Submitted to The Astrophysical Journal Letters
Preprint typeset using LATEX style emulateapj v. 02/07/07

DIRECT EVIDENCE FOR ENHANCED AGN ACTIVITY IN ZCOSMOS GALAXY PAIRS
J. D. Silverman 1,2 , P. Kampczyk2 , S. Lilly2 , M. Elvis3 , F. Civano3 , V. Mainieri4 , C. Vignali 5 , G. Zamorani6 ,P. Nair6 , O. Le Fèvre7 , L. de Ravel8 , K. Jahnke9 , S. Bardelli6 , A. Bongiorno10 , M. Bolzonella6 , M. Brusa10 ,N. Cappelluti6 , A. Cappi6 , K. Caputi11 , C. M. Carollo2 , T. Contini12,13, G. Coppa6 , O. Cucciati7 , S. de la
Torre7 , P. Franzetti14 , B. Garilli14 , C. Halliday15 , G. Hasinger16 , A. Iovino17 , C. Knobel2 , K. Kovač2 ,F. Lamareille7 , J. -F. Le Borgne12,13 , V. Le Brun7 , C. Maier2 , M. Mignoli6 , R. Pello12,13 , E. Perez
Montero12,13 , E. Ricciardelli18 , Y. Peng2 , M. Scodeggio17 , M. Tanaka1 , L. Tasca17 , L. Tresse7 , D. Vergani6 ,E. Zucca6 , R. Andrae9 , A. Comastri6 , A. Finoguenov10 , H. Fu19 , R. Gilli6 , H. Hao3 , L. Ho20 , M. Salvato14
Submitted to The Astrophysical Journal Letters

ABSTRACT
Mergers of gas-rich galaxies are expected to play a significant role in the growth of supermassive
black holes as shown by numerical simulations. To test this scenario, we construct a sample of 563
galaxies (M∗ > 2.5 × 1010 M⊙ ) in close kinematic pairs over the redshift range 0.25 < z < 1.05
that are more likely to be in an early stage of a merger than a well-matched control sample of 2733
galaxies both from the zCOSMOS 20k spectroscopic catalog. Galaxies that harbor an active galactic
nucleus (AGN) are identified on the basis of their X-ray emission detected by Chandra. We find thatthe incidence of AGNs in galaxies with a close neighbor is higher than those without and the levelof activity increases with decreasing separation between the galaxies. Our findings are the result ofan enhancement of AGN activity by a factor of 2.1 (with a significance of 3.3σ) for pairs of physicalseparation less than 75 kpc (h = 0.7) and line-of-sight velocity offset less than 500 km s−1 , relative
to isolated galaxies of similar stellar mass. Our study provides observational evidence that galaxy
mergers are likely to induce gas inflow to the nuclear region that subsequently fuels a supermassive
black hole, and mergers account for about 22% of all black hole growth during these early encounters.
Subject headings: galaxies: active — galaxies: interactions — quasars: general — X-rays: galaxies
1. INTRODUCTION

There has been a long-standing question in astrophysics, ever since quasars and AGN were firmly be1 Institute for the Physics and Mathematics of the Universe(IPMU), University of Tokyo, Kashiwanoha 5-1-5, Kashiwa-shi,
Chiba 277-8568, Japan
2 Institute of Astronomy, ETH Zürich, CH-8093, Zürich,
Switzerland.
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA, 02138
4 Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, I-40127, Bologna, Italy
5 European Southern Observatory, Karl-Schwarzschild-Strasse 2,
Garching, D-85748, Germany
6 INAF Osservatorio Astronomico di Bologna, via Ranzani 1, I40127, Bologna, Italy
7 Laboratoire d’Astrophysique de Marseille, Marseille, France
8 Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH93HJ, UK
9 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117
Heidelberg, Germany
10 Max-Planck-Institut für extraterrestrische Physik, D-84571
Garching, Germany
11 SUPA, Institute for Astronomy, The University of Edinburgh,
Royal Observatory, Edinburgh EH9 3HJ, UK
12 IRAP, Universit de Toulouse, UPS-OMP, Toulouse, France
13 CNRS, IRAP, 14, avenue Edouard Belin, F-31400 Toulouse,
France14 INAF - IASF Milano, 20133 Milan, Italy
15 INAF Osservatorio Astronomico di Arcetri, 50125 Firenze,
Italy
16 Max-Planck-Institut fuer Plasmaphysik, Boltzmannstrasse 2,85748 Garching bei Muenchen, Germany
17 INAF Osservatorio Astronomico di Brera, Milan, Italy
18 Dipartimento di Astronomia, Universita di Padova, Padova,
Italy
19 California Institute of Technology, Pasadena, CA, USA.
20 The Observatories of the Carnegie Institution for Science, 813
Santa Barbara Street, Pasadena, CA 91101, USA

lieved to be powered by accretion (Salpeter 1964) onto
supermassive black holes (SMBH); what physical mechanism(s) is (are) responsible for the loss of angular momentum of gas that is initially rotationally-supported
and that can then be available to fuel a central SMBH
(e.g., Lynden-Bell & Rees 1971)? There have been anumber of proposed mechanisms that have been investigated, over the past few decades, such as disk instabilities, galaxy mergers, or supernova-driven winds, just to
name a few. More recently, the merger scenario has beena leading contender since numerical simulations demonstrate that such events (Mihos & Hernquist 1996) cangenerate large mass inflow rates to the nuclear region
thus potentially fueling both AGNs and central starbursts (e.g., Hopkins et al. 2008).
In support of the merger scenario, the by-products ofmajor mergers of gas-rich galaxies, the ultra-luminous infrared galaxies (ULIRGs), have been known for decades
to be conducive to black hole growth (Sanders & Mirabel1996). ULIRGS have been shown to have high nuclear
concentrations of gas (Scoville et al. 1989) that likely provide an ample fuel reservoir for accretion onto a SMBH.
Indeed, the incidence of AGN in ULIRGs (e.g., Kartaltepe et al. 2010) rises substantially with IR luminosity
to an unprecedented level (> 50%) for the brightest thatreach bolometric luminosities equivalent to quasars.
There has been much effort (e.g., Bahcall et al. 1997;
Canalizo & Stockton 2001) over the past decade to determine whether galaxy mergers play a role in fuelingquasars by carrying out imaging campaigns of their hostgalaxies with HST to look for signs of morphological disturbances. Such efforts are challenging because of theglaring light of a nuclear point source and the lack of2Silverman et al.

adequate control samples. Fortunately, new samples of
AGNs have been constructed that appear to alleviate
some of these difficulties. In particular, X-ray surveys
are identifying an obscured population that makes it
easier to discern the morphology of their host. Largearea surveys, with HST coverage and multi-wavelength
support such as COSMOS (Scoville et al. 2007), are enabling the construction of adequate control samples. To
date, studies based on X-ray selected AGN have not yet
demonstrated the role of major mergers of galaxies as
triggering AGN activity (Grogin et al. 2005; Gabor etal. 2009; Cisternas et al. 2010), even though remarkable
examples of a binary AGN have been found in the COSMOS field (Comerford et al. 2009; Civano et al. 2010) andelsewhere (e.g., Green et al. 2010; Shen et al. 2010; Fu etal. 2010). One explanation is that deep surveys mainly
detect moderate-luminosity AGNs (i.e., “Seyfert” population) thus the fueling mechanisms may differ from themore luminous quasars (Hopkins & Hernquist 2009).
We present an alternative test of the merger hypothesis with respect to moderate-luminosity AGN activity
that does not consider host galaxy morphology. We instead identify galaxies in the zCOSMOS spectroscopic
survey that are in kinematic pairs. These are two galaxies within a given projected separation and line-of-sightvelocity difference that likely indicates an early stage ina galaxy merger. We determine the frequency that thesegalaxies host an AGN based on X-ray emission detectedby Chandra and compare to a well-defined control sample. We highlight that we are investigating the impact ofinteractions on scales between ∼ 10 − 150 kpc thus ourstudy will provide constraints on merger-driven models of
black hole growth before a coalescence phase. Throughout this work, we assume H0 = 70 km s−1 Mpc−1 ,ΩΛ = 0.75, ΩM = 0.25, and AB magnitudes.
2. ZCOSMOS 20K GALAXY REDSHIFT SURVEYzCOSMOS (Lilly et al. 2007, 2009) is a ESO programto acquire spectroscopic redshifts for a large sample ofgalaxies in the COSMOS field (Scoville et al. 2007) withVIMOS on the VLT. A ’bright’ sample of 20,000 galaxies(i < 22.5) is observed with a red grism to provide a wavelength coverage of 5500 − 9500 Å ideal for identifying L∗
galaxies up to z ∼ 1.2. We use this catalog to construct
a well-defined sample of galaxies. Specifically, we identify 15,807 galaxies with iACS ≤ 22.5 and spectroscopic
redshifts between 0.25 < z < 1.05, each having a quality
flag (≥ 1.5) that amounts to a confidence of ∼ 99% in
the redshift measurements for the overall sample. We do
not include any serendipitous objects that happen to fallwithin a slit. Full details on data acquisition, reduction,and redshift measurements can be found in Lilly et al.
(2007, 2009).
We further isolate 10,964 zCOSMOS galaxies that fallwithin the Chandra survey area (see below for details,
Elvis et al. 2009) since we use the X-rays from this data
set to identify those hosting an AGN. Of these, there
are 3481 with a stellar mass above M∗ > 2.5 × 1010 M⊙
and 0.25 < z < 1.05. These limits are chosen to ensurea fairly complete representation of both blue and red
galaxies over the given range in redshift (see Silverman
et al. 2009b). Derived properties such as stellar mass
and rest-frame color (Figure 1) are determined for eachgalaxy as detailed in Bolzonella et al. (2010) and Pozzetti

Fig. 1.— Rest-frame color U − V versus stellar mass for zCOSMOS galaxies with 0.25 < z < 1.05 (small circles). Galaxies in kinematic pairs are shown in black while all others aremarked in grey. The AGNs are further marked with colored circles(filled=kinematic pair; open=all others). The vertical line marks
our chosen mass limit.

et al. (2010).
2.1. Kinematic pairs
A major focus of the zCOSMOS survey is the studyof galaxies in close kinematic pairs as a laboratory to
determine the redshift evolution of the galaxy merger
rate (de Ravel et al. 2011) and the physical propertiesof the galaxies themselves (Kampczyk et al. 2011). We
use the zCOSMOS 20k catalog to identify those galaxies
with a nearby neighbor. Galaxies in kinematic pairs are
specifically selected to have a projected separation dr <100 kpc h−1 (143 kpc for our chosen h = 0.7) and a lineof-sight velocity difference dV < 500 km s−1 (Kampczyk
et al. 2011). Based on this selection, we identify 753
galaxies, out of the 3481 (selected by stellar mass andredshift) that are associated with a galaxy pair. Of these,
there are 330 with a mass ratio less than 3 to 1, while
563 have a mass ratio between 10 and 1. In Figure 1, weshow the stellar mass and rest-frame color of the galaxies
in kinematic pairs.
3. CHAN DRA X-RAY OBSERVATIONS AND AGNIDENTIFICATION
We match our zCOSMOS galaxy sample to the catalog of Chandra X-ray sources (Elvis et al. 2009; Puccetti
et al. 2009) using a maximum-likelihood routine as detailed in Civano et al. (2011). The Chandra observations
cover the central 0.9 sq. deg and result in the detectionof 1761 point sources above a flux level of 5.7 × 10−16
erg cm−2 s−1 in the 0.5-10 keV band. The tiling scheme
not only provides a fairly uniform sensitivity but an optimal PSF across the field. We highlight that the resolving power of Chandra is important with respect toour study since galaxy pairs have been found to residein regions of heightened galaxy overdensities (Lin et al.

AGNs in zCOSMOS galaxy pairs
2010; de Ravel et al. 2011; Kampczyk et al. 2011) where
extended X-ray emission, if present, may hamper thedetection of point sources. The Chandra observations
over the C-COSMOS region are sensitive to AGNs with
L0.5−10 keV > 3 × 1042 ergs s−1 out to z ∼ 1.
We identify 337 zCOSMOS galaxies with X-ray
emission that primarily falls within the range
1042 .L0.5−10 keV . 1044 erg s−1 . In terms of Eddington ratio, we are sensitive to SMBHs having
10−3 . LBol /LEdd . 10−1 given the stellar massesof their hosts, an assumption of their bulge-to-total
mass ratio (B/T ∼ 0.5), and the local MBH − Mbulge
relation. The majority of AGNs have optical emission
dominated by their host galaxies and lack broad opticalemission lines, both highlighting the importance of
X-ray selection to account for the moderately-obscured
population at these redshifts (Mainieri et al. 2007;
Brusa et al. 2010). It is also possible, although beyond
the scope of this paper, to determine the prevalenceof heavily-obscured or Compton-thick AGN, missed in
X-ray surveys, within the galaxy pair population thatcan only be identified through optical-line emission (e.g.,
Bongiorno et al. 2010; Yan et al. 2010) or excess infraredemission (e.g., Fiore et al. 2009).
We compare the properties of our AGN hosts to theunderlying galaxy population by selecting those with
1042.0 .L0.5−10 keV . 1043.7 erg s−1 , a range that likely
results in a purely AGN-dominated sample, in termsof X-ray emission, and avoids the inclusion of more
optically-luminous AGNs that can impact the derived
rest-frame colors and stellar mass estimates (e.g., Silverman et al. 2008). In Figure 1, we see that their hostgalaxies are massive (M∗ > 2.5 × 1010 M⊙ ) and residual
star formation may be present as evident by the slightly
bluer colors (< U − V >= 1.6) of AGN hosts as compared to non-active galaxies (< U − V >= 1.7) of similarstellar mass (e.g., Silverman et al. 2009b; Schawinski etal. 2010; Xue et al. 2010). This is in agreement with
Kampczyk et al. (2011) who find that galaxies in zCOSMOS kinematic pairs have elevated star formation rates,
heightened post-starburst signatures and an increase in
the numbers of galaxies with irregular morphologies.
4. RESULTS

We first highlight some examples of AGNs associatedwith galaxies in close kinematic pairs that illustrate thediversity in their optical properties and environments.
In Figure 2, we show Chandra and HST/ACS (F814W)
images for three cases. In the top panel, a system ofthree galaxies at z ∼ 0.659 (right) is identified to be
kinematically linked. An X-ray source (lef t) is clearly
associated with one of the galaxies undergoing a merger,
as is evident from the low-level optical emission linking
it to a neighbor (towards the south-east) about 20 kpc
away. In addition to a point-like hard X-ray source, diffuse emission is present, as seen in the 2-7 keV image;the extended emission is more prominent in the soft energy band as expected (not shown). In Fig. 2 (middle),
two spiral galaxies separated by ∼ 15 kpc are likely inthe process of merging and one of them hosts a luminous
AGN while the other is a post-starburst galaxy based on
spectral features (i.e., strong Balmer absorption series)
evident in a zCOSMOS optical spectrum (not shown).
A third example (bottom) clearly shows a barred spiral3

Fig. 2.— AGNs in close kinematic pairs: T op An AGN(CID=450) at z = 0.658 is part of a system of three galaxies
within the central potential of an X-ray emitting galaxy group.
The redshifts of the individual galaxies are labelled. On the left,
the Chandra hard-band (2-7 keV) image clearly shows the detection of an X-ray point source. On the right, the HST/ACS i′ image
is displayed. M iddle An AGN (CID=1711) is associated with apair of interacting, spiral galaxies at z = 0.77. Bottom A system
at z = 0.371 with a barred spiral hosting an AGN (CID=3083). In
all panels, the optical positions of the zCOSMOS galaxies in pairs
are marked with a small cross (red=AGN), and north is up while
east is to the left.

galaxy with an AGN and a companion 24 kpc away.
Our objective is to measure the fraction of galaxies
that host AGN as a function of whether they are associated with a kinematic pair or not to assess whether earlyencounters between galaxies induce such nuclear activity.
We specifically determine whether a zCOSMOS galaxy
with M∗ > 2.5 × 1010 M⊙ is identified with an X-ray
detection by Chandra. We allow a galaxy to be associated with a kinematic pair if it has a neighbor satisfying the criteria, given above, on the projected separationand line-of-sight velocity difference. We further restrict
the sample to pairs with a mass ratio less than 10 to 1
that allows for a companion to have a mass below ourthreshold. This mass ratio is chosen to isolate a sample
for which strong gravitational interactions are capable ofdestabilizing gas that can subsequently fuel an AGN.
We follow the technique discussed in Silverman et al.
(2009a,b) to determine the AGN fraction of our parent
population of galaxies. This method accounts for the
spatially-varying sensitivity limits of the Chandra observations over the COSMOS field (see Figure 4 of Elvis etal. 2009). The total fraction is the sum of all individual
AGN weighted by the effective number of galaxies ca-

4Silverman et al.
TABLE 1
Sample statistics-kinematic pair analysis
Type

Redshift

dr
kpc

dv
km s−1# of AGN(galaxies)

AGN
fraction (%)

Pairs
ControlPairs

0.25-1.05
0.25-1.05
0.25-0.65
0.65-1.05
0.25-0.65
0.65-1.05
0.25-1.05

< 75
>143
< 75
< 75
> 143
> 143
< 143

<500
>500
< 500
< 500
> 500
> 500
< 500

21 (217)120 (2733)
6a (89)14a (128)24a (907)
93a (1826)
40 (563)

11.20 ± 2.25
5.32 ± 0.44
6.51 ± 2.63
11.34 ± 2.89
2.63 ± 0.54
6.08 ± 0.57
8.16 ± 1.20

ControlPairsalog L0.5−10 keV >42.5 for the high redshift bin and log
L0.5−10 keV >42.2 for the low redshift bin.

pable of detecting the respective AGN of a given X-ray
luminosity. The procedure is fully described in Silverman et al. (2009a,b) with the appropriate equations. We
remark that the effective number of galaxies, in manycases, falls below the total number of galaxies reported
in Table 1. The sampling rates of the random galaxies and AGN are incorporated in this estimate, as anadditional weight, since these differ by a factor of 1.74,
because 29% of the Chandra sources are designated as’compulsory targets’ when designing masks for VIMOS.
Therefore, the AGN fraction is not a simple division of
two numbers. We estimate the associated 1σ error using
binomial statistics.
We then compare the fraction of galaxies, in pairs, thathost an AGN to those, not in pairs, over the redshift
range 0.25 < z < 1.05 (Table 1). We define an AGN
here as an X-ray point source with log L0.5−10 keV > 42.2
(units of erg s−1 ), unless otherwise noted; the inclusion of a small number (13%) of more luminous AGNs
(Log LX ∼ 43.7 − 44.5), that may have less accurate
stellar mass estimates, has no impact on our results. As
motivated by the analysis given below, we limit the sample of galaxies in pairs to those having a close neighborthat satisfies the criteria that dr < 75 kpc and dv < 500
km s−1 . We find that over the entire redshift range theAGN fraction for the control sample is 5.3 ± 0.4%, while
the pair sample has a higher AGN fraction of 11.2±2.2%.
We estimate the significance of the difference in the AGN
fraction (∆ = fA − fB ) by numerically integrating the
product of the beta distribution (likelihood) of fA andfB shown in the top and middle panels of Figure 3. Confidence levels (68%, 99%) are estimated by numericallyintegrating the likelihood of ∆. We find that the enhancement (2.1×) is significant at the 3.3σ level. In
Figure 3 (bottom panel), we show the AGN fraction ofgalaxies in kinematic pairs and the control sample asmeasured in two redshift bins of equivalent width. We
set the minimum X-ray luminosity to roughly the meansensitivity limit for each redshift bin (0.25 < z < 0.65:
0.65 < z < 1.05: log LX > 42.5). We find that thefraction of galaxies in pairs hosting an AGN is higher inboth redshift intervals even when implementing a constant luminosity cut (i.e., log LX > 42.5) over the full
redshift range. The increase in the AGN fraction with
redshift is as expected based on the overall evolution of
the luminosity function of AGN.
We expect that AGN activity is more pronounced as
two galaxies approach each other with smaller physicalseparations based on simulations of galaxy mergers. The

Fig. 3.— AGN fraction of galaxies in close kinematic pairs (dr <75 kpc and dv < 500 km s−1 ) as compared to galaxies with noneighbor within a projected separation of 143 kpc and a velocity
offset less than 500 km s−1 . The beta distribution (likelihood) of
the AGN fraction in pairs (top panel; fA ), not in pairs (top panel;fB ), and the the difference (∆ = fA − fB ; middle panel) is shown.
bottom AGN fraction of galaxies in pairs is given by the filled black
circles while the open circle denotes galaxies not in pairs for two
redshift intervals. The horizontal bars indicate the redshift range
for each value while the vertical bars are the 1σ error.

first signs of accretion, such as those of an AGN, are predicted (Hopkins et al. 2008) to be evident after the firstpassage of the interacting galaxies with a physical separation of ∼ 30 − 50 kpc and about ∼1 Gyr prior to thefinal coalescence of both galactic nuclei. Conclusive evidence for such an enhancement of moderate-luminosity
AGN activity has been elusive in SDSS samples at lower
redshifts (e.g., Li et al. 2008; Ellison et al. 2008).
To search for this behavior in the zCOSMOS pair sample, we measure the AGN fraction while separating thesample into bins of projected separation (dr) and lineof-sight velocity difference (dv). In Figure 4, we compare the results of this exercise with the AGN fractionof galaxies not in pairs that acts as our control sample.
Two sets of data points in panel a are shown only to illustrate the results and the associated errors for different
bin sizes; in no way are the two sets of data independent

AGNs in zCOSMOS galaxy pairs

5Fig. 4.— Fraction of galaxies hosting an AGN shown as a function of both the projected physical separation (a) and line-of-sight velocity
difference (b). The open circles in panel a are the measurements based on a finer binning in separation. The right-most data point in bothpanels gives the AGN fraction of galaxies not in pairs with the horizontal lines showing the 1σ error and extended along the abscissa forvisual comparison.

of each other. We find that the AGN fraction of galaxies
is highest on the smallest scales, in terms of both projected separation (dr . 75 kpc) and velocity (dv . 500
km s−1 ) difference, and rises with both decreasing dr
and dv. The trend is stronger when considering the projected physical separation with the velocity separation.
It is clear that the highest incidence (11.2%) of AGN
activity occurs in galaxies in kinematic pairs with projected physical separation less than 75 kpc. This physicalscale is in remarkable agreement with clustering studies(Serber et al. 2006) of SDSS quasars and L⋆ galaxies at
z < 0.4 that show that quasars reside within regions ofgalaxy overdensity on scales of less than 100 kpc.
We recognize that these observed trends may be driven
by the dependence of AGN activity on stellar mass andnot the close proximity of a neighboring galaxy. To address this, we performed a K-S test to determine whether
the stellar-mass distribution of the pairs and the control
sample differ in any of the bins (i.e., redshift, dr, dv)
implemented in this study. We find that there is no noticeable difference that can be responsible for the resultspresented herein.
5. SUMMARY AND CONCLUSIONS

We have performed a simple test of the merger scenario for triggering moderate-luminosity AGNs. To doso, we have utilized a sample of kinematic pairs identified
from the zCOSMOS 20k ’bright’ catalog and Chandra
observations that indicate those harboring AGNs. The
X-ray selection of AGN enables us to include those that
may be cloaked in star formation that can hamper optical selection. Based on these multi-wavelength observations, we have found that (1) galaxies (0.25 < z < 1.05;
M∗ > 2.5 × 1010 M⊙ ) in kinematic pairs, with physical separations less than 75 kpc and line-of-sight velocity difference less than 500 km s−1 , exhibit heightenedlevels of AGN activity (11.1%) relative to galaxies notwithin these associations (5.3%) by a factor of 2.1 (witha significance of 3.3σ), and (2) the incidence of AGNs
in pairs increases with decreasing projected separationand velocity difference. These results demonstrate thatgalaxy mergers are conducive environments for AGNs,
even those of moderate luminosity (LX ∼ 1043 erg s−1 ).
Our measurement of the fraction of galaxies hosting
AGN, either associated with kinematic pairs or not, canbe used to determine the relative importance of galaxy
mergers, as compared to other secular (i.e., internal)
mechanisms, for black hole growth. We need to consider
that zCOSMOS samples ∼ 50% of all galaxies down to
iACS = 22.5 that results in a completeness of 25% (Kampczyk et al. 2011) with respect to the identification of
all pairs to this limiting magnitude falling within the
Chandra footprint. While we observe 25% (40/160) of
AGNs to be in kinematic pairs, this fraction rises to 48%
after accounting for the efficiency of pair identification.
The true impact of mergers on AGN activity requiresthat we remove the contribution of non-merger activity
within the kinematic pair sample (i.e., background signal). Therefore, we estimate an excess AGN fraction
fx = (fA − fB )/(fA + NB × fB /NA ), where fA (8.2%)
and fB (4.5%) are the AGN fractions in each sample fora given number of galaxies (NA = 1126, NB = 2170)
in the zCOSMOS 20k catalog. We find fx = 22% that
represents the contribution of early stage encounters toblack hole growth. The overall importance of mergers
is likely to be higher since those in a late stage or coalescence phase have not been included in our analysis.
Indeed, a visual inspection of the HST images of AGNs in
our control sample reveals a non-negligible number of interacting or disturbed systems. We conclude that galaxy
mergers, even at early stages, are a vital mechanism forwhich SMBHs grow. Finally, we note that Chandra coverage of the full COSMOS area is imperative to tie together the formation of bulges and their black holes inan environmental context.

6

Silverman et al.

We thank Charles Steinhardt and Yen-Ting Lin for
useful discussions. This work was supported by World
Premier International Research Center Initiative (WPI

Initiative), MEXT, Japan.
Facilities: CXO(ACIS), VLT:Melipal (VIMOS).

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