Draft version October 27, 2018
Preprint typeset using LATEX style emulateapj v. 08/22/09
THE MID-INFRARED EMISSION OF M87
Eric S. Perlman
, R. E. Mason
, Christopher Packham
, N. A. Levenson
, Moshe Elitzur
, Justin J.
Schaefer
, Masatoshi Imanishi
, William B. Sparks
, James Radomski
Draft version October 27, 2018
ABSTRACT
We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared
from 5-35 µm. These observations allow us to investigate mid-IR emission mechanisms in the core
of M87 and to establish that the flaring, variable jet component HST-1 is not a major contributor
to the mid-IR flux. The Spitzer data include a high signal-to-noise 15-35 µm spectrum of the knot
A/B complex in the jet, which is consistent with synchrotron emission. However, a synchrotron model
cannot account for the observed nuclear spectrum, even when contributions from the jet, necessary
due to the degrading of resolution with wavelength, are included. The Spitzer data show a clear excess
in the spectrum of the nucleus at wavelengths longer than 25 µm, which we model as thermal emission
from cool dust at a characteristic temperature of 55 ± 10 K, with an IR luminosity ∼ 1039 erg s−1.
Given Spitzer’s few-arcsecond angular resolution, the dust seen in the nuclear spectrum could be
located anywhere within ∼ 5′′ (390 pc) of the nucleus. In any case, the ratio of AGN thermal to
bolometric luminosity indicates that M87 does not contain the IR-bright torus that classical unified
AGN schemes invoke. However, this result is consistent with theoretical predictions for low-luminosity
AGNs.
Subject headings:
1. INTRODUCTION
M87, the dominant galaxy in the Virgo cluster, is one of
the nearest (distance=16 Mpc, 1′′= 78 pc) radio galax-
ies. Since 1918, when Heber Curtis observed a “curi-
ous straight ray” extending from its nucleus, it has been
known to host a bright jet that is visible at radio through
X-ray wavelengths. The jet, one of the primary hall-
marks of M87’s nuclear activity, has a complex, knotty
structure (see Perlman et al. 2001b [hereafter P01b] and
references therein). M87 also exhibits energetic line emis-
sion from a disk that is seen to be in Keplerian mo-
tion (Ford et al. 1994, Harms et al. 1994). Longslit
spectroscopy of the material within the disk has allowed
the mass in the inner 3.5 pc of M87 to be measured at
≈ 3 × 109M⊙ (Marconi et al. 1997), implying the pres-
ence of one of the most massive known black holes.
M87’s proximity allows us to obtain particularly high
spatial resolution. Unfortunately, M87 is quite faint in
the infrared, with a core flux of only 16 mJy at 10.8µm
(Perlman et al. 2001a, hereafter P01a). This makes
Spitzer well-suited to sensitive observations of the in-
1 Joint Center for Astrophysics, Physics Department, University
of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore,
MD 21250
2 Current Address: Department of Physics & Space Sciences,
Florida Institute of Technology, 150 W. University Boulevard, Mel-
bourne, FL 32901
3 Gemini Observatory, Northern Operations Center, 670 N.
A’Ohoku Place, Hilo, HI 96720, USA
4 Department of Astronomy, University of Florida, 211 BRSC,
Gainesville, FL 32611, USA
5 Department of Physics and Astronomy, University of Kentucky,
177 Chem.-Phys. Building, Lexington, KY 40506-0055, USA
6 National Astronomical Observatory, Mitaka, Tokyo 181-8588,
Japan
7 Space Telescope Science Institute, 3700 San Martin Drive, Bal-
timore,MD 21218, USA
8 Gemini Observatory, Southern Operations Center, c/o AURA,
Casilla 603, La Serena, Chile
TABLE 1
Known knots in the jet of M87
Knot Dist. from Nuc.1 Approx. Extent2 10.8 µm Flux3
Name (arcsecs) (arcsecs) (mJy)
HST-14 0.8 0.5 × 0.1 < 2.3
D 2.7 1.8 × 0.2 1.2± 0.2
E5 6.0 0.9 × 0.3 0.1
F 8.2 1.4 × 0.4 1.3± 0.2
I5 11.2 0.6 × 0.5 0.25
A 12.4 2.2 × 1.3 6.4± 0.6
B 14.3 2.4 × 1.3 6.3± 0.6
C 17.4 1.9 × 1.7 4.0± 0.6
1 Location of the flux maximum, rather than centroid.
2 As measured in optical HST images.
3 Except where noted, as given in P01a.
4 Knot flux is highly variable; the flux maximum is at the upstream
end. The 2 σ flux limit given here is derived from these Subaru data.
5 Not detected in Gemini images (P01a); fluxes are extrapolated from
the radio-optical spectrum (P01b).
frared emission from this object, while at the same time
the small spatial scales that can be probed make M87 an
appropriate mid-infrared (mid-IR) target for the largest
ground-based telescopes – albeit one requiring long inte-
grations. The knots in the jet are significantly fainter
than the nucleus. Table 1 reviews the locations and
extents of major knots in the jet, along with N-band
fluxes either directly measured or extrapolated from ra-
dio/optical data.
Previous ground-based observations showed that most
of the mid-IR emission from the core of M87 can be
attributed to synchrotron radiation from the innermost
regions of the jet (P01a, Whysong & Antonucci 2004),
although a minor contribution from dust at T <∼ 150
K could not be ruled out. These results provided
strong evidence that the dusty torus of classical unified
AGN schemes (Antonucci 1993) may be absent in low-
http://arxiv.org/abs/0704.1156v2
luminosity AGNs. Two recent papers have used Spitzer
data to address the mid-IR emission processes of M87.
Bressan et al. (2006) used 5-20 µm IRS spectra to ex-
amine the issue of silicate emission from Virgo Cluster
galaxies. They showed that M87, like other Virgo el-
lipticals, exhibits a silicate emission feature at 10 µm,
but that this feature is not spatially resolved by Spitzer.
Shi et al. (2007) used IRAC and MIPS imaging data to
re-examine the origin of M87’s mid-IR emission. They
found that their photometry of the nucleus plus jet and
lobes can be fit by a combination of two synchrotron
power laws (usually breaking at optical or higher ener-
gies), but that in larger (1′) apertures an excess is present
over that model. This excess is ascribed to dust in the
host galaxy with a luminosity similar to that observed in
other brightest cluster galaxies.
In this paper we discuss new mid-IR imaging and spec-
troscopy of M87 using the Subaru observatory and Spitzer
Space Telescope. We combine data from the IRAC, MIPS
and IRS instruments (the latter covering the entire 5-
35µm range) to obtain a more detailed picture of the
emission processes in the nucleus and jet. In §2 we
present the observations and data reduction procedures,
while in §3 we discuss M87’s line and continuum emis-
sion. We close in §4 with a summary and discussion.
2. OBSERVATIONS AND DATA
2.1. Ground-Based Observations
Mid-IR imaging and spectroscopy of M87 were ob-
tained on the nights of UT 2005 April 27 and 28 us-
ing the COMICS camera (Kataza et al. 2000) attached
to the Cassegrain focus of the Subaru Telescope (Iye et
al. 2004). The 320 × 240 SiAs IBC array has a plate
scale of 0.129′′/pixel, delivering a 41× 31′′ field of view.
The detector was read-out in correlated quadruple sam-
pling (CQS) mode (Sako et al. 2003). The observing
conditions on both nights were not photometric, and the
precipitable water vapor was variable, as high as 3-5 mm
on April 27, but reaching 5-7mm on April 28.
For the acquisition images, we used the N11.7 filter
(central wavelength 11.67µm, bandwidth 1.05µm, 50%
cut-on/off). To remove time-variable sky background,
telescope thermal noise and so-called “1/f” detector
noise, we used the standard chop-nod technique, with
a 10′′ chop throw at a position angle of −69.5 degrees
(measured north through east), to project the jet along
the detector rows and place the reference beam between
minima in the jet emission. The chop frequency was 0.45
Hz (standard for Subaru/COMICS observations) and the
total on-source time was 700s.
We used a similar setup for the spectroscopic ob-
servations, with similar chop parameters, but without
nodding, as is standard practice with COMICS, and
the broad N band order blocking filter. The grating
and 0.33′′ slit provided a spectral resolution of ∼ 250
(0.02µm/pix), dispersing the entire N band across the
array. The superb design and capabilities of COMICS
permit on-slit mid-IR imaging of the polished slit jaws,
simultaneous with the spectroscopic observations. Thus
we were able to ensure that the emission from the nucleus
of M87 passed through the slit and into the spectroscopy
arm of COMICS. This proved to be crucial as differen-
tial refraction between the telescope’s optical autoguider
Fig. 1.— Subaru acquisition image of M87. North is up and east
is to the left. The high spatial resolution of the COMICS data is
apparent, despite its limited sensitivity. A 3-pixel Gaussian was
used to smooth these data. We do not detect emission from knot
HST-1, 0.8′′ from the nucleus (location of the circle on this figure).
See §§2,3 for details.
and the MIR science beam moved the object off the slit
in as little as 20 minutes at some airmasses. The varying,
suboptimal conditions meant that M87 was not detected
in many of the spectroscopy frames, and only those in
which the continuum was visible were stacked and used
in the final analysis. The data presented in this paper
represent 2100 s of on-source time.
The data were reduced using IRAF and in-house devel-
oped IDL routines. The difference for each chopped pair
(and for each nod-set for imaging), was calculated, and
the results combined until a single frame was created.
During the reduction process, chopped pairs obviously
compromised by cirrus, high electronic noise, or other
problems were discarded.
The PSF was measured from observations of the pho-
tometric standard. The measured full-width at half-
maximum (FWHM) of the standard was 0.55′′, which
agrees well with the FWHM of the spectral trace. Flux
calibration was achieved using HD108985 as a flux stan-
dard (Cohen et al. 1992, 1999; Tokunaga 1984) and inter-
polating the Cohen models to the proper bandpass. Ab-
solute errors in flux calibration were estimated from the
variations in the counts through the course of the nights.
Given the non-photometric conditions under which the
data were obtained, we estimate an accuracy of at best
15% for the flux calibration.
A 5-pixel extraction aperture was used for the spec-
troscopy data, and spectral flux calibration was with ref-
erence to the telluric standard star, Vega. No attempt
was made to correct for slit losses in the spectrum of M87,
but we note that the flux density in the calibrated spec-
trum is comparable to that of both the Subaru/COMICS
image and the Gemini/OSCIR photometry of P01a.
In Figure 1 we show the Subaru image, while in Figure
2 we show the Subaru spectrum.
2.2. Spitzer Observations
Spitzer Space Telescope observations of M87 were ob-
tained from the Spitzer archives. This includes IRS (In-
Fig. 2.— The Subaru/COMICS spectrum of the nucleus, showing
the [Ne II] 12.81 µm emission line. See §§2,3 for details.
frared Spectrograph, Houck et al. 2004) spectra taken on
UT 2004 January 4, as well as MIPS (Multi-band Imag-
ing Photometer for Spitzer, Rieke et al. 2004) images
taken on UT 2004 December 26. Both datasets were ac-
quired as part of PID 82 (PI Rieke). We also obtained
data taken on 2005 June 11 with the IRAC (Infrared
Array Camera) as part of PID 224 (PID Forman). The
advantage of this particular IRS dataset is that it cov-
ers the full wavelength range of the short-low and long-
low modules. We also comment below on other Spitzer
datasets.
The IRAC and MIPS data were processed according to
the standard pipeline recipes. In both, we used the post-
basic calibrated data (post-BCD). Fluxes were extracted
using circular apertures appropriate to the diffraction
limited resolution at a given wavelength. Background
subtraction was done using annular apertures that in-
cluded all the flux from the galaxy itself. The Spitzer
imaging data (shown in Figure 3) clearly show emission
from both the nucleus and the knot A/B complex, as well
as extended emission from the galaxy and the south-west
hotspot, a feature thought to be associated with the un-
seen counterjet (Hines et al. 1989, Sparks et al. 1992,
Stiavelli et al. 1992). However, individual knots are not
resolved at Spitzer’s angular resolution (≈ 3′′ at 10µm).
In this paper, we discuss only the emission from the core
and jet knots A and B. We subtracted from the core flux
measured on the IRAC data any emission that might
be due to galactic emission, using an annulus between
5 − 15′′; we also subtracted flux due to the jet by gaug-
ing independently the flux in the four quadrants. For this
reason our fluxes differ somewhat from those reported in
Shi et al. (2007). The 24µm flux derived in this way
agrees fully (within 1σ) with that found in the IRS spec-
trum of the core (§3.1).
The IRS spectra were acquired using both orders of the
low-resolution short-wavelength (“short-low”) and low-
resolution long-wavelength (“long-low”) modules of the
IRS. Total integration times of 28 s and 60 s were used
for the short- and long-wavelength modules, respectively,
and spectra were obtained in two nod positions separated
by 1/3 of the slit length, translating into nod throws
of 19′′ and 56′′ for the short-low and long-low modules
respectively. The spacecraft roll angle and relative slit
position angles were such that the 3.6− 3.7′′-wide short-
4.5 micron
SW Hotspot
Nucleus
24 micron
Fig. 3.— Spitzer imaging of M87, at 4.5 and 24.5 µm. We have
indicated the core, jet and SW hotspot for reference. See §2 for
details.
low slits were oriented perpendicular to the jet, while the
wider (10.5− 10.7′′) long-low slits were aligned along the
jet axis and therefore contain emission from the brightest
region of the jet (primarily knots A and B, see Table 1).
After standard pipeline processing, thermal back-
ground emission was removed by subtracting spectra
taken at the two nod positions, and 1D spectra then ex-
tracted using the SPICE package. The default extraction
apertures (7.2′′ at 6 µm, 14.4′′at 12 µm, scaling linearly
with wavelength) were employed for the extraction of the
short-low data, whereas to better separate the nuclear
and jet emission in the long-low data, we used a smaller
aperture that varied in size with wavelength, increasing
linearly from 5.1” at 17µm to 10.2” at 34 µm, compara-
ble to the FWHM of Spitzer’s PSF at those wavelengths.
As the long-low slits covered both the nucleus and the
knot A/B region, we extracted two 1D spectral datasets.
No significant fringing is visible in any of the spectra,
and no defringing was attempted.
As M87 is an extended source, it is non-trivial to equal-
ize the flux calibration of the orders in both the short-low
and long-low modules, or between the two modules, due
to the different extraction apertures. As a result, the
spectra from different orders may contain differing con-
tributions from the various extended structures, e.g., the
various jet components. The nucleus is essentially unaf-
fected by this problem at short wavelengths; however, at
longer wavelengths it does have a significant effect. The
fact that the nucleus is essentially unresolved at short
wavelengths allowed us to use the flux scale from the
pipeline for the SL data. We then made use of the over-
lap and “bonus segment” data to match up the flux and
slope of the spectral segments blueward and redward of
each join. No correction was found to be necessary to
scale the SL1 data to that from SL2. However, a scaling
factor of 1.67 ±0.03 was found to be necessary to scale
the LL2 data to the SL1 data, while a scaling factor of
1.38±0.03 was necessary to scale the two long-low orders
of the nuclear spectrum.
The knot A/B region, by contrast, is highly extended
(about 1.3′′×5′′, as measured on the optical and Gemini-
N+OSCIR data). This makes the flux calibration process
much more complicated, since the SPICE package is not
optimized (or intended) to deal with extended sources of
this nature. In this case we used the Gemini-N + OSCIR
result (P01a) to set the flux scale at 10.8µ m, and then
use the best-fit spectral index to extrapolate from the
LL spectra down to that wavelength (note that we do
not have SL data for the knot A/B region). We then
used the overlap and “bonus segment” data to match up
the flux and slope of the spectral segments blueward and
redward of the LL1/LL2 join (similarly to what we did for
the nucleus). Because the structural details of the knot
A/B region are rather different from the nuclear region, a
smaller scaling factor (1.04± 0.03) was required to scale
the LL1 data to the LL2 flux scale once it was fixed
using the Gemini-N 10.8µm flux point. The resulting
IRS spectra of M87’s nucleus and jet are shown in Fig.
3. RESULTS
The reduced Subaru acquisition image is shown in Fig-
ure 1. The core is clearly detected, with a flux of 20.8 ±
3.5 mJy, consistent with the findings of P01b to within
the 1σ errors The error in the flux calibration is dom-
inated by the variable weather conditions. We do not
detect any jet features. Even for the knot A/B complex
this is not unexpected given the extended nature of the
knot, our short integration time, fairly narrow band and
the poor sky conditions. Surprisingly, however, we do
not detect the flaring jet component HST-1 (Harris et
al. 2003, 2005; Perlman et al. 2003), 0.8′′ from the core,
despite the fact that at its peak in 2005 March (just a
few weeks before the Subaru observations), its 0.8 µm
flux was brighter than that of the core itself (Biretta et
al., in prep.). We place a limit of 2.3 mJy at 2σ on the
flux from knot HST-1.
The COMICS nuclear spectrum is shown in Figure 2.
The signal-to-noise ratio of the spectrum is low, but we
detect continuum emission plus a single emission line at
12.81 µm (rest frame). This spectrum contains only flux
from the nucleus; the extraction aperture would have ex-
cluded any significant contribution from HST-1 which
in any case is faint. In Figure 4, we show the Spitzer
spectrum of the knot A/B region (blue) as well as the
nucleus (black). The signal-to-noise ratio of the Spitzer
data, while still modest (∼ 10 per pixel) is significantly
better than that of the Subaru spectrum. We have over-
plotted on Figure 4 a binned version of the Subaru nu-
clear spectrum, which matches well with that obtained
by Spitzer. Despite the difference in angular resolution
between the Spitzer (∼ 3′′ at 10 µm) and Subaru (∼ 0.3′′)
Fig. 4.— The Spitzer IRS spectrum of the nucleus of M87 (black)
as well as knots A and B (blue). For comparison, the binned Subaru
COMICS data are shown in red (error bars reflect the standard
error on the mean of the points in each bin). To better separate the
two IRS spectra, the data for knots A and B have been multipled
by 0.75. Several low-ionization emission lines can be seen in the
spectrum of the nucleus, as well as hints of silicate emission around
10 and 18 µm. See §§3, 3.1 for details.
data, the nuclear spectra are similar in flux density and
spectral slope in both datasets. This confirms that HST-
1 (which in the Spitzer data would not be resolved from
the nucleus) makes no more than a minimal contribution
to the nuclear spectrum. Both the noise in the Subaru
data, however, and the known optical variability of the
nucleus and HST-1 (measured to be as large as 50% and
500%, respectively; Tsvetanov et al. 1998, Perlman et
al. 2003) prevent us from setting stricter limits on the
mid-IR flux of HST-1.
The overall similarity of the spectral shapes of the nu-
cleus and the knot A/B complex, at least below 25 µm,
confirms the synchrotron nature of the majority of the
nuclear emission, first shown by P01a. A close exami-
nation of the nuclear and jet spectra does, however, re-
veal differences. The first of these is the presence of five
obvious emission lines in the spectrum of the nucleus,
whereas only one weak emission line is seen in the knot
A/B spectrum. A more subtle difference is that the nu-
clear spectrum contains a sharper upturn towards longer
wavelengths. We discuss the spectral data in more depth
in the following subsections. Note that in the discussion
that follows, we use the terms “nuclear region” and “nu-
clear spectrum” to refer to the Spitzer spectrum taken
at the position of the nucleus (as opposed to that of the
knot A/B region), while what we term as the core refers
only to the regions within ∼ 0.3′′ of the central black hole
(and thus unresolvable in ground-based N-band images).
3.1. IR Continuum Emission
The infrared continuum of a galaxy can contain emis-
sion from a number of different components. In the case
of M87, the most likely sources are thermal emission from
warm or cool dust, synchrotron emission from the jet,
and (at shorter wavelengths) starlight from K and M
stars. In order to determine the nature and origin of this
IR emission, it is necessary to correctly account for all
possible sources that could fall within the slit and beam
of Spitzer at any given wavelength. At Spitzer’s angular
Fig. 5.— Spitzer IRS spectrum of jet knots A and B, modeled
as discussed in §3.2. In blue, we show the contribution due to
knots A and B (summed); in red, the contribution from the core,
and in green the contribution from all other knots. The black line
represents the sum of the contributions from the core and all knots.
The observed spectrum for knots A and B can be satisfactorily
reproduced with these synchrotron components.
resolution the nuclear spectrum will contain a contribu-
tion from emission in the jet (and vice versa; see Table 1),
and, because of the decrease in spatial resolution towards
longer wavelengths, the magnitude of that contribution
will increase with wavelength. We have modeled both
the nuclear and knot A/B spectra, and accounted for
this effect as follows.
We model each component as a 1-dimensional Gaus-
sian of width equal to Spitzer’s diffraction limit (which
we take to be a Gaussian of FWHM 2.96′′ at 10 µm,
located at its flux maximum position (Table 1)). We
then integrate each Gaussian across the slit (see §2.2 for
details). We use the fluxes of jet knots C, D and F of
P01a and take their positions and radio-optical spectral
indices αRO from P01b [Fν ∝ ν
−α]. For fainter knots,
we take 2.1 µm fluxes and positions from P01b and ex-
trapolate to 10µm using the αRO values in P01b. For
knots A and B, we fix the fluxes to be equal to those
observed by P01a but allow their spectral index to vary.
We do not include a contribution from HST-1 given that
we do not detect it in our 11.7 µm image (but see be-
low). The modeling of the knot A/B region spectrum
is shown in Figure 5. We show separate curves for the
contributions of the core, knot A/B and other knots to
the observed spectrum. This modeling procedure can
satisfactorily account for the observed spectrum of the
knot A/B region. We fit a power-law to the spectrum
of the knot A/B emission by holding constant the con-
tributions from all other components to the fluxes and
spectral indices from P01a and P01b, with the exception
of the core (for which we used the power-law fit we detail
below). Once this is done, we obtain a best fit spectral
index for knots A and B of αIR = 0.75 ± 0.04. This is
within 1σ of the radio-optical spectral indices given in
P01b for these knots. Thus the spectrum of the knot
A/B region is entirely consistent with synchrotron emis-
sion, as previously modeled by P01a, P01B and Shi et
al. (2007).
In Figures 6 and 7, we show the modeling of the nu-
Fig. 6.— Spitzer IRS spectrum of the nucleus of M87, modeled as
discussed in §3.1. We follow Figure 5 by showing the contributions
of the core and the knot A/B region as red and blue respectively,
while in green we show the contribution from all other knots. This
figure shows the sum of these components as purple, and then op-
tically thin and thick thermal dust emission models as the lower
black and orange curves respectively. The top set of black curves
represents the sum of all synchrotron emission plus the optically
thin dust model. The upper and lower black dotted curves repre-
sent the result of multiplyin the 55K thermal model by 1.2 and 0.8
respectively; this illustrates the range of normalizations that the
data are sensitive to.
clear spectrum, with Figure 6 showing the 5 − 35 µm
region alone and Figure 7 extending it to longer wave-
lengths. To fit the nuclear spectrum, we need to add to
the jet components an increase of flux of 15% over that
observed at 10.8µm by P01a. Significant nuclear vari-
ability in M87 has previously been noted in the optical
by Tsvetanov et al. (1998) and Perlman et al. (2003).
In addition, our data require a core spectral index that is
quite different from that predicted by P01a, specifically
αIR = 0.41 ± 0.05 instead of α ∼ 1.0 (as used also in
other papers, including Shi et al. 2007). This indicates
a synchrotron peak frequency >∼ 5× 10
13 Hz (and likely
> 1014 Hz), at least 1-2 decades higher in frequency than
that fit by P01a. The spectral index was fit only over the
spectral range 7.5-15 µm where no departure is observed
from a pure power-law form (see below), but the method
was otherwise identical to the one above for the knot
A/B spectrum. The different spectral index could be
the result of variability, as in blazars (of which M87 is a
somewhat misaligned example), where increases in flux
are often accompanied by spectral hardenings and large
increases in peak frequency (see e.g., Pian et al. 1997).
As already noted, the nucleus of M87 is known to be
variable in the optical (Tsvetanov et al. 1998, Perlman
et al. 2003) on timescales of ∼ 1 month, and on even
shorter timescales in the gamma-rays (Aharonian et al.
2006).
In contrast to the knot A/B spectrum, we cannot
model the observed nuclear emission with the purely
power-law emission characteristic of synchrotron radia-
tion. The pure power-law model has discrepancies both
at the short-wavelength and long-wavelength end of the
IRS spectrum. There are two likely causes of the appar-
ent short-wavelength excess. The first possibility is that
it is due to M and K stars in the nuclear regions of M87,
a component that P01a speculated might account for the
faint (7% of nuclear flux), extended emission found in the
much deeper OSCIR image. A second possibility is that
the excess might be the low-frequency tail of the emis-
sion from knot HST-1, which Spitzer would be unable
to resolve from the nucleus itself. If we were to use a
power-law to extrapolate the observed excess at ∼ 5µm
down to a wavelength of 0.8 µm, the flux from this ad-
ditional component would equal or slightly exceed that
of the core, consistent with the observed 0.8 µm flux of
knot HST-1 in January 2005 (Biretta et al., in prep.).
At longer wavelengths, beyond about 23 µm, we see
a much more pronounced excess (Figure 6). By 35 µm
the pure synchrotron model – which includes emission
from the core plus all jet knots (i.e., all known sources
of synchrotron emission in M87) – underestimates the
observed nuclear spectrum by about 20%. This excess
could be fit by adding an additional power-law compo-
nent with spectral index αexcess ∼ 6. However, such a
component would be unphysical given known models of
synchrotron radiation (e.g., Leahy 1991); and moreover,
our model has already accounted for all known sources
of synchrotron emission. Also, the addition of such a
steep power-law component would dominate over all the
other emission sources by ∼ 50 µm, producing a huge
excess by 100 µm, something which is ruled out by the
longer-wavelength MIPS and IRAS data (Figure 7)9 Al-
ternatively, we can fit this excess with thermal emission
(Figure 6), which is not unexpected in the nuclear regions
of M87 given the dust lanes found in HST and ground-
based images (e.g., Sparks et al. 1993, Ford et al. 1994).
We believe that thermal emission from cool dust in the
nucleus is the most physically realistic explanation for
the long-wavelength excess in the few arcsecond-aperture
IRS data (distinct from the host galaxy dust detected at
large radii by Shi et al. (2007) in 1′-aperture photome-
try). In addition, the data and modelling presented here
allow us to to place constraints on the mass and temper-
ature of dust in the nucleus of M87.
A single blackbody fit to this component yields ad-
equate fits with temperatures between 45-65K, with
higher temperatures being unable to reproduce the rise
at 23-35 µm, and lower temperatures resulting in an over-
production of the long-wavelength fluxes from IRAS and
Spitzer. We are unable to constrain whether the dust
is optically thin or thick (both curves are shown in Fig-
ure 6, for a characteristic temperature of 55K); however,
given that the dust filaments in the nuclear regions of
M87 are also typically marked by Hα emission (Sparks
et al. 1993, Ford et al. 2004), and also given the low
X-ray absorbing column (Perlman & Wilson 2005) the
optically thin model may be more likely to be correct.
Figure 7 shows only a 55K thermal component, as does
Figure 6. Although temperatures as low as 45K are com-
patible with the data, and in fact a single 45K thermal
emission component can account for all the IRS, MIPS
and IRAS data, a single dust temperature this low would
mean that the dust in the innermost 5′′ of the galaxy
contributes much more flux than dust at radii between
5 − 20′′. As M87 is extended at mid-IR wavelengths
9 Note that the ISO 60µm and 100µm fluxes (Haas et al. 2004)
are more than a factor two above all other photometry at similar
wavelengths.
Fig. 7.— Spitzer IRS spectrum and models of the nucleus of
M87, here plotted with a logarithmic y axis and the wavelength
axis continued up to 105µm. All curves are as in Figure 6, with the
exception of the optically thin thermal emission component, shown
in the dash-dot-dot-dot curves (optically thick thermal emission
models are not shown). Note that longward of 45µm, thermal
emission becomes dominant, a situation that continues to be the
case up to much longer wavelengths. Individual points are shown
representing data from Spitzer/MIPS (triangle), IRAS (diamonds),
and ISO (asterisks). Also shown are (squares) flux points from
Perlman et al. (2001a) and Whysong & Antonucci (2004), which
most likely lie below our data because of variability. We do not
attempt to fit data at longer wavelengths; however, if all the dust
were at T < 45K, it would overpredict the IRAS and Spitzer/MIPS
70µm and 160µm fluxes.
(e.g., Figure 3), we consider this unlikely. However, sen-
sitive, higher angular resolution observations at 30-100
µm would be required to put tighter constraints on dust
temperatures and optical thicknesses in the nucleus of
The total luminosity of the thermal component re-
vealed by the Spitzer spectra is, for T = 55K, 9.7× 1038
erg/s (2.6 × 105L⊙). This figure is highly temperature
dependent – for T = 45 K we find a thermal luminos-
ity of 3.1 × 1039 erg/s, while for T = 65K, we find a
thermal luminosity of 2.9× 1038 erg/s. (By comparison,
the error in the normalization of the thermal component,
indicated on Figure 6, is significantly smaller, approxi-
mately 20%). Thermal radiation from dust is therefore
a significant contributor to M87’s IR emission, being 20-
150% as bright as the core’s synchrotron emission be-
tween 1-300 µm. It is interesting to consider how much
dust could account for this thermal emission, and where
the dust would be located. If the dust is heated entirely
by the AGN, it must be within a few parsecs of the core.
If it is at larger distances it would have to be heated by
other mechanisms, for instance within the star-forming
disk tentatively detected by Tan et al. (2007). Using
standard formulae (Draine 2003 and refs. therein), and
assuming an optically thin dust component we derive a
dust mass of 83M⊙ for a temperature of T = 55K. This
figure is similarly temperature dependent: for T = 45
K we find a dust mass of 180 M⊙, while for T = 65
K we find a dust mass of 19 M⊙. This is a tiny mass
compared to that found in the dusty Hα filaments that
extend throughout the galaxy (Sparks et al. 1993). It is
also tiny compared to the molecular gas mass derived by
Tan et al. (2007) from 230 GHz observations.
3.2. Spectral Features
The 5-35 µm spectral region covers a range of dust
and molecular emission and absorption features, as well
as many fine structure lines. As mentioned above, a line
at 12.8 µm is detected in the COMICS spectrum of M87.
We ascribe this feature to [NeII] rather than the 12.7 µm
PAH band because of the lack of other molecular fea-
tures (including the stronger 11.2 µm PAH feature) in
both the COMICS and IRS spectra. In addition, the
IRS spectrum of the nucleus (Figure 4, 6a) also contains
lines of [Ne III] 15.6 µm, [S III] 18.7 µm, [O IV]+[Fe
II] 25.9 µm and [S III] 33.5µm, while in the knot A/B
spectrum (Figures 4, 5), we detect only one emission line
at low significance, namely [S III] 33.5µm. (The [Ar II]
7.0 µm line in the spectrum of Bressan et al. 2006 is
not clearly detected in our lower S/N spectrum). Table
2 shows the fluxes of the detected lines in the nuclear
spectrum, obtained by direct integration under each line
and using a continuum level determined by the mean of
several points at either side.
With the exception of [O IV], which may in principle
contribute to the 25.9 µm line, the ionization energies of
all of the lines are low enough (< 50 eV) that they can
be excited by massive stars and ionizing shocks. High-
excitation lines such as [Ne V] 24.3 µm, which can be
strong in AGN (e.g. Sturm et al. 2002), are not ob-
served in the spectrum of M87. Comparison of line fluxes
in different apertures suggest that the line-emitting re-
gion may be extended. For instance, the [Ne II] line in
the COMICS spectrum is a factor of about three weaker
than the same line in the larger-aperture Spitzer spec-
trum. Although the continuum level for the [Ne II] line in
the COMICS spectrum cannot be determined with great
accuracy, such a pronounced difference implies that the
line-emitting material extends beyond the compact cen-
tral source observed with COMICS. This interpretation
is supported by the fact that we observe a factor of about
2.5 lower flux in the [O IV]/[Fe II] line than in the [Fe
II] line in a 14′′ x 20′′-aperture ISO spectrum of M87
(Sturm et al. 2002). The ISO spectrum had sufficient
spectral resolution to separate the [O IV] 25.89 µm and
[Fe II] 25.99 µm lines but no lines other than [Fe II] were
detected. As expected, all the lines are unresolved in the
R ∼100 IRS spectrum.
The lack of high-excitation lines is consistent with the
classification of M87’s nucleus as a LINER (a suggestion
originally made by Willner et al. 1985). This might be
considered surprising given the luminosity of M87’s jet.
However, recent radio observations suggest that relativis-
tic jets may be quite common among LINERs (Nagar et
al. 2000, Falcke et al. 2000, Filho et al. 2004). More-
over, due to relativistic beaming, only a very small solid
angle of the emission-line regions would be exposed to
significant high-energy radiation from the jet.
The [S III] 33.5 µm seen weakly in the knot A/B spec-
trum (Figures 4, 5) is unlikely to come from knots A
and B, since spectra of M87’s jet have been taken many
times in other bands, most recently by HST in the UV
(Gelderman et al. 2005), and have failed to detect line
emission, placing severe limits on the presence of non-
stripped nuclei entrained within the jet. It is possible
that the [SIII] emission originates in the Hα filaments
within a few arcseconds of the knot A/B complex (see
TABLE 2
Fluxes of the fine structure lines detected in
Line, λrest
a Flux Eion
µm 10−21 W cm−2 (eV)
[Ne II] 12.81 1.9 b 21.6
........ 5.4 c 21.6
[Ne III] 15.56 4.1 41.0
[S III] 18.71 2.0 23.3
[O IV]/[Fe II] 25.89/25.99 1.9 54.9/7.9
[S III] 33.48 4.5 d 23.3
a From Sturm et al. (2002)
b Subaru/COMICS data
c Spitzer/IRS data
d See text for discussion
Sparks et al. 1993). Alternatively, the emission may
originate from material close to the nucleus, whose light
contributes to the jet spectrum particularly at longer
wavelengths where the PSF is largest. This is consis-
tent with the relative contribution of the core to the knot
A/B spectrum at 33.5µm, as computed in our modeling
procedure (§ 3.1). That only [SIII] 33.5 µm, the longest-
wavelength line in the nuclear spectrum, is also observed
in the jet spectrum suggests that this may be the more
likely explanation.
In highly obscured objects, such as Seyfert 2 galaxies,
the AGN unified model predicts absorption features near
10 and 18 µm due to the Si-O bond stretch in silicate dust
grains. Such features are indeed observed quite widely in
Seyfert 2 galaxies (e.g. Roche et al. 1991). Such a feature
would not necessarily be expected in M87, however, due
to the fact that its jet (which would be perpendicular to
any dusty torus under unified schemes) is seen at a fairly
small angle to the line of sight ( <∼ 15 − 20
◦, Biretta et
al. 1999). Consideration of the difference between means
in points at the peak and either side of a typical silicate
absorption profile in the COMICS spectrum allows us to
put a 95% confidence limit of τ(9.7) . 0.34 on any broad
silicate absorption feature in the nucleus of M87. Thus,
while a strong absorption feature can be ruled out, this
limit is comparable to the silicate optical depth observed
in the circumnuclear material of Seyfert 2 galaxies such
as NGC 1068 (∼ 0.4; Roche et al. 1984; Jaffe et al.
2004; Mason et al. 2006). It is also consistent with the
best available limit (NH < 6 × 10
20 cm−2, much lower
than that seen in typical Seyfert 2 galaxies) on nuclear
absorption from Chandra X-ray data (Perlman & Wilson
2005), given the recently found correlation between X-ray
N(H) figures and silicate absorption (Shi et al. 2006).
While the COMICS spectrum sets weak limits on the
presence of a 9.7 µm absorption band in the central
<0.65′′ of M87, the IRS spectrum can be used to search
for dust features which may exist on somewhat larger
scales. In fact, there are hints of weak silicate emis-
sion features around 10 and 18 µm in the IRS spectrum
(the 10µm feature would be well within the noise of the
COMICS spectrum). The 10 µm emission feature is also
evident in the IRS spectrum of M87 presented by Bressan
et al. (2006; their spectrum does not cover wavelengths
> 20µm). Previously thought to be rare in AGN, sili-
cate emission features have now been seen in the LINER
NGC 3998 (Sturm et al. 2005) as well as in several dis-
tant quasars (e.g., Hao et al. 2005; Siebenmorgen et al.
2005). Although the AGN unified scheme in its most
basic form predicts a silicate emission feature from the
hot inner edge of a dusty circumnuclear torus, the aper-
tures so far used to detect the silicate feature do not place
strong constraints on the location of the silicate-emitting
material in these galaxies. Furthermore, the cool (∼ 200
K) temperature implied by the strengths and profiles of
the features in the LINER, NGC 3998, suggests an origin
in narrow-line-region dust rather than the inner regions
of the torus (Sturm et al. 2005). Although Bressan et
al. point out that the 10µm feature in their data is not
spatially extended, the PSF of Spitzer at 10 µm (3′′) is
several times larger than the FWHM of the nucleus itself
in higher-resolution ground-based data. Therefore it is
entirely possible that the silicate emission could be pro-
duced by dust as far as 200 pc from the nucleus. HST and
ground-based optical observations show the existence of
a significant amount of warm dust in the inner 3” of M87,
as evidenced by theHα emission in many of these regions
(Sparks et al. 1993, Ford et al. 1994). Therefore, the
silicate emission in M87 does not necessarily imply the
existence of a nuclear torus.
4. SUMMARY AND CONCLUSIONS
We have presented mid-IR imaging and spectroscopy
of M87, using the Subaru observatory and the Spitzer
Space Telescope. The Spitzer spectroscopy covered both
the nucleus and the knot A/B complex in the jet. The
knot A/B spectrum can be well modeled by power-law
spectra, as expected for synchrotron emission from jet
components, with mid-IR fluxes consistent with those
published by P01a and spectral indices consistent with
the optical-radio spectra of P01b. However, the Spitzer
spectrum of the nucleus cannot be modeled by power-
law synchrotron emission from the core plus components
in the jet. We see clear signs of an infrared excess in
the core, which can be well modeled by a thermal spec-
trum with a characteristic temperature of 55± 10K. The
low temperature and luminosity of this component (§3.2)
are consistent with the limits found by Perlman et al.
(2001a).
Whysong & Antonucci (2004) made a similar argument
based on 11.7 µm photometry. The present data sup-
port and enhance this result as the thermal contribu-
tion is isolated and measured separately from the syn-
chrotron contribution. The measured luminosity is an
upper limit to the amount of emission from a dusty torus
in M87; the thermal contribution measured in the 10.7′′-
wide IRS slit could arise anywhere within 420 pc of the
nucleus and need not be associated with a torus at all.
Moreover, given that the bolometric luminosity of M87
is ∼ 1042 erg s−1 (e.g., Reynolds et al. 1996), classi-
cal torus models would predict a mid-IR luminosity at
least that high (e.g., Risaliti & Elvis 2005). By com-
parison we observe a much smaller thermal IR luminos-
ity (∼ 1039 erg s−1), making our observations difficult to
reconcile with standard unified models of AGN.
What is the overall significance of the weakness of the
torus emission in M87, both for this galaxy and for FR I
radio galaxies as a whole? One strong possibility is that
the weakly-emitting or absent torus in M87 may be a con-
sequence of the AGN’s low luminosity. In the disk wind
scenario, optically thick regions of an outflowing wind
comprise the geometrically and optically thick “torus.”
In the model of Elitzur & Shlosman (2006), the torus
disappears at low luminosities ( <∼ 10
42 erg s−1), because
accretion onto the central black hole can no longer sus-
tain the large-scale outflows. That we do not detect
the mid-IR signature of the classical torus is entirely
consistent with the predictions of the disk wind model.
An alternative to the disk-wind scenario was offered by
Reynolds et al. (1996) and di Matteo et al. (2003) who
modeled the nucleus of M87 with ADAF models. Both
of those papers derive accretion rates as much as 4 orders
of magnitude below the Eddington rate, which di Matteo
et al. speculate may signal that M87’s nuclear activity is
at a very late evolutionary stage. It is important to note,
however, that ADAF models do not in and of themselves
incorporate the notion of a torus. Finally, it is important
to point out that the lack of significant torus emission in
M87 does not yet constitute strong evidence that FR Is,
as a class, lack luminous tori, although Spitzer observa-
tions indicate that a significant number may lack this
component (Ogle et al. 2006, Birkinshaw et al. in prep).
Cen A, for example, shows significant thermal dust emis-
sion in the mid-IR, on scales ranging from the unresolved
sub-pc scale (likely the torus) to much larger-scale emis-
sion (Whysong & Antonucci 2004, Hardcastle et al. 2006
Radomski et al. 2007).
Based in part on data collected at Subaru Telescope,
which is operated by the National Astronomical Obser-
vatory of Japan. We wish to thank the Subaru TAC
for their support and the Subaru staff, particularly T.
Fujiyoshi, M. Lemmen and M. Letawsky for support-
ing our observations. ESP acknowledges support from
NASA LTSA grants NAG5-9997 and NNG05-GD63G,
as well as HST grant GO-9705. CP acknowledges sup-
port from NSF grant no. 0206617. NAL acknowledges
work supported by the NSF under grant no. 0237291.
ME acknowledges support from NSF grant no. 0507421
and NASA grant NNG05-GC38G. RM and JR were sup-
ported by the Gemini Observatory, which is operated by
the Association of Universities for Research in Astron-
omy, Inc., on behalf of the international Gemini part-
nership of Argentina, Australia, Brazil, Canada, Chile,
the United Kingdom, and the United States of America.
MI is supported by Grants-in-Aid for Scientific Research
(16740117).
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http://arxiv.org/abs/astro-ph/0610488

  We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87
in the mid-infrared from 5-35 um. These observations allow us to investigate
mid-IR emission mechanisms in the core of M87 and to establish that the
flaring, variable jet component HST-1 is not a major contributor to the mid-IR
flux. The Spitzer data include a high signal-to-noise 15-35 $\mu$m spectrum of
the knot A/B complex in the jet, which is consistent with synchrotron emission.
However, a synchrotron model cannot account for the observed {\it nuclear}
spectrum, even when contributions from the jet, necessary due to the degrading
of resolution with wavelength, are included. The Spitzer data show a clear
excess in the spectrum of the nucleus at wavelengths longer than 25 um, which
we model as thermal emission from cool dust at a characteristic temperature of
55 \pm 10 K, with an IR luminosity \sim 10^{39} {\rm ~erg ~s^{-1}}. Given
Spitzer's few-arcsecond angular resolution, the dust seen in the nuclear
spectrum could be located anywhere within ~5'' (390 pc) of the nucleus. In any
case, the ratio of AGN thermal to bolometric luminosity indicates that M87 does
not contain the IR-bright torus that classical unified AGN schemes invoke.
However, this result is consistent with theoretical predictions for
low-luminosity AGNs

Draft version October 27, 2018
Preprint typeset using LATEX style emulateapj v. 08/22/09
THE MID-INFRARED EMISSION OF M87
Eric S. Perlman
, R. E. Mason
, Christopher Packham
, N. A. Levenson
, Moshe Elitzur
, Justin J.
Schaefer
, Masatoshi Imanishi
, William B. Sparks
, James Radomski
Draft version October 27, 2018
ABSTRACT
We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid-infrared
from 5-35 µm. These observations allow us to investigate mid-IR emission mechanisms in the core
of M87 and to establish that the flaring, variable jet component HST-1 is not a major contributor
to the mid-IR flux. The Spitzer data include a high signal-to-noise 15-35 µm spectrum of the knot
A/B complex in the jet, which is consistent with synchrotron emission. However, a synchrotron model
cannot account for the observed nuclear spectrum, even when contributions from the jet, necessary
due to the degrading of resolution with wavelength, are included. The Spitzer data show a clear excess
in the spectrum of the nucleus at wavelengths longer than 25 µm, which we model as thermal emission
from cool dust at a characteristic temperature of 55 ± 10 K, with an IR luminosity ∼ 1039 erg s−1.
Given Spitzer’s few-arcsecond angular resolution, the dust seen in the nuclear spectrum could be
located anywhere within ∼ 5′′ (390 pc) of the nucleus. In any case, the ratio of AGN thermal to
bolometric luminosity indicates that M87 does not contain the IR-bright torus that classical unified
AGN schemes invoke. However, this result is consistent with theoretical predictions for low-luminosity
AGNs.
Subject headings:
1. INTRODUCTION
M87, the dominant galaxy in the Virgo cluster, is one of
the nearest (distance=16 Mpc, 1′′= 78 pc) radio galax-
ies. Since 1918, when Heber Curtis observed a “curi-
ous straight ray” extending from its nucleus, it has been
known to host a bright jet that is visible at radio through
X-ray wavelengths. The jet, one of the primary hall-
marks of M87’s nuclear activity, has a complex, knotty
structure (see Perlman et al. 2001b [hereafter P01b] and
references therein). M87 also exhibits energetic line emis-
sion from a disk that is seen to be in Keplerian mo-
tion (Ford et al. 1994, Harms et al. 1994). Longslit
spectroscopy of the material within the disk has allowed
the mass in the inner 3.5 pc of M87 to be measured at
≈ 3 × 109M⊙ (Marconi et al. 1997), implying the pres-
ence of one of the most massive known black holes.
M87’s proximity allows us to obtain particularly high
spatial resolution. Unfortunately, M87 is quite faint in
the infrared, with a core flux of only 16 mJy at 10.8µm
(Perlman et al. 2001a, hereafter P01a). This makes
Spitzer well-suited to sensitive observations of the in-
1 Joint Center for Astrophysics, Physics Department, University
of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore,
MD 21250
2 Current Address: Department of Physics & Space Sciences,
Florida Institute of Technology, 150 W. University Boulevard, Mel-
bourne, FL 32901
3 Gemini Observatory, Northern Operations Center, 670 N.
A’Ohoku Place, Hilo, HI 96720, USA
4 Department of Astronomy, University of Florida, 211 BRSC,
Gainesville, FL 32611, USA
5 Department of Physics and Astronomy, University of Kentucky,
177 Chem.-Phys. Building, Lexington, KY 40506-0055, USA
6 National Astronomical Observatory, Mitaka, Tokyo 181-8588,
Japan
7 Space Telescope Science Institute, 3700 San Martin Drive, Bal-
timore,MD 21218, USA
8 Gemini Observatory, Southern Operations Center, c/o AURA,
Casilla 603, La Serena, Chile
TABLE 1
Known knots in the jet of M87
Knot Dist. from Nuc.1 Approx. Extent2 10.8 µm Flux3
Name (arcsecs) (arcsecs) (mJy)
HST-14 0.8 0.5 × 0.1 < 2.3
D 2.7 1.8 × 0.2 1.2± 0.2
E5 6.0 0.9 × 0.3 0.1
F 8.2 1.4 × 0.4 1.3± 0.2
I5 11.2 0.6 × 0.5 0.25
A 12.4 2.2 × 1.3 6.4± 0.6
B 14.3 2.4 × 1.3 6.3± 0.6
C 17.4 1.9 × 1.7 4.0± 0.6
1 Location of the flux maximum, rather than centroid.
2 As measured in optical HST images.
3 Except where noted, as given in P01a.
4 Knot flux is highly variable; the flux maximum is at the upstream
end. The 2 σ flux limit given here is derived from these Subaru data.
5 Not detected in Gemini images (P01a); fluxes are extrapolated from
the radio-optical spectrum (P01b).
frared emission from this object, while at the same time
the small spatial scales that can be probed make M87 an
appropriate mid-infrared (mid-IR) target for the largest
ground-based telescopes – albeit one requiring long inte-
grations. The knots in the jet are significantly fainter
than the nucleus. Table 1 reviews the locations and
extents of major knots in the jet, along with N-band
fluxes either directly measured or extrapolated from ra-
dio/optical data.
Previous ground-based observations showed that most
of the mid-IR emission from the core of M87 can be
attributed to synchrotron radiation from the innermost
regions of the jet (P01a, Whysong & Antonucci 2004),
although a minor contribution from dust at T <∼ 150
K could not be ruled out. These results provided
strong evidence that the dusty torus of classical unified
AGN schemes (Antonucci 1993) may be absent in low-
http://arxiv.org/abs/0704.1156v2
luminosity AGNs. Two recent papers have used Spitzer
data to address the mid-IR emission processes of M87.
Bressan et al. (2006) used 5-20 µm IRS spectra to ex-
amine the issue of silicate emission from Virgo Cluster
galaxies. They showed that M87, like other Virgo el-
lipticals, exhibits a silicate emission feature at 10 µm,
but that this feature is not spatially resolved by Spitzer.
Shi et al. (2007) used IRAC and MIPS imaging data to
re-examine the origin of M87’s mid-IR emission. They
found that their photometry of the nucleus plus jet and
lobes can be fit by a combination of two synchrotron
power laws (usually breaking at optical or higher ener-
gies), but that in larger (1′) apertures an excess is present
over that model. This excess is ascribed to dust in the
host galaxy with a luminosity similar to that observed in
other brightest cluster galaxies.
In this paper we discuss new mid-IR imaging and spec-
troscopy of M87 using the Subaru observatory and Spitzer
Space Telescope. We combine data from the IRAC, MIPS
and IRS instruments (the latter covering the entire 5-
35µm range) to obtain a more detailed picture of the
emission processes in the nucleus and jet. In §2 we
present the observations and data reduction procedures,
while in §3 we discuss M87’s line and continuum emis-
sion. We close in §4 with a summary and discussion.
2. OBSERVATIONS AND DATA
2.1. Ground-Based Observations
Mid-IR imaging and spectroscopy of M87 were ob-
tained on the nights of UT 2005 April 27 and 28 us-
ing the COMICS camera (Kataza et al. 2000) attached
to the Cassegrain focus of the Subaru Telescope (Iye et
al. 2004). The 320 × 240 SiAs IBC array has a plate
scale of 0.129′′/pixel, delivering a 41× 31′′ field of view.
The detector was read-out in correlated quadruple sam-
pling (CQS) mode (Sako et al. 2003). The observing
conditions on both nights were not photometric, and the
precipitable water vapor was variable, as high as 3-5 mm
on April 27, but reaching 5-7mm on April 28.
For the acquisition images, we used the N11.7 filter
(central wavelength 11.67µm, bandwidth 1.05µm, 50%
cut-on/off). To remove time-variable sky background,
telescope thermal noise and so-called “1/f” detector
noise, we used the standard chop-nod technique, with
a 10′′ chop throw at a position angle of −69.5 degrees
(measured north through east), to project the jet along
the detector rows and place the reference beam between
minima in the jet emission. The chop frequency was 0.45
Hz (standard for Subaru/COMICS observations) and the
total on-source time was 700s.
We used a similar setup for the spectroscopic ob-
servations, with similar chop parameters, but without
nodding, as is standard practice with COMICS, and
the broad N band order blocking filter. The grating
and 0.33′′ slit provided a spectral resolution of ∼ 250
(0.02µm/pix), dispersing the entire N band across the
array. The superb design and capabilities of COMICS
permit on-slit mid-IR imaging of the polished slit jaws,
simultaneous with the spectroscopic observations. Thus
we were able to ensure that the emission from the nucleus
of M87 passed through the slit and into the spectroscopy
arm of COMICS. This proved to be crucial as differen-
tial refraction between the telescope’s optical autoguider
Fig. 1.— Subaru acquisition image of M87. North is up and east
is to the left. The high spatial resolution of the COMICS data is
apparent, despite its limited sensitivity. A 3-pixel Gaussian was
used to smooth these data. We do not detect emission from knot
HST-1, 0.8′′ from the nucleus (location of the circle on this figure).
See §§2,3 for details.
and the MIR science beam moved the object off the slit
in as little as 20 minutes at some airmasses. The varying,
suboptimal conditions meant that M87 was not detected
in many of the spectroscopy frames, and only those in
which the continuum was visible were stacked and used
in the final analysis. The data presented in this paper
represent 2100 s of on-source time.
The data were reduced using IRAF and in-house devel-
oped IDL routines. The difference for each chopped pair
(and for each nod-set for imaging), was calculated, and
the results combined until a single frame was created.
During the reduction process, chopped pairs obviously
compromised by cirrus, high electronic noise, or other
problems were discarded.
The PSF was measured from observations of the pho-
tometric standard. The measured full-width at half-
maximum (FWHM) of the standard was 0.55′′, which
agrees well with the FWHM of the spectral trace. Flux
calibration was achieved using HD108985 as a flux stan-
dard (Cohen et al. 1992, 1999; Tokunaga 1984) and inter-
polating the Cohen models to the proper bandpass. Ab-
solute errors in flux calibration were estimated from the
variations in the counts through the course of the nights.
Given the non-photometric conditions under which the
data were obtained, we estimate an accuracy of at best
15% for the flux calibration.
A 5-pixel extraction aperture was used for the spec-
troscopy data, and spectral flux calibration was with ref-
erence to the telluric standard star, Vega. No attempt
was made to correct for slit losses in the spectrum of M87,
but we note that the flux density in the calibrated spec-
trum is comparable to that of both the Subaru/COMICS
image and the Gemini/OSCIR photometry of P01a.
In Figure 1 we show the Subaru image, while in Figure
2 we show the Subaru spectrum.
2.2. Spitzer Observations
Spitzer Space Telescope observations of M87 were ob-
tained from the Spitzer archives. This includes IRS (In-
Fig. 2.— The Subaru/COMICS spectrum of the nucleus, showing
the [Ne II] 12.81 µm emission line. See §§2,3 for details.
frared Spectrograph, Houck et al. 2004) spectra taken on
UT 2004 January 4, as well as MIPS (Multi-band Imag-
ing Photometer for Spitzer, Rieke et al. 2004) images
taken on UT 2004 December 26. Both datasets were ac-
quired as part of PID 82 (PI Rieke). We also obtained
data taken on 2005 June 11 with the IRAC (Infrared
Array Camera) as part of PID 224 (PID Forman). The
advantage of this particular IRS dataset is that it cov-
ers the full wavelength range of the short-low and long-
low modules. We also comment below on other Spitzer
datasets.
The IRAC and MIPS data were processed according to
the standard pipeline recipes. In both, we used the post-
basic calibrated data (post-BCD). Fluxes were extracted
using circular apertures appropriate to the diffraction
limited resolution at a given wavelength. Background
subtraction was done using annular apertures that in-
cluded all the flux from the galaxy itself. The Spitzer
imaging data (shown in Figure 3) clearly show emission
from both the nucleus and the knot A/B complex, as well
as extended emission from the galaxy and the south-west
hotspot, a feature thought to be associated with the un-
seen counterjet (Hines et al. 1989, Sparks et al. 1992,
Stiavelli et al. 1992). However, individual knots are not
resolved at Spitzer’s angular resolution (≈ 3′′ at 10µm).
In this paper, we discuss only the emission from the core
and jet knots A and B. We subtracted from the core flux
measured on the IRAC data any emission that might
be due to galactic emission, using an annulus between
5 − 15′′; we also subtracted flux due to the jet by gaug-
ing independently the flux in the four quadrants. For this
reason our fluxes differ somewhat from those reported in
Shi et al. (2007). The 24µm flux derived in this way
agrees fully (within 1σ) with that found in the IRS spec-
trum of the core (§3.1).
The IRS spectra were acquired using both orders of the
low-resolution short-wavelength (“short-low”) and low-
resolution long-wavelength (“long-low”) modules of the
IRS. Total integration times of 28 s and 60 s were used
for the short- and long-wavelength modules, respectively,
and spectra were obtained in two nod positions separated
by 1/3 of the slit length, translating into nod throws
of 19′′ and 56′′ for the short-low and long-low modules
respectively. The spacecraft roll angle and relative slit
position angles were such that the 3.6− 3.7′′-wide short-
4.5 micron
SW Hotspot
Nucleus
24 micron
Fig. 3.— Spitzer imaging of M87, at 4.5 and 24.5 µm. We have
indicated the core, jet and SW hotspot for reference. See §2 for
details.
low slits were oriented perpendicular to the jet, while the
wider (10.5− 10.7′′) long-low slits were aligned along the
jet axis and therefore contain emission from the brightest
region of the jet (primarily knots A and B, see Table 1).
After standard pipeline processing, thermal back-
ground emission was removed by subtracting spectra
taken at the two nod positions, and 1D spectra then ex-
tracted using the SPICE package. The default extraction
apertures (7.2′′ at 6 µm, 14.4′′at 12 µm, scaling linearly
with wavelength) were employed for the extraction of the
short-low data, whereas to better separate the nuclear
and jet emission in the long-low data, we used a smaller
aperture that varied in size with wavelength, increasing
linearly from 5.1” at 17µm to 10.2” at 34 µm, compara-
ble to the FWHM of Spitzer’s PSF at those wavelengths.
As the long-low slits covered both the nucleus and the
knot A/B region, we extracted two 1D spectral datasets.
No significant fringing is visible in any of the spectra,
and no defringing was attempted.
As M87 is an extended source, it is non-trivial to equal-
ize the flux calibration of the orders in both the short-low
and long-low modules, or between the two modules, due
to the different extraction apertures. As a result, the
spectra from different orders may contain differing con-
tributions from the various extended structures, e.g., the
various jet components. The nucleus is essentially unaf-
fected by this problem at short wavelengths; however, at
longer wavelengths it does have a significant effect. The
fact that the nucleus is essentially unresolved at short
wavelengths allowed us to use the flux scale from the
pipeline for the SL data. We then made use of the over-
lap and “bonus segment” data to match up the flux and
slope of the spectral segments blueward and redward of
each join. No correction was found to be necessary to
scale the SL1 data to that from SL2. However, a scaling
factor of 1.67 ±0.03 was found to be necessary to scale
the LL2 data to the SL1 data, while a scaling factor of
1.38±0.03 was necessary to scale the two long-low orders
of the nuclear spectrum.
The knot A/B region, by contrast, is highly extended
(about 1.3′′×5′′, as measured on the optical and Gemini-
N+OSCIR data). This makes the flux calibration process
much more complicated, since the SPICE package is not
optimized (or intended) to deal with extended sources of
this nature. In this case we used the Gemini-N + OSCIR
result (P01a) to set the flux scale at 10.8µ m, and then
use the best-fit spectral index to extrapolate from the
LL spectra down to that wavelength (note that we do
not have SL data for the knot A/B region). We then
used the overlap and “bonus segment” data to match up
the flux and slope of the spectral segments blueward and
redward of the LL1/LL2 join (similarly to what we did for
the nucleus). Because the structural details of the knot
A/B region are rather different from the nuclear region, a
smaller scaling factor (1.04± 0.03) was required to scale
the LL1 data to the LL2 flux scale once it was fixed
using the Gemini-N 10.8µm flux point. The resulting
IRS spectra of M87’s nucleus and jet are shown in Fig.
3. RESULTS
The reduced Subaru acquisition image is shown in Fig-
ure 1. The core is clearly detected, with a flux of 20.8 ±
3.5 mJy, consistent with the findings of P01b to within
the 1σ errors The error in the flux calibration is dom-
inated by the variable weather conditions. We do not
detect any jet features. Even for the knot A/B complex
this is not unexpected given the extended nature of the
knot, our short integration time, fairly narrow band and
the poor sky conditions. Surprisingly, however, we do
not detect the flaring jet component HST-1 (Harris et
al. 2003, 2005; Perlman et al. 2003), 0.8′′ from the core,
despite the fact that at its peak in 2005 March (just a
few weeks before the Subaru observations), its 0.8 µm
flux was brighter than that of the core itself (Biretta et
al., in prep.). We place a limit of 2.3 mJy at 2σ on the
flux from knot HST-1.
The COMICS nuclear spectrum is shown in Figure 2.
The signal-to-noise ratio of the spectrum is low, but we
detect continuum emission plus a single emission line at
12.81 µm (rest frame). This spectrum contains only flux
from the nucleus; the extraction aperture would have ex-
cluded any significant contribution from HST-1 which
in any case is faint. In Figure 4, we show the Spitzer
spectrum of the knot A/B region (blue) as well as the
nucleus (black). The signal-to-noise ratio of the Spitzer
data, while still modest (∼ 10 per pixel) is significantly
better than that of the Subaru spectrum. We have over-
plotted on Figure 4 a binned version of the Subaru nu-
clear spectrum, which matches well with that obtained
by Spitzer. Despite the difference in angular resolution
between the Spitzer (∼ 3′′ at 10 µm) and Subaru (∼ 0.3′′)
Fig. 4.— The Spitzer IRS spectrum of the nucleus of M87 (black)
as well as knots A and B (blue). For comparison, the binned Subaru
COMICS data are shown in red (error bars reflect the standard
error on the mean of the points in each bin). To better separate the
two IRS spectra, the data for knots A and B have been multipled
by 0.75. Several low-ionization emission lines can be seen in the
spectrum of the nucleus, as well as hints of silicate emission around
10 and 18 µm. See §§3, 3.1 for details.
data, the nuclear spectra are similar in flux density and
spectral slope in both datasets. This confirms that HST-
1 (which in the Spitzer data would not be resolved from
the nucleus) makes no more than a minimal contribution
to the nuclear spectrum. Both the noise in the Subaru
data, however, and the known optical variability of the
nucleus and HST-1 (measured to be as large as 50% and
500%, respectively; Tsvetanov et al. 1998, Perlman et
al. 2003) prevent us from setting stricter limits on the
mid-IR flux of HST-1.
The overall similarity of the spectral shapes of the nu-
cleus and the knot A/B complex, at least below 25 µm,
confirms the synchrotron nature of the majority of the
nuclear emission, first shown by P01a. A close exami-
nation of the nuclear and jet spectra does, however, re-
veal differences. The first of these is the presence of five
obvious emission lines in the spectrum of the nucleus,
whereas only one weak emission line is seen in the knot
A/B spectrum. A more subtle difference is that the nu-
clear spectrum contains a sharper upturn towards longer
wavelengths. We discuss the spectral data in more depth
in the following subsections. Note that in the discussion
that follows, we use the terms “nuclear region” and “nu-
clear spectrum” to refer to the Spitzer spectrum taken
at the position of the nucleus (as opposed to that of the
knot A/B region), while what we term as the core refers
only to the regions within ∼ 0.3′′ of the central black hole
(and thus unresolvable in ground-based N-band images).
3.1. IR Continuum Emission
The infrared continuum of a galaxy can contain emis-
sion from a number of different components. In the case
of M87, the most likely sources are thermal emission from
warm or cool dust, synchrotron emission from the jet,
and (at shorter wavelengths) starlight from K and M
stars. In order to determine the nature and origin of this
IR emission, it is necessary to correctly account for all
possible sources that could fall within the slit and beam
of Spitzer at any given wavelength. At Spitzer’s angular
Fig. 5.— Spitzer IRS spectrum of jet knots A and B, modeled
as discussed in §3.2. In blue, we show the contribution due to
knots A and B (summed); in red, the contribution from the core,
and in green the contribution from all other knots. The black line
represents the sum of the contributions from the core and all knots.
The observed spectrum for knots A and B can be satisfactorily
reproduced with these synchrotron components.
resolution the nuclear spectrum will contain a contribu-
tion from emission in the jet (and vice versa; see Table 1),
and, because of the decrease in spatial resolution towards
longer wavelengths, the magnitude of that contribution
will increase with wavelength. We have modeled both
the nuclear and knot A/B spectra, and accounted for
this effect as follows.
We model each component as a 1-dimensional Gaus-
sian of width equal to Spitzer’s diffraction limit (which
we take to be a Gaussian of FWHM 2.96′′ at 10 µm,
located at its flux maximum position (Table 1)). We
then integrate each Gaussian across the slit (see §2.2 for
details). We use the fluxes of jet knots C, D and F of
P01a and take their positions and radio-optical spectral
indices αRO from P01b [Fν ∝ ν
−α]. For fainter knots,
we take 2.1 µm fluxes and positions from P01b and ex-
trapolate to 10µm using the αRO values in P01b. For
knots A and B, we fix the fluxes to be equal to those
observed by P01a but allow their spectral index to vary.
We do not include a contribution from HST-1 given that
we do not detect it in our 11.7 µm image (but see be-
low). The modeling of the knot A/B region spectrum
is shown in Figure 5. We show separate curves for the
contributions of the core, knot A/B and other knots to
the observed spectrum. This modeling procedure can
satisfactorily account for the observed spectrum of the
knot A/B region. We fit a power-law to the spectrum
of the knot A/B emission by holding constant the con-
tributions from all other components to the fluxes and
spectral indices from P01a and P01b, with the exception
of the core (for which we used the power-law fit we detail
below). Once this is done, we obtain a best fit spectral
index for knots A and B of αIR = 0.75 ± 0.04. This is
within 1σ of the radio-optical spectral indices given in
P01b for these knots. Thus the spectrum of the knot
A/B region is entirely consistent with synchrotron emis-
sion, as previously modeled by P01a, P01B and Shi et
al. (2007).
In Figures 6 and 7, we show the modeling of the nu-
Fig. 6.— Spitzer IRS spectrum of the nucleus of M87, modeled as
discussed in §3.1. We follow Figure 5 by showing the contributions
of the core and the knot A/B region as red and blue respectively,
while in green we show the contribution from all other knots. This
figure shows the sum of these components as purple, and then op-
tically thin and thick thermal dust emission models as the lower
black and orange curves respectively. The top set of black curves
represents the sum of all synchrotron emission plus the optically
thin dust model. The upper and lower black dotted curves repre-
sent the result of multiplyin the 55K thermal model by 1.2 and 0.8
respectively; this illustrates the range of normalizations that the
data are sensitive to.
clear spectrum, with Figure 6 showing the 5 − 35 µm
region alone and Figure 7 extending it to longer wave-
lengths. To fit the nuclear spectrum, we need to add to
the jet components an increase of flux of 15% over that
observed at 10.8µm by P01a. Significant nuclear vari-
ability in M87 has previously been noted in the optical
by Tsvetanov et al. (1998) and Perlman et al. (2003).
In addition, our data require a core spectral index that is
quite different from that predicted by P01a, specifically
αIR = 0.41 ± 0.05 instead of α ∼ 1.0 (as used also in
other papers, including Shi et al. 2007). This indicates
a synchrotron peak frequency >∼ 5× 10
13 Hz (and likely
> 1014 Hz), at least 1-2 decades higher in frequency than
that fit by P01a. The spectral index was fit only over the
spectral range 7.5-15 µm where no departure is observed
from a pure power-law form (see below), but the method
was otherwise identical to the one above for the knot
A/B spectrum. The different spectral index could be
the result of variability, as in blazars (of which M87 is a
somewhat misaligned example), where increases in flux
are often accompanied by spectral hardenings and large
increases in peak frequency (see e.g., Pian et al. 1997).
As already noted, the nucleus of M87 is known to be
variable in the optical (Tsvetanov et al. 1998, Perlman
et al. 2003) on timescales of ∼ 1 month, and on even
shorter timescales in the gamma-rays (Aharonian et al.
2006).
In contrast to the knot A/B spectrum, we cannot
model the observed nuclear emission with the purely
power-law emission characteristic of synchrotron radia-
tion. The pure power-law model has discrepancies both
at the short-wavelength and long-wavelength end of the
IRS spectrum. There are two likely causes of the appar-
ent short-wavelength excess. The first possibility is that
it is due to M and K stars in the nuclear regions of M87,
a component that P01a speculated might account for the
faint (7% of nuclear flux), extended emission found in the
much deeper OSCIR image. A second possibility is that
the excess might be the low-frequency tail of the emis-
sion from knot HST-1, which Spitzer would be unable
to resolve from the nucleus itself. If we were to use a
power-law to extrapolate the observed excess at ∼ 5µm
down to a wavelength of 0.8 µm, the flux from this ad-
ditional component would equal or slightly exceed that
of the core, consistent with the observed 0.8 µm flux of
knot HST-1 in January 2005 (Biretta et al., in prep.).
At longer wavelengths, beyond about 23 µm, we see
a much more pronounced excess (Figure 6). By 35 µm
the pure synchrotron model – which includes emission
from the core plus all jet knots (i.e., all known sources
of synchrotron emission in M87) – underestimates the
observed nuclear spectrum by about 20%. This excess
could be fit by adding an additional power-law compo-
nent with spectral index αexcess ∼ 6. However, such a
component would be unphysical given known models of
synchrotron radiation (e.g., Leahy 1991); and moreover,
our model has already accounted for all known sources
of synchrotron emission. Also, the addition of such a
steep power-law component would dominate over all the
other emission sources by ∼ 50 µm, producing a huge
excess by 100 µm, something which is ruled out by the
longer-wavelength MIPS and IRAS data (Figure 7)9 Al-
ternatively, we can fit this excess with thermal emission
(Figure 6), which is not unexpected in the nuclear regions
of M87 given the dust lanes found in HST and ground-
based images (e.g., Sparks et al. 1993, Ford et al. 1994).
We believe that thermal emission from cool dust in the
nucleus is the most physically realistic explanation for
the long-wavelength excess in the few arcsecond-aperture
IRS data (distinct from the host galaxy dust detected at
large radii by Shi et al. (2007) in 1′-aperture photome-
try). In addition, the data and modelling presented here
allow us to to place constraints on the mass and temper-
ature of dust in the nucleus of M87.
A single blackbody fit to this component yields ad-
equate fits with temperatures between 45-65K, with
higher temperatures being unable to reproduce the rise
at 23-35 µm, and lower temperatures resulting in an over-
production of the long-wavelength fluxes from IRAS and
Spitzer. We are unable to constrain whether the dust
is optically thin or thick (both curves are shown in Fig-
ure 6, for a characteristic temperature of 55K); however,
given that the dust filaments in the nuclear regions of
M87 are also typically marked by Hα emission (Sparks
et al. 1993, Ford et al. 2004), and also given the low
X-ray absorbing column (Perlman & Wilson 2005) the
optically thin model may be more likely to be correct.
Figure 7 shows only a 55K thermal component, as does
Figure 6. Although temperatures as low as 45K are com-
patible with the data, and in fact a single 45K thermal
emission component can account for all the IRS, MIPS
and IRAS data, a single dust temperature this low would
mean that the dust in the innermost 5′′ of the galaxy
contributes much more flux than dust at radii between
5 − 20′′. As M87 is extended at mid-IR wavelengths
9 Note that the ISO 60µm and 100µm fluxes (Haas et al. 2004)
are more than a factor two above all other photometry at similar
wavelengths.
Fig. 7.— Spitzer IRS spectrum and models of the nucleus of
M87, here plotted with a logarithmic y axis and the wavelength
axis continued up to 105µm. All curves are as in Figure 6, with the
exception of the optically thin thermal emission component, shown
in the dash-dot-dot-dot curves (optically thick thermal emission
models are not shown). Note that longward of 45µm, thermal
emission becomes dominant, a situation that continues to be the
case up to much longer wavelengths. Individual points are shown
representing data from Spitzer/MIPS (triangle), IRAS (diamonds),
and ISO (asterisks). Also shown are (squares) flux points from
Perlman et al. (2001a) and Whysong & Antonucci (2004), which
most likely lie below our data because of variability. We do not
attempt to fit data at longer wavelengths; however, if all the dust
were at T < 45K, it would overpredict the IRAS and Spitzer/MIPS
70µm and 160µm fluxes.
(e.g., Figure 3), we consider this unlikely. However, sen-
sitive, higher angular resolution observations at 30-100
µm would be required to put tighter constraints on dust
temperatures and optical thicknesses in the nucleus of
The total luminosity of the thermal component re-
vealed by the Spitzer spectra is, for T = 55K, 9.7× 1038
erg/s (2.6 × 105L⊙). This figure is highly temperature
dependent – for T = 45 K we find a thermal luminos-
ity of 3.1 × 1039 erg/s, while for T = 65K, we find a
thermal luminosity of 2.9× 1038 erg/s. (By comparison,
the error in the normalization of the thermal component,
indicated on Figure 6, is significantly smaller, approxi-
mately 20%). Thermal radiation from dust is therefore
a significant contributor to M87’s IR emission, being 20-
150% as bright as the core’s synchrotron emission be-
tween 1-300 µm. It is interesting to consider how much
dust could account for this thermal emission, and where
the dust would be located. If the dust is heated entirely
by the AGN, it must be within a few parsecs of the core.
If it is at larger distances it would have to be heated by
other mechanisms, for instance within the star-forming
disk tentatively detected by Tan et al. (2007). Using
standard formulae (Draine 2003 and refs. therein), and
assuming an optically thin dust component we derive a
dust mass of 83M⊙ for a temperature of T = 55K. This
figure is similarly temperature dependent: for T = 45
K we find a dust mass of 180 M⊙, while for T = 65
K we find a dust mass of 19 M⊙. This is a tiny mass
compared to that found in the dusty Hα filaments that
extend throughout the galaxy (Sparks et al. 1993). It is
also tiny compared to the molecular gas mass derived by
Tan et al. (2007) from 230 GHz observations.
3.2. Spectral Features
The 5-35 µm spectral region covers a range of dust
and molecular emission and absorption features, as well
as many fine structure lines. As mentioned above, a line
at 12.8 µm is detected in the COMICS spectrum of M87.
We ascribe this feature to [NeII] rather than the 12.7 µm
PAH band because of the lack of other molecular fea-
tures (including the stronger 11.2 µm PAH feature) in
both the COMICS and IRS spectra. In addition, the
IRS spectrum of the nucleus (Figure 4, 6a) also contains
lines of [Ne III] 15.6 µm, [S III] 18.7 µm, [O IV]+[Fe
II] 25.9 µm and [S III] 33.5µm, while in the knot A/B
spectrum (Figures 4, 5), we detect only one emission line
at low significance, namely [S III] 33.5µm. (The [Ar II]
7.0 µm line in the spectrum of Bressan et al. 2006 is
not clearly detected in our lower S/N spectrum). Table
2 shows the fluxes of the detected lines in the nuclear
spectrum, obtained by direct integration under each line
and using a continuum level determined by the mean of
several points at either side.
With the exception of [O IV], which may in principle
contribute to the 25.9 µm line, the ionization energies of
all of the lines are low enough (< 50 eV) that they can
be excited by massive stars and ionizing shocks. High-
excitation lines such as [Ne V] 24.3 µm, which can be
strong in AGN (e.g. Sturm et al. 2002), are not ob-
served in the spectrum of M87. Comparison of line fluxes
in different apertures suggest that the line-emitting re-
gion may be extended. For instance, the [Ne II] line in
the COMICS spectrum is a factor of about three weaker
than the same line in the larger-aperture Spitzer spec-
trum. Although the continuum level for the [Ne II] line in
the COMICS spectrum cannot be determined with great
accuracy, such a pronounced difference implies that the
line-emitting material extends beyond the compact cen-
tral source observed with COMICS. This interpretation
is supported by the fact that we observe a factor of about
2.5 lower flux in the [O IV]/[Fe II] line than in the [Fe
II] line in a 14′′ x 20′′-aperture ISO spectrum of M87
(Sturm et al. 2002). The ISO spectrum had sufficient
spectral resolution to separate the [O IV] 25.89 µm and
[Fe II] 25.99 µm lines but no lines other than [Fe II] were
detected. As expected, all the lines are unresolved in the
R ∼100 IRS spectrum.
The lack of high-excitation lines is consistent with the
classification of M87’s nucleus as a LINER (a suggestion
originally made by Willner et al. 1985). This might be
considered surprising given the luminosity of M87’s jet.
However, recent radio observations suggest that relativis-
tic jets may be quite common among LINERs (Nagar et
al. 2000, Falcke et al. 2000, Filho et al. 2004). More-
over, due to relativistic beaming, only a very small solid
angle of the emission-line regions would be exposed to
significant high-energy radiation from the jet.
The [S III] 33.5 µm seen weakly in the knot A/B spec-
trum (Figures 4, 5) is unlikely to come from knots A
and B, since spectra of M87’s jet have been taken many
times in other bands, most recently by HST in the UV
(Gelderman et al. 2005), and have failed to detect line
emission, placing severe limits on the presence of non-
stripped nuclei entrained within the jet. It is possible
that the [SIII] emission originates in the Hα filaments
within a few arcseconds of the knot A/B complex (see
TABLE 2
Fluxes of the fine structure lines detected in
Line, λrest
a Flux Eion
µm 10−21 W cm−2 (eV)
[Ne II] 12.81 1.9 b 21.6
........ 5.4 c 21.6
[Ne III] 15.56 4.1 41.0
[S III] 18.71 2.0 23.3
[O IV]/[Fe II] 25.89/25.99 1.9 54.9/7.9
[S III] 33.48 4.5 d 23.3
a From Sturm et al. (2002)
b Subaru/COMICS data
c Spitzer/IRS data
d See text for discussion
Sparks et al. 1993). Alternatively, the emission may
originate from material close to the nucleus, whose light
contributes to the jet spectrum particularly at longer
wavelengths where the PSF is largest. This is consis-
tent with the relative contribution of the core to the knot
A/B spectrum at 33.5µm, as computed in our modeling
procedure (§ 3.1). That only [SIII] 33.5 µm, the longest-
wavelength line in the nuclear spectrum, is also observed
in the jet spectrum suggests that this may be the more
likely explanation.
In highly obscured objects, such as Seyfert 2 galaxies,
the AGN unified model predicts absorption features near
10 and 18 µm due to the Si-O bond stretch in silicate dust
grains. Such features are indeed observed quite widely in
Seyfert 2 galaxies (e.g. Roche et al. 1991). Such a feature
would not necessarily be expected in M87, however, due
to the fact that its jet (which would be perpendicular to
any dusty torus under unified schemes) is seen at a fairly
small angle to the line of sight ( <∼ 15 − 20
◦, Biretta et
al. 1999). Consideration of the difference between means
in points at the peak and either side of a typical silicate
absorption profile in the COMICS spectrum allows us to
put a 95% confidence limit of τ(9.7) . 0.34 on any broad
silicate absorption feature in the nucleus of M87. Thus,
while a strong absorption feature can be ruled out, this
limit is comparable to the silicate optical depth observed
in the circumnuclear material of Seyfert 2 galaxies such
as NGC 1068 (∼ 0.4; Roche et al. 1984; Jaffe et al.
2004; Mason et al. 2006). It is also consistent with the
best available limit (NH < 6 × 10
20 cm−2, much lower
than that seen in typical Seyfert 2 galaxies) on nuclear
absorption from Chandra X-ray data (Perlman & Wilson
2005), given the recently found correlation between X-ray
N(H) figures and silicate absorption (Shi et al. 2006).
While the COMICS spectrum sets weak limits on the
presence of a 9.7 µm absorption band in the central
<0.65′′ of M87, the IRS spectrum can be used to search
for dust features which may exist on somewhat larger
scales. In fact, there are hints of weak silicate emis-
sion features around 10 and 18 µm in the IRS spectrum
(the 10µm feature would be well within the noise of the
COMICS spectrum). The 10 µm emission feature is also
evident in the IRS spectrum of M87 presented by Bressan
et al. (2006; their spectrum does not cover wavelengths
> 20µm). Previously thought to be rare in AGN, sili-
cate emission features have now been seen in the LINER
NGC 3998 (Sturm et al. 2005) as well as in several dis-
tant quasars (e.g., Hao et al. 2005; Siebenmorgen et al.
2005). Although the AGN unified scheme in its most
basic form predicts a silicate emission feature from the
hot inner edge of a dusty circumnuclear torus, the aper-
tures so far used to detect the silicate feature do not place
strong constraints on the location of the silicate-emitting
material in these galaxies. Furthermore, the cool (∼ 200
K) temperature implied by the strengths and profiles of
the features in the LINER, NGC 3998, suggests an origin
in narrow-line-region dust rather than the inner regions
of the torus (Sturm et al. 2005). Although Bressan et
al. point out that the 10µm feature in their data is not
spatially extended, the PSF of Spitzer at 10 µm (3′′) is
several times larger than the FWHM of the nucleus itself
in higher-resolution ground-based data. Therefore it is
entirely possible that the silicate emission could be pro-
duced by dust as far as 200 pc from the nucleus. HST and
ground-based optical observations show the existence of
a significant amount of warm dust in the inner 3” of M87,
as evidenced by theHα emission in many of these regions
(Sparks et al. 1993, Ford et al. 1994). Therefore, the
silicate emission in M87 does not necessarily imply the
existence of a nuclear torus.
4. SUMMARY AND CONCLUSIONS
We have presented mid-IR imaging and spectroscopy
of M87, using the Subaru observatory and the Spitzer
Space Telescope. The Spitzer spectroscopy covered both
the nucleus and the knot A/B complex in the jet. The
knot A/B spectrum can be well modeled by power-law
spectra, as expected for synchrotron emission from jet
components, with mid-IR fluxes consistent with those
published by P01a and spectral indices consistent with
the optical-radio spectra of P01b. However, the Spitzer
spectrum of the nucleus cannot be modeled by power-
law synchrotron emission from the core plus components
in the jet. We see clear signs of an infrared excess in
the core, which can be well modeled by a thermal spec-
trum with a characteristic temperature of 55± 10K. The
low temperature and luminosity of this component (§3.2)
are consistent with the limits found by Perlman et al.
(2001a).
Whysong & Antonucci (2004) made a similar argument
based on 11.7 µm photometry. The present data sup-
port and enhance this result as the thermal contribu-
tion is isolated and measured separately from the syn-
chrotron contribution. The measured luminosity is an
upper limit to the amount of emission from a dusty torus
in M87; the thermal contribution measured in the 10.7′′-
wide IRS slit could arise anywhere within 420 pc of the
nucleus and need not be associated with a torus at all.
Moreover, given that the bolometric luminosity of M87
is ∼ 1042 erg s−1 (e.g., Reynolds et al. 1996), classi-
cal torus models would predict a mid-IR luminosity at
least that high (e.g., Risaliti & Elvis 2005). By com-
parison we observe a much smaller thermal IR luminos-
ity (∼ 1039 erg s−1), making our observations difficult to
reconcile with standard unified models of AGN.
What is the overall significance of the weakness of the
torus emission in M87, both for this galaxy and for FR I
radio galaxies as a whole? One strong possibility is that
the weakly-emitting or absent torus in M87 may be a con-
sequence of the AGN’s low luminosity. In the disk wind
scenario, optically thick regions of an outflowing wind
comprise the geometrically and optically thick “torus.”
In the model of Elitzur & Shlosman (2006), the torus
disappears at low luminosities ( <∼ 10
42 erg s−1), because
accretion onto the central black hole can no longer sus-
tain the large-scale outflows. That we do not detect
the mid-IR signature of the classical torus is entirely
consistent with the predictions of the disk wind model.
An alternative to the disk-wind scenario was offered by
Reynolds et al. (1996) and di Matteo et al. (2003) who
modeled the nucleus of M87 with ADAF models. Both
of those papers derive accretion rates as much as 4 orders
of magnitude below the Eddington rate, which di Matteo
et al. speculate may signal that M87’s nuclear activity is
at a very late evolutionary stage. It is important to note,
however, that ADAF models do not in and of themselves
incorporate the notion of a torus. Finally, it is important
to point out that the lack of significant torus emission in
M87 does not yet constitute strong evidence that FR Is,
as a class, lack luminous tori, although Spitzer observa-
tions indicate that a significant number may lack this
component (Ogle et al. 2006, Birkinshaw et al. in prep).
Cen A, for example, shows significant thermal dust emis-
sion in the mid-IR, on scales ranging from the unresolved
sub-pc scale (likely the torus) to much larger-scale emis-
sion (Whysong & Antonucci 2004, Hardcastle et al. 2006
Radomski et al. 2007).
Based in part on data collected at Subaru Telescope,
which is operated by the National Astronomical Obser-
vatory of Japan. We wish to thank the Subaru TAC
for their support and the Subaru staff, particularly T.
Fujiyoshi, M. Lemmen and M. Letawsky for support-
ing our observations. ESP acknowledges support from
NASA LTSA grants NAG5-9997 and NNG05-GD63G,
as well as HST grant GO-9705. CP acknowledges sup-
port from NSF grant no. 0206617. NAL acknowledges
work supported by the NSF under grant no. 0237291.
ME acknowledges support from NSF grant no. 0507421
and NASA grant NNG05-GC38G. RM and JR were sup-
ported by the Gemini Observatory, which is operated by
the Association of Universities for Research in Astron-
omy, Inc., on behalf of the international Gemini part-
nership of Argentina, Australia, Brazil, Canada, Chile,
the United Kingdom, and the United States of America.
MI is supported by Grants-in-Aid for Scientific Research
(16740117).
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http://arxiv.org/abs/astro-ph/0610488
