Astronomy & Astrophysics manuscript no. paper˙fin c© ESO 2018
October 30, 2018
Dust covering factor, silicate emission and star formation in
luminous QSOs
R. Maiolino1, O. Shemmer2, M. Imanishi3, Hagai Netzer4, E. Oliva5, D. Lutz6, and E. Sturm6
1 INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
2 Department of Astronomy and Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802
3 National Astronomical Observatory, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
4 School of Physics and Astronomy and the Wise Observatory , Tel-Aviv University, Tel-Aviv 69978, Israel
5 INAF - Telescopio Nazionale Galileo, PO Box 565, 38700 Santa Cruz de La Palma, Tenerife, Spain
6 Max-Planck-Institut für Extraterrestrische Physik, D-85741 Garching, Germany
Received ; accepted
ABSTRACT
We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high luminosity QSOs at 2<z<3.5. When combined with
archival IRS observations of local, low luminosity type-I active galactic nuclei (AGNs), the sample spans five orders of magnitude
in luminosity. We find that the continuum dust thermal emission at λrest = 6.7µm is correlated with the optical luminosity, following
the non-linear relation λLλ(6.7µm) ∝ λLλ(5100Å)
0.82 . We also find an anti correlation between λLλ(6.7µm)/λLλ(5100Å) and the
[Oiii]λ5007 line luminosity. These effects are interpreted as a decreasing covering factor of the circumnuclear dust as a function of
luminosity. Such a result is in agreement with the decreasing fraction of absorbed AGNs as a function of luminosity recently found in
various surveys. In particular, while X-ray surveys find a decreasing covering factor of the absorbing gas as a function of luminosity,
our data provides an independent and complementary confirmation by finding a decreasing covering factor of dust. We clearly detect
the silicate emission feature in the average spectrum, but also in four individual objects. These are the Silicate emission in the most
luminous objects obtained so far. When combined with the silicate emission observed in local, low luminosity type-I AGNs, we
find that the silicate emission strength is correlated with luminosity. The silicate strength of all type-I AGNs also follows a positive
correlation with the black hole mass and with the accretion rate. The Polycyclic Aromatic Hydrocarbon (PAH) emission features,
expected from starburst activity, are not detected in the average spectrum of luminous, high-z QSOs. The upper limit inferred from the
average spectrum points to a ratio between PAH luminosity and QSO optical luminosity significantly lower than observed in lower
luminosity AGNs, implying that the correlation between star formation rate and AGN power saturates at high luminosities.
Key words. infrared: galaxies – galaxies: nuclei – galaxies: active – galaxies: Seyfert – galaxies: starburst – quasars: general
1. Introduction
The mid-IR (MIR) spectrum of AGNs contains a wealth of in-
formation which is crucial to the understanding of their inner
region. The observed prominent continuum emission is due to
circumnuclear dust heated to a temperature of several hundred
degrees by the nuclear, primary optical/UV/X-ray source (pri-
marily the central accretion disk); therefore, the MIR contin-
uum provides information on the amount and/or covering factor
of the circumnuclear dust. The MIR region is also rich of sev-
eral emission features which are important tracers of the ISM.
Among the dust features, the Polycyclic-Aromatic-Hydrocarbon
bands (PAH, whose most prominent feature is at ∼ 7.7µm)
are emitted by very small carbon grains excited in the Photo
Dissociation Regions, that are tracers of star forming activity
(although PAHs may not be reliable SF tracers for compact
HII regions or heavily embedded starbursts, Peeters et al., 2004;
Förster Schreiber et al., 2004). Additional MIR dust features are
the Silicate bands at ∼ 10µm and at ∼ 18µm, often seen in ab-
sorption in obscured AGNs and in luminous IR galaxies.
Major steps forward in this field were achieved thanks to
the Spitzer Space Observatory, and to its infrared spectrome-
ter, IRS, which allows a detailed investigation of the MIR spec-
tral features in a large number of sources. In particular, IRS
Send offprint requests to: R. Maiolino
allowed the detection of MIR emission lines in several AGNs
(e.g. Armus et al., 2004; Haas et al., 2005; Sturm et al., 2006a;
Weedman et al., 2005), the detection of PAHs in local PG QSOs
(Schweitzer et al., 2006), the first detection of the Silicate fea-
ture in emission (Siebenmorgen et al., 2005; Hao et al., 2005), as
well as detailed studies of the silicate strength in various classes
of sources (Spoon et al., 2007; Hao et al., 2007; Shi et al., 2006;
Imanishi et al., 2007).
However, most of the current Spitzer IRS studies have fo-
cused on local and modest luminosity AGNs (including low
luminosity QSOs), with the exception of a few bright, lensed
objects at high redshift (Soifer et al., 2004; Teplitz et al., 2006;
Lutz et al., 2007). We have obtained short IRS integrations of a
sample of 25 luminous AGNs (hereafter QSOs) at high redshifts
with the goal of extending the investigation of the MIR prop-
erties to the high luminosity range. The primary goals were to
investigate the covering factor of the circumnuclear dust and the
dependence of the star formation rate (SFR), as traced by the
PAH features, on various quantities such as metallicity, narrow
line luminosity, accretion rate and black hole mass. In combi-
nation with lower luminosity AGNs obtained by previous IRS
studies, our sample spans about 5 orders of magnitude in lumi-
nosity. This allows us to look for the dependence of the covering
factor on luminosity and black hole mass. We also search for the
silicate emission and PAH-related properties although the inte-
http://arxiv.org/abs/0704.1559v1
2 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
gration times were too short, in most cases, to unveil the proper-
ties of individual sources.
In Sect. 2 we discuss the sample selection, the observations
and the data reduction. In Sect. 3.1 we describe the spectral
analysis and the main observational results, and in Sect. 3.2 we
include additional data on local, low luminosity sources from
the literature and from the Spitzer archive. The dust covering
factor is discussed in Sect. 4.1, the properties of the Silicate
emission feature in Sect. 4.2 and the constraints on the star
formation in Sect. 4.3. The conclusion are outlined in Sect. 5.
Throughout the paper we assume a concordance Λ-cosmology
with H0 = 71 km s
−1 Mpc−1, Ωm = 0.27 and ΩΛ = 0.73
(Spergel et al., 2003).
2. Sample selection, observations and data
reduction
High redshift, high luminosity QSOs in our sample were
mostly drawn from Shemmer et al. (2004) and from Netzer et al.
(2004). The latter papers presented near-IR spectra (optical rest-
frame) of a large sample of QSOs at 2<z<3.5, which were used
to obtain detailed information on the black hole (BH) mass
(by means of the width of the Hβ line), on the accretion rate
and on the strength of the narrow emission line [OIII]λ5007.
The sample contains also infrared data on two sources from
Dietrich et al. (2002) and a few additional QSOs in the same
redshift range, for which near-IR spectra where obtained after
Shemmer et al. (2004), but unpublished yet. This sample allows
us not only to extend the investigation of the MIR properties
as a function of luminosity, but also to relate those properties
to other physical quantities such as BH mass, accretion rate
and luminosity of the narrow line region. In total our sample
includes 25 sources which are listed in Table 1. Note that the
QSOs in Shemmer et al. (2004) and in Netzer et al. (2004) were
extracted from optically or radio selected catalogs, without any
pre-selection in terms of mid- or far-IR brightness. Therefore,
the sample is not biased in terms of star formation or dust con-
tent in the host galaxy.
We observed these QSOs with the Long-Low resolution
module of the Spitzer Infrared Spectrograph IRS (Houck et al.,
2004), covering the wavelength range 22–35µm, in staring
mode. Objects were acquired by a blind offset from a nearby,
bright 2MASS star, whose location and proper motion were
known accurately from the Hipparcos catalog. We adopted the
“high accuracy” acquisition procedure, which provides a slit
centering good enough to deliver a flux calibration accuracy bet-
ter than 5%. The integration time was of 12 minutes on source,
with the exception of seven which were observed only 4 minutes
each1
We started our reduction from the Basic Calibrated Data
(BCD). For each observation, we combined all images with the
same position on the slit. Then the sky background was sub-
tracted by using pairs of frames where the sources appears at
two different positions along the slit. The spectra were cleaned
for bad, hot and rogue pixels by using the IRSCLEAN algorithm.
The monodimensional spectra were then extracted by means of
the SPICE software.
1 More specifically: LBQS0109+0213, [HB89]1318-113,
[HB89]1346-036,SBS1425+606,[HB89]2126-158, 2QZJ222006.7-
280324,VV0017.
Fig. 1. Average spectrum of all high-z, luminous QSOs in our
sample, normalized to the flux at 6.7µm (black solid line). The
blue dashed line indicates the power-law fitted to the data at
λ < 8µm; the green solid line is the fitted silicate emission and
the red, dot-dashed line is the resulting fit to the stacked spec-
trum (sum of the power-law and silicate emission). The bottom
panel indicates the number of objects contributing to the stacked
spectrum at each wavelength.
3. Analysis
3.1. Main observational results
All of the objects were clearly detected. In Tab. 2 we list the
observed continuum flux densities at the observed wavelength
corresponding to λrest = 6.7µm. This wavelength was chosen
both because it is directly observed in the spectra of all objects
and because it is far from the Silicate feature and in-between
PAH features. Thus the determination of L(6.7µm) should be
little affected by uncertainties in the subtraction of the star-
burst component (see below). For two of the radio-loud objects
([HB89]0123+257 and TON618) the MIR flux lies on the ex-
trapolation of the synchrotron radio emission and therefore the
former is also probably non-thermal. Since in this paper we are
mostly interested in the thermal emission by dust, the latter two
objects are not used in the statistical analysis. For the other two
radio loud QSOs, the extrapolation of the radio spectrum falls
below the observed MIR emission and the latter is little affected
by synchrotron contamination.
Fig. 1 shows the mean spectrum of all sources in the sample,
except for the two which are likely dominated by synchrotron
emission. Each spectrum has been normalized to 6.7µm prior
to averaging. The bottom panel shows the number of sources
contributing to the mean spectrum in different spectral regions.
We only consider the rest frame spectral range where at least
5 objects contribute to the mean spectrum. The spectrum at
λ < 8µm has been fitted with a simple power-law. While other
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 3
Fig. 2. IRS spectra of four individual high-z luminous QSOs showing evidence for silicate emission. The black solid lines indicate
the IRS spectra smoothed with a 5 pixels boxcar. The shaded areas indicate the flux uncertainty. The blue dashed line and the green
solid line are the power-law and the silicate emission components of the fits. The black dotted line shows the starburst component,
which is formally required by the fit, but statistically not significant. The red dot-dashed lines are the global fits to the observed
spectra.
workers in this field assumed more complicated continuum (e.g.
spline, polynomial) we do not consider it justified given the
limited wavelength range of our spectra. The extrapolation of
the continuum to 10µm clearly reveals an excess identified with
Silicate emission. Fitting and measuring the strength of this fea-
ture is not easy given the limited rest-frame spectral coverage
of our spectra. Therefore, we resort to the use of templates.
In particular, we fit the Silicate feature by using as a template
the (continuum-subtracted) silicate feature observed in the aver-
age spectrum of local QSOs as obtained by the QUEST project
(Schweitzer et al., 2006) and kindly provided by M. Schweitzer.
The template Silicate spectrum, with the best fitting scaling fac-
tor is shown in green in Fig. 1, while the red dot-dashed line
shows the resulting fit including the power-law. We adopt the
definition of “silicate strength” given in (Shi et al., 2006) which
is the ratio between the maximum of the silicate feature and the
interpolated featureless continuum at the same wavelength. In
the QSO-QUEST template the maximum of the Silicate feature
is at 10.5µm. This wavelength is slightly outside the band cov-
ered by our spectra but the uncertainty on the extrapolation is
not large (the latter is included in the error estimate of the sil-
icate strength). The silicate strength in the mean spectrum is
0.58±0.10 (Tab. 2).
We note that the average spectrum does not show evidence
for PAH features at 7.7µm and 6.2µm. Such features are ob-
served in lower luminosity AGNs. More specifically, a starburst
4 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
template (Sturm et al., 2000) is not required by the fit shown in
Fig. 1. In Sect. 4.3 we will infer an upper limit on the PAH lu-
minosity and discuss its implication.
We clearly detect the blue wing of the silicate feature in four
individual spectra, which are shown in Fig. 2. These spectra were
fitted with a power-law and a silicate template exactly as the
stacked spectrum. The resulting values for the Silicate strength
are given in Tab. 2. The presence of silicate emission in all other
cases is poorly constrained (or totally unconstrained) either be-
cause of low signal-to-noise (S/N) or because of a lack of spec-
tral coverage. The one exception is Ton 618 which has a high
S/N spectrum and a redshift (z=2.22) appropriate to observe the
Silicate 10.5 µm feature. No silicate emission is detected in this
case, but note that this is not expected since the MIR radiation
of this source is probably dominated by synchrotron emission.
Tables 1 and 2 list the more important MIR information on
the sources and physical properties deduced from the rest-frame
optical spectra and obtained from Shemmer et al. (2004): opti-
cal continuum luminosity λLλ(5100Å), [OIII]λ5007 line lumi-
nosity, BH mass and Eddington accretion rate L/LEdd.
3.2. A comparison with MIR properties of lower luminosity
To compare the MIR properties of our luminous QSOs with
those of lower luminosity sources we have included in our study
the IRS/MIR spectra of various low redshift, lower luminosity
type-I AGNs. We purposely avoid type-II sources because of the
additional complication due to absorption along the line of sight.
We have used data from Shi et al. (2006) who analyze the in-
tensity of the silicate features in several, local AGNs with lumi-
nosities ranging from those of nearby Seyfert 1s to intermediate
luminosity QSOs. We discarded BAL QSOs (which are known
to have intervening gas and dust absorption) as well as dust red-
dened type-I nuclei (e.g. 2MASS red QSOs). We also discard
those cases (e.g. 3C273) where the optical and MIR continuum
is likely dominated by synchrotron radiation. Note that Shi et al.
(2006) selected type-I objects with “high brightness” and, there-
fore, low-luminosity AGNs tend to be excluded from their sam-
Shi et al. (2006) provide a measure of the silicate feature
strength (whose definition was adopted also by us). The con-
tinuum emission at 6.7µm was measured by us from the archival
spectra. We also subtracted from the 6.7µm emission the pos-
sible contribution of a starburst component by using the M82
template. We estimate the host galaxy contribution (stellar pho-
tospheres) in all sources to be negligible.
We include in the sample of local Sy1s also some IRS spectra
taken from the sample of Buchanan et al. (2006), whose spectral
parameters were determined by us from the archival spectral,
in the same manner as for the Shi et al. (2006) spectra. As for
the previous sample, we discarded reddened/absorbed sources as
well as those affected by synchrotron emission. As discussed in
Buchanan et al. (2006), these spectra are affected by significant
flux calibration uncertainties, due to the adopted mapping tech-
nique. Therefore, the spectra were re-calibrated by using IRAC
photometric images. We discarded objects for which IRAC data
are not available or not usable (e.g. because saturated). Finally,
we also discarded data for which optical spectroscopic data are
not available (see below).
The mid-IR parameters of the sources in both samples are
listed in Tab. 2.
Optical data were mostly taken from Marziani et al. (2003)
and BH masses and Eddington accretion rates inferred as in
Shemmer et al. (2004). The resulting parameters are listed in
Table 1.
The type-I sources in Shi et al. (2006) and Buchanan et al.
(2006) are only used for the investigation of the covering fac-
tor and silicate strength, which are the main aims of our work.
The Shi et al. (2006) and Buchanan et al. (2006) samples are not
suitable for investigating the PAH features because most of these
objects are at small distances and the IRS slit misses most of the
star formation regions in the host galaxy. For what concerns the
the PAH emission, we use the data in Schweitzer et al. (2006)
who performed a detailed analysis of the PAH features in their
local QSOs sample. The slit losses in those sources are minor.
The Schweitzer et al. (2006) sample is also used for the investi-
gation of the MIR-to-optical luminosity ratio. The mid-IR data
of this sample are not listed in Tab. 2, since such data are already
reported in Schweitzer et al. (2006) and in Netzer et al. (2007).
4. Discussion
4.1. Dust covering factor
4.1.1. Covering factor as a function of source luminosity and
BH mass
The main assumption used here is that the covering factor of the
circumnuclear dust is given by the ratio of the thermal infrared
emission to the primary AGN radiation. The latter is mostly the
“big blue bump” radiation with additional contribution from the
optical and X-ray wavelength ranges (Blandford et al., 1990).
Determining the integral of the AGN-heated dust emission,
and disentangling it from other spectral components is not sim-
ple. The FIR emission in type-I AGNs is generally dominated by
a starburst component, even in QSOs (Schweitzer et al., 2006).
In lower luminosity AGNs, the near-IR emission may be affected
by stellar emission in the host galaxy, while in QSOs the near-
IR light is often contributed also by the direct primary radia-
tion. The MIR range (∼ 4 − 10µm) is where the contrast be-
tween AGN-heated dust emission and other components is max-
imal. This spectral region contains various spectral features, like
PAHs and silicate emission, yet MIR spectra allow us to disen-
tangle and remove these components, and determine the hot dust
continuum. In particular, by focusing on the continuum emis-
sion at 6.7µm, the uncertainty in the removal of PAH emission
is minimized, while the contribution from the Silicate emission
is totally negligible at this wavelength (note that such a spectral
decomposition is unfeasible with photometric data). If the spec-
tral shape of the AGN-heated dust does not change from object
to object (and in particular it does not change significantly with
luminosity), then the 6.7µm emission is a proxy of the global
circumnuclear hot dust emission. It is possible to infer a quanti-
tative relation between L(6.7µm) and the total AGN-heated hot
dust emission through the work of Silva et al. (2004), who use
observations of various nearby AGNs to determine their average,
nuclear IR SED (divided into absorption classes). From their
type I AGNs SED, we find that the integrated nuclear, thermal
IR bump is about ∼ 2.7 λLλ(6.7µm). This ratio is also consistent
with that found in the QUEST QSO sample, once the contribu-
tion by the silicate features is subtracted.
Regarding the primary optical-UV radiation, determining its
integrated flux would require observations of the entire intrin-
sic spectral energy distribution (SED) from the far-UV to the
near-IR. This is not available for most sources in our sample.
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 5
Fig. 3. MIR continuum luminosity at 6.7µm versus optical con-
tinuum luminosity at 5100Å. Red triangles mark high-z, lumi-
nous QSOs and blue diamonds mark low redshift type-I AGNs.
The black solid line is a fit to the data, which has a slope with a
power index of 0.82.
Moreover, there are indications that the SED is luminosity de-
pendent (e.g. Scott et al., 2004; Shang et al., 2005) and thus an
estimate of the bolometric luminosity of the primary continuum
based on the observed luminosity in a certain band is somewhat
uncertain. Notwithstanding these limitations, we assume in this
work that the optical continuum luminosity can be used as a
proxy of the bolometric luminosity of the primary continuum.
We use the continuum luminosity at the rest frame wavelength
of 5100Å, (λLλ, hereafter L5100) because it is directly measured
for all sources in our sample. We can infer the ratio between
the bolometric luminosity and L5100 from the mean spectrum of
QSOs obtained in recent studies, mostly the results of Scott et al.
(2004) and Richards et al. (2006). These studies indicate a bolo-
metric correction in the range of 5–9. Here we chose, rather ar-
bitrarily, the mean value of 7.
The comparison of L(6.7µm) and L5100 is our way of deduc-
ing the hot dust covering factor. Fig. 3 shows the λLλ(6.7µm)
versus L5100 for our sample. Red triangles are the high-z, lumi-
nous QSOs and blue diamonds are local type-I AGNs. Not sur-
prisingly, the two quantities show a good correlation. However,
the very large luminosity range spanned by our sample allows us
to clearly state that the correlation is not linear, but has a slope
α = 0.82 ± 0.02 defined by
log[λLλ(6.7µm)] = K + α log[λLλ(5100Å)] (1)
where K = 8.36±0.80 and luminosities are expressed in erg s−1.
This indicating that the MIR, reprocessed emission increases
more slowly than the primary luminosity.
The same phenomenon is observed in a cleaner way in
Fig. 4a, where the ratio between the two continuum luminosi-
ties is plotted as a function of L5100. There is a clear anti-
correlation between the MIR–to–optical ratio and optical lumi-
nosity. Fig. 4b shows the same MIR–to–optical ratio as a func-
tion of L([OIII]λ5007) as an alternative tracer of the global AGN
luminosity (although the latter is not a linear tracer of the nu-
clear luminosity, as discussed in Netzer et al., 2006), which dis-
plays the same anti-correlation as for the continuum optical flux.
Spearman-rank coefficients and probabilities for these correla-
tions are given in Tab. 3.
According to the above discussion, an obvious interpretation
of the decreasing MIR–to–optical ratio is that the covering factor
of the dust surrounding the AGN decreases with luminosity. In
particular, if the covering factor is proportional to the MIR–to–
optical ratio, then Figs. 4a-b indicate that the dust covering factor
decreases by about a factor 10 over the luminosity range probed
by us.
It is possible to convert the MIR–to–optical luminosity ra-
tio into absolute dust covering factor by assuming ratios of
broad band to monochromatic continuum luminosities observed
in AGNs, as discussed above. In particular, by using the 6.7µm–
to–MIR and 5100Å–to–bolometric luminosity ratios reported
above, we obtain that the absolute value of the dust covering
factor (CF) can be written as:
CF(dust) ≈ 0.39 ·
λLλ(6.7µm)
λLλ(5100Å)
. (2)
In Fig. 4 the axes on the right hand side provide the dust
covering factor inferred from the the equation above. A fraction
of objects have covering factor formally larger than one, these
could be due to uncertainties in the observational data, or nu-
clear SED differing from the ones assumed above, or to optical
variability, as discussed in Sect. 4.1.2. The dust covering fac-
tor ranges from about unity in low luminosity AGNs to about
10% in high luminosity QSOs. As it will be discussed in detail
in Sect. 4.1.3, the dust covering factor is expected to be equal
to the fraction of type 2 (obscured) AGNs relative to the total
AGN population. The finding of a large covering factor in low
luminosity AGNs is in agreement with the large fraction of type
2 nuclei observed in local Seyferts (∼ 0.8, Maiolino & Rieke,
1995).
A similar result has been obtained, in an independent way,
through the finding of a systemic decrease of the the obscured
to unobscured AGN ratio as a function of luminosity in various
surveys. The comparison with these results will be discussed in
more detail in the next section.
The physical origin of the decreasing covering factor is still
unknown. One possibility is that higher luminosities imply a
larger dust sublimation radius: if the obscuring medium is dis-
tributed in a disk with constant height, then a larger dust subli-
mation radius would automatically give a lower covering factor
of dust at higher luminosities (Lawrence, 1991). However, this
effect can only explain the decreasing covering factor with lu-
minosity for the dusty medium, and not for the gaseous X-ray
absorbing medium. Moreover, Simpson (2005) showed that the
simple scenario of such a “receding torus” is unable to account
for the observed dependence of the type 2 to type 1 AGN ratio
as a function of luminosity.
Another possibility is that the radiation pressure on dust is
stronger, relative to the BH gravitational potential, in luminous
AGNs (e.g. Laor & Draine, 1993; Scoville & Norman, 1995),
thus sweeping away circumnuclear dust more effectively. In this
scenario a more direct relation of the covering factor should be
with L/LEdd, rather than with luminosity. Our sample does not
show such a relation, as illustrated in Fig. 4c. However, the un-
certainties on the accretion rates (horizontal error bar in Fig. 4c)
may hamper the identification of such a correlation.
Lamastra et al. (2006) proposed that, independently of lumi-
nosity, the BH gravitational potential is responsible for flatten-
6 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 4. MIR–to–optical continuum luminosity [λLλ(6.7µm)]/[λLλ(5100Å)] versus (a) continuum optical luminosity, (b) [OIII]λ5007
line luminosity, (c) accretion rate L/LEdd and (d) black hole mass. Symbols are the same as in Fig. 3. The magenta square indicates
the location of the mean SDSS QSO SED in Richards et al. (2006). The horizontal error bars in panels (c) and (d) indicate con-
servative uncertainties on the accretion rates and BH masses. The right hand side axis on each panel shows the circumnuclear dust
covering factor inferred from Eq. 2. The dashed lines in panels (a) and (b) shows the fit resulting from the analytical forms in Eqs. 3
and 6, respectively.
ing the circumnuclear medium, so that larger BH masses effec-
tively produce a lower covering factor. According to this sce-
nario, the relation between covering factor and luminosity is
only an indirect one, in the sense that more luminous AGN tend
to have larger BH masses (if the Eddington accretion rate does
not change strongly on average). Fig. 4d shows the MIR–to–
optical ratio (and dust covering factor) as a function of BH mass,
indicating a clear (anti-)correlation between these two quanti-
ties. However, the correlation is not any tighter than the relation
with luminosity in Fig. 4a-b, as quantified by the comparison of
the Spearman-rank coefficients and probabilities in Tab. 3. The
degeneracy between luminosity and BH mass prevents us to dis-
criminate which of the two is the physical quantity driving the
relation.
4.1.2. Model uncertainties
In this section we discuss some possible caveats in our interpre-
tation of the MIR–to–optical ratio as an indicator of the hot dust
covering factor.
Our analysis assumes that the shape of the hot dust IR spec-
trum SED is not luminosity dependent. However, an alternative
interpretation of the trends observed in Fig. 4 could be that the
dust temperature distribution changes with luminosity. Yet, to
explain the decreasing 6.7µm to 5100Å flux ratio in terms of dust
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 7
Fig. 5. Mid-IR spectral slope (5–8µm) versus
λLλ(6.7µm)/λLλ(5100Å) ratio. Symbols are as in Fig. 3.
No clear correlation is observed between these two quan-
tities (see also Tab. 3). Note that the exceptional object
SDSSJ173352.22+540030.5 is out of scale, and it is discussed
in the text.
Fig. 6. Optical-to-UV spectral slope (1450–5100Å) versus
λLλ(6.7µm)/λLλ(5100Å) ratio. Symbols are as in Fig. 3. The
arrow indicates the effect of dust reddening with AV = 2 mag.
The data do not show any evidence for dust reddening effects.
temperature would require that the circumnuclear dust is cooler
at higher luminosities. This, besides being contrary to expecta-
tions, is ruled out by the observations which show no clear corre-
lation between the mid-IR continuum slope (which is associated
with the average dust temperature) and the MIR–to–optical ratio,
as illustrated in Fig. 5 and in Tab. 3. The one remarkable excep-
tion is SDSSJ173352.22+540030.5,which has the most negative
mid-IR continuum slope (αMIR = −2.88, i.e. an inverted spec-
trum in λLλ) and the lowest MIR-to-optical ratio of the whole
sample (λLλ(6.7µm)/λLλ(5100Å) = 0.22), which is out of scale
in Fig. 5. This QSO may be totally devoid of circumnuclear hot
dust, and its MIR emission may simply be the continuation of the
optical “blue-bump”. Similar high-z QSOs, with an exceptional
deficiency of mir-MIR flux, have been reported by Jiang et al.
(2006).
As explained earlier, there are indications that the UV-optical
SED, and hence the bolometric correction based to the observed
L5100, are luminosity dependent. If correct, this would mean a
smaller bolometric correction for higher L5100 sources which
would flatten the relationship found here (i.e. will result in a
slower decrease of the covering factor with increasing L5100).
However, the expected range (a factor of at most 2 in bolometric
correction) is much smaller than the deduced change in covering
factor.
Variability is an additional potential caveat because of the
time delay between the original L5100 “input” and the response
of the dusty absorbing “torus”. While we do not have the obser-
vations to test this effect (multi-epoch, high-quality optical spec-
troscopic data are available only for a few sources in our sam-
ple), we note that the location of the 6.7µm emitting gas from
the central accretion disk is at least several light years and thus
L(MIR) used here reflects the mean L5100 in most sources. We
expect that the average luminosity of a large sample will not be
affected much by individual source variations. Moreover, in the
high luminosity sources of our sample we do not expect much
variability (since luminous QSOs are known to show little or no
variability).
An alternative, possible explanation of the variation of the
optical-to-MIR luminosity ratio could be dust extinction affect-
ing the observed optical flux. Optical dust absorption increasing
towards low luminosities may in principle explain the trends ob-
served in Fig. 4. However, we have pre-selected the sample of
local QSOs and Sy1s to avoid objects showing any indication
of absorption, thus probably shielding us from such spurious ef-
fects. Yet, we have further investigated the extinction scenario
by analyzing the optical-to-UV continuum shape of our sample.
The optical-UV continuum slope does not necessarily trace dust
reddening, since intrinsic variations of the continuum shape are
known to occur, as discussed above. Variability introduce ad-
ditional uncertainties, since optical and UV data are not simul-
taneous. However, if the variations of λLλ(6.7µm)/λLλ(5100Å)
are mostly due to dust reddening, one would expect the MIR-
to-optical ratio to correlate with the optical-UV slope, at least
on average. We have compiled UV rest-frame continuum fluxes
(at λrest ∼ 1450Å) from spectra in the literature or in the HST
archive. By combining such data with the continuum luminosi-
ties at λrest = 5100Å we inferred the optical-UV continuum
slope αopt−UV defined as
2 Lλ ∝ λ
αopt−UV , as listed in Tab. 1.
Fig. 6 shows αopt−UV versus λLλ(6.7µm)/λLλ(5100Å). The ar-
row indicates the effect of dust reddening with AV = 2 mag
(by assuming a SMC extinction curve, as appropriate for type
1 AGNs, Hopkins et al., 2004), which would be required to ac-
count for the observed variations of λLλ(6.7µm)/λLλ(5100Å).
Fig. 6 does not show evidence for any (positive) correlation be-
tween αopt−UV and λLλ(6.7µm)/λLλ(5100Å) (see also Tab 3).
In particular, if the variation of MIR-to-optical ratio (spanning
more than a factor of ten) was due to dust reddening, we would
expect to find ∆αopt−UV > 5 (as indicated by the arrow in Fig. 6),
which is clearly not observed. If any, the data show a marginal
2 Our definition of power law index is linked to the αν given in
Vanden Berk et al. (2001) by the relation αopt−UV = −(αν + 2). Our dis-
tribution of αopt−UV is roughly consistent (within the uncertainties and
the scatter) with αν = −0.44 obtained by Vanden Berk et al. (2001) for
the SDSS QSO composite spectrum (see also Shemmer et al., 2004).
8 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
anti-correlation between αopt−UV and λLλ(6.7µm)/λLλ(5100Å)
(Tab. 3), i.e. opposite to that expected from dust reddening.
Finally, possible evolutionary effects on the dust covering
factor have not been considered. We have been comparing lo-
cal objects with QSOs at z∼2–3 yet assumed that only lumi-
nosity or BH mass plays a role. La Franca et al. (2005) and
Akylas et al. (2006) find evidence for an increasing fraction of
obscured AGNs as a function of redshift, a result which is still
debated (see Ueda et al., 2003; Gilli et al., 2007). If the ab-
sorbing medium covering factor really increases with redshift,
then the actual dependence of the covering factor on luminos-
ity would be stronger than shown in Fig. 4. Indeed, according to
La Franca et al. (2005) and Akylas et al. (2006), putative low-z
counterparts of our high-z QSOs (matching the same luminosi-
ties) should be affected by an even lower covering factor. As a
consequence, the diagrams in Figs. 4 and 7 should have even
steeper trends once the data are corrected for such putative evo-
lutionary effects, thus strengthening our conclusions.
4.1.3. Comparison with previous works
Wang et al. (2005) used IRAS and ISO mid-IR data of (mostly
local) PG quasars to infer their dust covering factor. They find
that the covering factor decreases as a function of X-ray lumi-
nosity. They probe a narrower luminosity range with respect to
our work, nonetheless their results are generally consistent with
ours, although with significant scatter.
More recently Richards et al. (2006) derived the broad band
SED of a large sample of SDSS QSOs by including Spitzer pho-
tometric data. Although such data do not have spectroscopic in-
formation, they can be used to obtain a rough indication of the
dust covering factor in their QSO sample, to be compared with
our result. The average optical luminosity of the Richards et al.
(2006) sample is 〈log[λLλ(5100Å)]〉 ∼ 45.5 erg s
−1. From their
mean SED we derive λLλ(6.7µm)/λLλ(5100Å) = 1.17. The cor-
responding location on the diagram of Fig. 4a is marked with a
magenta square, and it is in agreement with the general trend of
our data.
In a companion paper, Gallagher et al. (2007) use the same
set of data to investigate the MIR-to-optical properties as a func-
tion of luminosity. They find a result similar to ours, i.e. the
MIR-to-optical ratio decreases with luminosity. However, they
interpret such a result as a consequence of dust reddening in the
optical, since the effect is stronger in QSOs with redder opti-
cal slope. As discussed in the previous section, our data do no
support this scenario, at least for our sample. In particular, the
analysis of the optical-UV slope indicates that dust reddening
does not play a significant role in the variations of the MIR-
to-optical luminosity ratio. The discrepancy between our and
Gallagher et al. (2007) results may have various explanations.
The QSOs in the Gallagher et al. (2007) sample span about two
orders of magnitudes in luminosity, while we have seen that
to properly quantify the effect a wider luminosity range is re-
quired. Moreover, the majority of their sources are clustered
around the mean luminosity of 1045.5 erg s−1. In addition, the
the lack of spectroscopic information makes it difficult to allow
for the presence of other spectral features such as the silicate
emission which, as we show later, is luminosity dependent. The
lack of spectroscopic information may be an issue specially for
samples spanning a wide redshift range (as in Gallagher et al. ,
2007), where the photometric bands probe different rest-frame
bands. The same concerns applies for the optical luminosities.
Our rest-frame continuum optical luminosities are always in-
ferred through rest-frame optical spectra, even at high-z (through
near-IR spectra). Gallagher et al. (2007) do not probe directly
the optical continuum luminosity of high-z sources (at high-z
they only have optical and Spitzer data, which probe UV and
near-IR rest-frame, respectively). Finally, differences between
our and Gallagher et al. (2007) results may be simply due to
the different samples. As discussed in the previous section, we
avoided dust reddened targets, thus making us little sensitive to
extinction effects, while Gallagher et al. (2007) sample may in-
clude a larger fraction of reddened objects.
A decreasing dust covering factor as a function of lumi-
nosity must translate into a decreasing fraction of obscured
AGNs as a function of luminosity. The effect has been noted
in various X-ray surveys (Ueda et al., 2003; Steffen et al., 2003;
La Franca et al., 2005; Akylas et al., 2006; Barger et al., 2005;
Tozzi et al., 2006; Simpson, 2005), although the results have
been questioned by other authors (e.g. Dwelly & Page, 2006;
Treister & Urry, 2005; Wang et al., 2007). The X-ray based
studies do not distinguish between dust and gas and thus probe
mostly trends of the gaseous absorption. Our result provides an
independent confirmation of these trends. Moreover, our find-
ings are complementary to those obtained in the X-rays since,
instead of the covering factor of gas, we probe the covering fac-
tor of dust.
In order to compare our findings with the results obtained
from X-ray surveys, we have derived the expected fraction of
obscured AGNs by fitting the dust covering factor versus lumi-
nosity relation with an analytical function. Instead of using the
simple power-law illustrated in Fig. 3 (Eq. 1) we fit the depen-
dence of the covering factor on luminosity with a broken power-
law. The latter analytical function is preferred both because it
provides a statistically better fit and because a simple power-law
would yield a covering factor larger than unity at low luminosi-
ties. As a result we obtain the following best fit for the fraction
of obscured AGNs as a function of luminosity:
fobsc =
1 +Lopt
0.414
where fobsc is the fraction of obscured AGNs relative to the total
Lopt =
λLλ(5100Å) [erg s
1045.63
The resulting fit is shown with a dashed line in Fig. 4a. The frac-
tion of obscured AGNs as a function of luminosity is also re-
ported with a blue, solid line in Fig. 7a. The shaded area reflects
the uncertainty in the bolometric correction discussed above.
The most recent and most complete investigation on the frac-
tion of X-ray obscured AGNs as a function of luminosity has
been obtained by Hasinger (2007, in prep., see also Hasinger,
2004) who combined the data from surveys of different areas
and limiting fluxes to get a sample of ∼700 objects. We convert
from X-ray to optical luminosity by using the non-linear relation
obtained by Steffen et al. (2006). The latter use flux densities at
2 keV and 2500Å; we adapt their relation to our reference optical
wavelength (5100Å) by assuming the optical-UV spectral slope
obtained by Vanden Berk et al. (2001), and to the 2–10 keV inte-
grated luminosity (adopted in most X-ray surveys) by assuming
a photon index of –1.7, yielding the relation
log[L(2 − 10 keV)] = 0.721 · log[λLλ(5100Å)]+ 11.78 (5)
(where luminosities are in units of erg s−1). Fig. 7a compares the
fraction of obscured AGN obtained by Hasinger (2007) through
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 9
Fig. 7. a) Fraction of obscured AGNs (relative to total) as a function of optical continuum luminosity. The blue line shows the
fraction of obscured AGNs inferred from the hot dust covering factor with the analytical form of Eq. 3. The shaded area is the
uncertainty resulting from the plausible range of the bolometric correction (see text). Points with error bars are the fraction of X-ray
obscured AGNs as a function of X-ray luminosity inferred by Hasinger (2007). The upper scale of the diagram gives the intrinsic
hard X-ray luminosity; the (non-linear) correspondence between X-ray and optical luminosity is obtained from Eq. 5. b) Fraction of
obscured AGNs as a function of L([OIII]λ5007). The blue line is the fraction of obscured AGNs inferred from the hot dust covering
factor with the analytical form of Eq. 6. Points with error bars show the fraction of type 2 AGNs as a function of L([OIII]λ5007)
inferred by Simpson (2005).
X-ray surveys with our result based on the covering factor of
hot dust. Both have the same trends with luminosity, but the
fraction of obscured AGN expected from the hot dust cover-
ing factor is systematically higher. Such an offset is however ex-
pected. Indeed, current high redshift X-ray surveys do not probe
the Compton thick population of obscured AGNs since these
are heavily absorbed even in the hard X-rays. In local AGNs,
Compton thick nuclei are about as numerous as Compton thin
ones (Risaliti et al., 1999; Guainazzi et al., 2005; Cappi et al.,
2006). The fraction of Compton thick, high luminosity, high
redshift AGNs is still debated, but their contribution certainly
makes the fraction of X-ray obscured AGNs higher than in-
ferred by Hasinger (2007), who can only account for Compton
thin sources. The ratio between the dust covering factor curve in
Fig. 7a and the X-ray data from Hasinger (2007), indicates that
the ratio between the total number of obscured AGNs (includ-
ing Compton thick ones) and Compton thin ones is about 2 even
at high luminosities, i.e. consistent (within uncertainties) with
local, low-luminosity AGNs.
An analogous result on the decreasing fraction of obscured
AGNs as a function of luminosity was obtained by Simpson
(2005) who compared the numbers of type 2 and type 1 AGNs
at a given L([OIII]λ5007). To compare with Simpson (2005), we
used our sample to derive the following analytical description for
the fraction of obscured AGN as a function of L([OIII]λ5007):
fobsc =
1 +L[OIII]
0.409
where fobsc is the fraction of obscured AGNs relative to the total
L[OIII] =
L([OIII]) [erg s−1]
1043.21
The corresponding fit is shown with a dashed line in Fig. 4b, and
the fraction of obscured AGNs as a function of L([OIII]) is also
shown with a blue line in Fig. 7b. In the latter figure we also
compare the fraction of obscured AGNs obtained by Simpson
(2005) with our result based on the covering factor of the hot
dust. There is a good agreement (within uncertainties) between
the fraction of obscured AGNs inferred through the two meth-
ods, as expected since both optical surveys and our method probe
the covering factor of dust. However, we shall also mention that
the results obtained by Simpson (2005) have been questioned by
Haas et al. (2005), by arguing that at high luminosities, the de-
rived L([OIII]λ5007) may be affected by a large scale absorber.
4.2. Silicate emission
In this paper we have presented the most luminous (type 1)
QSOs where Silicate emission has been detected so far. When
combined with MIR spectra of lower luminosity sources, it is
possible to investigate the properties and behavior of this feature
over a wide luminosity range.
The discovery of silicate emission in the spectrum of (mostly
type 1) AGNs obtained by Spitzer was regarded as the so-
lution of a long standing puzzle on the properties of the cir-
10 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 8. Silicate strength versus (a) L5100, (b) L([OIII]λ5007), (c) normalized accretion rate (L/LEdd) and (d) black hole mass. Symbols
are the same as in Fig. 3. The red square indicates the silicate strength measured in the average spectrum. The horizontal, black error
bars in panels (c) and (d) indicate conservative uncertainties on the accretion rates and BH masses.
cumnuclear dusty medium. Indeed, silicate emission was ex-
pected by various models of the dusty torus. However, the ab-
sence of clear detections prior to the Spitzer epoch induced
various authors to either postulate a very compact and dense
torus (e.g. Pier & Krolik, 1993) or different dust compositions
(Laor & Draine, 1993; Maiolino et al., 2001a,b). Initial Spitzer
detections of silicate emission relaxed the torus model assump-
tions (Fritz et al., 2006), but more detailed investigations re-
vealed a complex scenario. The detection of silicate emission
even in type 2 AGNs (Sturm et al., 2006b; Teplitz et al., 2006;
Shi et al., 2006) suggested that part of the silicate emission may
originate in the Narrow Line Region (NLR) (Efstathiou, 2006).
Further support for a NLR origin of the silicate emission comes
from the temperature inferred for the Silicate features, which is
much lower (<200 K) than for the circumnuclear dust emitting
the featureless MIR continuum (>500 K), as well as from MIR
high resolution maps spatially resolving the silicate emission on
scales of 100 pc (Schweitzer et al. in prep.).
If most of the observed silicate emission originates in the
NLR, then the effects of circumnuclear hot dust covering fac-
tor should be amplified when looking at the “silicate strength”
(which we recall is defined as the ratio of the silicate maximum
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 11
intensity and the featureless hot dust continuum). Indeed, if the
covering factor of the circumnuclear dusty torus decreases, it im-
plies that the MIR hot dust continuum decreases and the silicate
emission increases because a larger volume of the NLR is il-
luminated. Both effects go in the same direction of increasing
the “silicate strength”. This scenario is made more complex by
the tendency of the NLR to disappear at very high luminosities,
or to get very dense and not to scale linearly with the nuclear
luminosity (Netzer et al., 2004, 2006). Moreover, the schematic
division of a silicate feature totally emitted by the NLR and a
MIR featureless continuum totally emitted by the inner side of
the obscuring torus is probably too simplistic. There must be
at least a small contribution to the featureless MIR continuum
from dust in the NLR, while some silicate emission is probably
also coming from the obscuring torus. However, from a general
qualitative point of view we expect a monotonic behavior of the
“silicate strength” with the physical quantity responsible for the
changes in the hot dust covering factor.
Before investigating the various trends of the Silicate
strength, we mention that by using the four silicate detections
shown in Fig. 2 and listed in Tab. 2, we may in principle intro-
duce a bias against weak silicate emitters. Indeed, although we
cannot set useful upper limits on the silicate strength in most of
the other objects, we have likely missed objects with low sili-
cate strength. However, the mean spectrum in Fig. 1 includes all
QSOs in our sample, and therefore its silicate strength should
be representative of the average Silicate emission in the sample
(at least for the objects at z<2.5, which are the ones where the
observed band includes the silicate feature, and which are the
majority).
Figs. 8a-b show the silicate strength of the objects in our
combined sample as functions of L5100 and L([OIII]λ5007). The
red square indicates the silicate strength in the high-z QSO mean
spectrum, while its horizontal bar indicates the range of lumi-
nosities spanned by the subsample of objects at z<2.5 (i.e. those
contributing to the silicate feature in the mean spectrum). Low
redshift AGNs and high redshift QSOs show an apparently clear
correlation between silicate strength and luminosity. Although
with a significant spread, the Silicate strength is observed to pos-
itively correlate also with the accretion rate L/LEdd and with the
BH mass, as shown in Figs. 8c-d. Essentially, the correlations
observed for the Silicate strength reflects the same correlation
observed for the L(6.7µm)/L(5100Å) (with the exception of the
accretion rate), in agreement with the idea that also the Silicate
strength is a proxy of the covering factor of the circumnuclear
hot dust, for the reasons discussed above.
Unfortunately, the correlations observed for the Silicate
strength do not improve our understanding on the origin of the
decreasing covering factor with luminosity, i.e. whether the driv-
ing physical quantity is the luminosity itself, the accretion rate or
the black hole mass. Formally, the correlation between Silicate
strength and optical continuum luminosity is tighter than the oth-
ers (Tab. 3), possibly hinting at the luminosity itself as the quan-
tity driving the dust covering factor. However, there are a few
low luminosity objects, such as a few LINERs, which have large
silicate strengths (Sturm et al., 2005) and which clearly deviate
from the correlation shown in Fig. 8a, thus questioning the role
of luminosity in determining the Silicate strength. In addition,
the apparently looser correlations of Silicate strength versus ac-
cretion rate and BH mass may simply be due to the additional
uncertainties affecting the latter two quantities (horizontal, black
error bars in Figs. 8c,d).
4.3. PAHs and star formation
The presence and intensity of star formation in QSOs has been
a hotly debated issue during the past few years. A major step
forward in this debate was achieved by Schweitzer et al. (2006)
through the Spitzer IRS detection of PAH features in a sample
of nearby QSOs, revealing vigorous star formation in these ob-
jects. The analysis also shows that the far-IR emission in these
QSOs is dominated by star formation and that the star forming
activity correlates with the nuclear AGN power. Here we show in
Fig. 9 the latter correlation in terms of PAH(7.7µm) luminosity
versus L5100 by using the PAH luminosities from the sample of
Schweitzer et al. (2006) and the corresponding optical data from
Marziani et al. (2003). Although the large fraction of upper lim-
its in the former sample prevents a careful statistical characteri-
zation, Fig. 9 shows a general correlation between QSO optical
luminosity and starburst activity in the host galaxy as traced by
the PAH luminosity.
The scale on the right hand side of Fig. 9 translates the 7.7µm
PAH luminosity into star formation rate (SFR). This was ob-
tained by combining the average L(PAH7.7µm)/L(FIR) ratio ob-
tained by Schweitzer et al. (2006) for the starburst dominated
QSOs in their sample with the SFR/L(FIR) given in Kennicutt
(1998), yielding
SFR [M⊙ yr
−1] = 3.46 10−42 L(PAH7.7µm) [erg s
−1] (8)
The average spectrum of high-z, luminous QSOs in Fig. 1
does not show evidence for the presence of PAH features and
can only provide an upper limit on the PAH flux relative to
the flux at 6.7µm (since all spectra were normalized to the lat-
ter wavelength prior to computing the average). However, we
can derive an upper limit on the PAH luminosity by assum-
ing the average distance of the sources in the sample. The in-
ferred upper limit on the PAH luminosity is reported with a red
square in Fig. 9a, and it is clearly below the extrapolation of
the L(PAH7.7µm) − λLλ(5100Å) relation found for local, low-
luminosity QSOs. This is shown more clearly in Fig. 9b which
shows the distribution of the ratio L(PAH7.7µm)/λLλ(5100Å) for
local QSOs (histogram) and the upper limit inferred from the av-
erage spectrum of high-z, luminous QSOs (red solid line). The
corresponding upper limit on the SFR is ∼ 700 M⊙ yr
Note that certainly there are luminous, high-z QSOs
with larger star formation rates (e.g. Bertoldi et al., 2003;
Beelen et al., 2006; Lutz et al., 2007). However, since our sam-
ple is not pre-selected in terms of MIR or FIR emission, our
result is not biased in terms of star formation and dust content,
and therefore it is representative of the general high-z, luminous
QSO population.
Our results indicate that the relation between star forma-
tion activity, as traced by the PAH features, and QSO power,
as traced by L5100, saturates at high luminosity. This result is
not surprising. Indeed, if high-z, luminous QSOs were char-
acterized by the same average L(PAH7.7µm)/λLλ(5100Å) ob-
served in local QSOs, this would imply huge star formation rates
of ∼ 7000 M⊙ yr
−1 at λLλ(5100Å) ∼ 10
47 erg s−1. On the
contrary, the few high-z QSOs detected at submm-mm wave-
lengths have far-IR luminosities corresponding to SFR of about
1000−3000 M⊙ yr
−1 (Omont et al., 2003). The majority of high-
z QSO (∼70%) are undetected at submm-mm wavelengths. The
mean mm-submm fluxes of QSOs in various surveys (includ-
ing both detections and non-detections) imply SFRs in the range
∼ 500−1500 M⊙ yr
−1 (Omont et al., 2003; Priddey et al., 2003),
in fair agreement with our finding, especially if we consider the
12 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 9. a) PAH(7.7µm) luminosity as a function of the QSO optical luminosity. Blue diamonds are data from Schweitzer et al. (2006).
The red square is the upper limit obtained by the average spectrum of luminous, high-z QSOs. b) Distribution of the PAH(7.7µm)
to optical luminosity ratio in the local QSOs sample of Schweitzer et al. (2006). The hatched region indicates upper limits. The red
vertical line indicate the upper limit inferred from the average spectrum of luminous QSOs at high-z.
uncertainties involved in the two different observational methods
to infer the SFR.
We also note that a similar, independent result was obtained
by Haas et al. (2003), who found that in a large sample of QSOs
the ratio between LFIR (powered by star formation) and LB de-
creases at high luminosities.
The finding of a “saturation” of the relation between star for-
mation activity and QSO power may provide an explanation for
the evolution of the relation between BH mass and galaxy mass
at high redshift. Indeed, Peng et al. (2006) and McLure et al.
(2006) found that, for a given BH mass, QSO hosts at z∼2 are
characterized by a stellar mass lower than expected from the
local BH-galaxy mass relation. In other terms, the BH growth
is faster, relative to star formation, in high-z, luminous QSOs.
Our result supports this scenario by independently showing that
the correlation between star formation and AGN activity breaks
down at high luminosities.
5. Conclusions
We have presented low resolution, mid-IR Spitzer spectra of a
sample of 25 luminous QSOs at high redshifts (2 < z < 3.5).
We have combined our data with Spitzer spectra of lower lumi-
nosity, type-I AGNs, either published in the literature or in the
Spitzer archive. The combined sample spans five orders of mag-
nitude in luminosity, and allowed us to investigate the dust prop-
erties and star formation rate as a function of luminosity. The
spectroscopic information allowed us to disentangle the various
spectral components contributing to the MIR band (PAH and sil-
icate emission) and to sample the continuum at a specific λrest, in
contrast to photometric MIR observations. The main results are:
– The mid-IR continuum luminosity at 6.7µm correlates with
the optical continuum luminosity but the correlation is not
linear. In particular, the ratio λLλ(6.7µm)/λLλ(5100Å) de-
creases by about a factor of ten as a function of lumi-
nosity over the luminosity range 1042.5 < λLλ(5100Å) <
1047.5 erg s−1. This is interpreted as a reduction of the cov-
ering factor of the circumnuclear hot dust as a function of
luminosity. This result is in agreement and provides an in-
dependent confirmation of the recent findings of a decreas-
ing fraction of obscured AGN as a function of luminosity,
obtained in X-ray and optical surveys. We stress that while
X-ray surveys probe the covering factor of the gas, our result
provides an independent confirmation by probing the cover-
ing factor of the dust. We have also shown that the dust cov-
ering factor, as traced by the λLλ(6.7µm)/λLλ(5100Å) ratio,
decreases also as a function of the BH mass. Based on these
correlations alone it is not possible to determine whether the
physical quantity primarily driving the reduction of the cov-
ering factor is the AGN luminosity or the BH mass.
– The mean spectrum of the luminous, high-z QSOs in our
sample shows a clear silicate emission at λrest ∼ 10µm.
Silicate emission is also detected in the individual spectra
of four high redshift QSOs. When combined with the spec-
tra of local, lower luminosity AGNs we find that the sili-
cate strength (defined as the ratio between the maximum of
the silicate feature and the extrapolated featureless contin-
uum) tend to increase as a function of luminosity. The sili-
cate strength correlates positively also with the accretion rate
and with the BH mass, albeit with a large scatter.
– The mean MIR spectrum of the luminous, high-z QSOs in
our sample does not show evidence for PAH emission. Our
sample is not pre-selected by the FIR emission and therefore
it is not biased in terms star formation. As a consequence, the
upper limit on the PAH emission in the total mean spectrum
provides a useful, representative upper limit on the SFR in
luminous QSOs at high redshifts. We find that the ratio be-
tween PAH luminosity and QSO optical luminosity is signifi-
cantly lower than observed in local, lower luminosity AGNs,
implying that the correlation between star formation rate and
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 13
AGN power probably saturates at high luminosities. This re-
sult may explain the evolution of the correlation between BH
mass and galaxy stellar mass recently observed in luminous
QSOs at high redshift.
Acknowledgements. We are grateful to M. Salvati for useful comments. We are
grateful to G. Hasinger for providing us with some of his results prior to publica-
tion. This work is based on observations made with the Spitzer Space Telescope,
which is operated by the Jet Propulsion Laboratory, California Institute of
Technology under a contract with NASA. Support for this work was provided
by NASA under contract 1276513 (O.S.). RM acknowledges partial support
from the Italian Space Agency (ASI). MI is supported by Grants-in-Aid for
Scientific Research (16740117). OS acknowledges support by the Israel Science
Foundation under grant 232/03.
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List of Objects
‘2QZJ002830.4-281706’ on page 16
‘LBQS0109+0213’ on page 16
http://arxiv.org/abs/astro-ph/0702272
http://arxiv.org/abs/astro-ph/0610939
http://arxiv.org/abs/astro-ph/0702136
http://arxiv.org/abs/astro-ph/0702701
14 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
‘[HB89]0123+257’ on page 16
‘HS0211+1858’ on page 16
‘2QZJ023805.8-274337’ on page 16
‘SDSSJ024933.42-083454.4’ on page 16
‘Q0256-0000’ on page 16
‘Q0302-0019’ on page 16
‘[HB89]0329-385’ on page 16
‘SDSSJ100428.43+001825.6’ on page 16
‘TON618’ on page 16
‘[HB89]1318-113’ on page 16
‘[HB89]1346-036’ on page 16
‘UM629’ on page 16
‘UM632’ on page 16
‘BS1425+606’ on page 16
‘[VCV01]J1649+5303’ on page 16
‘SDSSJ170102.18+612301.0’ on page 16
‘SDSSJ173352.22+540030.5’ on page 16
‘[HB89]2126-158’ on page 16
‘2QZJ221814.4-300306’ on page 16
‘2QZJ222006.7-280324’ on page 16
‘Q2227-3928’ on page 16
‘[HB89]2254+024’ on page 16
‘2QZJ234510.3-293155’ on page 16
‘Mrk335’ on page 16
‘IIIZw2’ on page 16
‘PG0050+124’ on page 16
‘PG0052+251’ on page 16
‘Fairall9’ on page 16
‘Mkr79’ on page 16
‘PG0804+761’ on page 16
‘Mrk704’ on page 16
‘PG0953+414’ on page 16
‘NGC3516’ on page 16
‘PG1116+215’ on page 16
‘NGC3783’ on page 16
‘PG1151+117’ on page 16
‘NGC4051’ on page 16
‘PG1211+143’ on page 16
‘NGC4593’ on page 16
‘PG1309+355’ on page 16
‘PG1351+640’ on page 16
‘IC4329a’ on page 16
‘NGC5548’ on page 16
‘Mrk817’ on page 16
‘Mrk509’ on page 16
‘Mrk926’ on page 16
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 15
Table 1. Combined sample of high-z luminous QSO, local QSO and Sy1, and physical properties inferred from optical-UV spectra.
Name RA(J2000) Dec(J2000) z log(λLλ(5100Å)) logL([OIII]) logM(BH)
d L/LdEdd αopt−UV Ref.
(erg/s) (erg/s) M⊙
High-z luminous QSOs
2QZJ002830.4-281706 00:12:21.18 −28:36:30.2 2.401 46.59 44.41 9.72 0.35 −1.42 1
LBQS0109+0213 01:12:16.91 +02:29:47.6 2.349 46.81 44.51 10.01 0.30 −1.75 1
[HB89]0123+257a 01:26:42.79 +25:59:01.3 2.369 46.58 44.32 9.10 1.40 −1.65 1
HS0211+1858 02:14:29.70 +19:12:37.0 2.470 46.63 44.38 10.11 0.16 −0.01 3,10
2QZJ023805.8-274337 02:38:05.80 −27:43:37.0 2.471 46.58 <43.71 9.41 0.69 −1.59 1
SDSSJ024933.42-083454.4 02:49:33.41 −08:34:54.4 2.491 46.39 44.12 9.67 0.25 −1.36 1
Q0256-0000 02:59:05.64 +00:11:21.9 3.377 46.99 44.55 10.11 0.19 −0.96 2
Q0302-0019 03:04:49.86 −00:08:13.4 3.286 46.83 45.01 10.11 0.30 −1.66 2
[HB89]0329-385 03:31:06.34 −38:24:04.8 2.435 46.72 44.31 10.11 0.18 −1.79 1
SDSSJ100428.43+001825.6 10:04:28.44 +00:18:25.6 3.040 46.45 44.47 9.34 0.70 −0.70 3,11
TON618a 12:28:24.97 +31:28:37.6 2.226 47.32 <44.12 10.81 0.14 −1.27 1
[HB89]1318-113 13:21:09.38 −11:39:31.6 2.306 46.90 44.32 9.76 0.62 −0.99 1
[HB89]1346-036 13:48:44.08 −03:53:24.9 2.370 46.89 43.73 9.95 0.41 −1.26 1
UM629 14:03:23.39 −00:06:06.9 2.460 46.57 44.41 9.17 1.16 −1.40 1
UM632b 14:04:45.89 −01:30:21.9 2.517 46.55 44.04 9.44 0.61 −1.27 1
SBS1425+606 14:26:56.10 +60:25:50.0 3.202 47.39 45.04 9.83 1.73 −1.45 1
[VCV01]J1649+5303 16:49:14.90 +53:03:16.0 2.260 46.70 44.19 9.99 0.24 −0.86 3,11
SDSSJ170102.18+612301.0 17:01:02.18 +61:23:01.0 2.301 46.35 <43.51 9.73 0.20 −1.48 1
SDSSJ173352.22+540030.5 17:33:52.23 +54:00:30.5 3.428 47.02 44.36 9.58 1.28 −1.51 1
[HB89]2126-158b 21:29:12.17 −15:38:41.0 3.282 47.27 44.66 9.73 1.60 0.72 1
2QZJ221814.4-300306 22:18:14.40 −30:03:06.0 2.389 46.55 43.95 9.28 0.89 −1.27 1
2QZJ222006.7-280324 22:20:06.70 −28:03:23.0 2.414 47.23 44.64 10.21 0.54 −1.28 1
Q2227-3928 22:30:32.95 −39:13:06.8 3.438 46.95 <44.02 10.31 0.19 −1.25 2
[HB89]2254+024 22:57:17.56 +02:43:17.5 2.083 46.46 43.95 9.10 1.08 −1.37 1
2QZJ234510.3-293155 23:45:10.36 −29:31:54.7 2.382 46.33 43.97 9.38 0.42 −1.26 1
High-z QSO aver. (z<2.5)c 46.63 44.07 9.68 0.51
Local QSOs and Sy1s
Mrk335 00:06:19.52 +20:12:10.4 0.025 43.62 41.29 7.10 0.28 −2.00 4,14
IIIZw2 00:10:30.80 +10:58:13.0 0.090 44.02 42.25 8.19 0.16 −1.56 4,13
PG0050+124 00:53:34.94 +12:41:36.2 0.058 44.36 41.87 7.09 0.96 −0.91 4,13
PG0052+251 00:54:52.10 +25:25:38.0 0.155 44.46 42.57 8.55 0.21 −2.27 4,13
Fairall9 01:23:45.78 −58:48:20.5 0.046 43.80 41.91 8.27 0.10 −1.99 4,13
Mrk79 07:42:32.79 +49:48:34.7 0.022 43.58 41.37 8.12 0.08 −0.83 9,14
PG0804+761 08:10:58.60 +76:02:42.0 0.100 44.42 42.03 8.08 0.33 −2.64 4,12
Mrk704 09:18:26.00 +16:18:19.2 0.029 43.44 41.18 7.97 0.08 – 4
PG0953+414 09:56:52.40 +41:15:22.0 0.234 44.96 42.69 8.39 0.56 −2.12 4,13
NGC3516 11:06:47.49 +72:34:06.8 0.009 42.81 40.52 7.39 0.06 −1.09 7,8,14
PG1116+215 11:19:08.60 +21:19:18.0 0.176 44.84 42.27 8.27 0.53 −2.58 4,13
NGC3783 11:39:01.72 −37:44:18.9 0.010 43.05 41.10 7.33 0.09 −1.46 4,13
PG1151+117 11:53:49.27 +11:28:30.4 0.176 44.48 42.09 8.31 0.28 −2.50 4,12
NGC4051 12:03:09.61 +44:31:52.8 0.002 41.39 39.64 5.32 0.06 −0.82 5,6,14
PG1211+143 12:14:17.70 +14:03:12.6 0.085 44.58 41.94 7.69 0.68 −1.32 4,13
NGC4593 12:39:39.42 −05:20:39.3 0.009 42.60 40.34 7.40 0.04 −1.26 4,14
PG1309+355 13:12:17.76 +35:15:21.2 0.184 44.50 42.18 8.29 0.30 −2.45 4,12
PG1351+640 13:53:15.80 +63:45:45.4 0.087 44.80 42.52 8.76 0.28 −0.86 4,13
IC4329a 13:49:19.26 −30:18:34.0 0.016 43.13 40.89 7.77 0.06 – 4
NGC5548 14:17:59.53 +25:08:12.4 0.017 43.10 41.15 7.78 0.06 −1.73 4,13
Mrk817 14:36:22.06 +58:47:39.3 0.033 43.96 41.65 8.11 0.16 −0.56 4,14
Mrk509 20:44:09.73 −10:43:24.5 0.034 44.01 42.13 7.87 0.22 −1.69 4,13
Mrk926 23:04:43.47 −08:41:08.6 0.047 43.83 42.29 8.55 0.08 – 4
The following quantities are reported in each column: column 1, object name; columns 2-3, coordinates (J2000); column 4, redshift; column 5, log
of the continuum luminosity λLλ (in units of erg/s) at the rest frame wavelength 5100Å; column 6, log of the [OIII]λ5007 emission line luminosity
(in units of erg/s); column 7, log of the black hole mass (in units of M⊙); column 8, Eddington ratio Lbol/LEdd; column 9, optical-to-UV (1450Å–
5100Å) continuum slope (Fλ ∝ λ
αopt−UV); column 11: reference for the optical and UV data: 1 - Shemmer et al. (2004), Netzer et al. (2004) and
therein references for UV data, 2 - Dietrich et al. (2002) and therein references for UV data, 3 - Juarez et al (in prep.), 4 - Marziani et al. (2003), 5 -
Suganuma et al. (2006), 6 - Peterson et al. (2000), 7 - Wanders et al. (1993), 8 - Ho & Ulvestad (2001), 9 - Peterson et al. (1998), 10 - Engels et al.
(1998), 11 - SDSS DR5 archive, 12- Baskin & Laor (2005), 13 - Evans & Koratkar (2004), 14 - Kaspi et al. (2005).
a Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is near or above the observed value, hence the
6.7µm flux is probably dominated by synchrotron emission; these objects will be excluded from statistical analyses.
b Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is well below the observed value, hence the 6.7µm
flux is likely thermal.
c Optical luminosities, black hole mass and Eddington ratio for the stacked spectrum refer to the average values of only the objects at z<2.5, i.e.
those who contribute to the Silicate feature observed in the stacked spectrum.
d As discussed in Shemmer et al. (2004), the uncertainties on the BH masses and accretion rate are no larger than a factor of two.
16 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Table 2. Infrared properties of the combined sample of high-z luminous QSO, local QSO and Sy1
Name FMIR αMIR
λLλ (6.7µm)
λLλ (5100Å)
Si strength Ref.
(mJy)
High-z luminous QSOs
2QZJ002830.4-281706 4.8 -0.59 0.75 1
LBQS0109+0213 8.3 -0.91 0.75 1
[HB89]0123+257a 4.9 -1.69 0.76 1
HS0211+1858 7.3 -1.36 1.09 1
2QZJ023805.8-274337 3.7 -1.04 0.62 0.81±0.16 1
SDSSJ024933.42-083454.4 2.2 -1.26 0.57 1
Q0256-0000 2.7 -1.34 0.30 1
Q0302-0019 3.6 -1.67 0.56 1
[HB89]0329-385 5.5 -1.74 0.65 1
SDSSJ100428.43+001825.6 2.3 -1.51 0.73 1
TON618a 20.1 -1.16 0.51 0.04±0.01 1
[HB89]1318-113 6.6 -1.70 0.47 1
[HB89]1346-036 12.9 -1.68 0.99 1
UM629 3.8 -0.68 0.65 1
UM632b 2.1 -1.77 0.40 1
BS1425+606 23.9 -1.56 0.96 1
[VCV01]J1649+5303 9.4 -1.28 1.03 1
SDSSJ170102.18+612301.0 3.3 -0.66 0.83 1
SDSSJ173352.22+540030.5 2.0 -2.88 0.21 1
[HB89]2126-158b 19.1 -1.18 1.08 1
2QZJ221814.4-300306 4.6 -1.03 0.78 1
2QZJ222006.7-280324 16.0 -1.52 0.58 0.65±0.05 1
Q2227-3928 3.0 -1.70 0.38 1
[HB89]2254+024 3.6 -1.12 0.60 0.63±0.15 1
2QZJ234510.3-293155 4.2 -1.33 1.16 0.92±0.15 1
High-z QSO aver. (z<2.5)c -1.57 0.58±0.10
Local QSOs and Sy1s
Mrk335 130. -1.38 1.90 0.25±0.06 3
IIIZw2 52. -1.09 4.02 0.05±0.03 2,3
PG0050+124 245. -0.74 3.55 0.38±0.05 2,3
PG0052+251 28. -1.35 2.40 0.33±0.06 2,3
Fairall9 146. -0.90 4.83 0.21±0.07 2,3
Mkr79 200. -1.03 2.44 0.10±0.06 3
PG0804+761 88. -1.81 3.38 0.60±0.05 2,3
Mrk704 190. -0.95 5.64 0.09±0.06 3
PG0953+414 26. -1.88 1.68 0.40±0.08 2,3
NGC3516 210. -1.05 2.40 0.06±0.05 3
PG1116+215 66. -1.71 3.08 0.22±0.05 2,3
NGC3783 315. -1.08 2.69 -0.01±0.03 2,3
PG1151+117 10. -2.19 1.08 0.36±0.13 2,3
NGC4051 230. -0.43 4.71 0.06±0.05 3
PG1211+143 100. -1.26 1.89 0.55±0.05 2,3
NGC4593 184. -1.14 3.61 0.08±0.05 2,3
PG1309+355 25. -1.24 2.82 0.41±0.07 2,3
PG1351+640 53. -0.89 0.64 1.25±0.05 2,3
IC4329a 487. -0.67 9.00 0.01±0.03 2,3
NGC5548 69. -1.06 1.53 0.27±0.05 2,3
Mrk817 140. -0.65 1.61 0.16±0.06 3
Mrk509 179. -1.29 1.98 0.11±0.04 2,3
Mrk926 55. -1.40 1.77 0.26±0.04 2,3
The following quantities are reported in each column: column 1, object name; column 2, continuum flux density at the observed wavelength
corresponding to λrest = 6.7µm (after removing starburst and stellar components, in units of mJy); column 3, power-law index (Fλ ∝ λ
α) fitted
to the continuum in the 5–8µm range (starburst component–subtracted); column 4, ratio of the continuum emission at 5100Å and at 6.7µm,
λLλ(5100Å)/λLλ(6.7µm); column 5, Silicate strength; column 6: reference for the infrared data: 1 - this work (from Spitzer program 20493) ; 2 -
Shi et al. (2006); 3 - this work (from Spitzer archival data).
a Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is near or above the observed value, hence the
6.7µm flux is probably dominated by synchrotron emission; these objects will be excluded from statistical analyses.
b Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is well below the observed value, hence the 6.7µm
flux is likely thermal.
c The ratio λLλ(5100Å)/λLλ(6.7µm) is not defined for the stacked spectrum, since all spectra were normalized to the 6.7µm flux before stacking.
As a consequence, only the Silicate strength (and more generally the continuum shape) has a physical meaning for the stacked spectrum.
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 17
Table 3. Spearman-rank coefficients for the correlations in Figs.4, 8, 5 and 6.
log(λLλ(5100Å)) log(L[OIII]) log(L/LEdd) log(MBH) αMIR αopt−UV
λLλ (6.7µm)
λLλ (5100Å)
−0.76 (< 10−6) −0.72 (< 10−6) −0.44 (3 10−4) −0.70 (< 10−6) 0.29 (0.05) −0.20 (0.19)
log(Si str.) 0.83 (2 10−5) 0.75 (6 10−6) 0.75 (8 10−6) 0.66 (8 10−4)
Numbers in parenthesis give the probability for the correlation coefficient to deviate from zero.
	Introduction
	Sample selection, observations and data reduction
	Analysis
	Main observational results
	A comparison with MIR properties of lower luminosity AGNs
	Discussion
	Dust covering factor
	Covering factor as a function of source luminosity and BH mass
	Model uncertainties
	Comparison with previous works
	Silicate emission
	PAHs and star formation
	Conclusions

  We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high
luminosity QSOs at 2<z<3.5. When combined with archival IRS observations of
local, low luminosity type-I AGNs, the sample spans five orders of magnitude in
luminosity. We find that the continuum dust thermal emission at
lambda(rest)=6.7um is correlated with the optical luminosity, following the
non-linear relation L(6.7um) propto L(5100A)^0.82. We also find an anti
correlation between the ratio L(6.7um)/L(5100A) and the [OIII]5007A line
luminosity. These effects are interpreted as a decreasing covering factor of
the circumnuclear dust as a function of luminosity. Such a result is in
agreement with the decreasing fraction of absorbed AGNs as a function of
luminosity recently found in various surveys. We clearly detect the silicate
emission feature in the average spectrum, but also in four individual objects.
These are the Silicate emission in the most luminous objects obtained so far.
When combined with the silicate emission observed in local, low luminosity
type-I AGNs, we find that the silicate emission strength is correlated with
luminosity.
  The silicate strength of all type-I AGNs also follows a positive correlation
with the black hole mass and with the accretion rate. The Polycyclic Aromatic
Hydrocarbon (PAH) emission features, expected from starburst activity, are not
detected in the average spectrum of luminous, high-z QSOs. The upper limit
inferred from the average spectrum points to a ratio between PAH luminosity and
QSO optical luminosity significantly lower than observed in lower luminosity
AGNs, implying that the correlation between star formation rate and AGN power
saturates at high luminosities.

Introduction
The mid-IR (MIR) spectrum of AGNs contains a wealth of in-
formation which is crucial to the understanding of their inner
region. The observed prominent continuum emission is due to
circumnuclear dust heated to a temperature of several hundred
degrees by the nuclear, primary optical/UV/X-ray source (pri-
marily the central accretion disk); therefore, the MIR contin-
uum provides information on the amount and/or covering factor
of the circumnuclear dust. The MIR region is also rich of sev-
eral emission features which are important tracers of the ISM.
Among the dust features, the Polycyclic-Aromatic-Hydrocarbon
bands (PAH, whose most prominent feature is at ∼ 7.7µm)
are emitted by very small carbon grains excited in the Photo
Dissociation Regions, that are tracers of star forming activity
(although PAHs may not be reliable SF tracers for compact
HII regions or heavily embedded starbursts, Peeters et al., 2004;
Förster Schreiber et al., 2004). Additional MIR dust features are
the Silicate bands at ∼ 10µm and at ∼ 18µm, often seen in ab-
sorption in obscured AGNs and in luminous IR galaxies.
Major steps forward in this field were achieved thanks to
the Spitzer Space Observatory, and to its infrared spectrome-
ter, IRS, which allows a detailed investigation of the MIR spec-
tral features in a large number of sources. In particular, IRS
Send offprint requests to: R. Maiolino
allowed the detection of MIR emission lines in several AGNs
(e.g. Armus et al., 2004; Haas et al., 2005; Sturm et al., 2006a;
Weedman et al., 2005), the detection of PAHs in local PG QSOs
(Schweitzer et al., 2006), the first detection of the Silicate fea-
ture in emission (Siebenmorgen et al., 2005; Hao et al., 2005), as
well as detailed studies of the silicate strength in various classes
of sources (Spoon et al., 2007; Hao et al., 2007; Shi et al., 2006;
Imanishi et al., 2007).
However, most of the current Spitzer IRS studies have fo-
cused on local and modest luminosity AGNs (including low
luminosity QSOs), with the exception of a few bright, lensed
objects at high redshift (Soifer et al., 2004; Teplitz et al., 2006;
Lutz et al., 2007). We have obtained short IRS integrations of a
sample of 25 luminous AGNs (hereafter QSOs) at high redshifts
with the goal of extending the investigation of the MIR prop-
erties to the high luminosity range. The primary goals were to
investigate the covering factor of the circumnuclear dust and the
dependence of the star formation rate (SFR), as traced by the
PAH features, on various quantities such as metallicity, narrow
line luminosity, accretion rate and black hole mass. In combi-
nation with lower luminosity AGNs obtained by previous IRS
studies, our sample spans about 5 orders of magnitude in lumi-
nosity. This allows us to look for the dependence of the covering
factor on luminosity and black hole mass. We also search for the
silicate emission and PAH-related properties although the inte-
http://arxiv.org/abs/0704.1559v1
2 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
gration times were too short, in most cases, to unveil the proper-
ties of individual sources.
In Sect. 2 we discuss the sample selection, the observations
and the data reduction. In Sect. 3.1 we describe the spectral
analysis and the main observational results, and in Sect. 3.2 we
include additional data on local, low luminosity sources from
the literature and from the Spitzer archive. The dust covering
factor is discussed in Sect. 4.1, the properties of the Silicate
emission feature in Sect. 4.2 and the constraints on the star
formation in Sect. 4.3. The conclusion are outlined in Sect. 5.
Throughout the paper we assume a concordance Λ-cosmology
with H0 = 71 km s
−1 Mpc−1, Ωm = 0.27 and ΩΛ = 0.73
(Spergel et al., 2003).
2. Sample selection, observations and data
reduction
High redshift, high luminosity QSOs in our sample were
mostly drawn from Shemmer et al. (2004) and from Netzer et al.
(2004). The latter papers presented near-IR spectra (optical rest-
frame) of a large sample of QSOs at 2<z<3.5, which were used
to obtain detailed information on the black hole (BH) mass
(by means of the width of the Hβ line), on the accretion rate
and on the strength of the narrow emission line [OIII]λ5007.
The sample contains also infrared data on two sources from
Dietrich et al. (2002) and a few additional QSOs in the same
redshift range, for which near-IR spectra where obtained after
Shemmer et al. (2004), but unpublished yet. This sample allows
us not only to extend the investigation of the MIR properties
as a function of luminosity, but also to relate those properties
to other physical quantities such as BH mass, accretion rate
and luminosity of the narrow line region. In total our sample
includes 25 sources which are listed in Table 1. Note that the
QSOs in Shemmer et al. (2004) and in Netzer et al. (2004) were
extracted from optically or radio selected catalogs, without any
pre-selection in terms of mid- or far-IR brightness. Therefore,
the sample is not biased in terms of star formation or dust con-
tent in the host galaxy.
We observed these QSOs with the Long-Low resolution
module of the Spitzer Infrared Spectrograph IRS (Houck et al.,
2004), covering the wavelength range 22–35µm, in staring
mode. Objects were acquired by a blind offset from a nearby,
bright 2MASS star, whose location and proper motion were
known accurately from the Hipparcos catalog. We adopted the
“high accuracy” acquisition procedure, which provides a slit
centering good enough to deliver a flux calibration accuracy bet-
ter than 5%. The integration time was of 12 minutes on source,
with the exception of seven which were observed only 4 minutes
each1
We started our reduction from the Basic Calibrated Data
(BCD). For each observation, we combined all images with the
same position on the slit. Then the sky background was sub-
tracted by using pairs of frames where the sources appears at
two different positions along the slit. The spectra were cleaned
for bad, hot and rogue pixels by using the IRSCLEAN algorithm.
The monodimensional spectra were then extracted by means of
the SPICE software.
1 More specifically: LBQS0109+0213, [HB89]1318-113,
[HB89]1346-036,SBS1425+606,[HB89]2126-158, 2QZJ222006.7-
280324,VV0017.
Fig. 1. Average spectrum of all high-z, luminous QSOs in our
sample, normalized to the flux at 6.7µm (black solid line). The
blue dashed line indicates the power-law fitted to the data at
λ < 8µm; the green solid line is the fitted silicate emission and
the red, dot-dashed line is the resulting fit to the stacked spec-
trum (sum of the power-law and silicate emission). The bottom
panel indicates the number of objects contributing to the stacked
spectrum at each wavelength.
3. Analysis
3.1. Main observational results
All of the objects were clearly detected. In Tab. 2 we list the
observed continuum flux densities at the observed wavelength
corresponding to λrest = 6.7µm. This wavelength was chosen
both because it is directly observed in the spectra of all objects
and because it is far from the Silicate feature and in-between
PAH features. Thus the determination of L(6.7µm) should be
little affected by uncertainties in the subtraction of the star-
burst component (see below). For two of the radio-loud objects
([HB89]0123+257 and TON618) the MIR flux lies on the ex-
trapolation of the synchrotron radio emission and therefore the
former is also probably non-thermal. Since in this paper we are
mostly interested in the thermal emission by dust, the latter two
objects are not used in the statistical analysis. For the other two
radio loud QSOs, the extrapolation of the radio spectrum falls
below the observed MIR emission and the latter is little affected
by synchrotron contamination.
Fig. 1 shows the mean spectrum of all sources in the sample,
except for the two which are likely dominated by synchrotron
emission. Each spectrum has been normalized to 6.7µm prior
to averaging. The bottom panel shows the number of sources
contributing to the mean spectrum in different spectral regions.
We only consider the rest frame spectral range where at least
5 objects contribute to the mean spectrum. The spectrum at
λ < 8µm has been fitted with a simple power-law. While other
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 3
Fig. 2. IRS spectra of four individual high-z luminous QSOs showing evidence for silicate emission. The black solid lines indicate
the IRS spectra smoothed with a 5 pixels boxcar. The shaded areas indicate the flux uncertainty. The blue dashed line and the green
solid line are the power-law and the silicate emission components of the fits. The black dotted line shows the starburst component,
which is formally required by the fit, but statistically not significant. The red dot-dashed lines are the global fits to the observed
spectra.
workers in this field assumed more complicated continuum (e.g.
spline, polynomial) we do not consider it justified given the
limited wavelength range of our spectra. The extrapolation of
the continuum to 10µm clearly reveals an excess identified with
Silicate emission. Fitting and measuring the strength of this fea-
ture is not easy given the limited rest-frame spectral coverage
of our spectra. Therefore, we resort to the use of templates.
In particular, we fit the Silicate feature by using as a template
the (continuum-subtracted) silicate feature observed in the aver-
age spectrum of local QSOs as obtained by the QUEST project
(Schweitzer et al., 2006) and kindly provided by M. Schweitzer.
The template Silicate spectrum, with the best fitting scaling fac-
tor is shown in green in Fig. 1, while the red dot-dashed line
shows the resulting fit including the power-law. We adopt the
definition of “silicate strength” given in (Shi et al., 2006) which
is the ratio between the maximum of the silicate feature and the
interpolated featureless continuum at the same wavelength. In
the QSO-QUEST template the maximum of the Silicate feature
is at 10.5µm. This wavelength is slightly outside the band cov-
ered by our spectra but the uncertainty on the extrapolation is
not large (the latter is included in the error estimate of the sil-
icate strength). The silicate strength in the mean spectrum is
0.58±0.10 (Tab. 2).
We note that the average spectrum does not show evidence
for PAH features at 7.7µm and 6.2µm. Such features are ob-
served in lower luminosity AGNs. More specifically, a starburst
4 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
template (Sturm et al., 2000) is not required by the fit shown in
Fig. 1. In Sect. 4.3 we will infer an upper limit on the PAH lu-
minosity and discuss its implication.
We clearly detect the blue wing of the silicate feature in four
individual spectra, which are shown in Fig. 2. These spectra were
fitted with a power-law and a silicate template exactly as the
stacked spectrum. The resulting values for the Silicate strength
are given in Tab. 2. The presence of silicate emission in all other
cases is poorly constrained (or totally unconstrained) either be-
cause of low signal-to-noise (S/N) or because of a lack of spec-
tral coverage. The one exception is Ton 618 which has a high
S/N spectrum and a redshift (z=2.22) appropriate to observe the
Silicate 10.5 µm feature. No silicate emission is detected in this
case, but note that this is not expected since the MIR radiation
of this source is probably dominated by synchrotron emission.
Tables 1 and 2 list the more important MIR information on
the sources and physical properties deduced from the rest-frame
optical spectra and obtained from Shemmer et al. (2004): opti-
cal continuum luminosity λLλ(5100Å), [OIII]λ5007 line lumi-
nosity, BH mass and Eddington accretion rate L/LEdd.
3.2. A comparison with MIR properties of lower luminosity
To compare the MIR properties of our luminous QSOs with
those of lower luminosity sources we have included in our study
the IRS/MIR spectra of various low redshift, lower luminosity
type-I AGNs. We purposely avoid type-II sources because of the
additional complication due to absorption along the line of sight.
We have used data from Shi et al. (2006) who analyze the in-
tensity of the silicate features in several, local AGNs with lumi-
nosities ranging from those of nearby Seyfert 1s to intermediate
luminosity QSOs. We discarded BAL QSOs (which are known
to have intervening gas and dust absorption) as well as dust red-
dened type-I nuclei (e.g. 2MASS red QSOs). We also discard
those cases (e.g. 3C273) where the optical and MIR continuum
is likely dominated by synchrotron radiation. Note that Shi et al.
(2006) selected type-I objects with “high brightness” and, there-
fore, low-luminosity AGNs tend to be excluded from their sam-
Shi et al. (2006) provide a measure of the silicate feature
strength (whose definition was adopted also by us). The con-
tinuum emission at 6.7µm was measured by us from the archival
spectra. We also subtracted from the 6.7µm emission the pos-
sible contribution of a starburst component by using the M82
template. We estimate the host galaxy contribution (stellar pho-
tospheres) in all sources to be negligible.
We include in the sample of local Sy1s also some IRS spectra
taken from the sample of Buchanan et al. (2006), whose spectral
parameters were determined by us from the archival spectral,
in the same manner as for the Shi et al. (2006) spectra. As for
the previous sample, we discarded reddened/absorbed sources as
well as those affected by synchrotron emission. As discussed in
Buchanan et al. (2006), these spectra are affected by significant
flux calibration uncertainties, due to the adopted mapping tech-
nique. Therefore, the spectra were re-calibrated by using IRAC
photometric images. We discarded objects for which IRAC data
are not available or not usable (e.g. because saturated). Finally,
we also discarded data for which optical spectroscopic data are
not available (see below).
The mid-IR parameters of the sources in both samples are
listed in Tab. 2.
Optical data were mostly taken from Marziani et al. (2003)
and BH masses and Eddington accretion rates inferred as in
Shemmer et al. (2004). The resulting parameters are listed in
Table 1.
The type-I sources in Shi et al. (2006) and Buchanan et al.
(2006) are only used for the investigation of the covering fac-
tor and silicate strength, which are the main aims of our work.
The Shi et al. (2006) and Buchanan et al. (2006) samples are not
suitable for investigating the PAH features because most of these
objects are at small distances and the IRS slit misses most of the
star formation regions in the host galaxy. For what concerns the
the PAH emission, we use the data in Schweitzer et al. (2006)
who performed a detailed analysis of the PAH features in their
local QSOs sample. The slit losses in those sources are minor.
The Schweitzer et al. (2006) sample is also used for the investi-
gation of the MIR-to-optical luminosity ratio. The mid-IR data
of this sample are not listed in Tab. 2, since such data are already
reported in Schweitzer et al. (2006) and in Netzer et al. (2007).
4. Discussion
4.1. Dust covering factor
4.1.1. Covering factor as a function of source luminosity and
BH mass
The main assumption used here is that the covering factor of the
circumnuclear dust is given by the ratio of the thermal infrared
emission to the primary AGN radiation. The latter is mostly the
“big blue bump” radiation with additional contribution from the
optical and X-ray wavelength ranges (Blandford et al., 1990).
Determining the integral of the AGN-heated dust emission,
and disentangling it from other spectral components is not sim-
ple. The FIR emission in type-I AGNs is generally dominated by
a starburst component, even in QSOs (Schweitzer et al., 2006).
In lower luminosity AGNs, the near-IR emission may be affected
by stellar emission in the host galaxy, while in QSOs the near-
IR light is often contributed also by the direct primary radia-
tion. The MIR range (∼ 4 − 10µm) is where the contrast be-
tween AGN-heated dust emission and other components is max-
imal. This spectral region contains various spectral features, like
PAHs and silicate emission, yet MIR spectra allow us to disen-
tangle and remove these components, and determine the hot dust
continuum. In particular, by focusing on the continuum emis-
sion at 6.7µm, the uncertainty in the removal of PAH emission
is minimized, while the contribution from the Silicate emission
is totally negligible at this wavelength (note that such a spectral
decomposition is unfeasible with photometric data). If the spec-
tral shape of the AGN-heated dust does not change from object
to object (and in particular it does not change significantly with
luminosity), then the 6.7µm emission is a proxy of the global
circumnuclear hot dust emission. It is possible to infer a quanti-
tative relation between L(6.7µm) and the total AGN-heated hot
dust emission through the work of Silva et al. (2004), who use
observations of various nearby AGNs to determine their average,
nuclear IR SED (divided into absorption classes). From their
type I AGNs SED, we find that the integrated nuclear, thermal
IR bump is about ∼ 2.7 λLλ(6.7µm). This ratio is also consistent
with that found in the QUEST QSO sample, once the contribu-
tion by the silicate features is subtracted.
Regarding the primary optical-UV radiation, determining its
integrated flux would require observations of the entire intrin-
sic spectral energy distribution (SED) from the far-UV to the
near-IR. This is not available for most sources in our sample.
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 5
Fig. 3. MIR continuum luminosity at 6.7µm versus optical con-
tinuum luminosity at 5100Å. Red triangles mark high-z, lumi-
nous QSOs and blue diamonds mark low redshift type-I AGNs.
The black solid line is a fit to the data, which has a slope with a
power index of 0.82.
Moreover, there are indications that the SED is luminosity de-
pendent (e.g. Scott et al., 2004; Shang et al., 2005) and thus an
estimate of the bolometric luminosity of the primary continuum
based on the observed luminosity in a certain band is somewhat
uncertain. Notwithstanding these limitations, we assume in this
work that the optical continuum luminosity can be used as a
proxy of the bolometric luminosity of the primary continuum.
We use the continuum luminosity at the rest frame wavelength
of 5100Å, (λLλ, hereafter L5100) because it is directly measured
for all sources in our sample. We can infer the ratio between
the bolometric luminosity and L5100 from the mean spectrum of
QSOs obtained in recent studies, mostly the results of Scott et al.
(2004) and Richards et al. (2006). These studies indicate a bolo-
metric correction in the range of 5–9. Here we chose, rather ar-
bitrarily, the mean value of 7.
The comparison of L(6.7µm) and L5100 is our way of deduc-
ing the hot dust covering factor. Fig. 3 shows the λLλ(6.7µm)
versus L5100 for our sample. Red triangles are the high-z, lumi-
nous QSOs and blue diamonds are local type-I AGNs. Not sur-
prisingly, the two quantities show a good correlation. However,
the very large luminosity range spanned by our sample allows us
to clearly state that the correlation is not linear, but has a slope
α = 0.82 ± 0.02 defined by
log[λLλ(6.7µm)] = K + α log[λLλ(5100Å)] (1)
where K = 8.36±0.80 and luminosities are expressed in erg s−1.
This indicating that the MIR, reprocessed emission increases
more slowly than the primary luminosity.
The same phenomenon is observed in a cleaner way in
Fig. 4a, where the ratio between the two continuum luminosi-
ties is plotted as a function of L5100. There is a clear anti-
correlation between the MIR–to–optical ratio and optical lumi-
nosity. Fig. 4b shows the same MIR–to–optical ratio as a func-
tion of L([OIII]λ5007) as an alternative tracer of the global AGN
luminosity (although the latter is not a linear tracer of the nu-
clear luminosity, as discussed in Netzer et al., 2006), which dis-
plays the same anti-correlation as for the continuum optical flux.
Spearman-rank coefficients and probabilities for these correla-
tions are given in Tab. 3.
According to the above discussion, an obvious interpretation
of the decreasing MIR–to–optical ratio is that the covering factor
of the dust surrounding the AGN decreases with luminosity. In
particular, if the covering factor is proportional to the MIR–to–
optical ratio, then Figs. 4a-b indicate that the dust covering factor
decreases by about a factor 10 over the luminosity range probed
by us.
It is possible to convert the MIR–to–optical luminosity ra-
tio into absolute dust covering factor by assuming ratios of
broad band to monochromatic continuum luminosities observed
in AGNs, as discussed above. In particular, by using the 6.7µm–
to–MIR and 5100Å–to–bolometric luminosity ratios reported
above, we obtain that the absolute value of the dust covering
factor (CF) can be written as:
CF(dust) ≈ 0.39 ·
λLλ(6.7µm)
λLλ(5100Å)
. (2)
In Fig. 4 the axes on the right hand side provide the dust
covering factor inferred from the the equation above. A fraction
of objects have covering factor formally larger than one, these
could be due to uncertainties in the observational data, or nu-
clear SED differing from the ones assumed above, or to optical
variability, as discussed in Sect. 4.1.2. The dust covering fac-
tor ranges from about unity in low luminosity AGNs to about
10% in high luminosity QSOs. As it will be discussed in detail
in Sect. 4.1.3, the dust covering factor is expected to be equal
to the fraction of type 2 (obscured) AGNs relative to the total
AGN population. The finding of a large covering factor in low
luminosity AGNs is in agreement with the large fraction of type
2 nuclei observed in local Seyferts (∼ 0.8, Maiolino & Rieke,
1995).
A similar result has been obtained, in an independent way,
through the finding of a systemic decrease of the the obscured
to unobscured AGN ratio as a function of luminosity in various
surveys. The comparison with these results will be discussed in
more detail in the next section.
The physical origin of the decreasing covering factor is still
unknown. One possibility is that higher luminosities imply a
larger dust sublimation radius: if the obscuring medium is dis-
tributed in a disk with constant height, then a larger dust subli-
mation radius would automatically give a lower covering factor
of dust at higher luminosities (Lawrence, 1991). However, this
effect can only explain the decreasing covering factor with lu-
minosity for the dusty medium, and not for the gaseous X-ray
absorbing medium. Moreover, Simpson (2005) showed that the
simple scenario of such a “receding torus” is unable to account
for the observed dependence of the type 2 to type 1 AGN ratio
as a function of luminosity.
Another possibility is that the radiation pressure on dust is
stronger, relative to the BH gravitational potential, in luminous
AGNs (e.g. Laor & Draine, 1993; Scoville & Norman, 1995),
thus sweeping away circumnuclear dust more effectively. In this
scenario a more direct relation of the covering factor should be
with L/LEdd, rather than with luminosity. Our sample does not
show such a relation, as illustrated in Fig. 4c. However, the un-
certainties on the accretion rates (horizontal error bar in Fig. 4c)
may hamper the identification of such a correlation.
Lamastra et al. (2006) proposed that, independently of lumi-
nosity, the BH gravitational potential is responsible for flatten-
6 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 4. MIR–to–optical continuum luminosity [λLλ(6.7µm)]/[λLλ(5100Å)] versus (a) continuum optical luminosity, (b) [OIII]λ5007
line luminosity, (c) accretion rate L/LEdd and (d) black hole mass. Symbols are the same as in Fig. 3. The magenta square indicates
the location of the mean SDSS QSO SED in Richards et al. (2006). The horizontal error bars in panels (c) and (d) indicate con-
servative uncertainties on the accretion rates and BH masses. The right hand side axis on each panel shows the circumnuclear dust
covering factor inferred from Eq. 2. The dashed lines in panels (a) and (b) shows the fit resulting from the analytical forms in Eqs. 3
and 6, respectively.
ing the circumnuclear medium, so that larger BH masses effec-
tively produce a lower covering factor. According to this sce-
nario, the relation between covering factor and luminosity is
only an indirect one, in the sense that more luminous AGN tend
to have larger BH masses (if the Eddington accretion rate does
not change strongly on average). Fig. 4d shows the MIR–to–
optical ratio (and dust covering factor) as a function of BH mass,
indicating a clear (anti-)correlation between these two quanti-
ties. However, the correlation is not any tighter than the relation
with luminosity in Fig. 4a-b, as quantified by the comparison of
the Spearman-rank coefficients and probabilities in Tab. 3. The
degeneracy between luminosity and BH mass prevents us to dis-
criminate which of the two is the physical quantity driving the
relation.
4.1.2. Model uncertainties
In this section we discuss some possible caveats in our interpre-
tation of the MIR–to–optical ratio as an indicator of the hot dust
covering factor.
Our analysis assumes that the shape of the hot dust IR spec-
trum SED is not luminosity dependent. However, an alternative
interpretation of the trends observed in Fig. 4 could be that the
dust temperature distribution changes with luminosity. Yet, to
explain the decreasing 6.7µm to 5100Å flux ratio in terms of dust
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 7
Fig. 5. Mid-IR spectral slope (5–8µm) versus
λLλ(6.7µm)/λLλ(5100Å) ratio. Symbols are as in Fig. 3.
No clear correlation is observed between these two quan-
tities (see also Tab. 3). Note that the exceptional object
SDSSJ173352.22+540030.5 is out of scale, and it is discussed
in the text.
Fig. 6. Optical-to-UV spectral slope (1450–5100Å) versus
λLλ(6.7µm)/λLλ(5100Å) ratio. Symbols are as in Fig. 3. The
arrow indicates the effect of dust reddening with AV = 2 mag.
The data do not show any evidence for dust reddening effects.
temperature would require that the circumnuclear dust is cooler
at higher luminosities. This, besides being contrary to expecta-
tions, is ruled out by the observations which show no clear corre-
lation between the mid-IR continuum slope (which is associated
with the average dust temperature) and the MIR–to–optical ratio,
as illustrated in Fig. 5 and in Tab. 3. The one remarkable excep-
tion is SDSSJ173352.22+540030.5,which has the most negative
mid-IR continuum slope (αMIR = −2.88, i.e. an inverted spec-
trum in λLλ) and the lowest MIR-to-optical ratio of the whole
sample (λLλ(6.7µm)/λLλ(5100Å) = 0.22), which is out of scale
in Fig. 5. This QSO may be totally devoid of circumnuclear hot
dust, and its MIR emission may simply be the continuation of the
optical “blue-bump”. Similar high-z QSOs, with an exceptional
deficiency of mir-MIR flux, have been reported by Jiang et al.
(2006).
As explained earlier, there are indications that the UV-optical
SED, and hence the bolometric correction based to the observed
L5100, are luminosity dependent. If correct, this would mean a
smaller bolometric correction for higher L5100 sources which
would flatten the relationship found here (i.e. will result in a
slower decrease of the covering factor with increasing L5100).
However, the expected range (a factor of at most 2 in bolometric
correction) is much smaller than the deduced change in covering
factor.
Variability is an additional potential caveat because of the
time delay between the original L5100 “input” and the response
of the dusty absorbing “torus”. While we do not have the obser-
vations to test this effect (multi-epoch, high-quality optical spec-
troscopic data are available only for a few sources in our sam-
ple), we note that the location of the 6.7µm emitting gas from
the central accretion disk is at least several light years and thus
L(MIR) used here reflects the mean L5100 in most sources. We
expect that the average luminosity of a large sample will not be
affected much by individual source variations. Moreover, in the
high luminosity sources of our sample we do not expect much
variability (since luminous QSOs are known to show little or no
variability).
An alternative, possible explanation of the variation of the
optical-to-MIR luminosity ratio could be dust extinction affect-
ing the observed optical flux. Optical dust absorption increasing
towards low luminosities may in principle explain the trends ob-
served in Fig. 4. However, we have pre-selected the sample of
local QSOs and Sy1s to avoid objects showing any indication
of absorption, thus probably shielding us from such spurious ef-
fects. Yet, we have further investigated the extinction scenario
by analyzing the optical-to-UV continuum shape of our sample.
The optical-UV continuum slope does not necessarily trace dust
reddening, since intrinsic variations of the continuum shape are
known to occur, as discussed above. Variability introduce ad-
ditional uncertainties, since optical and UV data are not simul-
taneous. However, if the variations of λLλ(6.7µm)/λLλ(5100Å)
are mostly due to dust reddening, one would expect the MIR-
to-optical ratio to correlate with the optical-UV slope, at least
on average. We have compiled UV rest-frame continuum fluxes
(at λrest ∼ 1450Å) from spectra in the literature or in the HST
archive. By combining such data with the continuum luminosi-
ties at λrest = 5100Å we inferred the optical-UV continuum
slope αopt−UV defined as
2 Lλ ∝ λ
αopt−UV , as listed in Tab. 1.
Fig. 6 shows αopt−UV versus λLλ(6.7µm)/λLλ(5100Å). The ar-
row indicates the effect of dust reddening with AV = 2 mag
(by assuming a SMC extinction curve, as appropriate for type
1 AGNs, Hopkins et al., 2004), which would be required to ac-
count for the observed variations of λLλ(6.7µm)/λLλ(5100Å).
Fig. 6 does not show evidence for any (positive) correlation be-
tween αopt−UV and λLλ(6.7µm)/λLλ(5100Å) (see also Tab 3).
In particular, if the variation of MIR-to-optical ratio (spanning
more than a factor of ten) was due to dust reddening, we would
expect to find ∆αopt−UV > 5 (as indicated by the arrow in Fig. 6),
which is clearly not observed. If any, the data show a marginal
2 Our definition of power law index is linked to the αν given in
Vanden Berk et al. (2001) by the relation αopt−UV = −(αν + 2). Our dis-
tribution of αopt−UV is roughly consistent (within the uncertainties and
the scatter) with αν = −0.44 obtained by Vanden Berk et al. (2001) for
the SDSS QSO composite spectrum (see also Shemmer et al., 2004).
8 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
anti-correlation between αopt−UV and λLλ(6.7µm)/λLλ(5100Å)
(Tab. 3), i.e. opposite to that expected from dust reddening.
Finally, possible evolutionary effects on the dust covering
factor have not been considered. We have been comparing lo-
cal objects with QSOs at z∼2–3 yet assumed that only lumi-
nosity or BH mass plays a role. La Franca et al. (2005) and
Akylas et al. (2006) find evidence for an increasing fraction of
obscured AGNs as a function of redshift, a result which is still
debated (see Ueda et al., 2003; Gilli et al., 2007). If the ab-
sorbing medium covering factor really increases with redshift,
then the actual dependence of the covering factor on luminos-
ity would be stronger than shown in Fig. 4. Indeed, according to
La Franca et al. (2005) and Akylas et al. (2006), putative low-z
counterparts of our high-z QSOs (matching the same luminosi-
ties) should be affected by an even lower covering factor. As a
consequence, the diagrams in Figs. 4 and 7 should have even
steeper trends once the data are corrected for such putative evo-
lutionary effects, thus strengthening our conclusions.
4.1.3. Comparison with previous works
Wang et al. (2005) used IRAS and ISO mid-IR data of (mostly
local) PG quasars to infer their dust covering factor. They find
that the covering factor decreases as a function of X-ray lumi-
nosity. They probe a narrower luminosity range with respect to
our work, nonetheless their results are generally consistent with
ours, although with significant scatter.
More recently Richards et al. (2006) derived the broad band
SED of a large sample of SDSS QSOs by including Spitzer pho-
tometric data. Although such data do not have spectroscopic in-
formation, they can be used to obtain a rough indication of the
dust covering factor in their QSO sample, to be compared with
our result. The average optical luminosity of the Richards et al.
(2006) sample is 〈log[λLλ(5100Å)]〉 ∼ 45.5 erg s
−1. From their
mean SED we derive λLλ(6.7µm)/λLλ(5100Å) = 1.17. The cor-
responding location on the diagram of Fig. 4a is marked with a
magenta square, and it is in agreement with the general trend of
our data.
In a companion paper, Gallagher et al. (2007) use the same
set of data to investigate the MIR-to-optical properties as a func-
tion of luminosity. They find a result similar to ours, i.e. the
MIR-to-optical ratio decreases with luminosity. However, they
interpret such a result as a consequence of dust reddening in the
optical, since the effect is stronger in QSOs with redder opti-
cal slope. As discussed in the previous section, our data do no
support this scenario, at least for our sample. In particular, the
analysis of the optical-UV slope indicates that dust reddening
does not play a significant role in the variations of the MIR-
to-optical luminosity ratio. The discrepancy between our and
Gallagher et al. (2007) results may have various explanations.
The QSOs in the Gallagher et al. (2007) sample span about two
orders of magnitudes in luminosity, while we have seen that
to properly quantify the effect a wider luminosity range is re-
quired. Moreover, the majority of their sources are clustered
around the mean luminosity of 1045.5 erg s−1. In addition, the
the lack of spectroscopic information makes it difficult to allow
for the presence of other spectral features such as the silicate
emission which, as we show later, is luminosity dependent. The
lack of spectroscopic information may be an issue specially for
samples spanning a wide redshift range (as in Gallagher et al. ,
2007), where the photometric bands probe different rest-frame
bands. The same concerns applies for the optical luminosities.
Our rest-frame continuum optical luminosities are always in-
ferred through rest-frame optical spectra, even at high-z (through
near-IR spectra). Gallagher et al. (2007) do not probe directly
the optical continuum luminosity of high-z sources (at high-z
they only have optical and Spitzer data, which probe UV and
near-IR rest-frame, respectively). Finally, differences between
our and Gallagher et al. (2007) results may be simply due to
the different samples. As discussed in the previous section, we
avoided dust reddened targets, thus making us little sensitive to
extinction effects, while Gallagher et al. (2007) sample may in-
clude a larger fraction of reddened objects.
A decreasing dust covering factor as a function of lumi-
nosity must translate into a decreasing fraction of obscured
AGNs as a function of luminosity. The effect has been noted
in various X-ray surveys (Ueda et al., 2003; Steffen et al., 2003;
La Franca et al., 2005; Akylas et al., 2006; Barger et al., 2005;
Tozzi et al., 2006; Simpson, 2005), although the results have
been questioned by other authors (e.g. Dwelly & Page, 2006;
Treister & Urry, 2005; Wang et al., 2007). The X-ray based
studies do not distinguish between dust and gas and thus probe
mostly trends of the gaseous absorption. Our result provides an
independent confirmation of these trends. Moreover, our find-
ings are complementary to those obtained in the X-rays since,
instead of the covering factor of gas, we probe the covering fac-
tor of dust.
In order to compare our findings with the results obtained
from X-ray surveys, we have derived the expected fraction of
obscured AGNs by fitting the dust covering factor versus lumi-
nosity relation with an analytical function. Instead of using the
simple power-law illustrated in Fig. 3 (Eq. 1) we fit the depen-
dence of the covering factor on luminosity with a broken power-
law. The latter analytical function is preferred both because it
provides a statistically better fit and because a simple power-law
would yield a covering factor larger than unity at low luminosi-
ties. As a result we obtain the following best fit for the fraction
of obscured AGNs as a function of luminosity:
fobsc =
1 +Lopt
0.414
where fobsc is the fraction of obscured AGNs relative to the total
Lopt =
λLλ(5100Å) [erg s
1045.63
The resulting fit is shown with a dashed line in Fig. 4a. The frac-
tion of obscured AGNs as a function of luminosity is also re-
ported with a blue, solid line in Fig. 7a. The shaded area reflects
the uncertainty in the bolometric correction discussed above.
The most recent and most complete investigation on the frac-
tion of X-ray obscured AGNs as a function of luminosity has
been obtained by Hasinger (2007, in prep., see also Hasinger,
2004) who combined the data from surveys of different areas
and limiting fluxes to get a sample of ∼700 objects. We convert
from X-ray to optical luminosity by using the non-linear relation
obtained by Steffen et al. (2006). The latter use flux densities at
2 keV and 2500Å; we adapt their relation to our reference optical
wavelength (5100Å) by assuming the optical-UV spectral slope
obtained by Vanden Berk et al. (2001), and to the 2–10 keV inte-
grated luminosity (adopted in most X-ray surveys) by assuming
a photon index of –1.7, yielding the relation
log[L(2 − 10 keV)] = 0.721 · log[λLλ(5100Å)]+ 11.78 (5)
(where luminosities are in units of erg s−1). Fig. 7a compares the
fraction of obscured AGN obtained by Hasinger (2007) through
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 9
Fig. 7. a) Fraction of obscured AGNs (relative to total) as a function of optical continuum luminosity. The blue line shows the
fraction of obscured AGNs inferred from the hot dust covering factor with the analytical form of Eq. 3. The shaded area is the
uncertainty resulting from the plausible range of the bolometric correction (see text). Points with error bars are the fraction of X-ray
obscured AGNs as a function of X-ray luminosity inferred by Hasinger (2007). The upper scale of the diagram gives the intrinsic
hard X-ray luminosity; the (non-linear) correspondence between X-ray and optical luminosity is obtained from Eq. 5. b) Fraction of
obscured AGNs as a function of L([OIII]λ5007). The blue line is the fraction of obscured AGNs inferred from the hot dust covering
factor with the analytical form of Eq. 6. Points with error bars show the fraction of type 2 AGNs as a function of L([OIII]λ5007)
inferred by Simpson (2005).
X-ray surveys with our result based on the covering factor of
hot dust. Both have the same trends with luminosity, but the
fraction of obscured AGN expected from the hot dust cover-
ing factor is systematically higher. Such an offset is however ex-
pected. Indeed, current high redshift X-ray surveys do not probe
the Compton thick population of obscured AGNs since these
are heavily absorbed even in the hard X-rays. In local AGNs,
Compton thick nuclei are about as numerous as Compton thin
ones (Risaliti et al., 1999; Guainazzi et al., 2005; Cappi et al.,
2006). The fraction of Compton thick, high luminosity, high
redshift AGNs is still debated, but their contribution certainly
makes the fraction of X-ray obscured AGNs higher than in-
ferred by Hasinger (2007), who can only account for Compton
thin sources. The ratio between the dust covering factor curve in
Fig. 7a and the X-ray data from Hasinger (2007), indicates that
the ratio between the total number of obscured AGNs (includ-
ing Compton thick ones) and Compton thin ones is about 2 even
at high luminosities, i.e. consistent (within uncertainties) with
local, low-luminosity AGNs.
An analogous result on the decreasing fraction of obscured
AGNs as a function of luminosity was obtained by Simpson
(2005) who compared the numbers of type 2 and type 1 AGNs
at a given L([OIII]λ5007). To compare with Simpson (2005), we
used our sample to derive the following analytical description for
the fraction of obscured AGN as a function of L([OIII]λ5007):
fobsc =
1 +L[OIII]
0.409
where fobsc is the fraction of obscured AGNs relative to the total
L[OIII] =
L([OIII]) [erg s−1]
1043.21
The corresponding fit is shown with a dashed line in Fig. 4b, and
the fraction of obscured AGNs as a function of L([OIII]) is also
shown with a blue line in Fig. 7b. In the latter figure we also
compare the fraction of obscured AGNs obtained by Simpson
(2005) with our result based on the covering factor of the hot
dust. There is a good agreement (within uncertainties) between
the fraction of obscured AGNs inferred through the two meth-
ods, as expected since both optical surveys and our method probe
the covering factor of dust. However, we shall also mention that
the results obtained by Simpson (2005) have been questioned by
Haas et al. (2005), by arguing that at high luminosities, the de-
rived L([OIII]λ5007) may be affected by a large scale absorber.
4.2. Silicate emission
In this paper we have presented the most luminous (type 1)
QSOs where Silicate emission has been detected so far. When
combined with MIR spectra of lower luminosity sources, it is
possible to investigate the properties and behavior of this feature
over a wide luminosity range.
The discovery of silicate emission in the spectrum of (mostly
type 1) AGNs obtained by Spitzer was regarded as the so-
lution of a long standing puzzle on the properties of the cir-
10 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 8. Silicate strength versus (a) L5100, (b) L([OIII]λ5007), (c) normalized accretion rate (L/LEdd) and (d) black hole mass. Symbols
are the same as in Fig. 3. The red square indicates the silicate strength measured in the average spectrum. The horizontal, black error
bars in panels (c) and (d) indicate conservative uncertainties on the accretion rates and BH masses.
cumnuclear dusty medium. Indeed, silicate emission was ex-
pected by various models of the dusty torus. However, the ab-
sence of clear detections prior to the Spitzer epoch induced
various authors to either postulate a very compact and dense
torus (e.g. Pier & Krolik, 1993) or different dust compositions
(Laor & Draine, 1993; Maiolino et al., 2001a,b). Initial Spitzer
detections of silicate emission relaxed the torus model assump-
tions (Fritz et al., 2006), but more detailed investigations re-
vealed a complex scenario. The detection of silicate emission
even in type 2 AGNs (Sturm et al., 2006b; Teplitz et al., 2006;
Shi et al., 2006) suggested that part of the silicate emission may
originate in the Narrow Line Region (NLR) (Efstathiou, 2006).
Further support for a NLR origin of the silicate emission comes
from the temperature inferred for the Silicate features, which is
much lower (<200 K) than for the circumnuclear dust emitting
the featureless MIR continuum (>500 K), as well as from MIR
high resolution maps spatially resolving the silicate emission on
scales of 100 pc (Schweitzer et al. in prep.).
If most of the observed silicate emission originates in the
NLR, then the effects of circumnuclear hot dust covering fac-
tor should be amplified when looking at the “silicate strength”
(which we recall is defined as the ratio of the silicate maximum
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 11
intensity and the featureless hot dust continuum). Indeed, if the
covering factor of the circumnuclear dusty torus decreases, it im-
plies that the MIR hot dust continuum decreases and the silicate
emission increases because a larger volume of the NLR is il-
luminated. Both effects go in the same direction of increasing
the “silicate strength”. This scenario is made more complex by
the tendency of the NLR to disappear at very high luminosities,
or to get very dense and not to scale linearly with the nuclear
luminosity (Netzer et al., 2004, 2006). Moreover, the schematic
division of a silicate feature totally emitted by the NLR and a
MIR featureless continuum totally emitted by the inner side of
the obscuring torus is probably too simplistic. There must be
at least a small contribution to the featureless MIR continuum
from dust in the NLR, while some silicate emission is probably
also coming from the obscuring torus. However, from a general
qualitative point of view we expect a monotonic behavior of the
“silicate strength” with the physical quantity responsible for the
changes in the hot dust covering factor.
Before investigating the various trends of the Silicate
strength, we mention that by using the four silicate detections
shown in Fig. 2 and listed in Tab. 2, we may in principle intro-
duce a bias against weak silicate emitters. Indeed, although we
cannot set useful upper limits on the silicate strength in most of
the other objects, we have likely missed objects with low sili-
cate strength. However, the mean spectrum in Fig. 1 includes all
QSOs in our sample, and therefore its silicate strength should
be representative of the average Silicate emission in the sample
(at least for the objects at z<2.5, which are the ones where the
observed band includes the silicate feature, and which are the
majority).
Figs. 8a-b show the silicate strength of the objects in our
combined sample as functions of L5100 and L([OIII]λ5007). The
red square indicates the silicate strength in the high-z QSO mean
spectrum, while its horizontal bar indicates the range of lumi-
nosities spanned by the subsample of objects at z<2.5 (i.e. those
contributing to the silicate feature in the mean spectrum). Low
redshift AGNs and high redshift QSOs show an apparently clear
correlation between silicate strength and luminosity. Although
with a significant spread, the Silicate strength is observed to pos-
itively correlate also with the accretion rate L/LEdd and with the
BH mass, as shown in Figs. 8c-d. Essentially, the correlations
observed for the Silicate strength reflects the same correlation
observed for the L(6.7µm)/L(5100Å) (with the exception of the
accretion rate), in agreement with the idea that also the Silicate
strength is a proxy of the covering factor of the circumnuclear
hot dust, for the reasons discussed above.
Unfortunately, the correlations observed for the Silicate
strength do not improve our understanding on the origin of the
decreasing covering factor with luminosity, i.e. whether the driv-
ing physical quantity is the luminosity itself, the accretion rate or
the black hole mass. Formally, the correlation between Silicate
strength and optical continuum luminosity is tighter than the oth-
ers (Tab. 3), possibly hinting at the luminosity itself as the quan-
tity driving the dust covering factor. However, there are a few
low luminosity objects, such as a few LINERs, which have large
silicate strengths (Sturm et al., 2005) and which clearly deviate
from the correlation shown in Fig. 8a, thus questioning the role
of luminosity in determining the Silicate strength. In addition,
the apparently looser correlations of Silicate strength versus ac-
cretion rate and BH mass may simply be due to the additional
uncertainties affecting the latter two quantities (horizontal, black
error bars in Figs. 8c,d).
4.3. PAHs and star formation
The presence and intensity of star formation in QSOs has been
a hotly debated issue during the past few years. A major step
forward in this debate was achieved by Schweitzer et al. (2006)
through the Spitzer IRS detection of PAH features in a sample
of nearby QSOs, revealing vigorous star formation in these ob-
jects. The analysis also shows that the far-IR emission in these
QSOs is dominated by star formation and that the star forming
activity correlates with the nuclear AGN power. Here we show in
Fig. 9 the latter correlation in terms of PAH(7.7µm) luminosity
versus L5100 by using the PAH luminosities from the sample of
Schweitzer et al. (2006) and the corresponding optical data from
Marziani et al. (2003). Although the large fraction of upper lim-
its in the former sample prevents a careful statistical characteri-
zation, Fig. 9 shows a general correlation between QSO optical
luminosity and starburst activity in the host galaxy as traced by
the PAH luminosity.
The scale on the right hand side of Fig. 9 translates the 7.7µm
PAH luminosity into star formation rate (SFR). This was ob-
tained by combining the average L(PAH7.7µm)/L(FIR) ratio ob-
tained by Schweitzer et al. (2006) for the starburst dominated
QSOs in their sample with the SFR/L(FIR) given in Kennicutt
(1998), yielding
SFR [M⊙ yr
−1] = 3.46 10−42 L(PAH7.7µm) [erg s
−1] (8)
The average spectrum of high-z, luminous QSOs in Fig. 1
does not show evidence for the presence of PAH features and
can only provide an upper limit on the PAH flux relative to
the flux at 6.7µm (since all spectra were normalized to the lat-
ter wavelength prior to computing the average). However, we
can derive an upper limit on the PAH luminosity by assum-
ing the average distance of the sources in the sample. The in-
ferred upper limit on the PAH luminosity is reported with a red
square in Fig. 9a, and it is clearly below the extrapolation of
the L(PAH7.7µm) − λLλ(5100Å) relation found for local, low-
luminosity QSOs. This is shown more clearly in Fig. 9b which
shows the distribution of the ratio L(PAH7.7µm)/λLλ(5100Å) for
local QSOs (histogram) and the upper limit inferred from the av-
erage spectrum of high-z, luminous QSOs (red solid line). The
corresponding upper limit on the SFR is ∼ 700 M⊙ yr
Note that certainly there are luminous, high-z QSOs
with larger star formation rates (e.g. Bertoldi et al., 2003;
Beelen et al., 2006; Lutz et al., 2007). However, since our sam-
ple is not pre-selected in terms of MIR or FIR emission, our
result is not biased in terms of star formation and dust content,
and therefore it is representative of the general high-z, luminous
QSO population.
Our results indicate that the relation between star forma-
tion activity, as traced by the PAH features, and QSO power,
as traced by L5100, saturates at high luminosity. This result is
not surprising. Indeed, if high-z, luminous QSOs were char-
acterized by the same average L(PAH7.7µm)/λLλ(5100Å) ob-
served in local QSOs, this would imply huge star formation rates
of ∼ 7000 M⊙ yr
−1 at λLλ(5100Å) ∼ 10
47 erg s−1. On the
contrary, the few high-z QSOs detected at submm-mm wave-
lengths have far-IR luminosities corresponding to SFR of about
1000−3000 M⊙ yr
−1 (Omont et al., 2003). The majority of high-
z QSO (∼70%) are undetected at submm-mm wavelengths. The
mean mm-submm fluxes of QSOs in various surveys (includ-
ing both detections and non-detections) imply SFRs in the range
∼ 500−1500 M⊙ yr
−1 (Omont et al., 2003; Priddey et al., 2003),
in fair agreement with our finding, especially if we consider the
12 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Fig. 9. a) PAH(7.7µm) luminosity as a function of the QSO optical luminosity. Blue diamonds are data from Schweitzer et al. (2006).
The red square is the upper limit obtained by the average spectrum of luminous, high-z QSOs. b) Distribution of the PAH(7.7µm)
to optical luminosity ratio in the local QSOs sample of Schweitzer et al. (2006). The hatched region indicates upper limits. The red
vertical line indicate the upper limit inferred from the average spectrum of luminous QSOs at high-z.
uncertainties involved in the two different observational methods
to infer the SFR.
We also note that a similar, independent result was obtained
by Haas et al. (2003), who found that in a large sample of QSOs
the ratio between LFIR (powered by star formation) and LB de-
creases at high luminosities.
The finding of a “saturation” of the relation between star for-
mation activity and QSO power may provide an explanation for
the evolution of the relation between BH mass and galaxy mass
at high redshift. Indeed, Peng et al. (2006) and McLure et al.
(2006) found that, for a given BH mass, QSO hosts at z∼2 are
characterized by a stellar mass lower than expected from the
local BH-galaxy mass relation. In other terms, the BH growth
is faster, relative to star formation, in high-z, luminous QSOs.
Our result supports this scenario by independently showing that
the correlation between star formation and AGN activity breaks
down at high luminosities.
5. Conclusions
We have presented low resolution, mid-IR Spitzer spectra of a
sample of 25 luminous QSOs at high redshifts (2 < z < 3.5).
We have combined our data with Spitzer spectra of lower lumi-
nosity, type-I AGNs, either published in the literature or in the
Spitzer archive. The combined sample spans five orders of mag-
nitude in luminosity, and allowed us to investigate the dust prop-
erties and star formation rate as a function of luminosity. The
spectroscopic information allowed us to disentangle the various
spectral components contributing to the MIR band (PAH and sil-
icate emission) and to sample the continuum at a specific λrest, in
contrast to photometric MIR observations. The main results are:
– The mid-IR continuum luminosity at 6.7µm correlates with
the optical continuum luminosity but the correlation is not
linear. In particular, the ratio λLλ(6.7µm)/λLλ(5100Å) de-
creases by about a factor of ten as a function of lumi-
nosity over the luminosity range 1042.5 < λLλ(5100Å) <
1047.5 erg s−1. This is interpreted as a reduction of the cov-
ering factor of the circumnuclear hot dust as a function of
luminosity. This result is in agreement and provides an in-
dependent confirmation of the recent findings of a decreas-
ing fraction of obscured AGN as a function of luminosity,
obtained in X-ray and optical surveys. We stress that while
X-ray surveys probe the covering factor of the gas, our result
provides an independent confirmation by probing the cover-
ing factor of the dust. We have also shown that the dust cov-
ering factor, as traced by the λLλ(6.7µm)/λLλ(5100Å) ratio,
decreases also as a function of the BH mass. Based on these
correlations alone it is not possible to determine whether the
physical quantity primarily driving the reduction of the cov-
ering factor is the AGN luminosity or the BH mass.
– The mean spectrum of the luminous, high-z QSOs in our
sample shows a clear silicate emission at λrest ∼ 10µm.
Silicate emission is also detected in the individual spectra
of four high redshift QSOs. When combined with the spec-
tra of local, lower luminosity AGNs we find that the sili-
cate strength (defined as the ratio between the maximum of
the silicate feature and the extrapolated featureless contin-
uum) tend to increase as a function of luminosity. The sili-
cate strength correlates positively also with the accretion rate
and with the BH mass, albeit with a large scatter.
– The mean MIR spectrum of the luminous, high-z QSOs in
our sample does not show evidence for PAH emission. Our
sample is not pre-selected by the FIR emission and therefore
it is not biased in terms star formation. As a consequence, the
upper limit on the PAH emission in the total mean spectrum
provides a useful, representative upper limit on the SFR in
luminous QSOs at high redshifts. We find that the ratio be-
tween PAH luminosity and QSO optical luminosity is signifi-
cantly lower than observed in local, lower luminosity AGNs,
implying that the correlation between star formation rate and
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 13
AGN power probably saturates at high luminosities. This re-
sult may explain the evolution of the correlation between BH
mass and galaxy stellar mass recently observed in luminous
QSOs at high redshift.
Acknowledgements. We are grateful to M. Salvati for useful comments. We are
grateful to G. Hasinger for providing us with some of his results prior to publica-
tion. This work is based on observations made with the Spitzer Space Telescope,
which is operated by the Jet Propulsion Laboratory, California Institute of
Technology under a contract with NASA. Support for this work was provided
by NASA under contract 1276513 (O.S.). RM acknowledges partial support
from the Italian Space Agency (ASI). MI is supported by Grants-in-Aid for
Scientific Research (16740117). OS acknowledges support by the Israel Science
Foundation under grant 232/03.
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List of Objects
‘2QZJ002830.4-281706’ on page 16
‘LBQS0109+0213’ on page 16
http://arxiv.org/abs/astro-ph/0702272
http://arxiv.org/abs/astro-ph/0610939
http://arxiv.org/abs/astro-ph/0702136
http://arxiv.org/abs/astro-ph/0702701
14 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
‘[HB89]0123+257’ on page 16
‘HS0211+1858’ on page 16
‘2QZJ023805.8-274337’ on page 16
‘SDSSJ024933.42-083454.4’ on page 16
‘Q0256-0000’ on page 16
‘Q0302-0019’ on page 16
‘[HB89]0329-385’ on page 16
‘SDSSJ100428.43+001825.6’ on page 16
‘TON618’ on page 16
‘[HB89]1318-113’ on page 16
‘[HB89]1346-036’ on page 16
‘UM629’ on page 16
‘UM632’ on page 16
‘BS1425+606’ on page 16
‘[VCV01]J1649+5303’ on page 16
‘SDSSJ170102.18+612301.0’ on page 16
‘SDSSJ173352.22+540030.5’ on page 16
‘[HB89]2126-158’ on page 16
‘2QZJ221814.4-300306’ on page 16
‘2QZJ222006.7-280324’ on page 16
‘Q2227-3928’ on page 16
‘[HB89]2254+024’ on page 16
‘2QZJ234510.3-293155’ on page 16
‘Mrk335’ on page 16
‘IIIZw2’ on page 16
‘PG0050+124’ on page 16
‘PG0052+251’ on page 16
‘Fairall9’ on page 16
‘Mkr79’ on page 16
‘PG0804+761’ on page 16
‘Mrk704’ on page 16
‘PG0953+414’ on page 16
‘NGC3516’ on page 16
‘PG1116+215’ on page 16
‘NGC3783’ on page 16
‘PG1151+117’ on page 16
‘NGC4051’ on page 16
‘PG1211+143’ on page 16
‘NGC4593’ on page 16
‘PG1309+355’ on page 16
‘PG1351+640’ on page 16
‘IC4329a’ on page 16
‘NGC5548’ on page 16
‘Mrk817’ on page 16
‘Mrk509’ on page 16
‘Mrk926’ on page 16
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 15
Table 1. Combined sample of high-z luminous QSO, local QSO and Sy1, and physical properties inferred from optical-UV spectra.
Name RA(J2000) Dec(J2000) z log(λLλ(5100Å)) logL([OIII]) logM(BH)
d L/LdEdd αopt−UV Ref.
(erg/s) (erg/s) M⊙
High-z luminous QSOs
2QZJ002830.4-281706 00:12:21.18 −28:36:30.2 2.401 46.59 44.41 9.72 0.35 −1.42 1
LBQS0109+0213 01:12:16.91 +02:29:47.6 2.349 46.81 44.51 10.01 0.30 −1.75 1
[HB89]0123+257a 01:26:42.79 +25:59:01.3 2.369 46.58 44.32 9.10 1.40 −1.65 1
HS0211+1858 02:14:29.70 +19:12:37.0 2.470 46.63 44.38 10.11 0.16 −0.01 3,10
2QZJ023805.8-274337 02:38:05.80 −27:43:37.0 2.471 46.58 <43.71 9.41 0.69 −1.59 1
SDSSJ024933.42-083454.4 02:49:33.41 −08:34:54.4 2.491 46.39 44.12 9.67 0.25 −1.36 1
Q0256-0000 02:59:05.64 +00:11:21.9 3.377 46.99 44.55 10.11 0.19 −0.96 2
Q0302-0019 03:04:49.86 −00:08:13.4 3.286 46.83 45.01 10.11 0.30 −1.66 2
[HB89]0329-385 03:31:06.34 −38:24:04.8 2.435 46.72 44.31 10.11 0.18 −1.79 1
SDSSJ100428.43+001825.6 10:04:28.44 +00:18:25.6 3.040 46.45 44.47 9.34 0.70 −0.70 3,11
TON618a 12:28:24.97 +31:28:37.6 2.226 47.32 <44.12 10.81 0.14 −1.27 1
[HB89]1318-113 13:21:09.38 −11:39:31.6 2.306 46.90 44.32 9.76 0.62 −0.99 1
[HB89]1346-036 13:48:44.08 −03:53:24.9 2.370 46.89 43.73 9.95 0.41 −1.26 1
UM629 14:03:23.39 −00:06:06.9 2.460 46.57 44.41 9.17 1.16 −1.40 1
UM632b 14:04:45.89 −01:30:21.9 2.517 46.55 44.04 9.44 0.61 −1.27 1
SBS1425+606 14:26:56.10 +60:25:50.0 3.202 47.39 45.04 9.83 1.73 −1.45 1
[VCV01]J1649+5303 16:49:14.90 +53:03:16.0 2.260 46.70 44.19 9.99 0.24 −0.86 3,11
SDSSJ170102.18+612301.0 17:01:02.18 +61:23:01.0 2.301 46.35 <43.51 9.73 0.20 −1.48 1
SDSSJ173352.22+540030.5 17:33:52.23 +54:00:30.5 3.428 47.02 44.36 9.58 1.28 −1.51 1
[HB89]2126-158b 21:29:12.17 −15:38:41.0 3.282 47.27 44.66 9.73 1.60 0.72 1
2QZJ221814.4-300306 22:18:14.40 −30:03:06.0 2.389 46.55 43.95 9.28 0.89 −1.27 1
2QZJ222006.7-280324 22:20:06.70 −28:03:23.0 2.414 47.23 44.64 10.21 0.54 −1.28 1
Q2227-3928 22:30:32.95 −39:13:06.8 3.438 46.95 <44.02 10.31 0.19 −1.25 2
[HB89]2254+024 22:57:17.56 +02:43:17.5 2.083 46.46 43.95 9.10 1.08 −1.37 1
2QZJ234510.3-293155 23:45:10.36 −29:31:54.7 2.382 46.33 43.97 9.38 0.42 −1.26 1
High-z QSO aver. (z<2.5)c 46.63 44.07 9.68 0.51
Local QSOs and Sy1s
Mrk335 00:06:19.52 +20:12:10.4 0.025 43.62 41.29 7.10 0.28 −2.00 4,14
IIIZw2 00:10:30.80 +10:58:13.0 0.090 44.02 42.25 8.19 0.16 −1.56 4,13
PG0050+124 00:53:34.94 +12:41:36.2 0.058 44.36 41.87 7.09 0.96 −0.91 4,13
PG0052+251 00:54:52.10 +25:25:38.0 0.155 44.46 42.57 8.55 0.21 −2.27 4,13
Fairall9 01:23:45.78 −58:48:20.5 0.046 43.80 41.91 8.27 0.10 −1.99 4,13
Mrk79 07:42:32.79 +49:48:34.7 0.022 43.58 41.37 8.12 0.08 −0.83 9,14
PG0804+761 08:10:58.60 +76:02:42.0 0.100 44.42 42.03 8.08 0.33 −2.64 4,12
Mrk704 09:18:26.00 +16:18:19.2 0.029 43.44 41.18 7.97 0.08 – 4
PG0953+414 09:56:52.40 +41:15:22.0 0.234 44.96 42.69 8.39 0.56 −2.12 4,13
NGC3516 11:06:47.49 +72:34:06.8 0.009 42.81 40.52 7.39 0.06 −1.09 7,8,14
PG1116+215 11:19:08.60 +21:19:18.0 0.176 44.84 42.27 8.27 0.53 −2.58 4,13
NGC3783 11:39:01.72 −37:44:18.9 0.010 43.05 41.10 7.33 0.09 −1.46 4,13
PG1151+117 11:53:49.27 +11:28:30.4 0.176 44.48 42.09 8.31 0.28 −2.50 4,12
NGC4051 12:03:09.61 +44:31:52.8 0.002 41.39 39.64 5.32 0.06 −0.82 5,6,14
PG1211+143 12:14:17.70 +14:03:12.6 0.085 44.58 41.94 7.69 0.68 −1.32 4,13
NGC4593 12:39:39.42 −05:20:39.3 0.009 42.60 40.34 7.40 0.04 −1.26 4,14
PG1309+355 13:12:17.76 +35:15:21.2 0.184 44.50 42.18 8.29 0.30 −2.45 4,12
PG1351+640 13:53:15.80 +63:45:45.4 0.087 44.80 42.52 8.76 0.28 −0.86 4,13
IC4329a 13:49:19.26 −30:18:34.0 0.016 43.13 40.89 7.77 0.06 – 4
NGC5548 14:17:59.53 +25:08:12.4 0.017 43.10 41.15 7.78 0.06 −1.73 4,13
Mrk817 14:36:22.06 +58:47:39.3 0.033 43.96 41.65 8.11 0.16 −0.56 4,14
Mrk509 20:44:09.73 −10:43:24.5 0.034 44.01 42.13 7.87 0.22 −1.69 4,13
Mrk926 23:04:43.47 −08:41:08.6 0.047 43.83 42.29 8.55 0.08 – 4
The following quantities are reported in each column: column 1, object name; columns 2-3, coordinates (J2000); column 4, redshift; column 5, log
of the continuum luminosity λLλ (in units of erg/s) at the rest frame wavelength 5100Å; column 6, log of the [OIII]λ5007 emission line luminosity
(in units of erg/s); column 7, log of the black hole mass (in units of M⊙); column 8, Eddington ratio Lbol/LEdd; column 9, optical-to-UV (1450Å–
5100Å) continuum slope (Fλ ∝ λ
αopt−UV); column 11: reference for the optical and UV data: 1 - Shemmer et al. (2004), Netzer et al. (2004) and
therein references for UV data, 2 - Dietrich et al. (2002) and therein references for UV data, 3 - Juarez et al (in prep.), 4 - Marziani et al. (2003), 5 -
Suganuma et al. (2006), 6 - Peterson et al. (2000), 7 - Wanders et al. (1993), 8 - Ho & Ulvestad (2001), 9 - Peterson et al. (1998), 10 - Engels et al.
(1998), 11 - SDSS DR5 archive, 12- Baskin & Laor (2005), 13 - Evans & Koratkar (2004), 14 - Kaspi et al. (2005).
a Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is near or above the observed value, hence the
6.7µm flux is probably dominated by synchrotron emission; these objects will be excluded from statistical analyses.
b Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is well below the observed value, hence the 6.7µm
flux is likely thermal.
c Optical luminosities, black hole mass and Eddington ratio for the stacked spectrum refer to the average values of only the objects at z<2.5, i.e.
those who contribute to the Silicate feature observed in the stacked spectrum.
d As discussed in Shemmer et al. (2004), the uncertainties on the BH masses and accretion rate are no larger than a factor of two.
16 R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs
Table 2. Infrared properties of the combined sample of high-z luminous QSO, local QSO and Sy1
Name FMIR αMIR
λLλ (6.7µm)
λLλ (5100Å)
Si strength Ref.
(mJy)
High-z luminous QSOs
2QZJ002830.4-281706 4.8 -0.59 0.75 1
LBQS0109+0213 8.3 -0.91 0.75 1
[HB89]0123+257a 4.9 -1.69 0.76 1
HS0211+1858 7.3 -1.36 1.09 1
2QZJ023805.8-274337 3.7 -1.04 0.62 0.81±0.16 1
SDSSJ024933.42-083454.4 2.2 -1.26 0.57 1
Q0256-0000 2.7 -1.34 0.30 1
Q0302-0019 3.6 -1.67 0.56 1
[HB89]0329-385 5.5 -1.74 0.65 1
SDSSJ100428.43+001825.6 2.3 -1.51 0.73 1
TON618a 20.1 -1.16 0.51 0.04±0.01 1
[HB89]1318-113 6.6 -1.70 0.47 1
[HB89]1346-036 12.9 -1.68 0.99 1
UM629 3.8 -0.68 0.65 1
UM632b 2.1 -1.77 0.40 1
BS1425+606 23.9 -1.56 0.96 1
[VCV01]J1649+5303 9.4 -1.28 1.03 1
SDSSJ170102.18+612301.0 3.3 -0.66 0.83 1
SDSSJ173352.22+540030.5 2.0 -2.88 0.21 1
[HB89]2126-158b 19.1 -1.18 1.08 1
2QZJ221814.4-300306 4.6 -1.03 0.78 1
2QZJ222006.7-280324 16.0 -1.52 0.58 0.65±0.05 1
Q2227-3928 3.0 -1.70 0.38 1
[HB89]2254+024 3.6 -1.12 0.60 0.63±0.15 1
2QZJ234510.3-293155 4.2 -1.33 1.16 0.92±0.15 1
High-z QSO aver. (z<2.5)c -1.57 0.58±0.10
Local QSOs and Sy1s
Mrk335 130. -1.38 1.90 0.25±0.06 3
IIIZw2 52. -1.09 4.02 0.05±0.03 2,3
PG0050+124 245. -0.74 3.55 0.38±0.05 2,3
PG0052+251 28. -1.35 2.40 0.33±0.06 2,3
Fairall9 146. -0.90 4.83 0.21±0.07 2,3
Mkr79 200. -1.03 2.44 0.10±0.06 3
PG0804+761 88. -1.81 3.38 0.60±0.05 2,3
Mrk704 190. -0.95 5.64 0.09±0.06 3
PG0953+414 26. -1.88 1.68 0.40±0.08 2,3
NGC3516 210. -1.05 2.40 0.06±0.05 3
PG1116+215 66. -1.71 3.08 0.22±0.05 2,3
NGC3783 315. -1.08 2.69 -0.01±0.03 2,3
PG1151+117 10. -2.19 1.08 0.36±0.13 2,3
NGC4051 230. -0.43 4.71 0.06±0.05 3
PG1211+143 100. -1.26 1.89 0.55±0.05 2,3
NGC4593 184. -1.14 3.61 0.08±0.05 2,3
PG1309+355 25. -1.24 2.82 0.41±0.07 2,3
PG1351+640 53. -0.89 0.64 1.25±0.05 2,3
IC4329a 487. -0.67 9.00 0.01±0.03 2,3
NGC5548 69. -1.06 1.53 0.27±0.05 2,3
Mrk817 140. -0.65 1.61 0.16±0.06 3
Mrk509 179. -1.29 1.98 0.11±0.04 2,3
Mrk926 55. -1.40 1.77 0.26±0.04 2,3
The following quantities are reported in each column: column 1, object name; column 2, continuum flux density at the observed wavelength
corresponding to λrest = 6.7µm (after removing starburst and stellar components, in units of mJy); column 3, power-law index (Fλ ∝ λ
α) fitted
to the continuum in the 5–8µm range (starburst component–subtracted); column 4, ratio of the continuum emission at 5100Å and at 6.7µm,
λLλ(5100Å)/λLλ(6.7µm); column 5, Silicate strength; column 6: reference for the infrared data: 1 - this work (from Spitzer program 20493) ; 2 -
Shi et al. (2006); 3 - this work (from Spitzer archival data).
a Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is near or above the observed value, hence the
6.7µm flux is probably dominated by synchrotron emission; these objects will be excluded from statistical analyses.
b Radio loud QSOs for which the extrapolation of the radio synchrotron emission to the MIR is well below the observed value, hence the 6.7µm
flux is likely thermal.
c The ratio λLλ(5100Å)/λLλ(6.7µm) is not defined for the stacked spectrum, since all spectra were normalized to the 6.7µm flux before stacking.
As a consequence, only the Silicate strength (and more generally the continuum shape) has a physical meaning for the stacked spectrum.
R. Maiolino et al.: Dust covering factor, silicate emission and star formation in luminous QSOs 17
Table 3. Spearman-rank coefficients for the correlations in Figs.4, 8, 5 and 6.
log(λLλ(5100Å)) log(L[OIII]) log(L/LEdd) log(MBH) αMIR αopt−UV
λLλ (6.7µm)
λLλ (5100Å)
−0.76 (< 10−6) −0.72 (< 10−6) −0.44 (3 10−4) −0.70 (< 10−6) 0.29 (0.05) −0.20 (0.19)
log(Si str.) 0.83 (2 10−5) 0.75 (6 10−6) 0.75 (8 10−6) 0.66 (8 10−4)
Numbers in parenthesis give the probability for the correlation coefficient to deviate from zero.
	Introduction
	Sample selection, observations and data reduction
	Analysis
	Main observational results
	A comparison with MIR properties of lower luminosity AGNs
	Discussion
	Dust covering factor
	Covering factor as a function of source luminosity and BH mass
	Model uncertainties
	Comparison with previous works
	Silicate emission
	PAHs and star formation
	Conclusions
