.55±0.06f · · · 1.00
+0.35
−0.26
· · · 85 · · · 73 61.0
NGC 2841 0.22±0.04f · · · 0.29±0.03f · · · 0.75
+0.26
−0.20
· · · 95 · · · 82 10.9
NGC 7213 0.47±0.05 · · · 0.59±0.06 · · · 0.80
+0.28
−0.21
· · · · · · · · · · · · 28.7
LINERs
NGC4579 0.32±0.06f <0.78g 0.24±0.03f <1.20g 1.33
+0.47
−0.35
· · · 75 · · · 65 17.3
NGC3031 0.61±0.03 · · · 0.09±0.09 · · · 0.67
+0.24
−0.18
· · · · · · · · · · · · 23.4
NGC6240 1.99b <4.00e 2.63b 4.50e 0.76
+0.27
−0.20
· · · 95 · · · 81 399b
NGC5194 1.06±0.05f 1.00d 1.48±0.03f 4.60d 0.72
+0.25
−0.19
· · · 96 · · · 83 26.4
MRK266∗∗ 1.00±0.13 · · · 4.65±0.09 · · · 0.21
+0.08
−0.06
25 138 22 118 23.1
NGC7552 17.11±0.08f 24.60d 13.38±0.41f 41.10d 1.28
+0.45
−0.33
· · · 77 · · · 66 872.0
NGC 4552 0.07±0.03f · · · 0.06±0.02f · · · 1.29
+0.45
−0.34
· · · 76 · · · 66 21.5
NGC 3079 1.25a 6.80g 6.08a 6.60g 0.21
+0.07
−0.05
· · · · · · · · · · · · 620.3
NGC 1614 9.63±0.27 · · · 11.60±0.43 · · · 0.83
+0.29
−0.15
· · · 91 · · · 79 508.5
NGC 3628 2.14±0.03 · · · 15.80±0.33 · · · 0.14
+0.05
−0.04
· · · · · · · · · · · · 430.4
NGC 2623 0.88±0.05 · · · 3.16±0.20 · · · 0.28
+0.10
−0.07
16 129 14 111 128.6
IRAS23128· · · 2.62±0.12 0.89e 2.11±0.18 2.80e 1.24
+0.44
−0.32
· · · 78 · · · 67 90.1
MRK273 1.24±0.07 <0.82e 3.88±0.40 2.30e 0.32
+0.11
−0.08
12 124 10 107 69.2
IRAS20551· · · 0.66±0.06 0.30e 1.18±0.13 1.40e 0.56
+0.20
−0.15
· · · 105 · · · 90 38.3
NGC3627 0.38±0.03f · · · 0.57±0.09f · · · 0.67
+0.24
−0.18
· · · 99 · · · 85 153.8
UGC05101 0.98b <1.40e 1.30b 2.50e 0.75
+0.27
−0.20
· · · 95 · · · 82 190b
NGC4125 · · ·f · · · 0.06±0.05f · · · · · · · · · · · · · · · · · · 14.9
NGC 4594 0.39±0.03 · · · 1.24±0.13 · · · 0.32
+0.11
−0.08
· · · · · · · · · · · · 14.2
Quasars
PG1351· · · 0.34±0.06 · · · <0.13 · · · >2.70 · · · · · · · · · · · · 29.6
PG1211· · · <0.06 · · · <0.08 · · · · · · · · · · · · · · · · · · 25.8
PG1119· · · <0.13 · · · 0.19±0.06 · · · <0.71 · · · · · · · · · · · · 8.7
PG2130· · · <0.19 · · · 0.34±0.06 · · · <0.55 · · · · · · · · · · · · 26.9
PG0804· · · <0.06 · · · <0.21 · · · · · · · · · · · · · · · · · · 27.4
PG1501· · · 0.67±0.15 · · · 0.41±0.05 · · · 1.64
+0.58
−0.43
· · · 68 · · · 59 19.2
Columns Explanation: Col(1):Common Source Names; Col(2): 18.71 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from
Spitzer; Col(3): 18.71 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(4): 33.48 µm [SIII] line flux and statistical
error in units of 10−20 W cm−2 from Spitzer; Col(5): 33.48 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(6):[SIII]
line flux ratio used for plots and calculations; Col(7): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL, calculated
using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µ (Lutz et
al. 1996) for those galaxies with distances greater than 55 Mpc that are not effected by aperture variations, Col(8): Extinction required to bring
ratios below the low-density limit (LDL) up to the high-density limit (HDL), calculated using the Draine (1989) extinction curve amended by the
more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µ (Lutz et al. 1996) for those galaxies with distances greater than 55
Mpc that are not effected by aperture variations, Col(9): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL,
calculated using the Chiar & Tielens (2006) extinction curve for the Galactic Center for those galaxies with distances greater than 55 Mpc that
are not effected by aperture variations, Col(10): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density limit
(HDL), calculated using the Chiar & Tielens (2006) extinction curve for the Galactic Center for those galaxies with distances greater than 55 Mpc
that are not effected by aperture variations, Col(11): 6.2 µm PAH line flux in units of 10−21 W cm−2 ∗∗ The [SIII] ratio for Mrk 266 is known
to be affected by aperture variations(See Section 6.2). For this reason it has been excluded from relevent plots and calculations. References for
Table 3:
a Weedman et al. 2005, b Armus et al. 2004 & 2006, c Sturm et al. 2002, d Verma et al. 2003, e Genzel et al. 1998, f Dale et al. 2006, g
Satyapal et al. 2004.
32 33 34 35 36 37 38
 = -0.01
log(24 m Specific Luminosity) (W m-1)
 14/24 < 0.50
 0.50 < 14/24 < 1.0
 14/24 > 1.0
Fig. 5.— The observed [NeV] line ratio as a function of the 24µm
specific luminosity for our sample of galaxies. The error bars shown
here represent the calibration uncertainties on the [NeV] line flux
ratio as in Figure 3. The symbol type indicates the 14µm/24µm
continuum ratio.
a role in lowering the [NeV] line flux ratio, we argue that
differential infrared extinction to the [NeV] emitting re-
gion due to dust in the obscuring torus is responsible
for the low line ratios in at least some AGN. Clearly,
this requires that there is significant extinction at mid-IR
wavelengths, and specifically toward the [NeV]-emitting
regions. Is this reasonable? If there is significant ex-
tinction, it is possible that: 1) the [NeV]-emitting region
originates much closer to the central source than previ-
ously recognized, close enough to be extinguished by the
central torus in some galaxies, 2) the [NeV]-emitting por-
tion of the NLR is obscured by dust in the host galaxy
or in the NLR itself, or 3) some combination of these
scenarios. We explore these possibilities in the following
analysis.
5.1. The [NeV] originates in gas interior to the central
torus.
In the conventional picture of an AGN, the broad line
region (BLR) is thought to exist within a small region
interior to a dusty molecular torus while the NLR origi-
nates further out. This of course is the paradigm invoked
to explain the Type 1/Type 2 dichotomy. However there
have been multiple optical spectroscopic studies that con-
tradict the assumption that the observational properties
of the NLR are not dependent on the viewing angle and
the inclination of the system, suggesting that some of
the narrow emission lines originate in gas interior to the
torus. For instance, Shuder and Osterbrock (1981) and
Cohen (1983) showed that narrow high ionization forbid-
den lines such as [Fe VII] λ 6374 (requiring photons with
energies ≥ 100eV to ionize) are stronger relative to the
low ionization lines in Seyfert 1 galaxies (including inter-
mediate Seyferts, 1.2, 1.5 etc.) than in Seyfert 2 galaxies,
suggesting that some of the emission is obscured by the
torus. In addition, [FeX] λ 6374 and [NeV] λ 3426 have
also been shown to be less luminous in Type 2 objects
than in Type 1 objects (Murayama & Taniguchi, 1998a;
Schmitt 1998, Nagao et al. 2000, 2001a, 2001b, 2003,
Tran et al. 2000, see also Jackson and Browne (1990) for
narrow line radio galaxies and quasars.) These findings
may imply that the emission lines of species with the
highest ionization potentials originate closer to the AGN
than those of lower ionization species such as [OII]λ3727,
[SII]λλ6716, 6731, [OI] λ6300 etc. and therefore may be
partially obscured by the central torus.
If there is considerable extinction to the line-emitting
regions due to the torus, one may expect the mid-infrared
continuum to be similarly obscured. To test this scenario
we divided our sample into Type 1 or Type 2 objects
based on the presence or absence of broad (full width at
half max (FWHM) exceeding 1000 km s−1) Balmer emis-
sion lines in the optical spectrum. The spectral classifi-
cation for the [NeV]-emitting galaxies is given in Table 1.
In Figure 6a, we plot the [NeV] 14µm/24µm line flux ra-
tio versus the 14µm/24µm continuum ratio of the [NeV]
emitting galaxies in our sample. Assuming there is no
correlation between the electron density and the contin-
uum shape, a correlation between the line flux and con-
tinuum ratios would suggest that the mid-IR extinction
associated with the torus (such as that found by Clavel et
al. 2000) affects the observed line flux ratios. As can be
seen, there is a correlation between the line and contin-
uum ratios for galaxies with [NeV] emission. Moreover
we note that the 3 nuclei with ratios significantly below
the LDL are all Type 2 AGNs, while the 2 that lie sig-
nificantly above this limit are Type 1 AGNs, suggestive
that the extinction of the [NeV]-emitting region in Type
2 AGNs may be due to the torus. We note that the
error bars displayed in Figure 6 are based on a conserva-
tive estimate (15%) of the absolute calibration error on
the flux (see Section 3). Moreover, we have adopted the
most conservative approach in propagating the error (see
Section 4) for each line ratio. We further note that two
of the three nuclei with ratios below the LDL were also
observed in high-accuracy peak-up mode, resulting in a
pointing accuracy on the continuum for these galaxies of
0.4”. The third galaxy, NGC 3627, was observed in high
resolution mapping mode over 15” X 22”. We extracted
the spectra and found that the full map and the single
slit fluxes agree to within 10%. Thus pointing errors do
not appear to be responsible for the low ratios in these
galaxies. Finally we find that the ratios for the all galax-
ies except Mrk 266 are not sensitive to the line-fitting or
flux extraction methods that we have employed.
The Spearman rank correlation coefficient for Figure 6
is 0.60 (with a probability of chance correlation of 0.008),
indicating a significant correlation between the [NeV] line
flux ratios and the mid-IR continuum ratio. We note
that some AGNs are known to contain prominent sili-
cate emission features (Hao et al. 2005, Sturm et al.
2006) which have not been disentangled from the under-
lying continuum in this study. Because of this, the 14µm
or 24µm flux may be overestimated in some cases mak-
ing intrinsic value of the continuum at 14µm and 24µm
somewhat uncertain. However only one galaxy plotted
in Figure 6 is currently known to contain such features
(PG1211+143, Hao et al. 2005). Variations in ne and
the underlying continuum shape will also add scatter to
the correlation, as will differences in extinction to the
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
 = 0.60
F (14 m) / F (24 m) Continuum
Type 1 Objects
Type 2 Objects
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
 D < 55 Mpc
 D > 55 Mpc
F (14 m) / F (24 m) Continuum
Fig. 6.— The [NeV] line ratio vs. the Fν(14µm)/Fν (24µm) continuum ratio for our sample. In both plots, the error bars mark the
calibration uncertainties on the line ratio. There is a correlation between the line and continuum ratios which suggests that extinction
affects the observed line flux ratios. 6a) The majority of galaxies with ratios below the LDL are Type 2 objects, implying that the extinction
toward the [NeV]-emitting region may be due to the torus. 6b) The correlation shown here is not an artifact of aperture vatiations between
the SH and LH slits. The correlation holds when only the most distant galaxies are considered.
line- and continuum-producing regions. We should note
that the correlation seen in Figure 6 is not an artifact
of aperture variations between the LH and SH slit. The
correlation holds when only the most distant galaxies
(closed symbols in Figure 6b) are considered.
Independent of this correlation, our most important
finding is that the [NeV] line flux ratio is significantly
lower for Type 2 AGNs than it is for Type 1 AGNs. Fig-
ure 7 shows the relative [NeV] flux ratios for the Type
1 and Type 2 objects in our sample. The mean ratios
are 0.97 and 0.72 for the eight Type 1 and ten Type 2
AGNs, respectively, with uncertainties in the mean of
about 0.08 for each. Interestingly, although the sample
size is limited, precluding us from drawing firm statisti-
cally significant conclusions, there is a similar suggestive
trend seen in the sample of AGNs observed by Sturm et
al. (2002) with ISO-SWS. That is, in their work, the two
galaxies with the lowest [NeV] flux ratios are NGC 1365
and NGC 7582, both Type 2 AGNs. The galaxy with
the highest ratio in their work is TOL 0109-383 , a Type
1 AGN.
If indeed the torus obscures the IR [NeV] emission
in Type 2 objects, one would expect the optical/UV
[NeV] emission in these objects to be obscured as well.
We searched the literature for optical/UV detections of
[NeV] λ3426 for all of the galaxies in our sample and
found five galaxies with observations at this wavelength.
Four of these galaxies (Mrk 463, Mrk 3, NGC 1566, and
NGC 4151) were detected at [NeV] λ3426; the other
(NGC 3031) was not detected (see, Kuraszkiewicz et al.
2002, 2004 and Forster et al. 2001 for optical/UV fluxes).
Of the four galaxies with optical/UV [NeV] detections,
two are Type 1 galaxies (NGC 1566 & NGC 4151) and,
surprisingly, two are Type 2 galaxies (Mrk 3 and Mrk
463). If the Type 2 galaxies Mrk 3 and Mrk 463 had
[NeV] emitting regions interior to the torus, then the op-
 Type 1 AGN
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75
[NeV]14 m/[NeV]24 m 
 Type 2 AGN
Fig. 7.— Histogram of the [NeV] 14µm/24µm line flux ratio as
a function of AGN type. The [NeV] line ratios for Type 2 AGNs
are consistently lower than those from Type 1 AGNs.
tical/UV lines in these objects should not be detected
due to severe obscuration. We note that Mrk 3 and Mrk
463 have some of the highest X-ray luminosities (both
∼ 1043 erg s−1 ) in the sample and mid-IR [NeV] ra-
tios that are comparable to similarly luminous Type 1
objects–consistent with little or no obscuration in the
mid-IR for these Type 2 galaxies. This finding may im-
ply that, in the most powerful AGNs, the [NeV] emitting
region is pushed beyond the torus because the radiation
field is so intense, while lines with higher ionization po-
tentials than [NeV] (such as [NeVI], [FeX] etc.) are still
concealed by the torus. More data, both from the mid-
IR and from the optical/UV, are needed to further test
this hypothesis.
5.2. The [NeV]-emitting region is obscured by the host
galaxy or dust in the NLR.
While the correlation in Figures 6a and 6b are very
promising explanations for the observed [NeV] line ra-
tios, it is not completely clear why some Type 2 galaxies
appear obscured and others may not. Perhaps the [NeV]
emission is attenuated by dust in the NLR itself or else-
where in the host galaxy. Indeed, it is well-known that
dust does exist in the NLR (e.g., Radomski et al. 2003,
Tran et al. 2000) and that it can be extended and patchy
(e.g Alloin et al. 2000; Galliano et al. 2005; Mason et
al. 2006). In addition, dust in the host galaxy could be
responsible for the extinction seen here. For complete-
ness, we have conducted a detailed archival analysis of
all of the galaxies in our sample with [NeV] ratios close
to or below the LDL in order to see if there is additional
evidence for high extinction either in the host galaxy or
within the NLR. We find that the majority of galaxies
with low densities do indeed have well-known dust lanes,
large X-ray inferred column densities, or other properties
indicative of extinction.
Cen A: This nearby (D = 3.4Mpc) early type (S0)
galaxy at one time devoured a smaller gas-rich spiral
galaxy (Israel 1998, Quillen 2006). There is clear evi-
dence for substantial obscuration toward the nucleus of
Cen A. For example, the central region is veiled by a well
known dense dust lane thought to be a warped thin disk
(Ebneter & Balick 1983, Bland et al. 1986, 1987, Nichol-
son et al. 1992, Sparke 1996, Israel 1998, Quillen et al.
2006). Schreier et al. (1996) find V-band extinction av-
eraging 4-5 mag and infrared observations by Alonso &
Minniti yield AV values exceeding 30 mag in some re-
gions. Thus, it is plausible that there are regions toward
the nucleus of Cen A that are obscured even at infrared
wavelengths.
NGC 1566: The optical nuclear spectrum of this
nearby galaxy is known to vary dramatically over a pe-
riod of months, changing its optical classification from a
Type 2 object to a Type 1 object and back again (Pastor-
iza & Gerola 1970, de Vaucouleurs 1973, Penfold 1979,
Alloin et al. 1985). The narrow optical lines in this ob-
ject also show prominent blue wings and the radio prop-
erties of this galaxy are more consistent with a Type 2
object than a Type 1 object (Alloin et al. 1985). HST
continuum imagery reveals spiral dust lanes within 1” of
the nucleus (Griffiths et al. 1997) which might be re-
sponsible for the Type 1/Type 2 variability. Baribaud et
al. (1992) find hot dust which lies just outside the broad
line region in this galaxy and a large covering factor that
might explain the steep continuum of the AGN. Ehle et
al. (1996) find NH ∼ 2.5 × 10
20 cm−2 from ROSAT
X-ray observations of this galaxy.
NGC 2623: This galaxy’s tidal tails are evidence of a
merger event, however infrared observations reveal a sin-
gle symmetric nucleus, implying that the merging galax-
ies have coalesced. Multi-color, near infrared observa-
tions reveal strong concentrations of obscuring material
in the central 500 pc.(Joy & Harvey 1987; Lipari et al.
2004). Lipari et al. (2004) also find an optically obscured
nucleus with V-band extinction ≥ 5 mag.
IRAS 23128-5919: This galaxy is also in the late
stages of a merger. The nuclei of the two galaxies are
4kpc apart and have not yet coalesced. The northern
nucleus is a starburst. The southern nucleus is a known
AGN, though its optical classification, Seyfert or LINER,
is unclear (Duc, Mirabel, & Maza 1997; Charmandaris
et al. 2002; Satyapal et al. 2004). IRAS 23128-5919 is
an ultraluminous infrared galaxy (ULIRG), clearly con-
sistent with the presence of substantial dust towards the
nucleus. Optical spectroscopy of the southern nucleus in-
dicates very large (1500 km s−1) blue asymmetries in the
Hβ and [OIII] lines. This blue wing could be a signature
of extinction toward the far side of an expanding region,
where the red wing is preferentially obscured. (Johans-
son & Bergvall 1988).
Mrk 273: This galaxy is also a ULIRG, so significant
dust obscuration toward the nucleus is expected. Near-
IR imaging and high resolution radio observations show
evidence for a double nucleus in this galaxy separated
by less than 1 kpc (Ulvestad & Wilson 1984; Mazzarella
et al. 1991; Majewski et al. 1993). However, high res-
olution Chandra observations reveal only the northern
of the two nuclei, suggesting that this galaxy is hosting
only one AGN and that perhaps the other ”nucleus” is
in fact a portion of the southern radio jet. The soft X-
ray emission from the northern nucleus is obscured by
column densities of at least 1023 cm−2 (Xia et al. 2002).
Although the X-ray-emitting regions are physically dis-
tinct from the NLR and some of the obscuration at X-ray
wavelengths likely arises in dust-free gas within the sub-
limation radius, the high column density derived may
be consistent with high extinction toward the central re-
gions of this galaxy. Though Xia et al (2002) find that
the X-ray morphology of the AGN in Mrk 273 is con-
sistent with a Seyfert, Colina et al. (1999) find that it
has a LINER optical spectrum, thus implying that some
LINER galaxies are in fact heavily absorbed powerful
AGN. The soft diffuse X-ray halo in combination with
the radio morphology found by Carilli & Taylor (2000)
may suggest a circumnuclear starburst surrounding the
northern AGN nucleus, again consistent with substantial
obscuration toward the AGN.
NGC 3627: This nearby galaxy (D ∼ 10Mpc) is
thought to have had tidal interactions with NGC 3628,
a neighboring galaxy in the Leo Triplet, some 8 × 108
years ago which caused an intense burst of star forma-
tion in the nuclear regions around the same time (Rots
1978, Zhang et al. 1993, Afanasiev & Sil’chenko 2005).
Zhang et al. (1993) also discovered an extremely dense
molecular bar (mass ≥ 4 × 108 M⊙) and Chemin et al.
(2003) uncovered a warped disk using Hα observations,
both evidence of the tidal interaction. In their spectral
fitting to the BeppoSAX observation of NGC 3627, Geor-
gantopoulos et al. (2002) find intrinsic column densities
of ∼ 1.5 × 1022 cm−2 which, like Mrk 273, may suggest
substantial extinction to other regions near the nucleus.
NGC 7469: This is a well-known, extensively-studied
galaxy with strong, active star formation surrounding
a Seyfert 1 nucleus. Meixner et al. (1990) find dense
molecular gas (2 × 1010 M⊙), two orders of magnitude
above the Galactic value, within the central 2.5kpc of
the nucleus. 3.3µm imaging of the galaxy reveals that
80% of the PAH emission comes from an annulus ∼ 1”-
3”in radius around the central nucleus, indicating that
there is an elongated region of material that shelters the
PAH from the harsh radiation field of the AGN (Cutri
et al. 1984, Mazzarella et al. 1994). [OIII] line asym-
metries may corroborate the presence of a dense obscur-
ing medium, revealing a blue wing resulting when the
redshifted gas is obscured by the star forming ring (Wil-
son et al. 1986). In addition, Genzel et al. (1995) find
variation in the NIR emission attributable to extinction
and estimate the extinction from the CO observations of
Meixner et al. (1990) to be AV ∼ 10 mag.
NGC 1365: This nearby (D = 18.6 Mpc) AGN is
known to be circumscribed by embedded young star clus-
ters. The galaxy also contains a prominent bar with a
dust lane that penetrates the nuclear region (Phillips et
al. 1983, Lindblad et al. 1996 & 1999, Galliano et al.
2005). Like NGC 7469, NGC 1365 shows a peak at 3.5µm
implying PAH emission in spite of the harsh AGN radia-
tion field (Galliano et al. 2005). The large Hα/Hβ ratio
found by Alloin et al. (1981) implies substantial extinc-
tion toward the emission line regions, ranging from 3-4
mag. Observations with ASCA and ROSAT imply high
intrinsic column densities toward the X-ray emitting re-
gions, suggesting possibly high obscuration towards other
regions near the nucleus (Iyomoto et al. 1997, Komossa
& Schulz (1998), see also Schulz et al. 1999). Komossa &
Schulz show that the ratio of Hα to both the mid-IR and
X-ray radiation is substantially different in NGC 1365
compared with typical Seyfert 1 galaxies, possibly sug-
gesting inhomogenous obscuration (Schultz et al. 1999).
In an XMM X-ray study of NGC 1365, Risaliti et al.
(2005) also find a heavily absorbed Seyfert nucleus. The
blueshifted X-ray spectral lines imply high column den-
sities of 1023 cm−2 or more.
Mrk 266 (NGC 5256): This luminous infrared
galaxy is the only galaxy for which aperture effects most
likely account for the low 14µm/24µm ratio. Mrk 266
contains a very complicated structure which includes at
least two bright nuclei, a Seyfert and a LINER, that are
10” apart–a signature of a merger in progress. The mor-
phology of the northeast LINER nucleus is extremely
controversial (Wang et al. 1997; Kollatschny & Kowatsch
1998; Satyapal 2004, 2005; Ishigaki et al. 2000; Davies,
Ward, & Sugai 2000). Mazzarella et al. (1988) find three
non-thermal radio structures, two that coincide with the
nuclei and one between the two nuclei. Mazzarella et
al. (1988) suggest that the two nuclear structures are
associated with classical AGN and are in the stage of a
violent interaction in which the center of gravity of the
collision produces a massive burst of star formation with
supernovae or shocks which are responsible for the third
nonthermal radio source. As can be seen in Figure 8,
the SH slit, which provides the 14µm flux, overlaps with
this third radio source, while the LH slit, responsible for
the 24µm flux, encompasses the southwestern nucleus,
the third radio source, and part of the northeastern nu-
cleus. In this case the two lines observed originate in
physically distinct regions that do not each encompass
all potential sources of [NeV] emission, resulting in an
unphysical 14µm/24µm ratio. This is not to say that
Mrk 266 does not suffer from extinction at all. Indeed
the possible presence of a circumnuclear starburst im-
plies that there may be substantial extinction (Ishigaki
et al. 2000; Davies, Ward, & Sugai 2000). We have ver-
ified that this is the only distant galaxy in our sample
Fig. 8.— 20 cm image of Mrk 266 taken from NED
(http://nedwww.ipac.caltech.edu/). As can be seen here, the SH
slit (from which the 14µm line is extracted) overlaps with a third
radio source, while the LH slit (from which the 24µm line is ex-
tracted) encompasses the southwestern nucleus and part of the
northeastern nucleus.
with a complicated nuclear structure that will result in
aperture effects.
5.3. Can the [NeV] line flux ratio be used as a density
diagnostic?
Our analysis reveals that extinction towards parts of
the NLR in some objects is significant and cannot be ig-
nored at mid-IR wavelengths. In fact, it is quite possible
that extinction affects the [NeV] line flux ratios of those
galaxies with ratios above the low density limit (LDL)
and the amount of extinction in all cases is highly un-
certain. In addition to extinction, the temperature of
the [NeV] emitting gas is unknown. If the [NeV] emis-
sion originates within the walls of the obscuring central
torus, which may be the source of extinction in many of
our galaxies, we might expect the temperature of the gas
to reach 106 K (Ferland et al. 2002). If, on the other
hand, the [NeV] emission comes from further out in the
NLR and is instead attenuated by the intervening mate-
rial, we might expect the temperature of the gas to be
closer to 104 K. As shown in Figure 1, the electron densi-
ties inferred from the [NeV] line flux ratios are sensitive
to temperature when such large temperature variations
are considered. Based on the calculations shown in Fig-
ure 1, the low ratios could indicate that the densities in
the [NeV] line emitting gas are typically ≤ 3000 cm−3 for
T = 104K. However, if the [NeV] gas is characterized by
temperatures as high as T = 105K to 106K, densities
as high as 105 cm−3 would be consistent with our mea-
surements. We note that the [NeV] line flux ratios for
the galaxies in our sample (especially the Type 1 AGNs)
http://nedwww.ipac.caltech.edu/
all cluster around a ratio of ≈ 1.0. Two separate con-
clusions may be drawn from this finding: 1) That the
temperatures of the gas are low (∼ 104K) and that the
electron density is relatively constant over many orders
of magnitude in X-ray Luminosity and Eddington Ratio
for these AGNs, or 2) That the temperature of the gas
is high (105K to 106 K) and that the AGNs here sam-
ple a wide range of electron densities (from 102 cm−3
to 105 cm−3). Since gas temperature, electron density,
mid-IR continuum, and extinction are all unknown for
these objects, the electron density cannot be determined
here.
6. THE SIII LINE FLUX RATIOS
In Figure 9 we plot the 18µm/33µm line ratio as a
function of electron density ne. As with [NeV], we only
consider the five levels of the ground configuration when
computing the line ratio and we plot the relationship for
gas temperatures of T = 104K and 105K. We adopt col-
lision strengths from Tayal & Gupta (1999) and radiative
transition probabilities from Mendoza & Zeippen (1982).
Fig. 9.— 18µm/33µm line flux ratio in S III versus electron
density ne, for gas temperatures T = 10
4 K and 105 K.
In Table 3, we list the observed [SIII] line flux ratios
for the galaxies in our sample. As with the [NeV] ratios,
the [SIII] ratios in many galaxies listed in Table 3 are
well below the theoretically allowed value of 0.45 for a
gas temperature of T = 104K (13/33 detections). Again
we explore the observational effects and the theoretical
uncertainties that could artificially lower these ratios.
Aperture Effects: The ionization potential of [SIII]
is ∼ 35 eV and therefore the [SIII] emission may arise
from gas ionized by either the AGN or young stars. In
Table 3 we list, in addition to our Spitzer [SIII] fluxes,
all available [SIII] fluxes from ISO. Unlike [NeV], the
[SIII] fluxes from ISO are significantly larger than the
Spitzer fluxes for most galaxies. In Figure 10 we plot
the ISO to Spitzer flux ratios for the 18µm and 33µm
the [SIII] lines. As can be seen here, the [SIII] emission
extends beyond the Spitzer slit for many galaxies (6 out
of 9 for [SIII] 18µm and 11 out of 13 for [SIII] 33µm).
Similarly, when we compare the [SIII] flux arising from
a single slit centered on the nucleus to the flux arising
from a more extended region obtained using mapping
observations (Dale et al. 2006), we find that in most cases
the fluxes from the extended region are much larger than
the nuclear single-slit fluxes. Galaxies with fluxes from
Dale et al. (2006) are not included in Figure 10 since
the extraction aperture for these galaxies is comparable
to the 18µm ISO slit. We point out that the value for
this ratio is dependent on the orientation of the Spitzer
slit relative to the ISO slit and on the distance of each
object. We also note that IRAS20551 and IRAS23128 are
point sources with Spitzer 18µm fluxes greater than the
ISO fluxes from Genzel et al. (1998), however they fall
within the Genzel et al. (1998) quoted errors of 30% and
the Spitzer calibration error of 15%. Figure 10 suggests
that the [SIII] emission may be produced in the extended,
circumnuclear star forming regions associated with many
AGNs and that aperture effects need to be considered in
our analysis of the [SIII] ratio for nearby objects.
[SIII]
(ISO) / F
[SIII]
(Spitzer)
 [SIII] 18 micron Ratio
0
 1
 2
 3
 4
 5
 6
 [SIII] 33 micron Ratio
Fig. 10.— The ratios of the [SIII] flux from ISO and Spitzer for
the 18µm and 33µm lines. The range indicated with arrows is that
corresponding to the absolute flux calibration for ISO (20%) and
Spitzer (15%). The [SIII] emission is indeed extended beyond the
Spitzer slit for many galaxies, suggesting that the [SIII] emission
may be produced in star forming regions. We note that IRAS20551
and IRAS23128 are point sources with Spitzer 18µm fluxes greater
than the ISO fluxes from Genzel et al. 1998, however they fall
within the Genzel et al. (1998) quoted errors of 30% and the Spitzer
calibration error of 15%. Galaxies with fluxes from Dale et al. 2006
are not included in this plot since the extraction aperture for these
galaxies is comparable to the 18µm ISO slit.
The contribution from star formation to the [SIII] lines
can be estimated using the strength of the PAH emis-
sion, one of the most widely used indicators of the star
formation activity in galaxies (e.g. Luhman et al. 2003;
Genzel et al. 1998; Roche et al. 1991; Rigopoulou et
al. 1999, Clavel et al. 2000; Peeters, Spoon, & Tielens
2004). We examined the [SIII] 18.71 µm/PAH 6.2 µm
and [SIII] 33.48 µm/PAH 6.2 µm line flux ratios in 7
starburst galaxies observed by Spitzer and found them
to be comparable to the analogous ratios in our entire
sample of AGNs as shown in Figure 11. This suggests
that the bulk of the [SIII] emission originates in gas ion-
ized by young stars. We note that the apertures of the
SH and LH IRS modules are smaller than that of the
SL2 module, which may artificially raise the line ratios
plotted in Figure 11 for nearby galaxies compared with
the more distant ones. However, the fact that the line
ratios plotted in Figure 11 span a very narrow range sug-
gests that the [SIII] line emission has a similar origin in
starbursts and in AGNs. Thus, we assume that the bulk
of the [SIII] emission originates in gas ionized by young
stars and that the electron densities derived using these
lines taken from slits of the same size (such those galaxies
coming from Dale et al. 2006 mapping observations) or
from the most distant galaxies are representative of the
gas density in star forming regions.
Extinction: We have shown that aperture effects are
the likely explanation for why many of the [SIII] ratios
for the galaxies in our sample fall below the LDL. How-
ever, there are three galaxies in the sample with ratios
below the LDL that are distant enough (D>55 Mpc, cor-
responding to projected distances greater than 1.2 by 3
kpc and 3 by 6 kpc for the SH and LH slits, respectively)
that aperture effects may not be as important (NGC 2623
& Mrk 273, Mrk 266 has been excluded since it is known
to be affected by aperture variations See Section 5.2).
Extinction may be the explanation for the low ratios in
these galaxies. However, even though the SH and LH slits
likely cover the entirety of the NLR at these distances, we
note that these three galaxies contain well-known, large
circumnuclear starbursts (See Section 5.2 for the indi-
vidual galaxy summaries) which may produce extremely
extended [SIII] emission. It is therefore still possible that
the line ratios in these galaxies are artificially lowered by
aperture variations between the SH and LH slits. How-
ever, in addition to these three distant galaxies, NGC
4725 from Dale et al. (2006) has a [SIII] ratio below the
low density limit. The low [SIII] ratio (<0.45) in this
case cannot be attributed to aperture variations since
the extraction region is the same for both the 18 and
33µm lines. Thus, for completeness, the extinction de-
rived using the extinction curves given in Section 4 from
the observed [SIII] line ratio for these four sources are
given in Table 3. The Draine (1989) and Lutz et al.
(1996) extinction curve calculations yield extinction val-
ues that range from ∼ 12 to 25 mag. The Chiar and
Tielens (2006) extinction curve for the Galactic Center
may also be used since, unlike [NeV], the extinction at
the longer wavelength line (33µm) is greater than that
at the shorter wavelength line (18µm). The values de-
rived from this method are quite similar, ranging from ∼
10 to 22 mag. The Chiar and Tielens (2006) extinction
curve from the local ISM cannot be used here since it
only extends to 27.0µm.
Computed Quantities: As with NeV, there may be
uncertainties in the computed SIII infrared collisional
rate coefficients. However, there is generally less contro-
versy surrounding the [SIII] coefficients and these values
are widely accepted.
Our analysis suggests that aperture effects severely in-
fluence the [SIII] line flux ratios in most cases and that
the observed flux is likely dominated by star forming
regions. Figure 12, a plot of the [SIII] line ratio as a
function of distance, illustrates the influence of aperture
effects on the [SIII] line ratio. Most of the galaxies at dis-
tances <55 Mpc with [SIII] fluxes extracted from aper-
tures of different sizes (i.e. NOT the Dale et al. (2006)
galaxies) are below the LDL. On the other hand, galax-
ies at larger distances and galaxies with fluxes from Dale
et al. (2006) are generally above the LDL. Thus, for the
most distant galaxies in our sample and the galaxies with
fluxes from Dale et al. (2006) where the aperture for the
18 and 33 µm lines are equal, aperture effects are not
problematic, but extinction, as can be seen from Mrk
273, NGC2623, and NGC 4725 in Figure 12, needs to be
considered. As with the [NeV] line ratio, the [SIII] line
ratio is NOT a tracer of the electron density in our sam-
ple. In conclusion, the ambiguity of the intrinsic [SIII]
line ratio is primarily the result of aperture variations.
However there is at least one case (NGC 4725) where
aperture effects cannot explain the low ratio, implying
that, in addition to aperture variations, extinction likely
plays a role in lowering the [SIII] line flux ratios.
7. SUMMARY
We report in this paper the [NeV] 14µm/24µm and
[SIII]18µm/33µm line flux ratios, traditionally used to
measure electron densities in ionized gas, in an archival
sample of 41 AGNs observed by the Spitzer Space Tele-
scope.
1. We find that the [NeV] 14µm/24µm line flux ratios
are low: approximately 70% of those measured are
consistent with the low density limit to within the
calibration uncertainties of the IRS.
2. We find that Type 2 AGNs have lower [NeV]
14µm/24µm line flux ratios than Type 1 AGNs.
The mean ratios are 0.97 and 0.72 for the eight
Type 1 and ten Type 2 AGNs, respectively, with
uncertainties in the mean of about 0.08 for each.
3. For several galaxies, the observed [NeV] line ratios
are below the theoretical low density limit. All of
these galaxies are Type 2 AGNs.
4. We discuss the physical mechanisms that may play
a role in lowering the line ratios: differential mid-
IR extinction, low density, high temperature, and
high mid-IR radiation density.
5. We argue that the [NeV]-emitting region likely
originates interior to the torus in many of these
AGNs and that differential infrared extinction due
to dust in the obscuring torus may be responsible
for the ratios below the low density limit. We sug-
gest that the ratio may be a tracer of the torus
inclination angle to our line of sight.
6. Our results imply that the extinction curve in these
galaxies must be characterized by higher extinction
at 14µm than at 24µm, contrary to recent studies
of the extinction curve toward the Galactic Center.
7. A comparison between the [NeV] line fluxes ob-
tained with Spitzer and ISO reveals that there are
systematic discrepancies in calibration between the
two instruments. However, our results are indepen-
dent of which instrument is used; [NeV] line flux
ratios are consistently lower in Type 2 AGNs than
in Type 1 and [NeV] line flux ratios below the LDL
are observed with both ISO and Spitzer.
0
 1
 2
 3
 4
 5
[SIII]18/F(PAH6.2)
 
Starbursts
0
 1
 2
 3
 4
 5
 
AGN
0
 1
 2
 3
 4
 5
[SIII](33)/F(PAH6.2)
 
Starbursts
0
 1
 2
 3
 4
 5
 
AGN
Fig. 11.— Distribution of the [SIII]33µm/PAH 6.2µm and the [SIII]18µm/PAH 6.2µm line flux ratios for our sample of AGNs and a
small sample of starburst galaxies observed by Spitzer. It is apparent that the line ratios of the AGNs are comparable to the corresponding
ratios in starbursts, suggesting that the bulk of the [SIII] emission originates in star forming regions and not the NLRs in our sample of
AGNs.
8. Our work provides strong motivation for investigat-
ing the mid-IR spectra of a larger sample of galaxies
with Spitzer in order to test our conclusions.
9. Finally, an analysis of the [SIII] emission reveals
that it is extended in many or all of the galaxies
and likely originates in star forming gas and NOT
the NLR. Since there is a variation in the aper-
tures between the SH and LH modules of the IRS,
we cannot use the [SIII] line flux ratios to derive
densities for the majority of galaxies in our sam-
We are extremely thankful for all of the invaluable data
analysis assistance from Dan Watson and Joel Green,
without which this work would not have been possible.
We are also very grateful to Davide Donato, Eli Dwek,
Frederic Galliano, Paul Martini, Kartik Sheth, Eckhard
Sturm, Peter van Hoof, and Dan Watson for their en-
lightening and thoughtful comments/expertise that sig-
nificantly improved this paper. Carissa Khanna was also
very helpful in providing assistance in the preliminary
data analysis. We are also grateful for the helpful and
constructive comments from the referee. This research
has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Labora-
tory, California Institute of Technology, under contract
with the National Aeronautics and Space Administra-
tion. SS gratefully acknowledges financial support from
NASA grant NAG5-11432 and NAG03-4134X. JCW
gratefully acknowledges support from Spitzer Space Tele-
scope Theoretical Research Program. JCW is a Cottrell
Scholar of Research Corporation. Research in infrared
astronomy at NRL is supported by 6.1 base funding.
RPD gratefully acknowledges financial support from the
NASA Graduate Student Research Program.
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  We present the first systematic investigation of the [NeV] (14um/24um) and
[SIII] (18um/33um) infrared line flux ratios, traditionally used to estimate
the density of the ionized gas, in a sample of 41 Type 1 and Type 2 active
galactic nuclei (AGNs) observed with the Infrared Spectrograph on board
Spitzer. The majority of galaxies with both [NeV] lines detected have observed
[NeV] line flux ratios consistent with or below the theoretical low density
limit, based on calculations using currently available collision strengths and
ignoring absorption and stimulated emission. We find that Type 2 AGNs have
lower line flux ratios than Type 1 AGNs and that all of the galaxies with line
flux ratios below the low density limit are Type 2 AGNs. We argue that
differential infrared extinction to the [NeV] emitting region due to dust in
the obscuring torus is responsible for the ratios below the low density limit
and we suggest that the ratio may be a tracer of the inclination angle of the
torus to our line of sight. Because the temperature of the gas, the amount of
extinction, and the effect of absorption and stimulated emission on the line
ratios are all unknown, we are not able to determine the electron densities
associated with the [NeV] line flux ratios for the objects in our sample. We
also find that the [SIII] emission from the galaxies in our sample is extended
and originates primarily in star forming regions. Since the emission from
low-ionization species is extended, any analysis using line flux ratios from
such species obtained from slits of different sizes is invalid for most nearby
galaxies.

Draft version August 6, 2018
Preprint typeset using LATEX style emulateapj v. 4/12/04
MID-INFRARED FINE STRUCTURE LINE RATIOS IN ACTIVE GALACTIC NUCLEI OBSERVED WITH
SPITZER IRS: EVIDENCE FOR EXTINCTION BY THE TORUS
R. P. Dudik
, J. C. Weingartner
, S. Satyapal
, J. Fischer
, C. C. Dudley
, & B. O’Halloran
Draft version August 6, 2018
ABSTRACT
We present the first systematic investigation of the [NeV] (14µm/24µm) and [SIII] (18µm/33µm)
infrared line flux ratios, traditionally used to estimate the density of the ionized gas, in a sample
of 41 Type 1 and Type 2 active galactic nuclei (AGNs) observed with the Infrared Spectrograph on
board Spitzer. The majority of galaxies with both [NeV] lines detected have observed [NeV] line flux
ratios consistent with or below the theoretical low density limit, based on calculations using currently
available collision strengths and ignoring absorption and stimulated emission. We find that Type 2
AGNs have lower line flux ratios than Type 1 AGNs and that all of the galaxies with line flux ratios
below the low density limit are Type 2 AGNs. We argue that differential infrared extinction to the
[NeV] emitting region due to dust in the obscuring torus is responsible for the ratios below the low
density limit and we suggest that the ratio may be a tracer of the inclination angle of the torus to our
line of sight. Because the temperature of the gas, the amount of extinction, and the effect of absorption
and stimulated emission on the line ratios are all unknown, we are not able to determine the electron
densities associated with the [NeV] line flux ratios for the objects in our sample. We also find that the
[SIII] emission from the galaxies in our sample is extended and originates primarily in star forming
regions. Since the emission from low-ionization species is extended, any analysis using line flux ratios
from such species obtained from slits of different sizes is invalid for most nearby galaxies.
Subject headings: Galaxies: Active— Galaxies: Starbursts— X-rays: Galaxies — Infrared: Galaxies
1. INTRODUCTION
Mid-infrared (mid-IR) emission-line spectroscopy of
active galactic nuclei (AGNs) is used to investigate the
physical conditions of the dust-enshrouded gas that is
in close proximity to the active nucleus. In particular,
many spectral lines are emitted in the so-called narrow-
line region (NLR) of these objects which typically ex-
tends between tens to at most a thousand parsecs from
the nucleus (Capetti, et al. 1995, 1997, Schmitt & Kin-
ney 1996, Falcke et al. 1998; Ferruit et al. 1999, Schmitt
et al. 2003).
The NLRs of AGNs have been studied extensively us-
ing optical spectroscopic observations. However, there
have been very few systematic studies of the NLR using
infrared spectroscopic observations. Infrared (IR) fine-
structure emission lines have a number of special char-
acteristics that have been regarded as distinct advan-
tages, particularly in determining the electron density of
the ionized gas very close to the central AGN. Infrared
spectroscopic observations allow access to fine-structure
lines from ions with higher ionization potentials than the
most widely used optical diagnostic lines. This is impor-
tant in many AGNs, where a significant fraction of the
line emission from lower ionization species can originate
in gas ionized by star forming regions. In addition, it
is generally assumed that the density-sensitive infrared
line ratios originate in gas with temperatures around 104
K and are less dependent on electron temperature vari-
ations, enabling a more straightforward determination
of the electron density in the ionized gas. Finally, it
1 George Mason University, Department of Physics & Astron-
omy, MS 3F3, 4400 University Drive, Fairfax, VA 22030
2 Naval Research Laboratory, Remote Sensing Division, 4555
Overlook Ave SW, Washington DC, 20375
has long been assumed that the IR diagnostic line ratios
are insensitive to reddening corrections–a serious impedi-
ment to optical and ultraviolet observations, particularly
in the NLRs of AGNs, where the dust composition and
spatial distribution are highly uncertain. For these rea-
sons, IR spectroscopic observations, especially since the
era of the Infrared Space Observatory (ISO), have pro-
vided us with some of the most reliable tools for studying
the NLRs in AGNs. However, while there are clear ad-
vantages of mid-IR fine-structure diagnostics in studying
the physical state of the ionized gas, very little work has
been done to investigate their robustness in determin-
ing the gas densities of the NLRs in a large sample of
AGNs. The Spitzer Space Telescope Infrared Spectrome-
ter (IRS), with its extraordinary sensitivity and spectral
resolution, offers the opportunity to examine for the first
time the physical state of NLR gas in a large sample of
AGNs.
The focus of most previous comparative studies of the
infrared fine-structure lines in AGNs has been on the
excitation state of the ionized gas, in an effort to de-
termine the existence and energetic importance of po-
tentially buried AGNs and to constrain their ionizing
radiation fields (Genzel et al. 1998, Lutz et al. 1999,
Alexander & Sternberg 1999, Sturm et al. 2002, Satya-
pal, Sambruna, & Dudik 2004, Spinoglio et al. 2005).
Remarkably, very little work has been done in the in-
frared on studying the line flux ratios traditionally used
to probe the NLR gas densities in a significant number of
AGNs. We present in this paper the first systematic in-
frared spectroscopic study of the line flux ratios of [NeV]
and [SIII] in order to 1) test the robustness of these line
ratios as density diagnostics and 2) if possible, to probe
the densities of the NLR gas in a large sample of AGNs.
http://arxiv.org/abs/0704.0547v2
2. THE SAMPLE
We searched the Spitzer archive for galaxies with an ac-
tive nucleus and both high- and low-resolution Infrared
Spectrometer (IRS; Houck et al. 2004) observations cur-
rently available. Only those galaxies with indisputable
optical, X-ray, or radio signatures of active nuclei (such
as broad Hα or X-ray or radio point sources) were in-
cluded in our sample. The sample includes three AGN
subclasses: Seyferts, LINERs, and Quasars. The galax-
ies in this sample span a wide range of distances (4 to
400 Mpc; median = 21 Mpc), Hubble types, bolomet-
ric luminosities (log (LBOL) ∼ 40 to 46, median = 43),
and Eddington Ratios (log(L/LEdd) ∼ -6.5 to 0.3; me-
dian= -2.5). The entire sample consists of 41 galaxies.
The basic properties of the sample are given in Table
1. The black hole masses listed in Table 1 were derived
using resolved stellar kinematics, if available, reverber-
ation mapping, or by applying the correlation between
optical bulge luminosity and central black hole mass de-
termined in nearby galaxies only when the host galaxy
was clearly resolved. Bolometric luminosities listed in
Table 1 were calculated from the X-ray luminosities for
most objects. For Seyferts, the relationship LBOL = 10 ×
LX was adopted (Elvis 1994). For LINERs
1 we assumed
LBOL = 34× LX , as derived from the spectral energy dis-
tribution of a sample of nearby LINERs from Ho (1999)
(see also Dudik et al. 2005 and Satyapal et al. 2005).
The bolometric luminosities and black hole masses for
quasars and radio galaxies were taken from Woo & Urry
(2002) and Marchesini, Celotti, & Ferrarese (2004), re-
spectively. A detailed discussion of our methodology and
justification of assumptions for determining black hole
masses and bolometric luminosities for the various AGN
classes represented in Table 1 can be found in Satyapal et
al. (2005) and Dudik et al. (2005). Table 1 also lists the
AGN type (1 or 2) for the galaxies in our sample based
on the presence or absence of broad (full width at half
max (FWHM) exceeding 1000 km s−1) Balmer emission
lines in the optical spectrum. We emphasize that the
selection basis for the objects in our sample was on the
availability of high resolution IRS Spitzer observations.
The sample should therefore not be viewed as complete
in any sense.
3. DATA ANALYSIS AND RESULTS
We extracted archival spectral data obtained us-
ing the short-wavelength, low-resolution module (SL2,
3.6”×57”, λ = 5.2-7.7µm) and both the short-
wavelength, high-resolution (SH, 4.7”×11.3”, λ = 9.9-
19.6µm) and long-wavelength, high-resolution (LH,
11.1”×22.3”, λ = 18.7-37.2µm) modules of IRS.
The data presented here were preprocessed by the
IRS pipeline (version 13.0) at the Spitzer Science Center
(SSC) prior to download. Preprocessing includes ramp
fitting, dark-sky subtraction, droop correction, linear-
ity correction, flat-fielding, and flux calibration2. The
Spitzer data were further processed using the SMART v.
5.5.7 analysis package (Higdon et al. 2004). The slit for
1 We include all galaxies that are classified as LINERs using
either the Heckman (1980) or Veilleux & Osterbrock (1987) diag-
nostic diagrams.
2 See Spitzer Observers Manual, Chapter 7,
http://ssc.spitzer.caltech.edu/documents/som/irs60.pdf
Table 1: Properties of the Sample
Galaxy Distance Hubble log log log AGN
Name (Mpc) Type (MBH) (LX ) (L/LEdd) Type
(1) (2) (3) (4) (5) (6) (7)
Seyferts
NGC4151 13 SABab 7.13a 42.7b -1.53 1r
NGC1365 19 SBb 7.64b 41.3d -3.42 2s
NGC1097 15 SBb · · · 40.7e · · · · · ·
NGC7469 65 SABa 6.84a 44.3a 0.34 1t
NGC4945 4 SBcd 7.35b 42.5f -1.97 2u
Circinus 4 SAb 7.72b 42.1g -2.74 2v
Mrk 231 169 SAc 7.24c 42.2h -2.16 · · ·
Mrk3 54 S0 8.65a 43.5a -2.21 2w
Cen A 3 S0 7.24b 41.8i -2.54 2x
Mrk463 201 Merger · · · 43.0j · · · 2y
NGC 4826 8 SAab 6.76b · · · · · · · · ·
NGC 4725 16 SABab 7.40b · · · · · · 2r
1 ZW 1 245 Sa · · · 43.9k · · · · · ·
NGC 5033 19 SAc 7.39b 41.4l -3.13 1r
NGC1566 20 SABbc 6.92a 43.5a -0.57 1t
NGC 2841 9 SAb 8.21a 42.7a -2.64 · · ·
NGC 7213 24 SA0 7.99a 43.30a -1.79 · · ·
LINERs
NGC4579 17 SABb 7.85b 41.0b -3.47 · · ·
NGC3031 4 SAab 7.79b 40.2b -4.16 · · ·
NGC6240 98 Merger 9.15b 44.2b -1.52 2z
NGC5194 8 SAbc 6.90b 41.0b -2.43 2r
MRK266NE 112 Merger · · · 40.9b · · · 2t
NGC7552 21 SBab 6.99b · · · · · · · · ·
NGC 4552 17 · · · 8.57b 39.6b -5.52 · · ·
NGC 3079 15 SBc 7.58b 40.1m -4.05 · · ·
NGC 1614 64 SBc 6.94b · · · · · · · · ·
NGC 3628 10 SAb 7.86b 39.9n -4.58 · · ·
NGC 2623 74 Pec 6.83b · · · · · · 2aa
IRAS23128-5919 178 Merger · · · 41.0b · · · 2bb
MRK273 151 Merger 7.74b 44.0o -0.31 2t
IRAS20551-4250 171 Merger 7.52c 40.9b -3.23 · · ·
NGC3627 10 SABb 7.16b 39.4p -4.33 2r
UGC05101 158 S · · · 40.9b · · · 1cc
NGC4125 18 E6 8.50b 38.6b -6.47 · · ·
NGC 4594 10 SAa 9.04b 40.1q -5.47 · · ·
Quasars
PG 1351+640 353 · · · 8.48a 44.5a -1.08 · · ·
PG 1211+143 324 · · · 7.49a 44.8a 0.22 1t
PG 1119+120 201 · · · · · · · · · · · · 1y
PG 2130+099 252 Sa 7.74a 44.47a -0.37 1y
PG 0804+761 400 · · · 8.24a 44.93a -0.41 1dd
PG 1501+106 146 E · · · · · · · · · 1y
Columns Explanation: Col(1):Common Source Names; Col(2): Dis-
tance (for H0= 75 km s
−1Mpc−1); Col(3): Morphological Class;
Col(4): Mass of central black hole in solar masses; Col(5): Log of
the hard X-ray luminosity (2-10keV) in erg s−1. Col(6): log of the
Eddington Ratio. (* = We include all galaxies that are classified as
LINERs using either the Heckman (1980) or Veilleux & Osterbrock
(1987) diagnostic diagrams. Col(6): AGN type based on the presence
or absence of broad Balmer emission lines.) References:aWoo & Urry
2002, b Satyapal et al. 2005, c Tacconi et al. 2002, d Risaliti et al.
2005, e Terashima et al. 2002, f Guainazzi et al. 2000, g Smith &
Wilson 2001, h Gallagher et al. 2002, i Evans et al. 2004, j Iman-
ishi & Terashima et al. 2004, kGallo et al. 2004 , l Terashima et al.
1999, m Cappi et al. 2006, n Roberts, Schurch, & Warwick 2001, o
Balestra et al. 2005, pGeorgantopoulos et al 2002, q Dudik et al. 2005,
r Ho et al. 1997, s Storchi-Bergmann, Mulchaey, & Wilson, 1992, t
Veron-Cetty & Veron 2003, u Marconi et al. 2000, vOliva et al. 1994,
w Khachikian & Weedman 1974, x Veron-Cetty & Veron 1986y Dahari
& De Robertis 1988, z Andreasian, Khachikian, & Ye, 1987, aa Laine
et al. 2003, bb Duc, Mirabel, & Maza 1997, cc Sanders et al. 1988, dd
Thompson 1992.
http://ssc.spitzer.caltech.edu/documents/som/irs60.pdf
the SH and LH modules is too small for background sub-
traction to take place and separate SH or LH background
observations do not exist for any of the galaxies in this
sample. For the SL2 module, background subtraction
was done using either a designated background file when
available or the interactive source extraction option. In
the case of the latter, the exact position of the slit on
the host galaxy was first checked using Leopard, the data
archive access tool available from the SSC. The source
was then carefully defined according to the boundary of
the slit and the edge of the host galaxy. The background
was defined at the edge of the slit, where no other obvious
source was present. In some cases, the slit was enveloped
in the host galaxy and background subtraction could not
take place. For both high and low resolution spectra, the
ends of each order were manually cut from the rest of the
spectrum.
The 41 observations presented in this work are archived
from various programs, including the SINGS Legacy Pro-
gram, and therefore contain both mapping and staring
observations. All of the staring observations were cen-
tered on the nucleus of the galaxy. The SH, LH, SL2
staring observations include data from two slit positions
overlapping by one third of a slit. In order to isolate
the nuclear region in the mapping observations so that
we might compare them to the staring observations, we
extracted only those 3 overlapping slit positions coin-
ciding with either radio or 2MASS nuclear coordinates.
Because the slits in both the mapping and staring obser-
vations occupy distinctly different regions of the sky, the
slits cannot be averaged unless the emission originates
from a compact source that is contained entirely in each
slit. Therefore the procedure for flux extraction was the
following: 1) If the fluxes measured from the two slits
differed by no more than the calibration error of the in-
strument, then the fluxes were averaged; otherwise, the
slit with the highest measured line flux was chosen. 2) If
an emission line was detected in one slit, but not in the
other, then the detection was selected. This is true for
all of the high and low resolution staring and mapping
observations.
In Tables 2 and 3 we list the line fluxes and statistical
errors from the SH and LH observations for the [NeV]
14.3µm and 24.3µm lines, the [SIII] 18.1µm and 33.5µm
lines, as well as the 6.2µm PAH emission feature. For
all galaxies with previously published fluxes, we list in
Tables 2 and 3 the published flux values. Our values dif-
fer by no more than a factor of 1.9, much less in most
cases, from the Weedman et al. (2005) or Armus et al.
(2004, 2006) published values. These differences can be
attributed to differences in the pipeline used for prepro-
cessing. In all cases detections were defined when the
line flux was at least 3σ. For the absolute photometric
flux uncertainty we conservatively adopt 15%, based on
the assessed values given by the Spitzer Science Center
(SSC) over the lifetime of the mission.3 This error is cal-
culated from multiple observations of various standard
stars throughout the Spitzer mission by the SSC. The
dominant component of the total error arises from the
3 See Spitzer Observers Manual, Chapter 7,
(http://ssc.spitzer.caltech.edu/documents/som/som7.1.irs.pdf
and IRS Data Handbook (http://ssc.spitzer.caltech.edu/irs/dh/dh20v2.pdf,
Chapter 7.2
uncertainty at mid-IR wavelengths in the stellar models
used in calibration and is systematic rather than Gaus-
sian in nature. We note that the spectral resolution of
the SH and LH modules of IRS (λ / ∆λ ∼ 600) is in-
sufficient to resolve the velocity structure for most of the
lines. There are a few galaxies which do show slightly
broadened [NeV] line profiles (FWHM ∼ 200 - 1200 km
s−1). These results will be discussed in a future paper.
Abundance-independent density estimates can readily
be obtained using infrared fine-structure transitions from
like ions in the same ionization state with different crit-
ical densities. The density diagnostics available in the
IRS spectra of our objects are: [NeV] 14.32µm, 24.32
µm (ncrit ∼ 4.9 × 10
4 cm−3, and 2.7 × 104 cm−3, where
ncrit = Aul/γul, with Aul the Einstein A coefficient and
γul the rate coefficient for collisional de-excitation from
the upper to the lower level), [NeIII] 15.55µm, 36.04 µm
(ncrit ∼ 3 × 10
5 cm−3, and 5 × 104 cm−3, Giveon et al.
2002), and [SIII]18.71µm, 33.48 µm (ncrit ∼ 1.5 × 10
cm−3, and 4.1 × 103 cm−3). The results are very insen-
sitive to the shape of the ionizing continuum. Since the
[NeIII] 36µm line was either not detected or was outside
the wavelength range of the LH module in virtually all
galaxies, we omit any analysis of the [NeIII] line ratio
from this work.
4. THE [NEV] LINE FLUX RATIOS
In Figure 1 we plot the calculated 14µm/24µm line lu-
minosity ratio as a function of electron density ne for gas
temperatures T = 104K, 105K, and 106K. We include
only the five levels of the ground 2s22p2 configuration and
neglect absorption and stimulated emission. The results
are nearly identical if only the lowest three levels of the
ground term are included. We adopt collision strengths
from Griffin & Badnell (2000) and radiative transition
probabilities from Galavis, Mendoza, & Zeippen (1997).
Fig. 1.— [NeV] 14µm/24µm line flux ratio versus electron den-
sity, ne, for gas temperatures T = 10
4 K, 105 K, 106 K
In Table 2, we list the observed [NeV] line flux ratios
http://ssc.spitzer.caltech.edu/documents/som/som7.1.irs.pdf
and their associated calibration uncertainties. In calcu-
lating the upper and lower limits on the ratios, RMAX
and RMIN , shown in Table 2, we did not propagate the
errors in quadrature as would be appropriate for statis-
tical uncertainties, but propagated them as follows:
RMAX =
F [NeV]14 + 0.15(F [NeV]14)
F [NeV]24 − 0.15(F [NeV]24)
RMIN =
F [NeV]14 − 0.15(F [NeV]14)
F [NeV]24 + 0.15(F [NeV]24)
We note that this is conservative, since some components
of the calibration errors should cancel in the ratio. Both
line fluxes were measured for 19 galaxies. In what fol-
lows we compare the line flux ratios measured in all but
one, MKN 266, for reasons that are discussed in detail
in Section 5.2. Of these 18 AGNs, 13 have ratios that
are consistent with the low density limit to within the
uncertainties, while only 2, both Type 1, have ratios sig-
nificantly above it. The remaining 3, all Type 2, have
ratios significantly below the low-density limit. Inter-
estingly, we note that a similar range of ratios was also
measured with the ISO SWS (Sturm et al. 2002, Alexan-
der et al. 1999). There are several possible explanations
for this finding. The observed, unphysically low ratios
could result from artifacts introduced by variations in
the slit sizes from which the line fluxes are obtained,
from calibration uncertainties, or from substantial mid-
IR extinction. Alternatively, perhaps important physical
processes were neglected in calculating the theoretical ra-
tios. In addition, errors in the collisional rate coefficients
for the [NeV] transitions associated with the mid-infrared
lines may be important. We explore these scenarios in
the following sections.
Observational Effects: Because the IRS LH slit is
larger than the SH slit, if the [NeV] emission is extended,
or multiple AGNs are present, the 14/24 µm line ratio
will be artificially reduced. However, since the ionization
potential of [NeV] is ∼ 97 eV, we expect that the [NeV]-
emitting gas is ionized by the AGN radiation field only
and is concentrated very close to the central source. Vir-
tually all of the [NeV] fluxes presented in this work were
obtained from IRS staring observations. Thus it is im-
possible to determine whether the emission is extended
using Spitzer observations alone. However, a number of
galaxies have been observed at 14 and 24 µm by ISO. In
Table 2 we list in addition to our Spitzer [NeV] fluxes, all
available [NeV] fluxes from ISO. The ISO aperture at 14
and 24 µm (14”×27”) is much larger than either the SH
or LH slits. In Figure 2 we plot the ratio of the [NeV] flux
measured by ISO to that measured by Spitzer for both
the 14 and 24 µm lines. The ranges of the [NeV] line
flux ratios are consistent with the instrument uncertain-
ties and are similar for all galaxies in the sample. Only
the 14µm ratio for Mrk 266 falls outside of the expected
range. This strongly suggests that the [NeV] emission is
indeed compact and originates in the NLR and that the
ratios are not affected by aperture variations, except for
Mrk 266 which is discussed in detail in Section 5.2.
If the data were affected by aperture variations we
would expect to see an overall systematic increase of
the 14µm/24µm line ratio with distance(See Figure 3).
The Spearman rank correlation coefficient (rS, Kendell
& Stuart 1976) corresponding to this plot is -0.069 (with
 [NeV] 14 micron Ratio
0.0 0.5 1.0 1.5 2.0 2.5
[NeV]
(ISO) / F
[NeV]
(Spitzer)
Mrk 266
[NeV] 24 micron Ratio
Fig. 2.— Ratio of the ISO to Spitzer [NeV] 14µm and 24µm
fluxes for those galaxies with overlapping observations. The range
indicated with arrows is that corresponding to the absolute flux
calibration for ISO (20%) and Spitzer (15%). Within the calibra-
tion uncertainties of the instrument, the [NeV] fluxes are virtually
the same for all of the galaxies except Mrk 266 (See Section 6.2).
This strongly suggests that the [NeV] emission is compact and orig-
inates in the NLR. We note that Sturm et al. 2002 find that the
[NeV] 24µm detection for NGC 7469 is questionable. The ISO to
Spitzer ratio for this galaxy (0.43) is the lowest shown here.
a probability of chance correlation of 0.78), where a co-
efficient of 1 or -1 indicates a strong correlation and a
coefficient of 0 indicates no correlation. Thus we find
that there is no correlation between the [NeV] ratio and
distance in our sample. However this does not completely
rule out aperture effects, if the size of the [NeV] emitting
region increases with the bolometric luminosity of the
AGN and the sample displays a significant trend in bolo-
metric luminosity with distance. In this case, a correla-
tion between the [NeV] ratio and distance would not be
apparent since aperture variations would affect all galax-
ies in the same way, regardless of distance. However this
scenario is unlikely since the size of the [NeV]-emitting
region would have to increase proportionately with dis-
tance in order to remain extended beyond the slit for all
galaxies. Nevertheless, we checked for this possibility,
both by examining the [NeV] ratio vs. bolometric lu-
minosity and by plotting the ratio vs. distance, binning
the galaxies according to their bolometric luminosity. We
find neither to be correlated over 5 orders of magnitude
in LBOL. Thus, in the case of the [NeV] line flux ratio,
we find no indication that ratios below the low density
limit are artifacts of aperture effects.
We point out that the [NeV] 24µm IRS line fluxes in the
small overlapping sample plotted in Figure 2 are system-
atically higher than the corresponding ISO-SWS fluxes,
despite the smaller IRS slit. This indicates that one or
both of the instruments is affected by systematic errors
more severe than are indicated by the calibration un-
certainty estimates. The SWS band 3D that includes
the [NeV] 24µm line was characterized by strong fring-
ing effects that when combined with the narrow range
of the line scan mode introduced sometimes large un-
0.5 1.0 1.5 2.0 2.5
Mrk 266
 = 0.069
log(Distance, Mpc)
 Seyferts
 Liners 
 Quasars 
Fig. 3.— The [NeV] 14µm/24µm ratio as a function of distance.
Open symbols signify Type 1 AGNs, Filled symbols signify Type 2
AGNs. The error bars shown here mark the calibration uncertain-
ties on the line ratio. If the ratio were indeed affected by aperture
variations we would expect a systematic increase of the ratio with
distance. As can be seen here, this is not the case, and we find no
indication that the low ratio is attribuable to aperture effects.
certainties in the baseline fitting, and therefore the line
flux measurement accuracy. In contrast, the baseline fit-
ting over the entire Spitzer IRS SH and LH full spectra
can be much more accurate. Moreover pointing accu-
racy and stability are an order of magnitude improved
over that obtained by ISO. We therefore assume in the
sections that follow that the adopted conservative Spitzer
IRS calibration uncertainties are accurate characteriza-
tions of the IRS measurements. Importantly, regardless
of which instrument is used, [NeV] ratios consistent with
the low density limit have been observed in a number of
sources with both ISO (e.g. Sturm et al. 2002 NGC 1365,
NGC 7582, NGC4151, NGC 5506; Alexander et al. 1999,
NGC 4151) and Spitzer (Weedman et al. 2005, Haas et
al. 2005, and this work).
Extinction: We consider the possibility that mid-IR
differential extinction toward the [NeV]-emitting regions
is responsible for the low [NeV] line ratios. Adopting the
low-density limit (LDL) for the intrinsic value of the ratio
([NeV]14µm/24µm ∼0.83 for ne≤200 cm
−3) for galaxies
with ratios below the LDL, the observed line ratio gives a
lower limit to the extinction, for a given MIR extinction
curve. We examined the visual extinctions correspond-
ing to the mid-IR differential extinction derived using
three separate extinction curves: 1) the Draine (1989)
extinction curve amended by the more recent ISO SWS
extinction curve toward the Galactic center for 2.5-10µm
(Lutz et al. 1996), 2) the Chiar & Tielens (2006) ex-
tinction curve for the Galactic Center using 2.38-40µm
ISO SWS observations of a bright IR source in the Quin-
tuplet cluster (GCS3-I) 3) the Chiar & Tielens (2006)
extinction curve for the local ISM using 2.38-40µm ISO
SWS observations of a WC-type Wolf-Rayet (WR) star
(WR98a). The Draine (1989) and Lutz et al. (1996)
extinction curve yields AV ∼ 3 to 99 mag (See Table
2). However, these values result from an extinction law
that is unexplored beyond 10µm. The Chiar & Tie-
lens (2006) Galactic center extinction curve cannot ex-
plain the observed [NeV] ratios since the extinction at
24µm is greater than the extinction at 14µm, so we do
not discuss it further. The visual extinction resulting
from their local ISM extinction curve is unrealistically
high (AV median=500mag). The calculated extinction ob-
tained using the Draine (1989), Lutz et al. (1996), and
Chiar & Tielens (2006) local ISM extinction curves are
given in Table 2.
The AV derived from the two extinction curves de-
scribed above are extremely high in many cases. Even
if the extinction is calculated from the upper limit on
the ratio to the LDL for the three galaxies whose upper
limits are below the LDL, the corresponding visual ex-
tinction is still very high (for the Draine 1989 and Lutz
1996 extinction curve AV = 21, 26, and 30 mag for these
three galaxies; for the Chiar and Tielens extinction curve
AV = 260, 330, and 370 mag). However, we caution the
reader that the actual value for extinction is highly un-
certain. Indeed very little is known about the 8-40µm
extinction curve in AGNs. Specifically, the 10 and 18
µm silicate features in this band are the source of in-
consistency. Even within the AGN class, extinction may
vary dramatically from 8-40µm because of variations in
the silicate features due to differences in grain size, poros-
ity, shape, composition, abundance, and location in each
galaxy. Hao et al. (2005) show that in five AGNs (4
of which are in our sample), both silicate features vary
considerably in strength and width. Sturm et al. (2005)
also show that the standard ISM silicate models do not
accurately fit NGC 3998, a LINER with silicate emis-
sion. Sturm et al. (2005) suggest that increased grain
size and possibly the presence of crystalline silicates such
as clino-pyroxenes may improve the fit, but that clearly
circumnuclear dust in AGNs has very different proper-
ties than dust in the Galactic ISM (see also Maiolino et
al. 2001a, 2001b, but Weingartner & Murray 2002 for
an alternative view). Chiar & Tielens (2006) even show
that the GC observations and the local ISM observations
within the Galaxy deviate from each other most dramat-
ically in the wavelength region between the two silicate
absorption features. In their observations, this is the re-
gion between ∼ 12-15µm -directly overlapping with the
14µm values in which we are interested. Because of ir-
regularity of the silicate features in the mid-IR, it is very
difficult to interpret the true extinction there. Moreover,
in addition to the uncertainty in the extinction law, the
geometry of the obscuring material is unknown and can
vary substantially from galaxy to galaxy. The most that
can be said here for the galaxies with ratios below the
LDL is that if extinction is responsible for the low ratios,
then the extinction must be less at 24µm than at 14µm.
Physical Processes: It is possible that important
physical processes have been neglected in calculating the
[NeV] line luminosity ratio as a function of electron den-
sity shown in Figure 1. We consider three physical pro-
cesses that may affect the line ratios:
(1) A source of gas heating in addition to photoioniza-
tion (e.g., shocks, turbulence) that may yield gas temper-
atures substantially higher than 104K. As can be seen
in Figure 1, higher gas temperatures do not yield sig-
nificantly lower line ratios in the low-density limit, but
could explain the generally low values of the ratios that
lie above the LDL.
(2) Pumping from the ground term to the first excited
term, e.g., by O III resonance lines. The specific energy
density required for this to significantly affect the line
ratio exceeds 10−14 erg cm−3 Hz−1, which is implausibly
large by orders of magnitude.
(3) Absorption and stimulated emission within the
ground term, which could be important if, e.g., a large
quantity of warm dust yielding copious 24µm continuum
emission is located close to the [NeV]-emitting region.
Figures 4a through 4e show the line ratio as a func-
tion of the specific energy density at 24µm, uν(24µm).
We display results for electron density ne = 10
2, 103,
104, 105, and 106 cm−3; gas temperature T = 104, 105,
and 106K; and ratio of the specific energy density at 14
and 24µm, uν(14µm)/uν(24µm) = 0.4, 1.0, and 1.8 (val-
ues were chosen to reproduce the observed range of the
14µm/24µm continuum flux ratios; see Section 5).
For the moment, assume that the NeV is located suf-
ficiently far from the source of the 14 and 24µm contin-
uum emission to treat the source as a point. If hot dust
within or near the inner edge of the torus is responsible
for this emission, then this assumption requires that the
distance to the NeV, rNe, be large compared with the
dust sublimation radius, rsub ∼ 1 pcL
bol, 46 (Ferland et
al. 2002); Lbol, 46 is the bolometric luminosity in units of
1046 erg s−1. In this case, we can obtain a simple estimate
of uν(24µm) at the location of the [NeV]-emitting region
from the observed specific flux Fν(24µm), the distance
to the galaxy D, and rNe:
uν(24µm) ∼
Fν(24µm)
. (3)
With rNe = 100 pc, uν(24µm) estimated in this way
ranges from ≈ 10−24 erg cm−3 Hz−1 to somewhat less
than 10−20 erg cm−3 Hz−1 for the galaxies in our sam-
ple. These can be compared to the results of Hönig
et al. (2006), who modeled the infrared emission from
clumpy tori. They presented plots of Fν at a distance
of 10Mpc from an AGN with bolometric luminosity
Lbol = 4 × 10
45 erg s−1. Extrapolating to a distance
of 100 pc, we find uν(24µm) as large as a few times
10−21 erg cm−3 Hz−1, close to the estimate for the most
luminous AGN in our sample.
From Figures 4a through 4e, we see that the infrared
continuum can only reduce the line ratio significantly at
rNe ≈ 100 pc if T & 10
5K when ne ∼ 10
2 cm−3 and
T & 106K when ne ∼ 10
3 cm−3. However, the NeV, as
a high-ionization species, may lie closer to the central
source than does the bulk of the narrow line region. If
rNe ≈ 10 pc, then uν(24µm) increases by a factor ∼ 100.
In this case, the observed low line ratios can be explained
by this mechanism with T ∼ 104K, if ne ∼ 10
2 cm−3.
Higher values of electron density would require higher
gas temperatures.
In Section 5.1, we suggest that the [NeV]-emitting re-
gion may lie within the torus. In this case, absorp-
tion and stimulated emission within the ground term
are probably important. For the high-luminosity objects,
these may even dominate over collisional excitation and
de-excitation. At these central locations, gas tempera-
tures T ∼ 106 K may be natural (Ferland et al. 2002).
Relatively high densities may also be expected, in which
case the infrared continuum may not appreciably depress
the line ratio (see Figure 4d).
Adopting the Mathews & Ferland (1987) spectrum and
T ≈ 106K, the ionization parameter U ≡ nγ/ne ∼ 10
in order for a substantial fraction of the Ne to be NeV;
nγ is the number density of H-ionizing photons. For
this spectrum, nγ ≈ 1.7×10
3Lbol, 46 r
Ne, 100 cm
−3, where
rNe, 100 = rNe/100 pc. If rNe = 1pc, then either (1)
ne ∼ 10
10Lbol,46 cm
−3 or (2) the nuclear continuum is
filtered through a far-UV/X-ray-absorbing medium be-
fore reaching the [NeV]-emitting region.
If absorption and stimulated emission are indeed rele-
vant processes in [NeV] line production, we might expect
a relationship between the [NeV] line flux ratio and the
24 µm continuum luminosity that is consistent with one
of the curves shown in Figures 4a through 4e. In Figure
5 we plot this relationship for the [NeV] emitting galax-
ies in our sample. As can be seen in Figure 5, we find
no relationship between the [NeV] line flux ratio and the
24µm continuum luminosity for our sample of galaxies.
The Spearman rank correlation coefficient for this plot
is -0.01 (probability of chance correlation = 0.95), indi-
cating no correlation. As a result, as can be seen from
Figures 4a and 4b, stimulated emission and absorption
at low densities can be ruled out as possible scenarios be-
cause the scatter plot shown in Figure 5 does not follow
the model predictions. We note that the location of the
[NeV]-emitting region relative to the source of the 24µm
continuum emission is uniform among the galaxies in the
sample. Variations in the location might obscure any
correlation in these plots. Figures 4c and 4d reveal that,
for some values of ne, T, and uν(14µm)/uν(24µm), the
line ratio is very insensitive to the value of uν(24µm). In
these cases, the line ratio remains above ∼ 0.8. Thus, al-
though absorption and stimulated emission may be con-
tributing processes to [NeV] production, another mecha-
nism is required to explain the low (<0.8) [NeV] line flux
ratios in our sample.
Computed Quantities: Finally, it is possible that
there is significant error in the adopted collisional rate
coefficients. The accuracy of collisional strengths of in-
frared atomic transitions has been a longstanding ques-
tion. We adopt the collisional rate coefficients from the
state of the art IRON project (Hummer et al. 1993)
which produced the most up-to-date and accurate colli-
sion strengths for a large database of atomic transitions.
While these calculations have been questioned based on
recent ISO observations of nebulae (Clegg et al. 1987,
Oliva et al. 1996, Rubin et al. 2002, Rubin 2004), it is
likely that the discrepancies between the observational
and theoretical values can be explained by inaccuracies
in the fluxes employed (van Hoof et al. 2000). Uncer-
tainties in the collisional rate coefficients for the [NeV]
transitions are unlikely to exceed 30% (van Hoof et al.
2000). It is therefore unlikely that the low critical den-
sities implied by our data can be attributed to uncer-
tainties in the theoretical values of the [NeV] collisional
strengths.
5. EXTINCTION EFFECTS OF THE TORUS AND AGN
UNIFICATION
Although low electron densities, high gas tempera-
tures, and/or high infrared radiation densities may play
Fig. 4.— The [NeV] line ratio as a function of the specific energy density at 24µm, uν(24µm), for temperatures, T = 10
4, 105, 106 K,
for 14µm/24µm continuum ratios of 0.4, 1.0, and 1.8, and finally for electron densites, ne = 10
2, 103, 104, 105, 106 cm−3.
Table 2: NeV Line Fluxes and Derived Extinction
Galaxy [NeV] [NeV] [NeV] [NeV] [NeV] AV AV AV AV
Source 14.32 14.32 24.32 24.32 Ratio Ratio to Ratio to Ratio to Ratio to
SH ISO LH ISO LDL (D&L) HDL (D&L) LDL (C&T) HDL (C&T)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Seyferts
NGC4151 7.77a 5.50c 6.77a 5.60c 1.15
+0.41
−0.30
· · · 126 · · · 1686
NGC1365 2.20±0.06 2.50c 5.36±0.06 3.90c 0.41
+0.14
−0.11
45 189 570 2519
NGC1097 <0.05 · · · <0.18 · · · · · · · · · · · · · · · · · ·
NGC7469 1.16a <1.50c 1.47a 0.63c∗ 0.79
+0.28
−0.21
3 149 41 1990
NGC4945 0.28±0.03 <0.50d <0.75 · · · >0.38 · · · · · · · · · · · ·
Circinus 23.94±0.61 31.70c 24.00±3.90 21.80c 1.00
+0.35
−0.26
· · · 135 · · · 1799
Mrk 231 <0.44a <1.50e <0.69a · · · · · · · · · · · · · · · · · ·
Mrk3 6.45a 4.60c 6.75a 3.40c 0.96
+0.34
−0.25
· · · 138 · · · 1835
Cen A 2.32a 2.70c 2.99a 2.00c 0.77
+0.27
−0.20
4 150 56 2005
Mrk463 1.83b 1.40c 2.04b · · · 0.90
+0.32
−0.23
· · · 141 · · · 1886
NGC 4826 · · · · · · · · · · · · · · · · · · · · · · · · · · ·
NGC 4725 <0.09 · · · 0.09±0.03 · · · <1.04 · · · · · · · · · · · ·
1 ZW 1 <0.11a 0.27c <0.10a · · · · · · · · · · · · · · · · · ·
NGC 5033 0.07±0.02 · · · 0.11±0.02 · · · 0.65+0.23
−0.17
16 161 198 2146
NGC1566 0.16±0.05 · · · 0.22±0.04 · · · 0.74+0.26
−0.19
7 153 92 2041
NGC 2841 <0.04 · · · <0.03 · · · · · · · · · · · · · · · · · ·
NGC 7213 <0.04 · · · <0.09 · · · · · · · · · · · · · · · · · ·
LINERs
NGC4579 <0.06 · · · <0.03 · · · · · · · · · · · · · · · · · ·
NGC3031 <0.06 · · · <0.04 · · · · · · · · · · · · · · · · · ·
NGC6240 0.51b <1.00e <0.39b · · · <1.31 · · · · · · · · · · · ·
NGC5194 0.41±0.04 <0.20c 0.39±0.09 · · · 1.06
+0.37
−0.28
· · · 131 · · · 1751
MRK266∗∗ 0.21±0.02 0.50f 1.19±0.06 · · · 0.18
+0.06
−0.05
100 240 1254 3203
NGC7552 <0.11 · · · <0.83 · · · · · · · · · · · · · · · · · ·
NGC 4552 <0.06 · · · <0.07 · · · · · · · · · · · · · · · · · ·
NGC 3079 <0.07a · · · <0.14a · · · · · · · · · · · · · · · · · ·
NGC 1614 <0.28 · · · <1.49 · · · · · · · · · · · · · · · · · ·
NGC 3628 <0.06 · · · <0.34 · · · · · · · · · · · · · · · · · ·
NGC 2623 0.30±0.04 · · · 0.47±0.07 · · · 0.63
+0.22
−0.14
17 163 218 2167
IRAS23128· · · 0.22±0.02 <0.40e 0.34±0.10 · · · 0.65
+0.23
−0.22
16 161 203 2152
MRK273 1.06±0.05 0.82e 2.74±0.19 · · · 0.39
+0.14
−0.10
49 192 617 2565
IRAS20551· · · <0.06 <0.25e <0.25 · · · · · · · · · · · · · · · · · ·
NGC3627 0.08±0.01 · · · 0.19±0.05 · · · 0.45+0.16
−0.12
40 184 504 2453
UGC05101 0.52b <1.50e 0.49b · · · 1.06+0.37
−0.28
· · · 131 · · · 1750
NGC4125 <0.03 · · · <0.07 · · · · · · · · · · · · · · · · · ·
NGC 4594 <0.03 · · · <0.04 · · · · · · · · · · · · · · · · · ·
Quasars
PG1351· · · <0.04 · · · <0.07 · · · · · · · · · · · · · · · · · ·
PG1211· · · 0.04±0.007 · · · <0.04 · · · >1.01 · · · · · · · · · · · ·
PG1119· · · 0.30±0.06 · · · 0.22±0.02 · · · 1.39
+0.49
−0.36
· · · 115 · · · 1531
PG2130· · · 0.42±0.03 · · · 0.42±0.05 · · · 1.00
+0.35
−0.26
· · · 135 · · · 1798
PG0804· · · <0.06 · · · <0.07 · · · · · · · · · · · · · · · · · ·
PG1501· · · 0.78±0.02 · · · 0.83±0.02 · · · 0.94
+0.33
−0.25
· · · 138 · · · 1846
Columns Explanation: Col(1):Common Source Names; Col(2): 14.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from
Spitzer; Col(3): 14.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(4): 24.31 µm [NeV] line flux and statistical
error in units of 10−20 W cm−2 from Spitzer; Col(5): 24.32 µm [NeV] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(6):
[NeV] Line Ratio used in plots and calculations; Col(7): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL,
calculated using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µm
(Lutz et al. 1996); Col(8): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density limit (HDL), calculated
using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µm (Lutz
et al. 1996), Col(9): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL, calculated using the Chiar & Tielens
(2006) extinction curve for the local ISM, Col(10): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density
limit (HDL), calculated using the Chiar & Tielens (2006) extinction curve for the local ISM.∗ Sturm et al. 2002 find that the [NeV] 24µm detection
for NGC 7469 is a questionable one, ∗∗ As discussed in detail in Section 6.2, Mrk 266 is the only galaxy in our sample where we find that aperture
variation may affect the observed [NeV] line flux ratio. For this reason it has been excluded from relevent plots and calculations. References for
Table 2:
a Weedman et al. 2005, b Armus et al. 2004 & 2006, c Sturm et al. 2002, d Verma et al. 2003, e Genzel et al. 1998, f Prieto & Viegas
Table 3: SIII Line Fluxes and Derived Extinction
Galaxy [SIII] [SIII] [SIII] [SIII] [SIII] Av Av Av Av PAH6.2
Source 18.71 18.71 33.48 33.48 Ratio Ratio to Ratio to Ratio to Ratio to SL2
SH ISO LH ISO LDL (D&L) HDL (D&L) LDL (C&T) HDL (C&T)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
Seyferts
NGC4151 7.50a 5.40c 6.57a 8.10c 1.14
+0.40
−0.30
· · · · · · · · · · · · 168.1
NGC1365 5.73±0.05 13.50c 27.20±0.38 36.10c 0.21
+0.07
−0.05
· · · · · · · · · · · · 132.3
NGC1097 2.18±0.02 · · · 11.40±0.23 · · · 0.19
+0.07
−0.05
· · · · · · · · · · · · 151.7
NGC7469 7.70a 9.20c 9.80a 10.40c 0.79
+0.28
−0.20
· · · · · · · · · · · · 415.0
NGC4945 3.18±0.03 6.30d 38.70±1.80 51.40d 0.08
+0.03
−0.02
· · · · · · · · · · · · 671.7
Circinus 19.10±0.70 35.20c 56.30±3.31 93.20c 0.37
+0.13
−0.10
· · · · · · · · · · · · 1018.4
Mrk 231 <0.47a <3.00e <2.30a <3.00e · · · · · · · · · · · · · · · 175.6
Mrk3 5.55a · · · 5.25a · · · 1.06
+0.37
−0.28
· · · 83 · · · 72 18.4
Cen A 4.54a 6.40c 14.80a 22.30c 0.31
+0.11
−0.08
· · · · · · · · · · · · 220.8
Mrk463 1.50b <0.80c 1.35b 1.20c 1.11
+0.39
−0.29
· · · 81 · · · 70 55.4
NGC 4826 3.39±0.03f · · · 4.61±0.08f · · · 0.74
+0.26
−0.19
· · · 96 · · · 82 66.2
NGC 4725 0.02±0.02f · · · 0.11±0.02f · · · 0.23
+0.08
−0.06
23 135 20 116 3.7
1 ZW 1 <0.11a <0.50c <0.18a <1.00c · · · · · · · · · · · · · · · 22.2
NGC 5033 0.85±0.11 · · · 2.42±0.10 · · · 0.35
+0.12
−0.09
· · · · · · · · · · · · 33.9
NGC1566 0.55±0.05f · · · 0.55±0.06f · · · 1.00
+0.35
−0.26
· · · 85 · · · 73 61.0
NGC 2841 0.22±0.04f · · · 0.29±0.03f · · · 0.75
+0.26
−0.20
· · · 95 · · · 82 10.9
NGC 7213 0.47±0.05 · · · 0.59±0.06 · · · 0.80
+0.28
−0.21
· · · · · · · · · · · · 28.7
LINERs
NGC4579 0.32±0.06f <0.78g 0.24±0.03f <1.20g 1.33
+0.47
−0.35
· · · 75 · · · 65 17.3
NGC3031 0.61±0.03 · · · 0.09±0.09 · · · 0.67
+0.24
−0.18
· · · · · · · · · · · · 23.4
NGC6240 1.99b <4.00e 2.63b 4.50e 0.76
+0.27
−0.20
· · · 95 · · · 81 399b
NGC5194 1.06±0.05f 1.00d 1.48±0.03f 4.60d 0.72
+0.25
−0.19
· · · 96 · · · 83 26.4
MRK266∗∗ 1.00±0.13 · · · 4.65±0.09 · · · 0.21
+0.08
−0.06
25 138 22 118 23.1
NGC7552 17.11±0.08f 24.60d 13.38±0.41f 41.10d 1.28
+0.45
−0.33
· · · 77 · · · 66 872.0
NGC 4552 0.07±0.03f · · · 0.06±0.02f · · · 1.29
+0.45
−0.34
· · · 76 · · · 66 21.5
NGC 3079 1.25a 6.80g 6.08a 6.60g 0.21
+0.07
−0.05
· · · · · · · · · · · · 620.3
NGC 1614 9.63±0.27 · · · 11.60±0.43 · · · 0.83
+0.29
−0.15
· · · 91 · · · 79 508.5
NGC 3628 2.14±0.03 · · · 15.80±0.33 · · · 0.14
+0.05
−0.04
· · · · · · · · · · · · 430.4
NGC 2623 0.88±0.05 · · · 3.16±0.20 · · · 0.28
+0.10
−0.07
16 129 14 111 128.6
IRAS23128· · · 2.62±0.12 0.89e 2.11±0.18 2.80e 1.24
+0.44
−0.32
· · · 78 · · · 67 90.1
MRK273 1.24±0.07 <0.82e 3.88±0.40 2.30e 0.32
+0.11
−0.08
12 124 10 107 69.2
IRAS20551· · · 0.66±0.06 0.30e 1.18±0.13 1.40e 0.56
+0.20
−0.15
· · · 105 · · · 90 38.3
NGC3627 0.38±0.03f · · · 0.57±0.09f · · · 0.67
+0.24
−0.18
· · · 99 · · · 85 153.8
UGC05101 0.98b <1.40e 1.30b 2.50e 0.75
+0.27
−0.20
· · · 95 · · · 82 190b
NGC4125 · · ·f · · · 0.06±0.05f · · · · · · · · · · · · · · · · · · 14.9
NGC 4594 0.39±0.03 · · · 1.24±0.13 · · · 0.32
+0.11
−0.08
· · · · · · · · · · · · 14.2
Quasars
PG1351· · · 0.34±0.06 · · · <0.13 · · · >2.70 · · · · · · · · · · · · 29.6
PG1211· · · <0.06 · · · <0.08 · · · · · · · · · · · · · · · · · · 25.8
PG1119· · · <0.13 · · · 0.19±0.06 · · · <0.71 · · · · · · · · · · · · 8.7
PG2130· · · <0.19 · · · 0.34±0.06 · · · <0.55 · · · · · · · · · · · · 26.9
PG0804· · · <0.06 · · · <0.21 · · · · · · · · · · · · · · · · · · 27.4
PG1501· · · 0.67±0.15 · · · 0.41±0.05 · · · 1.64
+0.58
−0.43
· · · 68 · · · 59 19.2
Columns Explanation: Col(1):Common Source Names; Col(2): 18.71 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from
Spitzer; Col(3): 18.71 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(4): 33.48 µm [SIII] line flux and statistical
error in units of 10−20 W cm−2 from Spitzer; Col(5): 33.48 µm [SIII] line flux and statistical error in units of 10−20 W cm−2 from ISO; Col(6):[SIII]
line flux ratio used for plots and calculations; Col(7): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL, calculated
using the Draine (1989) extinction curve amended by the more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µ (Lutz et
al. 1996) for those galaxies with distances greater than 55 Mpc that are not effected by aperture variations, Col(8): Extinction required to bring
ratios below the low-density limit (LDL) up to the high-density limit (HDL), calculated using the Draine (1989) extinction curve amended by the
more recent ISO SWS extinction curve toward the Galactic center for 2.5-10µ (Lutz et al. 1996) for those galaxies with distances greater than 55
Mpc that are not effected by aperture variations, Col(9): Extinction required to bring ratios below the low-density limit (LDL) up to the LDL,
calculated using the Chiar & Tielens (2006) extinction curve for the Galactic Center for those galaxies with distances greater than 55 Mpc that
are not effected by aperture variations, Col(10): Extinction required to bring ratios below the low-density limit (LDL) up to the high-density limit
(HDL), calculated using the Chiar & Tielens (2006) extinction curve for the Galactic Center for those galaxies with distances greater than 55 Mpc
that are not effected by aperture variations, Col(11): 6.2 µm PAH line flux in units of 10−21 W cm−2 ∗∗ The [SIII] ratio for Mrk 266 is known
to be affected by aperture variations(See Section 6.2). For this reason it has been excluded from relevent plots and calculations. References for
Table 3:
a Weedman et al. 2005, b Armus et al. 2004 & 2006, c Sturm et al. 2002, d Verma et al. 2003, e Genzel et al. 1998, f Dale et al. 2006, g
Satyapal et al. 2004.
32 33 34 35 36 37 38
 = -0.01
log(24 m Specific Luminosity) (W m-1)
 14/24 < 0.50
 0.50 < 14/24 < 1.0
 14/24 > 1.0
Fig. 5.— The observed [NeV] line ratio as a function of the 24µm
specific luminosity for our sample of galaxies. The error bars shown
here represent the calibration uncertainties on the [NeV] line flux
ratio as in Figure 3. The symbol type indicates the 14µm/24µm
continuum ratio.
a role in lowering the [NeV] line flux ratio, we argue that
differential infrared extinction to the [NeV] emitting re-
gion due to dust in the obscuring torus is responsible
for the low line ratios in at least some AGN. Clearly,
this requires that there is significant extinction at mid-IR
wavelengths, and specifically toward the [NeV]-emitting
regions. Is this reasonable? If there is significant ex-
tinction, it is possible that: 1) the [NeV]-emitting region
originates much closer to the central source than previ-
ously recognized, close enough to be extinguished by the
central torus in some galaxies, 2) the [NeV]-emitting por-
tion of the NLR is obscured by dust in the host galaxy
or in the NLR itself, or 3) some combination of these
scenarios. We explore these possibilities in the following
analysis.
5.1. The [NeV] originates in gas interior to the central
torus.
In the conventional picture of an AGN, the broad line
region (BLR) is thought to exist within a small region
interior to a dusty molecular torus while the NLR origi-
nates further out. This of course is the paradigm invoked
to explain the Type 1/Type 2 dichotomy. However there
have been multiple optical spectroscopic studies that con-
tradict the assumption that the observational properties
of the NLR are not dependent on the viewing angle and
the inclination of the system, suggesting that some of
the narrow emission lines originate in gas interior to the
torus. For instance, Shuder and Osterbrock (1981) and
Cohen (1983) showed that narrow high ionization forbid-
den lines such as [Fe VII] λ 6374 (requiring photons with
energies ≥ 100eV to ionize) are stronger relative to the
low ionization lines in Seyfert 1 galaxies (including inter-
mediate Seyferts, 1.2, 1.5 etc.) than in Seyfert 2 galaxies,
suggesting that some of the emission is obscured by the
torus. In addition, [FeX] λ 6374 and [NeV] λ 3426 have
also been shown to be less luminous in Type 2 objects
than in Type 1 objects (Murayama & Taniguchi, 1998a;
Schmitt 1998, Nagao et al. 2000, 2001a, 2001b, 2003,
Tran et al. 2000, see also Jackson and Browne (1990) for
narrow line radio galaxies and quasars.) These findings
may imply that the emission lines of species with the
highest ionization potentials originate closer to the AGN
than those of lower ionization species such as [OII]λ3727,
[SII]λλ6716, 6731, [OI] λ6300 etc. and therefore may be
partially obscured by the central torus.
If there is considerable extinction to the line-emitting
regions due to the torus, one may expect the mid-infrared
continuum to be similarly obscured. To test this scenario
we divided our sample into Type 1 or Type 2 objects
based on the presence or absence of broad (full width at
half max (FWHM) exceeding 1000 km s−1) Balmer emis-
sion lines in the optical spectrum. The spectral classifi-
cation for the [NeV]-emitting galaxies is given in Table 1.
In Figure 6a, we plot the [NeV] 14µm/24µm line flux ra-
tio versus the 14µm/24µm continuum ratio of the [NeV]
emitting galaxies in our sample. Assuming there is no
correlation between the electron density and the contin-
uum shape, a correlation between the line flux and con-
tinuum ratios would suggest that the mid-IR extinction
associated with the torus (such as that found by Clavel et
al. 2000) affects the observed line flux ratios. As can be
seen, there is a correlation between the line and contin-
uum ratios for galaxies with [NeV] emission. Moreover
we note that the 3 nuclei with ratios significantly below
the LDL are all Type 2 AGNs, while the 2 that lie sig-
nificantly above this limit are Type 1 AGNs, suggestive
that the extinction of the [NeV]-emitting region in Type
2 AGNs may be due to the torus. We note that the
error bars displayed in Figure 6 are based on a conserva-
tive estimate (15%) of the absolute calibration error on
the flux (see Section 3). Moreover, we have adopted the
most conservative approach in propagating the error (see
Section 4) for each line ratio. We further note that two
of the three nuclei with ratios below the LDL were also
observed in high-accuracy peak-up mode, resulting in a
pointing accuracy on the continuum for these galaxies of
0.4”. The third galaxy, NGC 3627, was observed in high
resolution mapping mode over 15” X 22”. We extracted
the spectra and found that the full map and the single
slit fluxes agree to within 10%. Thus pointing errors do
not appear to be responsible for the low ratios in these
galaxies. Finally we find that the ratios for the all galax-
ies except Mrk 266 are not sensitive to the line-fitting or
flux extraction methods that we have employed.
The Spearman rank correlation coefficient for Figure 6
is 0.60 (with a probability of chance correlation of 0.008),
indicating a significant correlation between the [NeV] line
flux ratios and the mid-IR continuum ratio. We note
that some AGNs are known to contain prominent sili-
cate emission features (Hao et al. 2005, Sturm et al.
2006) which have not been disentangled from the under-
lying continuum in this study. Because of this, the 14µm
or 24µm flux may be overestimated in some cases mak-
ing intrinsic value of the continuum at 14µm and 24µm
somewhat uncertain. However only one galaxy plotted
in Figure 6 is currently known to contain such features
(PG1211+143, Hao et al. 2005). Variations in ne and
the underlying continuum shape will also add scatter to
the correlation, as will differences in extinction to the
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
 = 0.60
F (14 m) / F (24 m) Continuum
Type 1 Objects
Type 2 Objects
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
 D < 55 Mpc
 D > 55 Mpc
F (14 m) / F (24 m) Continuum
Fig. 6.— The [NeV] line ratio vs. the Fν(14µm)/Fν (24µm) continuum ratio for our sample. In both plots, the error bars mark the
calibration uncertainties on the line ratio. There is a correlation between the line and continuum ratios which suggests that extinction
affects the observed line flux ratios. 6a) The majority of galaxies with ratios below the LDL are Type 2 objects, implying that the extinction
toward the [NeV]-emitting region may be due to the torus. 6b) The correlation shown here is not an artifact of aperture vatiations between
the SH and LH slits. The correlation holds when only the most distant galaxies are considered.
line- and continuum-producing regions. We should note
that the correlation seen in Figure 6 is not an artifact
of aperture variations between the LH and SH slit. The
correlation holds when only the most distant galaxies
(closed symbols in Figure 6b) are considered.
Independent of this correlation, our most important
finding is that the [NeV] line flux ratio is significantly
lower for Type 2 AGNs than it is for Type 1 AGNs. Fig-
ure 7 shows the relative [NeV] flux ratios for the Type
1 and Type 2 objects in our sample. The mean ratios
are 0.97 and 0.72 for the eight Type 1 and ten Type 2
AGNs, respectively, with uncertainties in the mean of
about 0.08 for each. Interestingly, although the sample
size is limited, precluding us from drawing firm statisti-
cally significant conclusions, there is a similar suggestive
trend seen in the sample of AGNs observed by Sturm et
al. (2002) with ISO-SWS. That is, in their work, the two
galaxies with the lowest [NeV] flux ratios are NGC 1365
and NGC 7582, both Type 2 AGNs. The galaxy with
the highest ratio in their work is TOL 0109-383 , a Type
1 AGN.
If indeed the torus obscures the IR [NeV] emission
in Type 2 objects, one would expect the optical/UV
[NeV] emission in these objects to be obscured as well.
We searched the literature for optical/UV detections of
[NeV] λ3426 for all of the galaxies in our sample and
found five galaxies with observations at this wavelength.
Four of these galaxies (Mrk 463, Mrk 3, NGC 1566, and
NGC 4151) were detected at [NeV] λ3426; the other
(NGC 3031) was not detected (see, Kuraszkiewicz et al.
2002, 2004 and Forster et al. 2001 for optical/UV fluxes).
Of the four galaxies with optical/UV [NeV] detections,
two are Type 1 galaxies (NGC 1566 & NGC 4151) and,
surprisingly, two are Type 2 galaxies (Mrk 3 and Mrk
463). If the Type 2 galaxies Mrk 3 and Mrk 463 had
[NeV] emitting regions interior to the torus, then the op-
 Type 1 AGN
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75
[NeV]14 m/[NeV]24 m 
 Type 2 AGN
Fig. 7.— Histogram of the [NeV] 14µm/24µm line flux ratio as
a function of AGN type. The [NeV] line ratios for Type 2 AGNs
are consistently lower than those from Type 1 AGNs.
tical/UV lines in these objects should not be detected
due to severe obscuration. We note that Mrk 3 and Mrk
463 have some of the highest X-ray luminosities (both
∼ 1043 erg s−1 ) in the sample and mid-IR [NeV] ra-
tios that are comparable to similarly luminous Type 1
objects–consistent with little or no obscuration in the
mid-IR for these Type 2 galaxies. This finding may im-
ply that, in the most powerful AGNs, the [NeV] emitting
region is pushed beyond the torus because the radiation
field is so intense, while lines with higher ionization po-
tentials than [NeV] (such as [NeVI], [FeX] etc.) are still
concealed by the torus. More data, both from the mid-
IR and from the optical/UV, are needed to further test
this hypothesis.
5.2. The [NeV]-emitting region is obscured by the host
galaxy or dust in the NLR.
While the correlation in Figures 6a and 6b are very
promising explanations for the observed [NeV] line ra-
tios, it is not completely clear why some Type 2 galaxies
appear obscured and others may not. Perhaps the [NeV]
emission is attenuated by dust in the NLR itself or else-
where in the host galaxy. Indeed, it is well-known that
dust does exist in the NLR (e.g., Radomski et al. 2003,
Tran et al. 2000) and that it can be extended and patchy
(e.g Alloin et al. 2000; Galliano et al. 2005; Mason et
al. 2006). In addition, dust in the host galaxy could be
responsible for the extinction seen here. For complete-
ness, we have conducted a detailed archival analysis of
all of the galaxies in our sample with [NeV] ratios close
to or below the LDL in order to see if there is additional
evidence for high extinction either in the host galaxy or
within the NLR. We find that the majority of galaxies
with low densities do indeed have well-known dust lanes,
large X-ray inferred column densities, or other properties
indicative of extinction.
Cen A: This nearby (D = 3.4Mpc) early type (S0)
galaxy at one time devoured a smaller gas-rich spiral
galaxy (Israel 1998, Quillen 2006). There is clear evi-
dence for substantial obscuration toward the nucleus of
Cen A. For example, the central region is veiled by a well
known dense dust lane thought to be a warped thin disk
(Ebneter & Balick 1983, Bland et al. 1986, 1987, Nichol-
son et al. 1992, Sparke 1996, Israel 1998, Quillen et al.
2006). Schreier et al. (1996) find V-band extinction av-
eraging 4-5 mag and infrared observations by Alonso &
Minniti yield AV values exceeding 30 mag in some re-
gions. Thus, it is plausible that there are regions toward
the nucleus of Cen A that are obscured even at infrared
wavelengths.
NGC 1566: The optical nuclear spectrum of this
nearby galaxy is known to vary dramatically over a pe-
riod of months, changing its optical classification from a
Type 2 object to a Type 1 object and back again (Pastor-
iza & Gerola 1970, de Vaucouleurs 1973, Penfold 1979,
Alloin et al. 1985). The narrow optical lines in this ob-
ject also show prominent blue wings and the radio prop-
erties of this galaxy are more consistent with a Type 2
object than a Type 1 object (Alloin et al. 1985). HST
continuum imagery reveals spiral dust lanes within 1” of
the nucleus (Griffiths et al. 1997) which might be re-
sponsible for the Type 1/Type 2 variability. Baribaud et
al. (1992) find hot dust which lies just outside the broad
line region in this galaxy and a large covering factor that
might explain the steep continuum of the AGN. Ehle et
al. (1996) find NH ∼ 2.5 × 10
20 cm−2 from ROSAT
X-ray observations of this galaxy.
NGC 2623: This galaxy’s tidal tails are evidence of a
merger event, however infrared observations reveal a sin-
gle symmetric nucleus, implying that the merging galax-
ies have coalesced. Multi-color, near infrared observa-
tions reveal strong concentrations of obscuring material
in the central 500 pc.(Joy & Harvey 1987; Lipari et al.
2004). Lipari et al. (2004) also find an optically obscured
nucleus with V-band extinction ≥ 5 mag.
IRAS 23128-5919: This galaxy is also in the late
stages of a merger. The nuclei of the two galaxies are
4kpc apart and have not yet coalesced. The northern
nucleus is a starburst. The southern nucleus is a known
AGN, though its optical classification, Seyfert or LINER,
is unclear (Duc, Mirabel, & Maza 1997; Charmandaris
et al. 2002; Satyapal et al. 2004). IRAS 23128-5919 is
an ultraluminous infrared galaxy (ULIRG), clearly con-
sistent with the presence of substantial dust towards the
nucleus. Optical spectroscopy of the southern nucleus in-
dicates very large (1500 km s−1) blue asymmetries in the
Hβ and [OIII] lines. This blue wing could be a signature
of extinction toward the far side of an expanding region,
where the red wing is preferentially obscured. (Johans-
son & Bergvall 1988).
Mrk 273: This galaxy is also a ULIRG, so significant
dust obscuration toward the nucleus is expected. Near-
IR imaging and high resolution radio observations show
evidence for a double nucleus in this galaxy separated
by less than 1 kpc (Ulvestad & Wilson 1984; Mazzarella
et al. 1991; Majewski et al. 1993). However, high res-
olution Chandra observations reveal only the northern
of the two nuclei, suggesting that this galaxy is hosting
only one AGN and that perhaps the other ”nucleus” is
in fact a portion of the southern radio jet. The soft X-
ray emission from the northern nucleus is obscured by
column densities of at least 1023 cm−2 (Xia et al. 2002).
Although the X-ray-emitting regions are physically dis-
tinct from the NLR and some of the obscuration at X-ray
wavelengths likely arises in dust-free gas within the sub-
limation radius, the high column density derived may
be consistent with high extinction toward the central re-
gions of this galaxy. Though Xia et al (2002) find that
the X-ray morphology of the AGN in Mrk 273 is con-
sistent with a Seyfert, Colina et al. (1999) find that it
has a LINER optical spectrum, thus implying that some
LINER galaxies are in fact heavily absorbed powerful
AGN. The soft diffuse X-ray halo in combination with
the radio morphology found by Carilli & Taylor (2000)
may suggest a circumnuclear starburst surrounding the
northern AGN nucleus, again consistent with substantial
obscuration toward the AGN.
NGC 3627: This nearby galaxy (D ∼ 10Mpc) is
thought to have had tidal interactions with NGC 3628,
a neighboring galaxy in the Leo Triplet, some 8 × 108
years ago which caused an intense burst of star forma-
tion in the nuclear regions around the same time (Rots
1978, Zhang et al. 1993, Afanasiev & Sil’chenko 2005).
Zhang et al. (1993) also discovered an extremely dense
molecular bar (mass ≥ 4 × 108 M⊙) and Chemin et al.
(2003) uncovered a warped disk using Hα observations,
both evidence of the tidal interaction. In their spectral
fitting to the BeppoSAX observation of NGC 3627, Geor-
gantopoulos et al. (2002) find intrinsic column densities
of ∼ 1.5 × 1022 cm−2 which, like Mrk 273, may suggest
substantial extinction to other regions near the nucleus.
NGC 7469: This is a well-known, extensively-studied
galaxy with strong, active star formation surrounding
a Seyfert 1 nucleus. Meixner et al. (1990) find dense
molecular gas (2 × 1010 M⊙), two orders of magnitude
above the Galactic value, within the central 2.5kpc of
the nucleus. 3.3µm imaging of the galaxy reveals that
80% of the PAH emission comes from an annulus ∼ 1”-
3”in radius around the central nucleus, indicating that
there is an elongated region of material that shelters the
PAH from the harsh radiation field of the AGN (Cutri
et al. 1984, Mazzarella et al. 1994). [OIII] line asym-
metries may corroborate the presence of a dense obscur-
ing medium, revealing a blue wing resulting when the
redshifted gas is obscured by the star forming ring (Wil-
son et al. 1986). In addition, Genzel et al. (1995) find
variation in the NIR emission attributable to extinction
and estimate the extinction from the CO observations of
Meixner et al. (1990) to be AV ∼ 10 mag.
NGC 1365: This nearby (D = 18.6 Mpc) AGN is
known to be circumscribed by embedded young star clus-
ters. The galaxy also contains a prominent bar with a
dust lane that penetrates the nuclear region (Phillips et
al. 1983, Lindblad et al. 1996 & 1999, Galliano et al.
2005). Like NGC 7469, NGC 1365 shows a peak at 3.5µm
implying PAH emission in spite of the harsh AGN radia-
tion field (Galliano et al. 2005). The large Hα/Hβ ratio
found by Alloin et al. (1981) implies substantial extinc-
tion toward the emission line regions, ranging from 3-4
mag. Observations with ASCA and ROSAT imply high
intrinsic column densities toward the X-ray emitting re-
gions, suggesting possibly high obscuration towards other
regions near the nucleus (Iyomoto et al. 1997, Komossa
& Schulz (1998), see also Schulz et al. 1999). Komossa &
Schulz show that the ratio of Hα to both the mid-IR and
X-ray radiation is substantially different in NGC 1365
compared with typical Seyfert 1 galaxies, possibly sug-
gesting inhomogenous obscuration (Schultz et al. 1999).
In an XMM X-ray study of NGC 1365, Risaliti et al.
(2005) also find a heavily absorbed Seyfert nucleus. The
blueshifted X-ray spectral lines imply high column den-
sities of 1023 cm−2 or more.
Mrk 266 (NGC 5256): This luminous infrared
galaxy is the only galaxy for which aperture effects most
likely account for the low 14µm/24µm ratio. Mrk 266
contains a very complicated structure which includes at
least two bright nuclei, a Seyfert and a LINER, that are
10” apart–a signature of a merger in progress. The mor-
phology of the northeast LINER nucleus is extremely
controversial (Wang et al. 1997; Kollatschny & Kowatsch
1998; Satyapal 2004, 2005; Ishigaki et al. 2000; Davies,
Ward, & Sugai 2000). Mazzarella et al. (1988) find three
non-thermal radio structures, two that coincide with the
nuclei and one between the two nuclei. Mazzarella et
al. (1988) suggest that the two nuclear structures are
associated with classical AGN and are in the stage of a
violent interaction in which the center of gravity of the
collision produces a massive burst of star formation with
supernovae or shocks which are responsible for the third
nonthermal radio source. As can be seen in Figure 8,
the SH slit, which provides the 14µm flux, overlaps with
this third radio source, while the LH slit, responsible for
the 24µm flux, encompasses the southwestern nucleus,
the third radio source, and part of the northeastern nu-
cleus. In this case the two lines observed originate in
physically distinct regions that do not each encompass
all potential sources of [NeV] emission, resulting in an
unphysical 14µm/24µm ratio. This is not to say that
Mrk 266 does not suffer from extinction at all. Indeed
the possible presence of a circumnuclear starburst im-
plies that there may be substantial extinction (Ishigaki
et al. 2000; Davies, Ward, & Sugai 2000). We have ver-
ified that this is the only distant galaxy in our sample
Fig. 8.— 20 cm image of Mrk 266 taken from NED
(http://nedwww.ipac.caltech.edu/). As can be seen here, the SH
slit (from which the 14µm line is extracted) overlaps with a third
radio source, while the LH slit (from which the 24µm line is ex-
tracted) encompasses the southwestern nucleus and part of the
northeastern nucleus.
with a complicated nuclear structure that will result in
aperture effects.
5.3. Can the [NeV] line flux ratio be used as a density
diagnostic?
Our analysis reveals that extinction towards parts of
the NLR in some objects is significant and cannot be ig-
nored at mid-IR wavelengths. In fact, it is quite possible
that extinction affects the [NeV] line flux ratios of those
galaxies with ratios above the low density limit (LDL)
and the amount of extinction in all cases is highly un-
certain. In addition to extinction, the temperature of
the [NeV] emitting gas is unknown. If the [NeV] emis-
sion originates within the walls of the obscuring central
torus, which may be the source of extinction in many of
our galaxies, we might expect the temperature of the gas
to reach 106 K (Ferland et al. 2002). If, on the other
hand, the [NeV] emission comes from further out in the
NLR and is instead attenuated by the intervening mate-
rial, we might expect the temperature of the gas to be
closer to 104 K. As shown in Figure 1, the electron densi-
ties inferred from the [NeV] line flux ratios are sensitive
to temperature when such large temperature variations
are considered. Based on the calculations shown in Fig-
ure 1, the low ratios could indicate that the densities in
the [NeV] line emitting gas are typically ≤ 3000 cm−3 for
T = 104K. However, if the [NeV] gas is characterized by
temperatures as high as T = 105K to 106K, densities
as high as 105 cm−3 would be consistent with our mea-
surements. We note that the [NeV] line flux ratios for
the galaxies in our sample (especially the Type 1 AGNs)
http://nedwww.ipac.caltech.edu/
all cluster around a ratio of ≈ 1.0. Two separate con-
clusions may be drawn from this finding: 1) That the
temperatures of the gas are low (∼ 104K) and that the
electron density is relatively constant over many orders
of magnitude in X-ray Luminosity and Eddington Ratio
for these AGNs, or 2) That the temperature of the gas
is high (105K to 106 K) and that the AGNs here sam-
ple a wide range of electron densities (from 102 cm−3
to 105 cm−3). Since gas temperature, electron density,
mid-IR continuum, and extinction are all unknown for
these objects, the electron density cannot be determined
here.
6. THE SIII LINE FLUX RATIOS
In Figure 9 we plot the 18µm/33µm line ratio as a
function of electron density ne. As with [NeV], we only
consider the five levels of the ground configuration when
computing the line ratio and we plot the relationship for
gas temperatures of T = 104K and 105K. We adopt col-
lision strengths from Tayal & Gupta (1999) and radiative
transition probabilities from Mendoza & Zeippen (1982).
Fig. 9.— 18µm/33µm line flux ratio in S III versus electron
density ne, for gas temperatures T = 10
4 K and 105 K.
In Table 3, we list the observed [SIII] line flux ratios
for the galaxies in our sample. As with the [NeV] ratios,
the [SIII] ratios in many galaxies listed in Table 3 are
well below the theoretically allowed value of 0.45 for a
gas temperature of T = 104K (13/33 detections). Again
we explore the observational effects and the theoretical
uncertainties that could artificially lower these ratios.
Aperture Effects: The ionization potential of [SIII]
is ∼ 35 eV and therefore the [SIII] emission may arise
from gas ionized by either the AGN or young stars. In
Table 3 we list, in addition to our Spitzer [SIII] fluxes,
all available [SIII] fluxes from ISO. Unlike [NeV], the
[SIII] fluxes from ISO are significantly larger than the
Spitzer fluxes for most galaxies. In Figure 10 we plot
the ISO to Spitzer flux ratios for the 18µm and 33µm
the [SIII] lines. As can be seen here, the [SIII] emission
extends beyond the Spitzer slit for many galaxies (6 out
of 9 for [SIII] 18µm and 11 out of 13 for [SIII] 33µm).
Similarly, when we compare the [SIII] flux arising from
a single slit centered on the nucleus to the flux arising
from a more extended region obtained using mapping
observations (Dale et al. 2006), we find that in most cases
the fluxes from the extended region are much larger than
the nuclear single-slit fluxes. Galaxies with fluxes from
Dale et al. (2006) are not included in Figure 10 since
the extraction aperture for these galaxies is comparable
to the 18µm ISO slit. We point out that the value for
this ratio is dependent on the orientation of the Spitzer
slit relative to the ISO slit and on the distance of each
object. We also note that IRAS20551 and IRAS23128 are
point sources with Spitzer 18µm fluxes greater than the
ISO fluxes from Genzel et al. (1998), however they fall
within the Genzel et al. (1998) quoted errors of 30% and
the Spitzer calibration error of 15%. Figure 10 suggests
that the [SIII] emission may be produced in the extended,
circumnuclear star forming regions associated with many
AGNs and that aperture effects need to be considered in
our analysis of the [SIII] ratio for nearby objects.
[SIII]
(ISO) / F
[SIII]
(Spitzer)
 [SIII] 18 micron Ratio
0
 1
 2
 3
 4
 5
 6
 [SIII] 33 micron Ratio
Fig. 10.— The ratios of the [SIII] flux from ISO and Spitzer for
the 18µm and 33µm lines. The range indicated with arrows is that
corresponding to the absolute flux calibration for ISO (20%) and
Spitzer (15%). The [SIII] emission is indeed extended beyond the
Spitzer slit for many galaxies, suggesting that the [SIII] emission
may be produced in star forming regions. We note that IRAS20551
and IRAS23128 are point sources with Spitzer 18µm fluxes greater
than the ISO fluxes from Genzel et al. 1998, however they fall
within the Genzel et al. (1998) quoted errors of 30% and the Spitzer
calibration error of 15%. Galaxies with fluxes from Dale et al. 2006
are not included in this plot since the extraction aperture for these
galaxies is comparable to the 18µm ISO slit.
The contribution from star formation to the [SIII] lines
can be estimated using the strength of the PAH emis-
sion, one of the most widely used indicators of the star
formation activity in galaxies (e.g. Luhman et al. 2003;
Genzel et al. 1998; Roche et al. 1991; Rigopoulou et
al. 1999, Clavel et al. 2000; Peeters, Spoon, & Tielens
2004). We examined the [SIII] 18.71 µm/PAH 6.2 µm
and [SIII] 33.48 µm/PAH 6.2 µm line flux ratios in 7
starburst galaxies observed by Spitzer and found them
to be comparable to the analogous ratios in our entire
sample of AGNs as shown in Figure 11. This suggests
that the bulk of the [SIII] emission originates in gas ion-
ized by young stars. We note that the apertures of the
SH and LH IRS modules are smaller than that of the
SL2 module, which may artificially raise the line ratios
plotted in Figure 11 for nearby galaxies compared with
the more distant ones. However, the fact that the line
ratios plotted in Figure 11 span a very narrow range sug-
gests that the [SIII] line emission has a similar origin in
starbursts and in AGNs. Thus, we assume that the bulk
of the [SIII] emission originates in gas ionized by young
stars and that the electron densities derived using these
lines taken from slits of the same size (such those galaxies
coming from Dale et al. 2006 mapping observations) or
from the most distant galaxies are representative of the
gas density in star forming regions.
Extinction: We have shown that aperture effects are
the likely explanation for why many of the [SIII] ratios
for the galaxies in our sample fall below the LDL. How-
ever, there are three galaxies in the sample with ratios
below the LDL that are distant enough (D>55 Mpc, cor-
responding to projected distances greater than 1.2 by 3
kpc and 3 by 6 kpc for the SH and LH slits, respectively)
that aperture effects may not be as important (NGC 2623
& Mrk 273, Mrk 266 has been excluded since it is known
to be affected by aperture variations See Section 5.2).
Extinction may be the explanation for the low ratios in
these galaxies. However, even though the SH and LH slits
likely cover the entirety of the NLR at these distances, we
note that these three galaxies contain well-known, large
circumnuclear starbursts (See Section 5.2 for the indi-
vidual galaxy summaries) which may produce extremely
extended [SIII] emission. It is therefore still possible that
the line ratios in these galaxies are artificially lowered by
aperture variations between the SH and LH slits. How-
ever, in addition to these three distant galaxies, NGC
4725 from Dale et al. (2006) has a [SIII] ratio below the
low density limit. The low [SIII] ratio (<0.45) in this
case cannot be attributed to aperture variations since
the extraction region is the same for both the 18 and
33µm lines. Thus, for completeness, the extinction de-
rived using the extinction curves given in Section 4 from
the observed [SIII] line ratio for these four sources are
given in Table 3. The Draine (1989) and Lutz et al.
(1996) extinction curve calculations yield extinction val-
ues that range from ∼ 12 to 25 mag. The Chiar and
Tielens (2006) extinction curve for the Galactic Center
may also be used since, unlike [NeV], the extinction at
the longer wavelength line (33µm) is greater than that
at the shorter wavelength line (18µm). The values de-
rived from this method are quite similar, ranging from ∼
10 to 22 mag. The Chiar and Tielens (2006) extinction
curve from the local ISM cannot be used here since it
only extends to 27.0µm.
Computed Quantities: As with NeV, there may be
uncertainties in the computed SIII infrared collisional
rate coefficients. However, there is generally less contro-
versy surrounding the [SIII] coefficients and these values
are widely accepted.
Our analysis suggests that aperture effects severely in-
fluence the [SIII] line flux ratios in most cases and that
the observed flux is likely dominated by star forming
regions. Figure 12, a plot of the [SIII] line ratio as a
function of distance, illustrates the influence of aperture
effects on the [SIII] line ratio. Most of the galaxies at dis-
tances <55 Mpc with [SIII] fluxes extracted from aper-
tures of different sizes (i.e. NOT the Dale et al. (2006)
galaxies) are below the LDL. On the other hand, galax-
ies at larger distances and galaxies with fluxes from Dale
et al. (2006) are generally above the LDL. Thus, for the
most distant galaxies in our sample and the galaxies with
fluxes from Dale et al. (2006) where the aperture for the
18 and 33 µm lines are equal, aperture effects are not
problematic, but extinction, as can be seen from Mrk
273, NGC2623, and NGC 4725 in Figure 12, needs to be
considered. As with the [NeV] line ratio, the [SIII] line
ratio is NOT a tracer of the electron density in our sam-
ple. In conclusion, the ambiguity of the intrinsic [SIII]
line ratio is primarily the result of aperture variations.
However there is at least one case (NGC 4725) where
aperture effects cannot explain the low ratio, implying
that, in addition to aperture variations, extinction likely
plays a role in lowering the [SIII] line flux ratios.
7. SUMMARY
We report in this paper the [NeV] 14µm/24µm and
[SIII]18µm/33µm line flux ratios, traditionally used to
measure electron densities in ionized gas, in an archival
sample of 41 AGNs observed by the Spitzer Space Tele-
scope.
1. We find that the [NeV] 14µm/24µm line flux ratios
are low: approximately 70% of those measured are
consistent with the low density limit to within the
calibration uncertainties of the IRS.
2. We find that Type 2 AGNs have lower [NeV]
14µm/24µm line flux ratios than Type 1 AGNs.
The mean ratios are 0.97 and 0.72 for the eight
Type 1 and ten Type 2 AGNs, respectively, with
uncertainties in the mean of about 0.08 for each.
3. For several galaxies, the observed [NeV] line ratios
are below the theoretical low density limit. All of
these galaxies are Type 2 AGNs.
4. We discuss the physical mechanisms that may play
a role in lowering the line ratios: differential mid-
IR extinction, low density, high temperature, and
high mid-IR radiation density.
5. We argue that the [NeV]-emitting region likely
originates interior to the torus in many of these
AGNs and that differential infrared extinction due
to dust in the obscuring torus may be responsible
for the ratios below the low density limit. We sug-
gest that the ratio may be a tracer of the torus
inclination angle to our line of sight.
6. Our results imply that the extinction curve in these
galaxies must be characterized by higher extinction
at 14µm than at 24µm, contrary to recent studies
of the extinction curve toward the Galactic Center.
7. A comparison between the [NeV] line fluxes ob-
tained with Spitzer and ISO reveals that there are
systematic discrepancies in calibration between the
two instruments. However, our results are indepen-
dent of which instrument is used; [NeV] line flux
ratios are consistently lower in Type 2 AGNs than
in Type 1 and [NeV] line flux ratios below the LDL
are observed with both ISO and Spitzer.
0
 1
 2
 3
 4
 5
[SIII]18/F(PAH6.2)
 
Starbursts
0
 1
 2
 3
 4
 5
 
AGN
0
 1
 2
 3
 4
 5
[SIII](33)/F(PAH6.2)
 
Starbursts
0
 1
 2
 3
 4
 5
 
AGN
Fig. 11.— Distribution of the [SIII]33µm/PAH 6.2µm and the [SIII]18µm/PAH 6.2µm line flux ratios for our sample of AGNs and a
small sample of starburst galaxies observed by Spitzer. It is apparent that the line ratios of the AGNs are comparable to the corresponding
ratios in starbursts, suggesting that the bulk of the [SIII] emission originates in star forming regions and not the NLRs in our sample of
AGNs.
8. Our work provides strong motivation for investigat-
ing the mid-IR spectra of a larger sample of galaxies
with Spitzer in order to test our conclusions.
9. Finally, an analysis of the [SIII] emission reveals
that it is extended in many or all of the galaxies
and likely originates in star forming gas and NOT
the NLR. Since there is a variation in the aper-
tures between the SH and LH modules of the IRS,
we cannot use the [SIII] line flux ratios to derive
densities for the majority of galaxies in our sam-
We are extremely thankful for all of the invaluable data
analysis assistance from Dan Watson and Joel Green,
without which this work would not have been possible.
We are also very grateful to Davide Donato, Eli Dwek,
Frederic Galliano, Paul Martini, Kartik Sheth, Eckhard
Sturm, Peter van Hoof, and Dan Watson for their en-
lightening and thoughtful comments/expertise that sig-
nificantly improved this paper. Carissa Khanna was also
very helpful in providing assistance in the preliminary
data analysis. We are also grateful for the helpful and
constructive comments from the referee. This research
has made use of the NASA/IPAC Extragalactic Database
(NED) which is operated by the Jet Propulsion Labora-
tory, California Institute of Technology, under contract
with the National Aeronautics and Space Administra-
tion. SS gratefully acknowledges financial support from
NASA grant NAG5-11432 and NAG03-4134X. JCW
gratefully acknowledges support from Spitzer Space Tele-
scope Theoretical Research Program. JCW is a Cottrell
Scholar of Research Corporation. Research in infrared
astronomy at NRL is supported by 6.1 base funding.
RPD gratefully acknowledges financial support from the
NASA Graduate Student Research Program.
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