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arXiv:1509.03629v1 [astro-ph.GA] 11 Sep 2015
Draft version September 15, 2015
Preprint typeset using L
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ARE COMPTON-THICK AGN THE MISSING LINK BETWEEN MERGERS AND BLACK HOLE GROWTH?
Dale D. Kocevski1, Murray Brightman2, Kirpal Nandra3, Anton M. Koekemoer4, Mara Salvato3, James Aird5,
Eric F. Bell6, Li-Ting Hsu3, Jeyhan S. Kartaltepe7, David C. Koo8, Jennifer M. Lotz4, Daniel H. McIntosh9,
Mark Mozena8, David Rosario3, Jonathan R. Trump10
Department of Physics and Astronomy, Colby College, Waterville, ME 04961
Draft version September 15, 2015
ABSTRACT
We examine the host morphologies of heavily obscured active galactic nuclei (AGN) at z∼1 to
test whether obscured supermassive black hole growth at this epoch is preferentially linked to galaxy
mergers. Our sample consists of 154 obscured AGN with NH>1023.5cm−2and z < 1.5. Using visual
classifications, we compare the morphologies of these AGN to control samples of moderately obscured
(1022 cm−2< NH<1023.5cm−2) and unobscured (NH<1022 cm−2) AGN. These control AGN
have similar redshifts and intrinsic X-ray luminosities to our heavily obscured AGN. We find that
heavily obscured AGN are twice as likely to be hosted by late-type galaxies relative to unobscured
AGN (65.3+4.1
−4.6% vs 34.5+2.9
−2.7%) and three times as likely to exhibit merger or interaction signatures
(21.5+4.2
−3.3% vs 7.8+1.9
−1.3%). The increased merger fraction is significant at the 3.8σlevel. If we exclude
all point sources and consider only extended hosts, we find the correlation between merger fraction
and obscuration is still evident, however at a reduced statistical significance (2.5σ). The fact that
we observe a different disk/spheroid fraction versus obscuration indicates that viewing angle cannot
be the only thing differentiating our three AGN samples, as a simple unification model would sug-
gest. The increased fraction of disturbed morphologies with obscuration supports an evolutionary
scenario, in which Compton-thick AGN are a distinct phase of obscured SMBH growth following a
merger/interaction event. Our findings also suggest that some of the merger-triggered SMBH growth
predicted by recent AGN fueling models may be hidden among the heavily obscured, Compton-thick
population.
Subject headings: galaxies: active — galaxies: evolution — X-rays: galaxies
1. INTRODUCTION
Observations over the past two decades have revealed
a tight correlation between the mass of a galaxy’s stellar
bulge and its central super-massive black hole (SMBH;
Magorrian et al. 1998; Gebhardt et al. 2000; Tremaine
et al. 2002, G¨ultekin et al. 2009; McConnell & Ma 2013).
This finding is commonly interpreted as evidence that
the growth of SMBHs and their host spheroids is con-
nected. Given the effectiveness of violent galaxy mergers
to dissipate angular momentum, mergers have long been
proposed as a means to forge this connection (Sanders et
al. 1988; Hernquist et al. 1989; Kauffmann & Haehnelt
2000). In this scenario, the strong gravitational torques
produced as a result of a merger funnel gas to the cen-
ter of a galaxy, triggering both accretion onto the central
black hole and star formation that grows the stellar bulge
(Barnes & Hernquist 1991; Mihos & Hernquist 1996).
Coupled with self-regulated black hole growth (i.e., AGN
feedback; Di Matteo et al. 2005; Hopkins et al. 2005,
2006), galaxy mergers provide an attractive mechanism
to both trigger AGN activity and help explain the co-
dale.kocevski@colby.edu
2California Institute of Technology
3Max-Planck-Institut f¨ur Extraterrestrische Physik
4Space Telescope Science Institute
5Institute of Astronomy, Cambridge
6University of Michigan
7National Optical Astronomy Observatories
8University of California, Santa Cruz
9University of Missouri, Kansas City
10 The Pennsylvania State University
evolution observed between SMBHs and their host galax-
ies.
However, observational attempts to tie AGN activity
to galaxy mergers have produced mixed results. Gas-
rich mergers are observed to fuel a substantial fraction
of bright quasars (e.g. Guyon et al. 2006; Bennert et
al. 2008; Veilleux et al. 2009; Koss et al. 2010, 2012)
and recent studies of kinematic galaxy pairs have demon-
strated that nuclear activity is indeed enhanced in galax-
ies with an interacting companion (Silverman et al. 2011;
Ellison et al. 2011). On the other hand, morphological
studies have consistently found that the bulk of the AGN
population does not appear to be triggered by major
galaxy mergers. Both at z∼1 (Grogin et al. 2005; Pierce
et al. 2007; Cisternas et al. 2011; Villforth et al. 2014)
and more recently at z∼2 (Schawinski et al. 2011; Ko-
cevski et al. 2012; Rosario et al. 2015), studies have found
that X-ray selected AGN hosts are no more likely to ex-
hibit morphological disturbances compared to similar in-
active galaxies. In fact, results from the CANDELS sur-
vey (Grogin et al. 2011; Koekemoer et al. 2011) indicate
that roughly half of moderate-luminosity (LX<1043 erg
s−1) AGN at z∼2 reside in disks and are likely fueled
stochastically by secular processes and/or disk instabili-
ties rather than ma jor mergers (Kocevski et al. 2012).
While the efficiency of stochastic fueling is expected
to increase with redshift, given the rapid rise in the
gas fraction of galaxies at z > 1 (see e.g. Tacconi et
al. 2010), AGN fueling models predict that only a small
fraction (∼30%) of the overall AGN luminosity density
2 Kocevski et al.
and BH mass density are the result of this fueling mode
(Hopkins, Kocevski, & Bundy 2014). Instead the ma-
jority of SMBH growth is predicted to be the result of
merger-induced fueling, especially at high luminosities
(LX>1044 erg s−1; Hopkins & Hernquist 2009; Draper
& Balantyne 2012). The low merger fraction observed
among X-ray selected AGN out to z∼2 appears to be
at odds with this prediction.
A major caveat associated with these findings is that
heavily obscured AGN are not well sampled by X-ray sur-
veys (see e.g. Treister et al. 2004). The most obscured,
Compton-thick AGN (hereafter CT-AGN) are hidden by
extreme column densities (NH>1024 cm−2) of obscur-
ing gas that can absorb even hard X-ray photons. Anal-
ysis of the diffuse X-ray background indicates a signif-
icant fraction (up to ∼50%) of AGN are hidden be-
hind Compton-thick obscuration (Comastri et al. 1995;
Ueda et al. 2003; Gilli et al. 2007); Akylas et al. 2012)1,
however much remains unknown about the demograph-
ics of their host galaxies. In the evolutionary sequence of
Sanders et al. (1998), heavily obscured AGN represent a
key phase in the life cycle of galaxies, as it is during this
period that SMBHs are predicted to accrete the bulk of
their mass (e.g., Fabian 1999; Gilli et al. 2007; Treister
et al. 2009; Draper & Ballantyne 2010). Furthermore,
hydrodynamical merger simulations predict that this ob-
scured phase should coincide with the most morpholog-
ically disturbed phase of a galaxy interaction (Cattaneo
et al. 2005; Hopkins et al. 2008). It is therefore acutely
possible that past studies have systematically missed the
AGN-merger connection by not sampling the obscured
AGN population well.
Several studies have attempted to overcome this bias
by selecting AGN at mid-infrared (IR) wavelengths,
where radiation absorbed by obscuring circumnuclear
dust is expected to be re-emitted (e.g. Lacy et al. 2004;
Stern et al. 2005; Daddi et al. 2007; Donley et al. 2007;
Soifer et al. 2008). However, the most recent work to
examine the morphologies of IR-selected AGN have pro-
duced conflicting results. Schawinski et al. (2012) exam-
ined the morphologies of 24µm-selected Dust Obscured
Galaxies (DOGs) at z∼2, a high fraction of which are
thought to host heavily obscured AGN based on X-ray
stacking analyses (Fiore et al. 2008; Treister et al. 2009).
The authors report a high disk fraction (90%) and a rel-
atively low merger fraction (4%) that is consistent with
studies of more unobscured AGN hosts (i..e., Schawinski
et al. 2011). On the other hand, Donley et al. (2015)
find that galaxies with a power-law spectral slope in the
mid-IR, a signature of hot dust near an obscured AGN’s
central engine (Donley et al. 2007, 2012), have a higher
fraction of disturbed morphologies compared to X-ray
detected AGN hosts that do not exhibit similar IR emis-
sion. This might be a result of the power-law technique
preferentially selecting relatively high luminosity AGN,
which may be more associated with galaxy mergers (e.g.,
Draper & Ballantyne 2012; Treister et al. 2012).
In this study, we re-examine the connection between
AGN obscuration and host morphology using a sample
of heavily obscured AGN identified by their X-ray spec-
1Studies of resolved X-ray sources estimate a Compton-thick
fraction of 35-40% at z > 1 (Brightman & Ueda 2012; Buchner et
al. 2015)
Fig. 1.— Redshift versus luminosity for X-ray sources detected
in our three target fields. X-ray luminosities are intrinsic rest-
frame 2-10 keV luminosities having been corrected for absorption
(i.e. setting NH=0 in our best-fit spectral model). The use of both
deep and wide survey data allows us to probe a wide range of
luminosities, including sources at LX>1044 erg s−1, which are
not well sampled in deep/narrow surveys such as the CDFS 4Ms
observations.
tral properties. Due to the differential absorption of hard
and soft X-ray photons, the shape of an AGN’s X-ray
spectrum reveals not only the presence of gas along the
line of sight, but it also provides a measure of its col-
umn density. CT-AGN, in particular, can be identified
by their X-ray spectra due to nuclear emission that is
Compton scattered into our line of sight even when the
direct emission is suppressed. This “reflected” emission
has a characteristic spectral shape consisting of a flat
continuum and a high equivalent width Fe Kαfluores-
cence line (Reynolds et al. 1994; Matt, Brandt & Fabian
1996).
Identifying CT-AGN using low energy (<10 keV) ob-
servations from Chandra or XMM-Newton is challenging
because the heavy attenuation suffered at these wave-
lengths often restricts the accuracy of any X-ray spec-
tral analysis. In addition, CT-AGN often appear softer
than expected at low energies due to their reflection-
dominated emission. As a result, a simple absorbed
power-law fit to the soft X-ray spectra of CT-AGN
will systematically underestimate their obscuring col-
umn density, as was recently shown using NuSTAR ob-
servations (Gandhi et al. 2014; Lansbury et al. 2015).
However, with sufficiently deep observations and proper
spectral modeling, even the most obscured CT-AGN
can be identified with relatively soft X-ray observations
(e.g. Brightman et al. 2014; Buchner et al. 2014). Sev-
eral studies have successfully employed X-ray spectral
modeling to identify heavily obscured AGN using both
deep Chandra (Tozzi et al. 2006; Georgantopoulos et
al. 2009; Feruglio et al. 2011; Gilli et al. 2011; Alexander
et al. 2011; Brightman et al. 2014; Buchner et al. 2014)
and XMM-Newton observations (Comastri et al. 2011;
Georgantopoulos et al. 2013; Lanzuisi et al. 2015).
For this work, we examine the host morphologies of
the CT-AGN sample of Brightman et al. (2014). These
sources were identified using the new spectral models of
Brightman & Nandra (2011) that correctly account for
emission from Compton scattering, the geometry of the
Host Morphologies of Compton-Thick AGN at z∼1 3
absorbing material, and include a self-consistent treat-
ment for Fe Kαemission. These models include all of
the signatures of Compton-thick obscuration in a sin-
gle model, allowing for the identification of CT-AGN in
lower signal-to-noise data than previously possible. Us-
ing visual classifications, we examine whether heavily ob-
scured AGN exhibit an enhancement of merger and/or
interaction signatures relative to their unobscured coun-
terparts with the same intrinsic X-ray luminosity and
redshift.
Our analysis is presented as follows. In §2 we de-
scribe the X-ray and optical data used for the study, as
well as discuss the methodology employed to select our
sample of obscured AGN and unobscured control AGN.
The details of our morphological classification scheme are
given in §3 and our primary results are presented in §4.
We discuss the implications of our findings in §5. Fi-
nally, our conclusions are summarized in §6. When nec-
essary, the following cosmological parameters are used:
H0= 70kms−1Mpc−1; Ωtot ,ΩΛ,Ωm= 1,0.3,0.7.
2. OBSERVATIONS AND SAMPLE SELECTION
2.1. X-ray Datasets
The AGN sample used for our analysis is drawn from
Chandra datasets in three fields: the Chandra Deep Field
South (CDFS; Alexander et al. 2003; Xue et al. 2011),
the AEGIS-XD dataset in the Extended Groth Strip
(EGS; Nandra et al. 2015), and the C-COSMOS obser-
vations (Elvis et al. 2009; Civano et al. 2011). These
datasets have characteristic exposure times of 4 Msec,
800 ksec, and 180 ksec and cover an area of roughly 0.13,
0.28, 0.98 degrees2, respectively. This combination of
deep and wide survey data was chosen to ensure that
both moderate and high luminosity (LX∼1043−45 erg/s)
AGN are well represented in our final sample.
X-ray source catalogs in the CDFS and AEGIS-XD
were created by processing the Chandra observations in
each field with the custom reduction and source detec-
tion pipeline of Laird et al. (2009). These catalogs were
matched to optical counterparts using the maximum-
likelihood technique described by Sutherland & Saun-
ders (1992). In the CDFS, the X-ray sources were cross-
matched to the CANDELS F160W-selected photometry
catalog of Guo et al. (2013), while the AEGIS-XD sources
were matched to the 3.6µm selected multi-waveband pho-
tometric catalog provided by the Rainbow Cosmological
Surveys Database (Barro et al. 2011a,b). For the C-
COSMOS dataset, we adopt the published X-ray source
and counterpart catalog of Civano et al. (2012). These
published counterparts were identified by cross-matching
the X-ray source catalog to the I-band optical sample of
Capak et al. (2007) and the 3.6µm sample of Sanders et
al. (2007).
Redshifts for the identified X-ray counterparts were
drawn from various spectroscopic datasets in each field.
For the CDFS, we used the compilation of Cardamone et
al. (2010) and Xue et al. (2011). For the EGS field, spec-
troscopic redshifts are drawn primarily from the DEEP2
(Newman et al. 2013) and DEEP3 (Cooper et al. 2012)
redshift surveys. For sources without spectroscopic red-
shifts in these fields, we use photometric redshifts from
Hsu et al. (2014) and Nandra et al. (2015), which are de-
rived through spectral energy distribution (SED) fitting
that employs a combination of galaxy and AGN tem-
plates to account for non-stellar emission (e.g., Salvato
et al. 2011). For the C-COSMOS sources, we adopt the
spectroscopic and photometric redshifts compiled by Ci-
viano et al. (2012); the former are drawn primarily come
from Lilly et al. (2009), Trump et al. (2009), and Brusa et
al. (2010), while the latter come from the work of Salvato
et al. (2011). The redshift and luminosity distribution of
the resulting AGN sample in all three fields is shown in
Figure 1.
2.2. Optical High-Resolution Imaging
To analyze the host morphologies of our AGN sam-
ple, we make use of the high resolution HST Advanced
Camera for Survey (ACS) optical imaging that is pub-
licly available in each of our three fields. In the CDFS,
we use the F850LP (z-band) imaging from the Great Ob-
servatories Origins Deep Survey (GOODS; Giavalisco et
al. 2004), which covers the central 10′×16′of the field.
This imaging has an exposure time of ∼18200 sec and
reaches a limiting magnitude of mAB = 28.3 (5σ, point
source, within a circular aperture of radius 0.
′′12; Grogin
et al. 2011). In COSMOS, we use the F814W (IF814W-
band) mosaic that covers an area of roughly 77′×77′with
an exposure time of ∼2000 sec and which reaches a lim-
iting magnitude of mAB = 27.2 (Koekemoer et al. 2007).
In the EGS, we make use of the AEGIS F814W mosaic
which covers a 10.1′×70.5′region. This imaging has
an exposure time of ∼2100 sec and reaches a limiting
magnitude of mAB = 27.5 (Davis et al. 2007). The ACS
imaging in all three fields have a pixel scale of 0.
′′03/pixel.
2.3. Identifying Obscured AGN
We select obscured AGN from our parent sample using
an X-ray spectral analysis that provides a measure of
the line-of-sight obscuration present in each source. The
details of this spectral fitting are presented in Brightman
et al. (2014); below we briefly summarize this analysis.
Individual source spectra were extracted using ACIS
Extract (Broose et al. 2010) and lightly grouped with a
minimum of one count per bin using the HEASARC tool
grppha. The spectral fitting was carried out with XSPEC
using the Cash statistic (c-stat; Cash 1979). The spec-
tral models we use are from Brightman & Nandra (2011),
which employ Monte Carlo simulations to account for
Compton scattering and the geometry of the obscuring
material. They also include a self-consistent treatment
of iron Kαemission and describe spherical and torus dis-
tributions of the circumnuclear material.
Four model combination are fit to each spectrum. The
first three models represent obscured emission with var-
ious torus geometries. In these models, the column den-
sity, NH, primary power-law index, Γ, and power-law
normalization are free parameters. Rather than attempt-
ing to constrain the torus opening angle from the spectra,
three different cases where tested where torus opening
angles were fixed at 60◦, 30◦and 0◦; here 0◦is essentially
a 4πspherical distribution. For opening angles >0◦
we include a secondary power-law component, Γscatt , in
the fit, which represents intrinsic scattered emission, re-
flected by hot electrons filling the cone of the torus. Here
Γscatt is set to the primary power law index. When the
opening angle is 0◦, we do not include this scattered com-
ponent as this model represents the case where there is
4 Kocevski et al.
Fig. 2.— X-ray spectra of three Compton-thick AGN detected in the CDFS, EGS, and COSMOS fields by Brightman et al. (2014).
The red dashed line shows the best-fit direct torus emission from the AGN, while the black dashed line shows the Thompson scattered
component. Due to heavy obscuration, the scattered component often dominates the emission at low energies, despite typically accounting
for <1% of the direct emission from these sources. All three sources exhibit strong Fe Kαemission characteristic of a Compton-thick AGN.
Fig. 3.— Redshift versus luminosity for X-ray sources in the
CDFS, EGS and COSMOS fields. Sources are color coded by their
level of nuclear obscuration, as determined by our X-ray spectral
modeling. X-ray luminosities are intrinsic rest-frame 2-10 keV lu-
minosities having been corrected for absorption.
no escape route for the primary radiation to be scattered
into the line of sight. The fourth model is a simple power-
law model with two free parameters, the power-law in-
dex, Γ, and its normalization, which represents purely
unobscured X-ray emission.
Each of the four model combinations is fit to the
source spectrum in turn with at least 100 iterations. We
adopted a critical ∆c-stat of 1 ×10−5as the minimum
decrease in the fit statistic required for XSPEC to say that
it has found the minimum. The best-fitting model com-
bination is chosen to be that which presents the lowest
c-stat value after penalizing the more complex models
(those with more free parameters). However, Brightman
& Ueda (2012) have shown that for sources with less
than 600 counts, large uncertainties in the spectral fits
can be reduced by fixing the power-law index in the fit.
Thus, for these sources, we use a fixed value of Γ = 1.7,
the mean spectral index of sources with more than 600
counts. We do, however, allow a consideration for intrin-
sically steep or flat spectra. If the best-fitting model for
sources with less than 600 counts, where Γ is free, is a sig-
nificantly better fit than the best-fitting model where Γ
is fixed, using the criterion of ∆c-stat >2.71, we choose
the model with Γ free as the best-fitting model. In total,
the fraction of sources where Γ is left free is 221/549,
220/937, and 232/1761 in the CDFS, EGS, and COS-
MOS fields, respectively. Examples of our X-ray spectral
fits in all three fields can be seen in Figure 2.
From our spectral fits we obtain a best-fit line-of-sight
column density, NH, for each source in our parent sample.
The resulting distribution of NHversus redshift and lu-
minosity is shown in Figure 3. As reported in Brightman
et al. (2014), we find that heavily obscured (NH>1024
cm−2) sources are best fit by the torus models with open-
ing angles of 30 or 60 degrees, i.e. not the 0 degree model,
whereas sources with 1023 cm−2< NH<1024 cm−2are
better fit by the 0 degree model. This is mostly due to
the models being degenerate below 1024 cm−2, so the
best fit model was chosen to be the simplest one, which
was the 0 degree model. Furthermore, for Compton-thin
sources, we find that the NHvalues obtained using our
torus models are in very good agreement with the result
obtained by Lanzuisi et al. (2013), who used simple ab-
sorption models, as would be expected for Compton-thin
sources.
For our morphology study, we define a primary sample
of heavily obscured AGN as those sources with z < 1.5
and NH>1023.5cm−2. We have chosen a column density
limit that is lower than the canonical cutoff for Compton-
thick AGN (NH>1.5×1024 cm−2) in order to increase
our sample size of heavily obscured AGN. In addition,
our upper redshift limit is motivated by the fact that
only 21% of the heavily obscured AGN at z > 1.5 in our
three target fields have been imaged with HST/WFC3.
Without this near-infrared imaging we can not properly
access the rest-frame optical morphology of galaxies be-
yond z∼1.5. Therefore, we limit our analysis to those
sources at z < 1.5 that fall within the EGS, GOODS and
COSMOS HST/ACS imaging.
Using these selection criteria results in a sample of 154
heavily obscured AGN at z < 1.5 in our three target
fields. For simplicity, we will refer to these sources as our
CT-AGN sample for the remainder of the paper, despite
our relaxed NHcut. Of this sample, 21 CT-AGN are
drawn from CDFS data, while 44 and 89 are detected in
the EGS and COSMOS fields, respectively.
Host Morphologies of Compton-Thick AGN at z∼1 5
Fig. 4.— Distribution of absorption corrected, rest-frame 2-10 keV luminosities (left ) and redshifts (right) of the CT-AGN sample
(NH>1023.5cm−2) and our control samples of moderately obscured (1022 cm−2< NH<1023.5cm−2) and unobscured (NH<1022
cm−2) AGN.
2.4. Control Sample Selection
In order to compare the host morphologies of CT-AGN
to that of their less obscured counterparts, we have con-
structed two control samples which are matched in red-
shift and X-ray luminosity to the CT-AGN sample, but
have lower measured absorbing column densities. These
two control samples consist of unobscured AGN with
NH<1022 cm−2and moderately obscured AGN with
1022 < NH<1023.5cm−2. In order to match the red-
shift and luminosity distributions of the samples, for each
CT-AGN we randomly select two unobscured and two
moderately obscured AGN that have a redshift within
∆z= 0.1 and an absorption corrected X-ray luminos-
ity within a factor of two (0.5≤LX,CT/LX,control ≤2)
of the CT-AGN. For this matching we use rest-frame 2-
10 keV absorption corrected luminosities, which are de-
rived by setting NH=0 in our best-fit spectral model. If
two unique comparison AGN could not be found within
this parameter range, the search range is iteratively in-
creased by 10%. Because of differences in the depth of
the GOODS-S, EGS and COSMOS HST/ACS imaging,
the control AGN were selected separately for each re-
gion. The resulting luminosity and redshift distribution
of the three subsamples are shown in Figure 4. The me-
dian obscuration-corrected luminosity of the CT-AGN,
moderately obscured, and unobscured subsamples are
< L2−10 keV >= 1043.69, 1043.40, and 1043.34 erg s−1,
respectively.
Selecting control AGN matched in luminosity is chal-
lenging because the large absorption corrections applied
to the luminosities of the CT-AGN make them among the
most luminous sources in our fields. Statistically (i.e. ac-
cording to a K-S test), the luminosity distributions of
the three subsamples are not perfectly matched, with the
CT-AGN having a longer tail toward higher X-ray lumi-
nosities, as evidenced by their slightly higher median 2-
10 keV luminosity. However, our methodology effectively
ensures that we have selected the most luminous moder-
ately obscured and unobscured sources in each field that
have similar redshifts as the CT-AGN. Unfortunately,
the only way to improve our luminosity matching would
be to increase the sample size of AGN available to draw
upon. A proper redshift and luminosity matching is vi-
tal since it has been proposed that galaxy mergers play
a greater role in triggering luminous AGN, while secular
processes trigger lower luminosity AGN (e.g., Triester
et al. 2012). That said, we do not believe the differ-
ence in the median luminosity of the three subsamples
is large enough to be the primary driver of the results
presented in §4 as we find no systematic trend between
disturbed morphologies and absorption-corrected lumi-
nosity among the CT-AGN that would indicate mergers
dominate the tail of the CT-AGN luminosity distribu-
tion.
Finally, since we are interested in assessing the mor-
phology of the AGN hosts, we follow Cisternas et
al. (2011) and apply a magnitude cut of IF814W <24 for
AGN in the EGS and COSMOS fields and zF850LP <24
for sources in the CDFS. This leaves 120, 279, 282 sources
in the Compton-thick, moderately obscured, and unob-
scured AGN samples, respectively10.
3. MORPHOLOGY CLASSIFICATION
Host morphologies of the CT-AGN and control AGN
were assessed through visual inspection using a classifi-
cation scheme similar to the one presented in Kocevski et
al. (2012) and Kartaltepe et al. (2015). These inspections
were carried out by the lead author, D.K., and performed
blind using the reddest HST/ACS bands available in each
field, namely the F814W band in the EGS and COSMOS
fields and the F850LP band in the CDFS. In addition,
F606W imaging was used to provide supplemental color
information for sources in the EGS and CDFS (similar
imaging is not available in the COSMOS field). The size
of each thumbnail image was set to cover roughly 100 kpc
10 Our NHcut of 1023.5cm−2was chosen to ensure that roughly
half (61/120) of our final CT-AGN sample have NH>1024 cm−2,
and are therefore truly Compton-thick. It should be noted, though,
that given the large uncertainties on our NHestimates, even sources
with NH∼1023.5cm−2could still be consistent with being
Compton-thick.
6 Kocevski et al.
Fig. 5.— Examples of AGN host galaxies in each morphology and disturbance class of our visual classification scheme. While the Disk,
Spheroid,Irregular,Point-like classifications are mutually exclusive, the Disturbed/Asymmetric class is a superset of the Merger/Interaction
class, as it includes train-wreck mergers and galaxies that exhibit only minor disturbances. See §3 for details.
on a side at the redshift of each AGN and ranged from
12′′ −16′′. Because of differences in the bands used in
each field and the depth of the available imaging, control
AGN were drawn from the same field as their matched
CT-AGN and the subsequent classifications were carried
out separately for each field.11
For each AGN host, we classified the morphological
type of the galaxy and the degree to which it is dis-
turbed. The possible morphologies were: Disk,Spheroid,
Irregular/Peculiar,Point-like. These classes are mutu-
ally exclusive and only the predominate morphology of
each galaxy was noted. For example, disk galaxies with
a substantial bulge component would simply be classi-
fied as disks in this system. This differs from the scheme
used in Kocevski et al. (2012), where bulge and disk dom-
inated late-type galaxies were differentiated. The change
was made to mitigate the effects of moderate AGN con-
tamination, which can mimic an increase in the bulge-
to-disk ratio of a galaxy. In this scheme, as long as an
extended disk is visible, regardless of the level of nuclear
AGN contamination, the galaxy is classified as a Disk.
This is physically motivated by the fact that disks are
easily destroyed in major mergers and take a consider-
able amount of time to reform (Robertson et al. 2006).
Therefore the presence of a disk constrains, to a certain
extent, the past merger history of a given galaxy12.
To gauge the degree to which a galaxy is disturbed,
three disturbance classifications were used:
11 Where possible, we have compared the classification of D.K.
to those of the CANDELS collaboration (where each galaxy was
inspected by an average of 4 unique classifiers; Kartaltepe et
al. 2015), and we find excellent (>90%) agreement between the
two.
12 It is important to note that while disks can reform following
a merger under the right circumstances (when they are sufficiently
gas rich and have favorable initial orbital parameters), disk sur-
vival is most efficient at low galaxy masses and generally require
conditions that suppress strong inflows toward the galaxy center
(see Springel & Hernquist 2005; Robertson et al. 2006; Hopkins et
al. 2009). This effectively prevents strong bulge growth and AGN
fueling; the opposite of the regime we are interested in here.
•Merger/Interaction: Two distinct galaxies showing
interaction features such as tidal arms or a single
train-wreck system exhibiting strong distortions.
•Disturbed/Asymmetric: All galaxies in the Merger/
Interaction class plus single asymmetric or disturbed
galaxies with no visible interacting companion.
•Undisturbed: None of the above.
In this scheme the Merger/Interaction class includes
train-wreck mergers that have multiple nuclei and/or
strong distortions in a single coalescing system, as well as
disturbed galaxies with an interacting companion. The
Disturbed/Asymmetric class, however, serves as a more
liberal selection of galaxies that may have experienced
an interaction in the recent past. This class includes any
galaxy which has a distorted or asymmetric light profile,
even those with no visible interacting companion. As a
result, these classes are not mutually exclusive. These
classes are similar to the Disturbed I and Disturbed II
classes used in Kocevski et al. (2012), respectively. Ex-
amples of AGN host galaxies in each of our morphology
classes can be seen in Figure 5.
It should be noted that unresolved AGN hosts (those
classified as having Point-like morphologies) are by def-
inition classified as Undisturbed in this system. As a
result, any AGN subsample that has a high Point-like
fraction will also have a high Undisturbed fraction. Al-
ternatively, one could argue that the disturbance level
of Point-like sources is not measurable and should not
be classified as Undisturbed. In the following section, we
present our results using both approaches: first including
Point-like/Undisturbed sources in our analysis and then
excluding them completely. While our primary results
do not change, the statistical significance of our findings
do change as a result of having fewer AGN in our final
sample.
4. RESULTS
The fraction of AGN hosts in each of our morphol-
ogy classes versus their level of nuclear obscuration is
Host Morphologies of Compton-Thick AGN at z∼1 7
Fig. 6.— Fraction of AGN hosts at 0.5< z < 1.5 assigned to various morphology and disturbance classes as a function of their nuclear
obscuration. We find that the hosts of heavily obscured AGN are more likely to be disks and have disturbed morphologies relative to the
hosts of unobscured AGN with the same redshift and absorption-corrected X-ray luminosity.
shown in Figure 6 and listed in Table 1. The er-
ror bars on each fraction reflect the 68.3% binomial
confidence limits given the number of sources in each
category, calculated using the method of Cameron et
al. (2010). For our sample of CT-AGN we find that
65.3+4.1
−4.6% have predominately disk-like morphologies.
This includes disks with and without a central bulge. A
smaller fraction, 16.5+3.9
−2.8%, are classified as spheroidal,
whereas 12.4+3.6
−2.4% are found to have peculiar or irreg-
ular morphologies such that neither a prominent disk
or spheroidal component could be discerned. Only a
small fraction, 5.0+2.8
−1.3%, of the CT-AGN are classified
as point-like. This may be expected if heavy nuclear ob-
scuration is blocking emission from the central engine in
these sources.
For our control sample of unobscured AGN (NH<1022
cm−2) we find a lower disk fraction (34.5+2.9
−2.7%) relative
to the CT-AGN hosts, a slightly higher spheroid fraction
(21.4+2.6
−2.2%), and a lower irregular fraction (6.4+1.8
−1.2%).
Unlike their heavily obscured counterparts, a much larger
fraction of the unobscured sources appear point-like in
the ACS imaging, accounting for 37.4+3.0
−2.8% of the host
morphologies. The hosts of the moderately obscured
(1022 cm−2< NH<1023.5cm−2) control sample have
morphologies that lie between the two extremes of the
Compton-thick and unobscured AGN. Here disks make
up 50.9+3.0
−3.0% of the population, spheroids 24.4+2.7
−2.4% and
irregulars 11.5+2.2
−1.6%. We find an increased point-like
fraction (13.6+2.3
−1.8%) relative the CT-AGN population,
however this fraction is lower than that found in the un-
obscured control sample.
The fraction of AGN with disturbed morphologies in
each of our three subsamples is also shown on the right
side of Figure 6. We find a statistically significant in-
crease in the Merger/Interaction fraction versus AGN
obscuration, rising from 7.8+1.9
−1.3% among the unobscured
AGN to 15.1+2.4
−1.9% for the moderately obscured AGN
and 21.5+4.2
−3.3% for the CT-AGN sample. The increase
in the merger fraction of the CT-AGN relative to their
unobscured counterparts is significant at the 3.8σlevel.
If we include any galaxy that has a distorted or asym-
metric light profile, the overall disturbed fraction in-
creases in all three samples, but the trend with ob-
scuration remains. The Disturbed/Asymmetric fraction
increases from 21.0+2.6
−2.2% among the unobscured AGN
to 34.1+2.9
−2.7% for the moderately obscured AGN and
43.0+4.6
−4.4% for the CT-AGN sample. Here the difference
in the disturbed fraction of the CT-AGN relative to the
unobscured AGN is significant at the 4.4σlevel.
As discussed in §3, the high point source fraction
among the unobscured AGN may artificially drive the
disturbed fraction down for that subsample since unre-
solved hosts are classified as Undisturbed by default. To
account for this, we have excluded all unresolved hosts
from our analysis and present the resulting morphology
and disturbance fractions in Figure 7. When consider-
ing only extended hosts, we find that the disk fraction of
the three subsamples is in much greater agreement, al-
though the CT-AGN are still found in disk hosts more of-
ten than the unobscured AGN. We find the disk fraction
steadily increases from 55.1+3.6
−3.8% among the unobscured
AGN to 58.9+3.1
−3.2% for the moderately obscured AGN and
68.7+4.0
−4.6% for the CT-AGN sample. This trend reverses
for the spheroid fraction, which steadily decreases with
obscuration. Here the spheroid fraction decreases from
34.1+3.7
−3.4% among the unobscured AGN to 28.2+3.0
−2.7% for
8 Kocevski et al.
Fig. 7.— Fraction of extended AGN hosts at 0.5< z < 1.5 assigned to various morphology and disturbance classes as a function of their
nuclear obscuration. This plot is the same as Figure 7, however point sources are now excluded from the analysis. We again find that the
hosts of heavily obscured AGN are more likely to be disks and show some morphological disturbance relative to our unobscured control
sample, albeit at a reduced statistical significance. See §4 for details.
the moderately obscured AGN and 17.4+4.1
−3.0% for the CT-
AGN sample. The irregular fraction is low for all three
subsamples and consistent with showing no trend with
obscuration.
The fraction of AGN in extended hosts that exhibit
a morphological disturbance is shown in Figures 8 and
9. The correlation between merger fraction and ob-
scuration is still evident when excluding point sources,
however the statistical significance of the increase drops
from 3.8σto 2.5σ. The Merger/Interaction fraction is
now 12.5+2.9
−2.1%, 17.4+2.7
−2.2%, and 22.6+4.3
−3.4%, for the un-
obscured, moderately obscured, and CT-AGN samples.
A similar trend is found for the Disturbed/Asymmetric
fraction, which increases from 33.5+3.7
−3.4% among the un-
obscured AGN to 39.4+3.2
−3.1% for the moderately obscured
AGN and 45.2+4.7
−4.5% for the CT-AGN sample. Here the
statistical significance of the increase is now 2.3σ.
In summary, we find an increasing disk fraction and a
decreasing spheroid fraction with increasing nuclear ob-
scuration among AGN at 0.5< z < 1.5. In addition, we
find that the fraction of AGN with disturbed host mor-
phologies increases as a function of obscuration. This
increase is found whether we consider only train-wreck
mergers and galaxies with clear interacting companions
or any galaxy showing an asymmetric light profile. It is
also present regardless of whether we exclude unresolved
host galaxies from our analysis, albeit at a reduced sta-
tistical significance.
5. DISCUSSION
Using a sample of heavily obscured AGN identified by
their X-ray spectral signatures, we find a correlation be-
tween disturbed host morphology and nuclear obscura-
tion at fixed AGN luminosity and redshift. In this sec-
tion we discuss the implications of this result in terms of
both the AGN unification model (§5.1) and the role that
mergers play in fueling SMBH growth (§5.2). In addi-
tion, we conclude the section with a discussion of several
important caveats to keep in mind when interpreting our
findings (§5.3).
5.1. Implications for the AGN Unification Model
The standard unification paradigm invokes a torus-
like structure that obscures the central engine for some
sight lines and not for others, producing the two ob-
served AGN types. In this scheme AGN obscuration is
largely dependent on the viewing angle of the observer
(Antonucci 1993; Urry & Padovani 1995; Tran 2003) and
therefore all AGN would sample the same parent popula-
tion of host galaxies, regardless of their level of obscura-
tion. In other words, there should be no correlation be-
tween heavy nuclear obscuration and disturbed host mor-
phologies. Alternatively, obscured SMBH growth may
be a distinct phase in the co-evolution of AGN and their
hosts, specifically one in which the central engine goes
through rapid growth phase following a merger event
(Sander et al. 1998; Hopkins et al. 2005, 2008). This is
supported by the findings of Draper & Ballantyne 2010,
who suggest that a vast majority of AGN accreting near
the Eddington limit must be hidden by Compton-thick
obscuration based on the observed space density of CT-
AGN and their contribution to the CXB. Furthermore,
hydrodynamical merger simulations predict that this ob-
scured phase should coincide with the most morpholog-
ically disturbed phase of a galaxy interaction (Cattaneo
et al. 2005; Hopkins et al. 2008). Therefore, merger-
driven co-evolution models predict that there should be
Host Morphologies of Compton-Thick AGN at z∼1 9
Fig. 8.— Fraction of AGN hosts in the Disturbed/Asymmetric
class as a function of nuclear obscuration, with point sources in-
cluded (open box) and excluded (filled box) from the analysis. In
both cases, the hosts of heavily obscured AGN are more likely to
be classified as disturbed relative to unobscured AGN with sim-
ilar redshifts and luminosities. However, when point sources are
excluded, the statistical significance of this difference drops from
4.4σto 2.3σ.
a strong dependence between obscuration and host prop-
erties such as morphology.
The fact that we observe a different disk/spheroid and
merger fraction versus obscuration indicates that viewing
angle cannot be the only thing differentiating our three
AGN samples, as the unification model would suggest.
That is not to say that viewing angle plays no part in
obscuring the CT-AGN in our sample, only that interac-
tions play a greater role in fueling their activity relative
to the unobscured AGN in our control samples. This
finding appears to support an evolutionary scenario, in
which an increased fraction of the CT-AGN are heav-
ily obscured as a result of a growth phase triggered by a
galaxy interaction in the recent past. Given that the CT-
AGN are hosted by largely disk-dominated galaxies, we
propose that we are catching these systems near the start
of this evolutionary sequence, before the disk structure
of these galaxies is substantially disturbed or destroyed.
This may be due to increasing obscuration levels as the
merger sequence progresses. For example, if the cover-
ing fraction of the obscuring torus is higher for sources
further along in the merger sequence, then the fraction
of X-ray photons scattered into our line-of-sight would
decrease and we would therefore not be sensitive to the
most disturbed sources. Our proposed location for the
CT-AGN sample in a possible evolutionary sequence is
illustrated in Figure 10.
Previous studies have reached similar conclusions re-
garding the transitional nature of obscured AGN. For
example, the hosts of dust-reddened quasars (Urrutia
et al. 2009; Glikman et al. 2004, 2012) show a high
incidence of merger activity and a disturbance fraction
that increases with increasing obscuration (Urrutia et al.
2008; Glikman et al. 2015). These quasars are intrin-
sically more luminous than the CT-AGN in our sample
and are preferentially found in spheroid-dominated hosts.
It is therefore thought that these quasars are detected in
the final stages of emerging from their dusty cocoons and
near the end of the evolutionary sequence outlined in Fig-
Fig. 9.— Fraction of AGN hosts in the Merger/Interaction class
as a function of nuclear obscuration, with point sources included
(open box) and excluded (filled box ) from the analysis. In both
cases, the hosts of heavily obscured AGN are more likely to be
classified as being involved in a merger or interaction relative to
unobscured AGN with similar redshifts and luminosities. However,
when point sources are excluded, the statistical significance of this
difference drops from 3.8σto 2.5σ.
ure 10 (Urrutia et al. 2012, Glikman et al. 2012, Banerji
et al. 2012).
Our results also agree with the recent findings of Don-
ley et al. (2015) and Juneau et al. (2013), who ex-
amined the morphologies and star formation activity,
respectively, of obscured AGN selected by their mid-
infrared colors and emission line properties. In the for-
mer, the hosts of IRAC power-law selected AGN (Donley
et al. 2008) are found to be more disturbed than their
X-ray selected, and presumably less obscured, counter-
parts. In the latter, the obscured AGN fraction is found
to be higher among galaxies with elevated specific star
formation rates, which the authors argue may be due to
recent galaxy interactions.
On the other hand, our findings are at odds with the
results of Schawinski et al. (2012). Here the authors ex-
amined the morphology of DOGs in the Extended CDFS,
which are thought to host heavily obscured quasars based
on the X-ray stacking analysis of Treister et al. (2009).
This study found that only a small fraction (∼4%) of
DOGs at 1 < z < 3 show signs of recent merger activ-
ity. It is worth noting, however, that only one of our
CT-AGN in the CDFS would be selected as an obscured
AGN via the infrared excess method employed by Schaw-
inski et al. (2012). This is consistent with the findings
of Comastri et al. (2011), who noted that X-ray detected
CT-AGN are not readily picked up by standard mid-
infrared selection techniques. We therefore suspect our
conflicting results are due to substantial differences in
our parent samples and we caution against direct com-
parisons of the two studies.
5.2. Implications for the AGN-Merger Connection
Galaxy mergers have long been proposed as a possible
triggering mechanism for AGN activity, however there is
a growing consensus that most moderate-luminosity, X-
ray selected AGN show no signs of recent merger activity
based on their host morphologies (Grogin et al. 2005,
Cisternas et al. 2011, Schawinski et al. 2011, Kocevski et
10 Kocevski et al.
Fig. 10.— AGN fueling models have proposed that obscured SMBH growth is a distinct phase in an evolutionary sequence following a
merger event. Given that our CT-AGN are largely hosted by disturbed, disk-dominated galaxies, we propose that we are catching these
systems near the start of this evolutionary sequence, before the disk structure of these galaxies is substantially disturbed or destroyed. The
previously proposed location of obscured quasars and infrared-selected AGN along this sequence is also shown. See §5.1 for details.
al. 2012, Villforth et al. 2014). In fact, recent results from
the CANDELS survey suggest that stochastic fueling by
secular processes or disk instabilities play a greater role in
fueling SMBH growth at z > 1 than previously predicted
by AGN fueling models (Kocevski et al. 2012). This is
likely due to the increasing gas fraction of galaxies at
high redshifts (e.g. Tacconi et al. 2010), which acts to
increase the duty cycle of distant, stochastically-fed AGN
(Hopkins et al. 2007, Johansson et al. 2009).
However, even with these new observational con-
straints, AGN fueling models continue to predict that
the integrated total SMBH growth in the Universe should
be dominated by merger-induced fueling. For example,
the semi-emprical fueling model of Hopkins, Kocevski,
& Bundy (2014), which incorporates both stochastic
and merger-induced fueling modes, finds that while non-
merger processes may dominate the AGN population by
numbers, only ∼30% of the total AGN luminosity den-
sity and SMBH mass density is the result of stochas-
tic fueling. The predicted contribution is strongly mass
and luminosity-dependent, with mergers playing an in-
creasingly important role in fueling high-mass (MBH >
107M⊙) SMBH growth and high luminosity (Lbol >
1012L⊙) AGN. A similar conclusion was reached by
Draper & Ballantyne (2012) using AGN population syn-
thesis modeling to determine the importance of different
AGN triggering mechanisms.
It is conceivable that some of the merger-induced fu-
eling that is predicted may have been missed by past
studies of the Chandra deep fields, given the few high-
luminosity AGN present in these fields (e.g. Triester et
al. 2012). However, our results indicate that a por-
tion of this merger-triggered activity may also be hidden
among heavily obscured AGN. This implies that past
studies may have missed the AGN population where the
AGN-merger connection is expected to be the strongest.
Whether there is sufficient merger-fueled SMBH growth
occurring among heavily obscured and high luminosity
AGN to match what is predicted by the Hopkins et
al. (2014) fueling model remains to be determined. A key
to testing this will be identifying additional CT-AGN at
z∼1−2 in order to determine what fraction of this
obscured growth remains undetected.
5.3. Caveats & Future Work
Having discussed the possible implications of our re-
sults, it is worth keeping in mind a couple of important
caveats. First, the difference in the disturbed fraction be-
tween the obscured and unobscured AGN is statistically
significant only when point-like hosts are included in our
analysis. When we consider only hosts with extended
morphologies, the elevated merger fraction among the
CT-AGN is significant at only the 2.5σlevel. A larger
sample of distant, obscured AGN will need to be identi-
fied in order to confirm our findings with greater statis-
tical confidence. This will soon be possible as a result of
the X-UDS Chandra Legacy Survey (co-PIs G. Hasinger
and D. Kocevski), which is obtaining deep (1.25 Msec)
and wide (22′×22′) X-ray observations of the UKIRT
Infrared Deep Sky Survey (UKIDSS) Ultra-deep Survey
field (UDS; Lawrence et al. 2007; Cirasuolo et al. 2007).
This dataset, when combined with the existing ACS and
WFC3 imaging from CANDELS, will substantially in-
crease the number of CT-AGN at z∼1 available for
study. It will also increase the number of CT-AGN at
z∼2 that have rest-frame optical imaging, allowing us
to extend our study to this redshift for the first time.
The second caveat relates to a possible connection be-
tween the elevated disk and disturbed fractions among
the CT-AGN. If morphological disturbances are easier
to visually detect in late-type systems, then it is pos-
sible that the increased Disturbed/Asymmetric fraction
among the CT-AGN is simply a reflection of their higher
Disk fraction relative to the control samples. We do not
believe this is the case, as the CT-AGN also show an in-
creased Merger/Interaction fraction, a classification that
requires a visible interacting neighbor or a train-wreck
morphology. In other words, highly disruptive events
that should be detectable in early-type galaxies as well as
their late-type counterparts. Nonetheless, further work
is needed to determine what fraction of interactions may
be missed among spheroidal hosts. The CANDELS team
is actively pursuing this work by visually classifying sim-
ulated interactions using a classification scheme similar
to the one used in this study.
Host Morphologies of Compton-Thick AGN at z∼1 11
TABLE 1
Visual Classification Results
Unobscured AGN Moderately-Obscured AGN Compton-Thick AGN
(NH<1022 cm−2) (1022 < NH<1023.5cm−2) (NH>1023.5cm−2)
Classification All Hosts/Extended Hosts All Hosts/Extended Hosts All Hosts/Extended Hosts
Disk 34.5+2.9
−2.7% 55.1+3.6
−3.8% 50.9+3.0
−3.0% 58.9+3.1
−3.2% 65.3+4.1
−4.6% 68.7+4.0
−4.6%
Spheroid 21.4+2.6
−2.2% 34.1+3.7
−3.4% 24.4+2.7
−2.4% 28.2+3.0
−2.7% 16.5+3.9
−2.8% 17.4+4.1
−3.0%
Irregular 06.4+1.8
−1.2% 10.2+2.7
−1.9% 11.5+2.2
−1.6% 13.3+2.5
−1.9% 12.4+3.6
−2.4% 13.0+3.8
−2.5%
Point-like 37.4+3.0
−2.8% ———— 13.6+2.3
−1.8% ———— 05.0+2.8
−1.3% ————
Disturbed/Asym 21.0+2.6
−2.2% 33.5+3.7
−3.4% 34.1+2.9
−2.7% 39.4+3.2
−3.1% 43.0+4.6
−4.4% 45.2+4.7
−4.5%
Merger/Interaction 07.8+1.9
−1.3% 12.5+2.9
−2.1% 15.1+2.4
−1.9% 17.4+2.7
−2.2% 21.5+4.2
−3.3% 22.6+4.3
−3.4%
6. CONCLUSIONS
We have used HST/ACS imaging to examine the mor-
phologies of galaxies hosting heavily obscured AGN at
z∼1 in order to test whether obscured SMBH growth
at this epoch is linked to major merger events. Using the
X-ray spectral analysis of Brightman et al. (2014), we se-
lect 154 heavily obscured AGN with NH>1023.5cm−2
and z < 1.5 in the CDFS, EGS, and COSMOS fields. To
determine if these AGN are triggered by galaxy interac-
tions more often than less obscured AGN, we construct
two control samples composed of moderately obscured
(1022 < NH<1023.5) and unobscured (NH<1023.5)
AGN. These samples are matched in redshift and intrin-
sic X-ray luminosity to the heavily obscured AGN sam-
ple. To determine the morphology of the host galaxies,
we employ a visual classification scheme similar to the
one used in Kocevski et al. (2012) and by the CANDELS
collaboration. We assess both the predominant morphol-
ogy of each host galaxy and the level of disturbance that
is visible. Based on our visual classifications, we find:
1. The heavily obscured AGN are predominantly
hosted by late-type galaxies; 65.3+4.1
−4.6% are clas-
sified as Disks, while only 16.5+3.9
−2.8% are classi-
fied as Spheroids. This disk fraction is elevated
relative to our control samples of moderately ob-
scured and unobscured AGN, which have disk frac-
tions of 50.9+3.0
−3.0% and 34.5+2.9
−2.7%, respectively. All
three samples have a low Irregular/Peculiar frac-
tion, which ranges from 6.4+1.8
−1.2% for the unob-
scured AGN to 16.5+3.9
−2.8% for the most heavily ob-
scured.
2. We find a statistically significant increase in the
fraction of disturbed hosts versus AGN obscura-
tion. Roughly 21.5+4.2
−3.3% of the Compton-thick
AGN have highly disturbed host morphologies and
fall in the Merger/Interaction class. This is true for
only 7.8+1.9
−1.3% of the unobscured AGN; a difference
that is significant at the 3.8σlevel. This trend with
obscuration remains when we include galaxies that
exhibit any minor disturbance or asymmetry in
their morphology. Here the Disturbed/Asymmetric
fraction increases from 21.0+2.6
−2.2% for the unob-
scured AGN to 34.1+2.9
−2.7% for the moderately ob-
scured AGN and 43.0+4.6
−4.4% for the Compton-thick
sample. The statistically significance of this in-
crease is 4.4σ.
3. We find that the incidence of Point-like morpholo-
gies is inversely proportional to obscuration, as
might be expected if heavy nuclear obscuration is
blocking emission from the central engine. To ac-
count for any biases this may introduce, we ex-
cluded all unresolved hosts from our samples and
repeated our analysis. When considering only ex-
tended hosts, we find that the disk fraction of the
three subsamples is in much better agreement, al-
though the heavily obscured AGN are still found
in disk hosts more often than their unobscured
counterparts (68.7+4.0
−4.6% versus 55.1+3.6
−3.8%). Fur-
thermore, the correlation between merger frac-
tion and obscuration is still evident when exclud-
ing point sources, however at a reduced statis-
tical significance. The Merger/Interaction frac-
tion increases from 12.5+2.9
−2.1% to 22.6+4.3
−3.4% for
the unobscured and heavily obscured samples, re-
spectively; a difference that is now significant at
the 2.5σlevel. A similar trend is found for the
Disturbed/Asymmetric fraction, which increases
from 33.5+3.7
−3.4% among the unobscured AGN to
45.2+4.7
−4.5% for the Compton-thick sample. Here the
statistical significance is 2.3σ.
The fact that we observe a different disk/spheroid frac-
tion versus obscuration indicates that viewing angle can-
not be the only thing differentiating our three AGN sam-
ples, as a simple unification model would suggest. The
increased fraction of disturbed morphologies with obscu-
ration would appear to support an evolutionary scenario,
in which Compton-thick AGN are a distinct phase where
the central SMBH undergoes rapid, obscured growth fol-
lowing a merger/interaction event. Given that our heav-
ily obscured AGN are hosted by disk-dominated galax-
ies, we propose that we are catching these systems near
the start of this evolutionary sequence, before their disk
structure is destroyed. Our findings also suggest that
some of the merger-triggered SMBH growth that is pre-
dicted by AGN fueling models may be hidden among
heavily obscured, Compton-thick AGN, as previous stud-
ies of dust-reddened quasars have proposed. That said,
a larger sample of distant, obscured AGN will need to
be studied in order to confirm our findings with greater
statistical confidence, especially among extended AGN
hosts. This will soon be possible as a result of the UDS
Chandra Legacy Survey, which will allow us to extend
this work to z∼2.
12 Kocevski et al.
Support for Program number HST-GO-12060 was pro-
vided by NASA through a grant from the Space Tele-
scope Science Institute, which is operated by the Asso-
ciation of Universities for Research in Astronomy, Incor-
porated, under NASA contract NAS5-26555.
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