To appear in The Astrophysical Journal Supplement Series
Preprint typeset using LATEX style emulateapj v. 02/07/07

arXiv:submit/0106303 [astro-ph.CO] 10 Sep 2010

THE EXTENDED CHAN DRA DEEP FIELD-SOUTH SURVEY: OPTICAL SPECTROSCOPY OF FAINT
X-RAY SOURCES WITH THE VLT∗ AND KECK†
J. D. Silverman 1,2,3 , V. Mainieri4 , M. Salvato5 , G. Hasinger5 , J. Bergeron6 , P. Capak7 , G. Szokoly8 , A.
Finoguenov2 , R. Gilli9 , P. Rosati4 , P. Tozzi10 , C. Vignali11 , D. M. Alexander12, W. N. Brandt13 , B. D.
Lehmer14,15, B. Luo13 , D. Rafferty13 , Y. Q. Xue13 , I. Balestra2 , F. E. Bauer16,17 , M. Brusa2 , A. Comastri9 , J.
Kartaltepe18 , A. M. Koekemoer19 , T. Miyaji20, D. P. Schneider13, E. Treister18 , L. Wisotski21 , M. Schramm22
To appear in The Astrophysical Journal Supplement Series

ABSTRACT
We present the results of a program to acquire high-quality optical spectra of X-ray sources detectedin the Extended-Chandra Deep Field - South (E-CDF-S) and its central 2 Ms area. New spectroscopic
redshifts, up to z = 4, are measured for 283 counterparts to Chandra sources with deep exposures(t ∼ 2 − 9 hr per pointing) using multi-slit facilities on both the VLT (VIMOS) and Keck (DEIMOS)
thus bringing the total number of spectroscopically-identified X-ray sources to over 500 in this survey
field. Since our new spectroscopic identifications are mainly associated with X-ray sources in the shallower 250 ks coverage, we provide a comprehensive catalog of X-ray sources detected in the E-CDF-S
including the optical and near-infrared counterparts, determined by a likelihood routine, and redshifts(both spectroscopic and photometric), that incorporate published spectroscopic catalogs, thus resulting in a final sample with a high fraction (80%) of X-ray sources having secure identifications. We
demonstrate the remarkable coverage of the luminosity-redshift plane now accessible from our data
while emphasizing the detection of AGNs that contribute to the faint end of the luminosity function(L0.5−8 keV ∼ 1043−44 erg s−1 ) at 1.5 . z . 3 including those with and without broad emission lines.
Our redshift catalog includes 17 type 2 QSOs at 1 . z . 3.5 that significantly increases such samples (2×). Based on our deepest (9hr) VLT/VIMOS observation, we identify “elusive” optically-faintgalaxies (Rmag ∼ 25) at z ∼ 2 − 3 based upon the detection of interstellar absorption lines (e.g.,
OII+SiIV, CII], CIV); we highlight one such case, an absorption-line galaxy at z = 3.208 having no
obvious signs of an AGN in its optical spectrum. In addition, we determine accurate distances to eightgalaxy groups with extended X-ray emission detected both by Chandra and XMM-N ewton. Finally,
we measure the physical extent of known large-scale structures (z ∼ 0.7) evident in the CDF-S. Whilea thick sheet (radial size of 67.7 Mpc) at z ∼ 0.67 extends over the full field, the z ∼ 0.73 structureis thin (18.8 Mpc) and filamentary as traced by both AGNs and galaxy groups. In the appendix, we
provide spectroscopic redshifts for 49 counterparts to fainter X-ray sources detected only in the 1 and2 Ms catalogs, and 48 V LA radio sources not detected in X-rays.
Subject headings:∗ BASEDON OBSERVATIONS MADE WITH ESO TELESCOPES AT PARANAL OBSERVATORIES UNDER PROGRAMME IDS 072.A-0139 AND 080.A-0411.
† “SOME OF THE DATA PRESENTED HEREIN WERE OBTAINED AT THE W.M. KECK OBSERVATORY, WHICH IS
OPERATED AS A SCIENTIFIC PARTNERSHIP AMONG THE
CALIFORNIA INSTITUTE OF TECHNOLOGY, THE UNIVERSITY OF CALIFORNIA AND THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION. THE OBSERVATORYWAS MADE POSSIBLE BY THE GENEROUS FINANCIALSUPPORT OF THE W.M. KECK FOUNDATION.”
1 Institute of Astronomy, ETH Zürich, CH-8093, Zürich,
Switzerland.
2 Max-Planck-Institut für extraterrestrische Physik, D-84571
Garching, Germany
3 Current address: Institute for the Physics and Mathematicsof the Universe (IPMU), University of Tokyo, Kashiwanoha 5-1-5,
Kashiwa-shi, Chiba 277-8568, Japan
4 European Southern Observatory, Karl-Schwarzschild-Strasse 2,
Garching, D-85748, Germany
5 Max-Planck-Institute for Plasma Physics, Boltzmannstrass 2,
Garching, D-85748, Germany
6 Institut d’Astrophysique de Paris, 98bis Boulevard, F-75014
Paris, France
7 California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125
8 Eötvös University, Institute of Physics, 1117 Budapest,
Pázmány P. s. 1/A, Hungary
9 Instituto Nazionale di Astrofisica (INAF) - Osservatorio Astro-

nomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
10 INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo
11, 34131 Trieste, Italy
11 Dipartimento di Astronomia, Universitá degli Studi diBologna, Via Ranzani 1, 40127 Bologna, Italy
12 Department of Physics, University of Durham, South Road,
Durham, DH1 3LE, UK
13 Department of Astronomy & Astrophysics, 525 Davey Lab,The Pennsylvania State University, University Park, PA 16802,
USA
14 The Johns Hopkins University, Homewood Campus, Baltimore, MD, 21218, USA
15 NASA Goddard Space Flight Centre, Code 662, Greenbelt,
MD, 20771, USA
16 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301
17 Pontificia Universidad Católica de Chile, Departamento de
Astronomı́a y Astrofı́sica, Casilla 306, Santiago 22, Chile
18 Institute of Astronomy, University of Hawaii, 2680 Woodlawn
Drive, Honolulu, HI, 96822
19 Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA
20 Instituto de Astronomia, Universidad Nacional Autonoma de
Mexico-Ensenada, Km 103 Carretera Tijuana-Ensenada, BC 22860
21 Astrophysikalisches Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany
22 Department of Astronomy, Kyoto University, Kyoto 606-8502,
Japan

2

Silverman et al.

1. INTRODUCTION
It has become clear that the remarkable capability of
the Chandra observatory, as demonstrated with the observations of the Chandra Deep Field - North (Brandt
et al. 2001; Alexander et al. 2003) and South (Giacconi
et al. 2002; Luo et al. 2008), to detect faint X-ray sources
such as obscured AGN (e.g., Rosati et al. 2002; Barger et
al. 2003; Szokoly et al. 2004; Tozzi et al. 2006; Daddi et al.
2007), star-forming and normal galaxies (e.g., Alexander
et al. 2002; Hornschemeier et al. 2003; Bauer et al. 2004;
Lehmer et al. 2007, 2008) and extended group/cluster
emission (e.g., Bauer et al. 2002) out to cosmological redshifts must be further exploited (see Brandt & Hasinger
2005; Brandt & Alexander 2010, for a review). For instance, the predominance of moderate-luminosity AGNs
(LX ∼ 1043 erg s−1 ) at z ∼ 1 (e.g. Hasinger, Miyaji &Schmidt 2005; Barger et al. 2005) in these surveys may
be due to the emergence of young, rapidly growing supermassive black holes (e.g. Marconi et al. 2004; Merloni& Heinz 2008; Shankar et al. 2009) or the tail end (e.g.,
Babić et al. 2007) of an earlier period of rapid growth.
The spatial clustering of these low-to-moderate luminosity AGNs (Gilli et al. 2005) indicate where SMBHs reside in the context of the large-scale mass distribution.
To constrain evolutionary models of SMBHs, it is imperative to detect these moderate-luminosity AGNs insignificant numbers over a cosmic time span similar tothat covered by luminous quasars (1 . z . 3) and onlarger spatial scales (> 10 Mpc) than has yet to be attained due to the limited area coverage of the deep fields.
The ‘Extended Chandra Deep Field-South’ (E-CDF-S;
Lehmer et al. 2005), COSMOS (Elvis et al. 2009; Puccetti et al. 2009), and the ’Extended Groth Strip’ (EGS;
Nandra et al. 2005; Laird et al. 2009) surveys have been
observed by Chandra for this purpose.
The E-CDF-S survey has completed a 1 Ms Chandra
Very Large Program that covers a wide area (0.33 deg2 ;3× area of the CDF-S) at depths reaching ∼ 1 × 10−16
and ∼ 7 × 10−16 ergs cm−2 s−1 for the 0.5–2.0 and 2–8
keV bands, respectively. This survey adds four Chandra
pointings (ACIS-I; texposure ∼250 ksec each), each flanking the deeper central CDF-S (Giacconi et al. 2002; Luoet al. 2008), that have generated a catalog of 762 X-ray
sources (Lehmer et al. 2005) with 559 falling outside the 2
Ms area (Luo et al. 2008). This combination of depth andarea is optimized to mitigate cosmic variance (Yang et al.
2005; Gilli et al. 2005) and detect moderate-luminosity(i.e., Seyfert-type) AGNs beyond z∼1.5 that can only beadequately accounted for by X-ray selection; severe hostgalaxy dilution in the optical and strong star formationin the IR hampers standard color selection techniques(e.g., Donley et al. 2007). As a bonus, wider area X-ray
surveys have the potential to detect rare and interesting sources in the distant universe such as double-peaked
broad emission-line quasars (Luo et al. 2009).
Deeper X-ray data have recently been obtained in thecentral CDF-S regions by Chandra and XMM-N ewton.
The 2 Ms Chandra observation of the CDF-S (Luo et al.
2008) has brought the exposure to the equivalent depthof the CDF-N (Alexander et al. 2003) thus effectively improving upon the limited statistics of faint X-ray sources
and spectral characterization of already known brighter
sources. Recently, an additional 2 Ms has been allocatedto the field with director’s discretionary time to reachan unprecedented flux limit (f0.5−2 keV ∼ 1 × 10−17 erg
cm−2 s−1 ) for X-ray surveys. Furthermore, an ongoingprogram is close to reaching a depth of 3 Ms with XMMN ewton that aims to perform X-ray spectral analysis of alarge sample of obscured AGN, including a handful thatare Compton-thick.
Both the Chandra and XMM-N ewton programs complement the extensive multi-wavelength observations of
the CDF-S and its extended area. The optical imaging
includes coverage in the U BV RI from the ESO Deep
Public Survey (e.g., Arnouts et al. 2001) and the 17 filter COMBO-17 survey (Wolf et al. 2004, 2008) that provides accurate photometric redshifts (σz ≈ 0.03). Highresolution imaging is available from the HST Advanced
Camera for Surveys (ACS) observations via the GEMS
(Rix et al. 2004; Caldwell et al. 2008), GOODS (Giavalisco et al. 2004) and HST Ultra Deep field (UDF;
Beckwith et al. 2006) projects. Extensive ground-based,
near-infrared imaging campaigns (Olsen et al. 2006;
Grazian et al. 2006; Taylor et al. 2009) have been completed in the J, H and K bands. Deep Spitzer IRAC
(Damen et al. 2009) and MIPS observations have facilitated the study of the relationship between AGNs andtheir host galaxies (i.e., stellar mass, star formation activity). Deep radio observations at 20 cm wavelength
have been acquired with the Very Large Array (Kellermann et al. 2008; Miller et al. 2008; Mainieri et al. 2008).
Over 1000 spectroscopic redshifts are available for multiwavelength selected galaxies via the CDF-S (Szokoly et
al. 2004), VVDS (Le Fèvre et al. 2004), K20 (Mignoli etal. 2005), GOODS (e.g., Popesso et al. 2009; Balestra etal. 2010) and MUSYC (Treister et al. 2009a) surveys, although many of the Chandra sources lack spectroscopic
redshifts and/or are very faint thus photometric redshifts
may have considerable uncertainty.
While ground-based spectroscopic campaigns havecontributed significantly to many successful studies of
the X-ray source population, they have struggled to acquire spectra of a large fraction of the X-ray sourcesin the deep fields due to the faint optical magnitudes(Rmag & 24) of the counterparts. As evident in Figure 6 of Brandt & Hasinger (2005), this has impacted
the identification of low-to-moderate luminosity AGN atz > 1.5, whose optical emission is dominated by their
host galaxy, thus presenting a challenge equivalent tothe ’redshift desert’ encountered by high-redshift galaxy
surveys. Even though photometric techniques have been
demonstrated to be successful at these redshifts (Zheng
et al. 2004; Mainieri et al. 2005; Salvato et al. 2009; Luoet al. 2010), their errors (∆z∼ 0.3 at z ∼ 2; see Fig. 12
of Salvato et al. 2009) are substantial at faint magnitudes (24 . R . 26) thus limiting their use where precise distances are required (e.g., spatial correlation analysis). Fortunately, strategies to identify these galaxies
spectroscopically have been successfully employed with
8 − 10m class telescopes (Steidel et al. 2004; Lilly et al.
2007; Popesso et al. 2009). These efforts require deep
spectroscopic observations with wavelength coverage below 5000 Å to identify faint AGN signatures (i.e., emission lines such as Lyα, CIV and CIII]) or absorption
features usually attributed to the interstellar medium inthe host galaxies. The interstellar absorption lines of
SiII, OI, OII+SiIV, CII, SiIV, CIV, FeII and AlII willOptical spectroscopy of X-ray sources
be available within the observable window (3800-6700Å)over z ∼ 1.5 − 3.0 (See Figure 2 of Shapley et al. 2003).
The detection of Lyα in emission or absorption is an important diagnostic at z > 2.
In this paper, we present results from our optical spectroscopic program of Chandra sources, mainly in the
E-CDF-S, but also including those lacking redshifts inthe central CDF-S, with the Very Large Telescope (VLT)
and Keck. We have measured redshifts for 248 X-ray
sources from the catalog of Lehmer et al. (2005) that
had been previously unidentified. In total, there are now
422 out of 762 X-ray sources that have a spectroscopic
redshift. We organize the paper as follows. In Section 2,
we provide specific details about the determination ofreliable optical and near-infrared counterparts to X-ray
sources in the E-CDF-S. Section 3 describes the processing and analysis of our VLT and Keck spectroscopic
observations including our redshift measurements andclassification scheme. In Section 4, we present a comprehensive catalog of E-CDF-S X-ray sources that havespectroscopic redshift measurements not only from ourown data but those previously reported by various programs. We include newly-derived photometric redshifts(Section 4.1), by our team (Rafferty et al. 2010) thatincorporate near-infrared photometry from Spitzer anddeeper ground-based imaging (e.g., Taylor et al. 2009),
for X-ray sources that lack a secure spectroscopic redshift. In Section 5, we discuss the X-ray and opticalproperties of the point sources and briefly describe thepopulations detected in these Chandra observations. In
Section 6, we use our compilation of AGNs to measure
the physical extent of the large-scale structures (z ∼ 0.7)
found in the CDF-S (Gilli et al. 2003) with better precision, and detect additional structures at higher redshifts. To explore the relationship between AGNs andgalaxy groups within these large-scale structures, we report on the identification of extended sources, primarily
X-ray emission associated with galaxy groups and clusters based on both the Chandra and XMM-N ewton observations of the E-CDF-S. Furthermore, we present in
the appendix additional spectroscopic redshifts for targets from our spectroscopic program including counterparts to 49 X-ray sources solely detected in the 1–2 Mscatalogs (Giacconi et al. 2002; Luo et al. 2008) and 48
V LA radio sources not detected with the current X-ray
data.
Throughout this work, we assume H◦ = 70 km s−1
Mpc−1 , ΩΛ = 0.7, and ΩM = 0.3. All magnitudes are
expressed in the AB system where RAB = RJohnson +0.055.
2. OPTICAL AND NEAR-INFRAREDIDENTIFICATION
We utilize published optical and near-infrared catalogs,
listed in Table 1, that cover the E-CDF-S to identify
counterparts to the 762 Chandra sources presented in
Lehmer et al. (2005). A higher priority is given to catalogs based on imaging with superior resolution and depth
at red optical wavelengths thus our weighting is set accordingly (HST/ACS i-band, WFI R-band, ISAAC Ks
SOFI Ks ). A likelihood-ratio technique (e.g., Sutherland& Saunders 1992; Rutledge et al. 2000; Ciliegi et al. 2003,
2005; Brusa et al. 2007) is implemented, since there canbe multiple possible counterparts within an X-ray error

3
TABLE 1
Public imaging catalogs

Name

Band

Reference

WFIGEMS
GOODS/ISAAC
EIS/SOFIRv,zJ, H, Ks
J, H, Ks

Arnouts et al. (2001)
Caldwell et al. (2008)
Retzlaff et al., in prep
Olsen et al. (2006)

circle mainly due to the faint (R ∼ 26) optical depths
required to identify the majority of the X-ray sources.
This method provides a quantitative measure of reliability for each counterpart essentially based on its angulardistance from the X-ray centroid (r) and its magnitude(m). Specifically, the likelihood ratio (LR; equation 1)
is the probability that a given optical or near-infrared
source is the true counterpart to the X-ray detection,
relative to the chance that the same object is a random
background source.
LR =q(m)f (r)
n(m)

(1)

The function q(m) describes the probability of a sourceof magnitude m being the likely counterpart; this probability distribution is generated by comparing the magnitude distribution of optical counterparts within a specified radius of the X-ray sources to the distribution of
background objects. Since we expect all true counterparts to be within a radius of 2′′ from their corresponding
X-ray centroids, the distribution function q(m) is determined by selecting only sources within this radius. The
probability distribution based on the radial offset f (r) is
modeled as a Gaussian function with a variance that depends both on the X-ray and optical uncertainties taken
in quadrature. We assume a fixed radial uncertainty inthe position of the optical/near-infrared counterparts tobe 0.5′′ (∼ 3σ). The likelihood ratio is normalized by
the surface density n(m) of objects clearly not associated with the X-ray counterparts (i.e., background objects). Further explicit details on the determination ofthese three distribution functions can be found in Ciliegi
et al. (2003). We further require the likelihood ratio LR
for each candidate to be above 0.2 and visually inspect
each case to generate a final catalog of high confidence
optical/near-infrared counterparts. The latter step enables accurate identification of counterparts in a few specific cases where there may be confusion between nearby
faint and bright optical candidates, crowded regions, orsources with complex optical morphologies.
We then measure the reliability (0 < Rj < 1; Sutherland & Saunders 1992) that a particular source (j) is thetrue counterpart.
LRjLR
i + (1 − Q)i

Rj = P(2)

The sum is over all counterparts (i) associated with asingle X-ray source. The probability that a counterpart
is above the magnitude limit of our optical/near-IR
catR mlim
alog is taken into consideration (Q= −∞q(m) dm).
Here, we fix Q= 0.8 that corresponds to the ratio of the
expected number of identifications (the integral of q(m))

4Silverman et al.

Fig. 1.— (a) Cumulative number of X-ray sources as a functionof the distance to their most likely optical/near-infrared counterpart. The vertical line marks the distance which includes 90%of the sources . (b) The distribution of the reliability index (R)for the likely counterparts. The value of R for which 90% of
the optical/near-infrared counterparts have a higher reliability isshown by the dashed line.

to the total number of X-ray sources. We note that our
results are insensitive to differences in Q within the rangeof 0.5-1.0.
Our final catalog contains counterparts to 677 X-ray
sources. We find that 90% of the counterparts have offsets less than 0.9′′ (Figure 1a) and have a reliability
parameter R > 0.68 (Fig 1b). Only 36% of the X-ray
sources have a near-infrared counterpart in the Ks band
lim
(KsAB
= 21.4; Olsen et al. 2006), thus highlighting thelimimportance of deeper observations (KsAB∼ 22; Taylor
et al. 2009) in this filter. For example, Brusa et al. (2010)
find counterparts to nearly 100% of the X-ray sourcesin the COSMOS field due to a deep KsAB band photometry (McCracken et al. 2010) coupled with the factthat the XMM-N ewton sources in the COSMOS field arebrighter in comparison to X-ray sources in the Chandra
Deep fields. We stress that our main objective in thepresent study is to provide optical spectroscopy of faint
optical counterparts and thus recent improvements withrespect to the depth of public near-infrared imaging is
not the focus of the present work. Highly complete catalogs of counterparts to X-ray sources based on deeper
near-infrared imaging can be found elsewhere such as
Cardamone et al. (2008) who find a higher detection rate(∼ 90%) when incorporating the Spitzer/IRAC observations of the E-CDF-S. Our final catalog is presented in
Section 4.
3. OPTICAL SPECTROSCOPY

There are over one thousand spectroscopic redshifts
available in the CDF-S region. The majority have been
supplied by galaxy redshift surveys on the VLT suchas VVDS (Le Fèvre et al. 2005), GOODS (Vanzella etal. 2005, 2006, 2008; Popesso et al. 2009; Balestra et al.
2010), and K20 (Mignoli et al. 2005). Most relevant forour study is the redshift survey of X-ray sources in the 1
Ms CDF-S (Szokoly et al. 2004) that provides redshifts
for 74 sources in the Lehmer et al. (2005) catalog. At
the outset of our program, there were published spectroscopic redshifts for only ∼ 100 E-CDF-S sources, thus
justifying the need for a dedicated spectroscopic program
specifically targeting the X-ray source population. We
note that an independent effort similar in scope to ourprogram has been undertaken (Treister et al. 2009a) on
Magellan and the VLT that acquired spectra of shallower depth and at red wavelengths; these spectroscopic
redshifts are incorporated into our catalog (Section 4) if

Fig. 2.— Footprints of our multi-aperture spectroscopic observations overlaid onto a smoothed X-ray image of the E-CDF-S.
(a) The four quadrants of our three VLT/VIMOS pointings aremarked in black with the central deep (9hr) exposure highlighted
by thick lines. (b) Fourteen Keck/DEIMOS pointings are shownby the black rectangular boxes. North is up and East is to the left.
In both panels, the jagged region marks the central 2Ms Chandra
region.

there is confidence that an improvement in the qualityof the overall catalog is assured. In addition, we include
newly available redshifts provided by the GOODS program (Balestra et al. 2010). As further described below, we have obtained optical spectroscopic observations
with both the VLT and Keck of the faint X-ray source
population detected not only in the E-CDF-S but also
in the 1-2 Ms catalogs (Giacconi et al. 2002; Luo et al.
2008). Additional slits have been placed on galaxies associated with X-ray selected clusters and non-X-ray targets
such as VLA radio sources, Spitzer IRAC/MIPS sources,and galaxies (RAB < 25) in the GOODS/MUSIC catalog (Grazian et al. 2006) having photometric redshifts
between 1.5 < z < 2.5. All processed flux-calibrated
spectra, in FITS and ASCII formats, are available online23 with the exception of the non-active galaxies (i.e.,
GOODS/MUSIC, Spitzer MIPS and IRAC) that will bepresented separately with publicly available spectra.
3.1. VLT/VIMOS observations, data reduction &analysis
We have acquired optical spectra with the VLT, using the Visible Multi-Object Spectrograph (VIMOS; Le
Fèvre et al. 2003) mounted on the Nasmyth focus of
UT3 Melipal. Our campaign, in service mode, primarily
targeted counterparts to Chandra X-ray sources in the
E-CDF-S that lacked spectroscopy. VIMOS is ideal forspectroscopy of optically faint targets due to its multi-slit
capability over a wide field of view that is well matched
to Chandra since an area of 7′ × 8′ is covered by each of
4 CCDs.
We have carried out two pointings, each with two different setups, in the northern half of the E-CDF-S field(Figure 2a). In many cases, we obtain spectra for each
X-ray source covering a wide wavelength range to detect
spectroscopic features in a variety of AGNs; this requires
the use of two different dispersive elements separately.
The difference in spectral resolution (as described below)
essentially prevents us from observing all sources withboth configurations. Since many of these AGN are obscured and reside in early-type galaxies at 0.4 < z < 1.2,
we used the medium-resolution (MR) grism in the red
23
http://member.ipmu.jp/john.silverman/CDFS.html
http://www.eso.org/˜vmainier/cdfs pub/and

Optical spectroscopy of X-ray sourcesthat allows a better subtraction of night-sky lines andhas higher transmission compared to the low-resolution(LR) red grism. The MR grism has the following characteristics: a wavelength range of 5500 − 9600 Å, R ∼ 580(∆v ∼ 500 km s−1 ) and dispersion of 2.5 Å pix−1 . To
cover shorter wavelengths (3700-6700 Å), we carry out
observations using the low-resolution blue grism (LRblue; R = 180, dispersion=5.3 Å pix−1 ) to detect emission lines (i.e., Lyα, CIV, CIII]) in AGN with 1.5 < z < 4or absorption lines mainly attributed to the ISM of their
host galaxy when no AGN signatures are present. Observations with the LR-blue grism are crucial to identify
the obscured AGN at these redshifts that are underrepresented in the identification of sources in Deep Chandra
surveys (e.g., See Figure 6 of Brandt & Hasinger 2005).
A wide spectral range greatly facilitates object classification, since many of these optically underluminous AGN
suffer from severe host dilution. It is worth highlighting that expanded wavelength coverage can detect broad
optical emission lines in sources previously classified as
normal or narrow emission-line galaxies (e.g., Page et al.
2003; Silverman et al. 2005). Our total exposure times(∼ 5 hours per mask) are set to identify faint opticalcounterparts to a limiting magnitude of R ≈ 25 for AGN
with strong emission lines and R = 24 for highly obscured
AGN having emission/absorption features of moderate
strength.
A single, deep VIMOS exposure was obtained in thecentral CDF-S to identify the obscured AGN population
at 1.5 < z < 3. The setup was configured with the LRblue grism and OS-blue filter that provided a wavelength
coverage sufficiently blue to identify spectral features ingalaxies at these redshifts. Initially, two separate pointings were scheduled with integration times of 18 hours
each, but scheduling difficulties resulted in a single pointing with a total on-source exposure time of 9hr. The
observation was split into 18 segments each having an
exposure time of 30 minutes. This single observation is
unique due to its depth and enables us to achieve goals
such as identifying the obscured AGN at z > 1.5 that
tend to have faint optical counterparts (R ∼ 25) andlack spectroscopic identification.
All masks were designed with the VIMOS Mask Preparation Software (VMMPS; Bottini et al. 2005) that optimizes the slit assignments based on our input catalog
and VIMOS pre-imaging. The coordinates of our targets, initially selected from our deep R-band WFI image,were transformed to the reference frame of the pre-image.
Optical counterparts to X-ray sources are designated as“Compulsory” in the input catalog and are given toppriority; additional targets, as mentioned above, are labelled as “Selectable”. All slits had a width of 1′′ . Twostellar-like objects were milled for each of the four quadrants for field acquisition.
We list the details of the VLT observations in Table 2.
Science exposures of 955-1800 seconds each were ditheredby ∼ 1.5′′ while keeping the object in the slit for optimal
removal of night-sky emission. Actual exposure times permask range from 3.2 to 9 hours. The typical seeing was∼1′′ and airmass . 1.2. All necessary calibration frames(e.g., dome flats, arc lamps) are taken including short
spectroscopic observations of standard stars required foradequate flux calibration. The data were reduced using

5

TABLE 2Log of VLT/VIMOS observations
Date
UT

RA
J2000

DEC
J2000

GrismaExposure timecsec (# of exp.)

2004-10-21
2004-10-23
2004-11-11
2004-11-13
2004-12-03
2004-12-04
2004-12-05
2004-12-07
2004-12-08
2005-01-02
2005-01-03
2005-01-05
2005-01-06
2005-10-04
2005-10-30
2007-12-12
2007-12-13
2007-12-14
2007-12-15
2008-01-04
2008-01-06
2008-01-07
2008-02-01

03:31:59
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-27:42:23
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-27:42:28”””””-27:48:36
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LRblueLRblueLRblue
MRred
MRred
MRred
MRred
MRred
MRred
MRred
MRred
MRred
MRred
LRblueLRblueLRblueLRblueLRblueLRblueLRblueLRblueLRblueLRblue5730.0 (6)955.0 (1)8595.0 (9)2865.0 (3)2865.0 (3)2865.0 (3)2865.0 (3)5730.0 (6)2865.0 (3)2865.0 (3)1910.0 (2)1910.0 (2)9550.0 (10)5730.0 (9c )5730.0 (6)7200 (4)3600 (2)3600 (2)3600 (2)3600 (2)3600 (2)3600 (2)3600 (2)a The filter OS-blue was used in conjunction with the LR observations and the GG475 for the MR.
b The exposure time is the sum of all science frames.
c Three observations were not used in the reduction due to a large
pixel shift that could not easily be rectified within the VIPGI environment.

the VIMOS Interactive Pipeline and Graphical Interface(VIPGI; Scodeggio et al. 2005) software.
3.2. Keck/Deimos spectroscopy
Optical spectra were obtained in January of 2007, 2008
and 2010 on Keck II with the Deep Imaging Multi-object
Spectrograph (DEIMOS; Faber et al. 2003). These observations have a substantially higher spectral resolution than those with the VLT. In Figure 2b, we show
the placement of fourteen slit masks with respect to the
E-CDF-S field. For the most part, we use the 600 l mm−1
grating and a blue blocking filter (GG455) to achieve a
wavelength coverage of 4600-9700 Å, a dispersion of 0.65
Å pix−1 and spectral resolution of ∆λF W HM ≈ 3.5 Å
(≈ 125 km s−1 ). Although, six masks were observedwith a slightly redder filter (GG495), and a single observation was performed using the 830 l mm−1 grating
due to a limitation on having multiple configurations ina single evening since the primary target during most of
the later half of the night was the COSMOS field. Thishigher resolution observation has a wavelength coverageof 5700-8000 Å, a dispersion of 0.47 Å pix−1 and spectral resolution of ∆λF W HM ≈ 2.5 Å. All slits were milledwith a width of 1′′ . Integration times ranged from 1 to
3.6 hours per mask. Wavelength calibration is achieved
using short arc lamp exposures. Observations of a single
standard star in the same evening and dome flats allow
flux calibration of our spectra. Due to the southern location of the CDF-S, all observations are carried out athigh airmass (1.5-2.0), although under photometric conditions. In Table 3, we provide a log of the observations
including some relevant details.
All data reduction was performed using the DEEP2

6

Silverman et al.
due to high S/N spectra and multiple spectral features

TABLE 3Log of Keck/DEIMOS observations
Date
UT

RA
J2000

DEC
J2000

Grism
l/mm

Filter(GG)

Exp time
sec(# exp)

2007-01-21
2007-01-18a
2007-01-14
2007-01-17
2007-01-15
2007-01-16
2008-01-08
2008-01-09
2008-01-10
2010-01-14
2010-01-15
2010-01-16
2010-01-17
2010-02-11

03:32:59.9
03:31:55.0
03:33:06.8
03:33:03.2
03:31:50.1
03:31:57.5
03:32:54.0
03:33:04:0
03:31:28.2
03:32:28.9
03:32:13.8
03:32:50.5
03:33:02.0
03:31:54.2-27:45:10-27:44:25-27:56:49-28:01:48-27:55:58-28:01:10-27:50:00-27:35:19-27:44:05-27:41:15-27:39:09-27:41:58-27:42:43-27:54:59

600
600
600
600
600
600
830
600
600
600
600
600
600
600

455
495
455
455
455
455
495
495
495
495
495
495
495
495

3600 (3)3600 (3)7200 (4)9000 (5)9000 (5)9000 (5)6000 (5)4800 (5)3600 (3)9000 (5)9000 (5)9000 (5)8707 (5)13016 (11)a Masks with position angle equal to 0 (N-S). All others are oriented in

the E-W direction.

software pipeline.24 This is essentially an automatedroutine that identifies slit positions, determines thetwo-dimensional wavelength solution based on the wellunderstood geometrical distortions of the instrument, removes cosmic rays, and traces the targets to provide onedimensional, sky-subtracted spectra. In a few specificcases mainly pertaining to standard-star observations, we
manually extracted the spectra with a modified versionof the DEEP2 script (’do extract’). Flux calibration isthen applied to all 1-D FITS spectra using IRAF.25
3.3. Redshift measurements and source classification
We measure redshifts using the KBRED and EZ26 software that cross-correlates each spectrum with a range ofspectral templates. KBRED and EZ have been developed by the VVDS consortium to optimize the analysisof large numbers of galaxy spectra being generated fromlarge surveys (e.g., VVDS, zCOSMOS). These templates
include stars, quasars, and galaxies with a varying degreeof emission/absorption line strength over the wavelength
range 1100–9000 Å. High-redshift (z > 1.5) templates
have been provided by the zCOSMOS and VVDS teams
that were generated from a sample of galaxies using asimilar observing configuration (i.e., spectral resolution,
wavelength coverage) with the LRblue grism (see Lilly et
al. 2007 for further details). Specifically, these templates
aid in the identification of galaxies with redshifts between
1.5 < z < 3.0 that are known to be observationally challenging. Each redshift is visually inspected in the VIPGI
environment; in some cases the input parameters such as
wavelength range or template must be further restricted.
For spectra with strong, narrow emission lines, we use
VIPGI to determine the centroid of each line and thencompute a mean redshift. We assign a quality flag toeach redshift measurement as defined here:• Flag 2: High-confidence redshift (> 95% secure)
24
Further
details
can
be
found
at
http://www2.keck.hawaii.edu/inst/deimos/pipeline.html
25 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with
the National Science Foundation.
26 http://cosmos.iasf-milano.inaf.it/pandora/EZ.html• Flag 1: Redshift with a significant amount of uncertainty at the ∼ 50% level due to either low S/N
spectra or the presence of only a single emissionline.
• Flag 0: No redshift measurement is achieved.
It is worth highlighting that the assignment of a redshift with a quality flag of 1 is a judgement call thatreflects a situation where a solution is likely to be correct but has considerable uncertainty due to usually either poor signal-to-noise, a lack of spectral features (i.e.,emission and absorption lines) or both. A single emission line present in a spectrum with a faint continuum
can be attributed to Lyα, CIII], MgII, [OII] or Hα. We
note that it is not possible to empirically determine our
success rate with respect to redshift measurements sincea coordinated effort to carry out repeat observations has
not been undertaken. Although, as described below (Section 4.1), we do compare our spectroscopic redshifts withlow quality flags to their photometric estimates and findthat ∼ 50% are likely to be correct.
We have measured new redshifts (flag ≥ 1) for counterparts to 283 X-ray sources detected in the combined
Lehmer et al. (2005) and Luo et al. (2008) catalogs. To
demonstrate our success rate, we show in Figure 3 the optical magnitude distribution of all X-ray sources targetedby our VLT and Keck observations and highlight thosefor which we have obtained a redshift (Filled histogram:f lag ≥ 1; dashed line: f lag = 2). These distributions
represent every attempt to measure a redshift based ona slit being placed on a counterpart to a X-ray source.
Therefore, a single target may be counted more thanonce depending on how many attempts were made toacquire an optical spectrum that provides a reliable redshift. For instance, a spectrum is likely to be acquired
using Keck if the VIMOS observation did not yield a secure redshift. As clearly evident (Fig. 3a), we have a highsuccess rate of 83.2% (f lag ≥ 1) for bright counterparts(RAB < 24) that drops to 75.1% for the most securecases (f lag = 2). At RAB ∼ 24, our success rate drops
to ∼50%; this is roughly a magnitude fainter than the
success rate achieved with previous campaigns (Szokoly
et al. 2004; Treister et al. 2009a) in the E-CDF-S. It
is worth highlighting that the sharp cutoff in the success rate with Keck/DEIMOS for RAB > 24 (Fig. 3c)
is significantly less severe for our deeper (texp ∼ 5 or 9hours) observations using the LR grism on VLT/VIMOS(Fig. 3b) thus resulting in a number of identifications atthese faint magnitudes that attests to the effectiveness ofour strategy; even at the extreme limit for spectroscopy(RAB ∼ 26), we also have a handful of identifications.
The number of independent spectroscopic programs in
the CDF-S enables us to further improve the quality of
the catalog. For example, there are 132 optical counterparts to X-ray sources in the E-CDF-S that have multiple redshift measurements. Of these, there are 35 casesthat are clearly discrepant. The majority of these can be
explained by the varying quality of the spectra. For example, the cases in common (24) with the MUSYC program (Treister et al. 2009a) are mainly due to a significant uncertainty based on their low-quality flags for theseOptical spectroscopy of X-ray sources

7

Fig. 3.— Number distribution of all observed X-ray counterparts (open histogram) and those with reliable spectroscopic redshifts(F lag ≥ 1: filled histogram; F lag = 2: dashed line): (a) all, (b) VLT-VIMOS (LR blue only), (c) Keck/DEIMOS.

objects most likely due to lower signal-to-noise spectra
acquired from smaller aperture telescopes (i.e., Magellan) than employed in this study. We have reconciledwith the MUSYC team a handful of cases (8), for which
both teams measured a different redshift and assigned a’secure’ quality flag, by inspecting all available spectra;
the conclusion was that these eight MUSYC redshifts
were incorrect. Also, three cases of discrepant redshiftswith the recent GOODS/VIMOS program (Balestra etal. 2010) were resolved by our higher quality flags andagreement with photometric redshifts.
Furthermore, we classify each object by the presenceof spectral features that fall within the observed optical
window as listed here:• Broad emission-line AGN (BLAGN): Presence ofat least one emission line having F W HM > 2000
km s−1• Narrow emission-line galaxy (NELG): Presence ofat least one emission line having F W HM < 2000
km s−1 and no BLAGN signatures.
• Absorption-line galaxy (ALG): No emission lines
with an observed EQW > 5 Å. Clear signs of absorption lines such as Ca H+K, MgII or G band.
• StarWe note that there are a handful of objects where theclassification depended on the observed spectral window.
For example, we initially classified the optical spectra ofa few cases as ALG or NELG based on VIMOS/MR coverage with observed wavelength above 5500 Å, but wechanged the classification to BLAGN due to the presence of broad MgII falling at shorter wavelengths in VIMOS/LR spectra.
Here, we further demonstrate that the observed spectral window not only impacts source classification but
the overall redshift distribution of specific types of objects. In Figure 4, we show the optical-magnitude andredshift distributions of the various classes of objects(BLAGN, NELG, ALG) that have fairly secure redshifts(f lag ≥ 1) from our program. We split the sample bythose with redshifts from either the VLT/VIMOS using the LR grism (solid histogram) that is more sensitive at blue wavelengths or Keck/DEIMOS (dashed histogram). We stress that these distributions are meant toillustrate the success rate of redshift determination andsource classification for X-ray sources (from both the ECDF-S and the central 2 Ms survey) using these two
observational configurations. Therefore, as in Figure 3,
an object can be included in two categories (e.g., ALG
and NELG) if different spectra cover different spectral
regions. For example, the X-ray source ECDFS #411 isincluded here as an ALG based on a VLT/VIMOS spectrum and also as an NELG from Keck observations withthe final classification determined to be NELG. We seethat our deep exposures with VLT/VIMOS (5 or 9 hours)
have provided the spectra to identify X-ray sources down
to faint optical magnitudes (Rmag ∼ 25.5) for all classes(Fig. 4a − c) as designed. In the bottom row, we showthat the redshift distribution, while similar for BLAGN(Fig. 4d) between the VLT and Keck observations, is dissimilar for non-BLAGN (Fig. 4e − f ). Both NELG and
ALG exhibit a more pronounced high redshift component(1.5 . z . 3) due to the bluer wavelength coverage that
effectively enables the detection of emission lines such as
Lyα and CIV, or interstellar absorption lines thus beginning to fill in the ’redshift desert’. We do recognizethat the target selection between the two strategies differs slightly and may bias this comparison in the sensethat the VIMOS observations were tuned towards fainterobjects. For example, the VIMOS mask preparation software does not allow one to tilt slits as was done for Keck.
Therefore, in a number of cases, a fainter source waspreferred over a brighter target where slit assignment
conflicts occurred. This was especially the case for the
single deep (9hr exposure) VIMOS pointing. Anothercontributing factor is that many of the brighter targets
were assigned redshifts based on prior Keck observations
and then subsequently removed from the input catalog ofthe deep VIMOS pointing. Such biases are likely responsible for slightly different magnitude distributions shownin Figure 3. In consideration of these selection effects,
we have compared the redshift distributions as shown in
Figure 4e − f for both NELGs and ALGs using a restrictive magnitude range (24 < Rmag < 26) and still findthat the VIMOS LR blue program results in a redshift
distribution that is shifted to higher values.
4. OPTICAL/NEAR-INFRARED CATALOG OF

X-RAY SOURCES IN THE E-CDF-S
In Table 4, we present a catalog of the most-likely
optical/near-infrared counterparts for each X-ray source

8

Silverman et al.

Fig. 4.— Magnitude and redshift distribution of successes (f lag ≥ 1) as shown by their optical classification (BLAGN, NELG, ALG).
The histograms represent the distribution of each quantity and object type for different instrument setups (solid=VLT/VIMOS-LRblue;
dashed: Keck/DEIMOS). We highlight that deep exposures (solid histogram) that cover the blue part of the spectrum result in a shift ofthe distribution to higher redshift for non-BLAGN.

listed in Lehmer et al. (2005) and provide parameters(i.e., likelihood ratio, reliability, and angular separation)that quantify the robustness of these multi-wavelength
associations. The spectroscopic information provided
here is based on a compilation of all such observations
undertaken in the E-CDF-S. In addition to the spectroscopic redshift, we have included a quality flag that gives
the confidence in the redshift measurement. We attempt
to place all quality flags given in the literature on a common scale. The flags given in Szokoly et al. (2004) aretranslated as follows: a ‘2’ or a ‘3’ is converted to a ‘2’
in our scale, a ‘1’ is kept as a ‘1’, and all others are replaced with a ‘0’. With respect to the GOODS-S catalogs(Vanzella et al. 2008; Popesso et al. 2009; Balestra et al.
2010), we convert an ‘A’ or ‘B’ into a ‘2’, a ‘C’ becomesa ‘1’ and all others have a ‘0’. Quality flags associatedwith the MUSYC redshifts (Treister et al. 2009a) havebeen incorporated as provided through private communication on the same scale (0,1,2) as used in this work. In
addition, a spectral classification is given mainly basedon the presence of emission lines and their width. Finally, a photometric redshift measurement is provided asdetermined by Rafferty et al. (2010) as further describedin Section 4.1.
The columns of Table 4 are as follows.
• Col. 1

XID from Lehmer et al. (2005)• Col. 2-3 Right Ascension and Declination (J2000)
of the optical/near-infrared counterpart.
• Col. 4
Catalog associated with the identified
counterpart: GEMS-z (z-band HST/ACS catalog),
WFI-R (WFI R-band catalog), SOFI-K (K-band
catalog from SOFI), SOFI-H (H-band catalog fromSOFI)

• Col. 5
Separation, in arcsec, between the X-ray
source and its counterpart.
• Col. 6
2.

Likelihood ratio (LR) as defined in Section• Col. 7
Reliability parameter (R) as defined in
Section 2.
• Col. 8

R-band magnitude (AB).

• Col. 9

Spectroscopic redshift of the counterpart.

• Col. 10
Quality flag for the spectroscopic redshifts: ‘2’ secure, ‘1’ some uncertainty, ‘0’ no redshift; +0.5 if in agreement with photometric redshift• Col.11
Spectroscopic catalog (1=Szokoly et
al. 2004; 2=FORS2/GOODS; 3=VVDS; 4=New
VLT; 5=VIMOS/GOODS; 6=K20; 7=New Keck;
8=Ravikumar et al. 2007; 9=New VLT/9hr;
10=Treister et al. 2009a; 11=Wisotzki/Schramm,
private communication; 15).
• Col. 12
Spectroscopic classification: BLAGN,
NELG, ALG, STAR.
• Col. 13
Photometric redshift (Rafferty et al.
2010; Luo et al. 2010)
We list in Table 5 the statistics regarding the full sample such as those having spectroscopic redshifts and classifications (e.g., BLAGN, NELG, ALG, STAR); all optical counterparts discussed from this point on have areliability, as given in Table 4, greater than 50%. We
highlight that our VLT and Keck programs have yielded

Optical spectroscopy of X-ray sources

9

TABLE 4
Optical and near-infrared counterparts to X-ray sources in the E-CDF-S
XID

RA

Dec

Catalog

dist
(′′ )

LR

Rel

Rmag
(AB)

zspec

Qualityflag

Spectroscopic
catalog

Class

zphot

-27.56042-27.93025-28.07248-27.78539-28.02890-28.00758-27.92183-27.82785-27.87511-27.63511-27.68926-27.71372-27.74238-27.78369-27.74502-27.94660-27.92499-27.72266-27.98900

WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
WFI-R
GEMS

1.3
0.8
0.8
0.5
0.6
0.5
0.0
0.1
3.2
0.2
0.1
1.1
0.1
0.2
0.5
0.2
0.1
0.2
0.2

3.9
11.4
1.3
17.1
9.9
0.8
52.9
2.3
3.2
2.0
40.4
18.3
24.5
3.7
5.7
9.9
35.2
4.0
48.4

0.70
0.98
0.64
0.99
0.98
0.79
1.00
0.92
0.94
0.87
0.99
0.99
0.99
0.95
0.97
0.95
0.99
0.95
1.00

21.4
23.2
23.9
22.9
23.5
26.0
20.8
25.3
18.5
24.3
21.6
20.3
22.8
24.9
24.5
23.9
22.1
25.0
22.2

—
—
—
—
—
—
1.368
—
—
—
—
0.400
1.218
—
1.033
—
0.760
—
0.683

—
—
—
—
—
—
2
—
—
—
—
2
2
—
1.5
—
2
—
2

—
—
—
—
—
—
7
—
—
—
—
7
10
—
4
—
7
—
10

—
—
—
—
—
—
BLAGN
—
—
—
—
ALG
BLAGN
—
NELG
—
NELG
—
NELG

—
—
—
—
0.745+0.003−0.017——–2.212+0.025−0.0220.864+0.004−0.0122.257+0.045−0.043
1.058+0.002−0.002—–—–2.049+0.011−0.012
1.058+0.007−0.0081.635+0.010−0.010—–1.356+0.028−0.017—–(J2000)
1
2
3
4
5
6
7
8
9
11
12
13
14
15
16
17
18
19
20

52.79726
52.80404
52.80854
52.80950
52.81118
52.81231
52.81267
52.81864
52.82344
52.82757
52.82790
52.83035
52.83099
52.83217
52.83272
52.83290
52.83515
52.83604
52.83634

Note. — An abbreviated version of the table is shown. The full table is provided online with 762 entries.

TABLE 5
Statistics of E-CDF-S X-ray point
sources
Type

ALL

Rmag < 24

X-raySpec + phot z
Spec z: Flag>0
=2.5
=2
=1.5
=1
Spec z; New VLTNew Keck
BLAGN
NELG
ALG
STAR

762
608 (80%)422 (55%)
278
42
26
45
101
147
123
188
48
14

397
372 (94%)310 (78%)
236
33
14
17
57
123
90
137
37
14

248 new spectroscopic redshifts thus reaching a completeness of 55% (422 out of 762 X-ray sources) and 78% for
the optically-bright population (RAB < 24).
4.1. Photometric redshifts
We provide in Table 4 and use through this work newphotometric redshifts (Luo et al. 2010; Rafferty et al.
2010) for 274 X-ray sources in the E-CDF-S that lack secure (f lag < 2) spectroscopic redshifts. Briefly, thesephotometric estimates are primarily based on opticalphotometry in 17 bands, (Wolf et al. 2004, 2008), the
GOODS-S MUSIC survey (Grazian et al. 2006), and new
data at UV (e.g., from GALEX27 ), optical (e.g., from
MUSYC; Gawiser et al. 2006), near-infrared (MUSYC
J, H, and Ks ; Taylor et al. 2009), and mid-infrared(e.g., from SIMPLE; Damen et al. 2009) wavelengths
that greatly aid in the derivation of photometric red27 See http://galex.stsci.edu/GR4/

shifts of fainter sources (mR & 24) and those at higher
redshift (z & 1.4). Based on these data sets, we constructa spectral energy distribution (SED) in up to 42 bands
for each optically selected source and then utilize thepublically-available Zurich Extragalactic Bayesian Redshift Analyzer (ZEBRA; Feldmann et al. 2006) code toderive photometric redshifts based on 259 galaxy templates constructed from PEGASE stellar population synthesis models (for details, see Grazian et al. 2006). We
further include a set of hybrid templates (galaxy+AGN)
and 10 empirical AGN templates as provided by Polletta
et al. (2007). The ZEBRA code enables us to correct
for systematics in the photometry by modifying the input templates to better represent the observed SEDs of
the 2651 sources with spectroscopic redshifts. A maximum likelihood technique is then used to identify themost probable redshift. We note that photometric redshifts are likely to be more accurate in the central CDF-S
region (Luo et al. 2010) due to the deeper data as compared to the extended region (Rafferty et al. 2010). We
refer the reader to Luo et al. (2010) and Rafferty et al.
(2010) for specific details on the procedure and a detailed quantitative analysis of their reliability especiallywith respect to the X-ray selected AGN population.
Photometric redshifts can also be used to check thevalidity of spectroscopic redshifts especially relevant for
the faintest sources having considerable uncertainty associated with their redshift determination mainly due
to low signal-to-noise spectra. Therefore, an assessmentof the difference between spectroscopic and photometricredshifts is carried out; the spectroscopic redshift quality
flag is increased by 0.5 if there is good agreement between
the two (∆z(1 + z)−1 < 0.12); this cutoff (2σN MAD ) isbased on blind tests (see Figure 12b in Luo et al. 2010).
Therefore, a low confidence redshift that agrees with its
photometric redshifts will have a quality flag 1.5 andlikely be correct. We find that 49% for the lower reli-10

Silverman et al.

ability sources (f lag = 1) have spectroscopic and photometric redshifts in agreement. Identifications, based ona single emission line, have a comparable success rate tothose determined by the faint continuum as detected in
low signal-to-noise spectra. We can also check the consistency for the secure spectroscopic redshifts (f lag = 2)
and find that 79% are in agreement but caution thereader that the photometric estimates for these sources
with reliable redshifts are not independent measurements
since their spectroscopic redshifts were used as a training
set for the estimate of photometric redshifts. We provide
a bit of caution that these success rates do not necessarily mean that the spectroscopic redshifts are incorrectsince spectra of AGN of these luminosities are known
to have complexities due to a blend of AGN and hostgalaxy emission further complicated by the presence ofdust for which may be either associated with the nuclear
region in the form of a dusty torus or more extended
thus attenuating the stellar light. We provide statistics,
in Table 5, regarding the fraction of the E-CDF-S X-ray
sources with redshift estimates. Remarkably, we haveattained a completeness of 80% based on both spectroscopic and photometric redshifts that reaches 94% for
those with RAB < 24.
5. RESULTS
5.1. Demographics of the X-ray point-source
population
Overall, we find similar optical source populations toprevious identification programs in the Chandra Deep
Fields (e.g., Barger et al. 2003; Szokoly et al. 2004).
With our deeper exposures, we are able to extend effectively the accessible optical luminosity and redshift range
as illustrated below. We show in Figure 5 the distribution of broad-band (0.5–8.0 keV) X-ray flux, as given
in Lehmer et al. (2005), versus optical magnitude (R)to illustrate the parameter space spanned by the various populations. Sources are marked by their method of
identification, either spectroscopic or photometric, andobject type if available based on their optical spectra.
As shown by the aforementioned identification programs,
the BLAGN follow the trend of 0 . log(fX /fR )28 . 1
with significant scatter. The NELG are more numerous
than the BLAGN, span a wider range in the fX − R
plane, and are the dominant population at fX /fR > 10
(Fiore et al. 2003). A significant amount of the dispersion
in this population comes from the fact that most NELGs
have significant absorbing columns; this is evident by thetighter relationship between X-ray flux and optical magnitude when considering the hard (2–8 keV) band (e.g.,
Barger et al. 2003; Silverman et al. 2005; Eckart et al.
2006). We further find that 64% of the X-ray bright, optically faint (fX /fR > 10) NELGs are AGNs of moderateluminosity (1042 . LX . 1044 erg s−1 ) at 0.5 . z . 1.5;
type 2 QSOs, defined by their lack of broad-emission lines(Section 3.3) and having L0.5−8.0 keV > 1044 erg s−1 ,comprise 30% of sources having fX /fR > 10 and R > 24
(7 of 24). At faint flux levels (f0.5−8.0 keV . 5 × 10−15
erg cm−2 s−1 ), a significant number of ALGs are evident; they span a wide range of optical magnitude (See28 Ratio between broad-band X-ray flux and optical magnitudeis determined by the equation log(fX /fR ) = log(fX ) + 0.4R + 5.69

Fig. 5.— Broad-band (0.5–8.0 keV) X-ray flux versus opticalmagnitude (RAB ). X-ray sources with identification through optical spectroscopy are marked in color with the symbol type denoting the object classification (BLAGN, NELG, ALG STAR; see section 3.3 for further details). Sources with photometric redshifts aremarked by filled black circles. Open red circles are sources having
spectroscopic redshifts but lack classification. X-ray sources withno optical counterparts are arbitrarily placed at R=27 (crosses).
Lines of constant X-ray-to-optical flux ratio (0.1,1,10) are given by
Equation 1 of Szokoly et al. (2004).

Sec. 5.1.2 for further details). Based on their X-ray luminosities (LX & 1041 erg s−1 ), their X-ray emission is
likely powered by an AGN in most cases (See Figure 6).
In addition, there are also a handful of stellar sourcesthat have been identified having faint X-ray fluxes andbright optical magnitudes (15 < R < 20).
Many of the optical counterparts lacking secure spectroscopic identification (filled black circles) are fainterthan R ∼ 24. Although, as demonstrated in Figure 4
and 5, our VLT and Keck observations have enabled theidentification of a significant number (56 with f lag ≥ 1;
36 with f lag ≥ 1.5) of optically faint (R > 24) counterparts by their emission lines, thus classified primarily as NELGs, although a fair number do exhibit faintbroad lines (See Figure 7b − c for some examples) thatare further discussed below. Finally, we highlight that
there is a significant number of X-ray sources having
R & 26 that can further be characterized with deeper
near-infrared imaging (Cardamone et al. 2008; Brusa et
al. 2009) that is beyond the scope of the current work;
although, we do confirm that all of the brightest X-ray
sources (f0.5−8 keV > 10−14 erg s−1 cm−2 ) without optical counterparts are clearly associated with bright, nearinfrared detections.
We further remark that many of the narrow emission
lines (e.g., [OIII]λ5007, Hβ, Hα, [NII]) usually present in
X-ray selected NELGs are likely powered by photoionization by the central AGN especially for those having LX > 1042 erg s−1 as demonstrated by COSMOS(Bongiorno et al. 2010) and ChaMP (Constantin et al.
2009). Even so, a significant fraction (∼ 10 − 60%) of the[OII]λ3727 emission-line flux has been attributed to starformation even for the X-ray luminous cases (Silverman
et al. 2009b). Such detailed emission-line diagnostics ofOptical spectroscopy of X-ray sources

Fig. 6.— Luminosity–redshift distribution of X-ray sources.
Symbols are the same as described in Figure 5 with the exception of the open squares that indicate those with new spectroscopic
redshifts from our own VLT and Keck programs. For illustrative
purposes, we plot the evolution of the break in the X-ray luminosity
function based on analytic model of Aird et al. (2010).

AGNs in deep X-ray surveys require further spectroscopy
in the near-infrared given their typical redshifts (z ∼ 1).
The effectiveness of probing moderate-luminosity AGN(LX ∼ 1043−44 erg s−1 ) at z > 1.5 is demonstrated by
the X-ray luminosity distribution as a function of redshift
shown in Figure 6. The X-ray luminosity has been estimated from the broad-band flux given in Lehmer et al.
(2005) with a k-correction based on a power law spectralenergy distribution (lE ∝ E 1−Γ ; Γ = 1.9; e.g., Silverman
et al. 2005; Mainieri et al. 2007). No correction for intrinsic absorption has been applied thus these luminosities
are likely to be underestimated. The solid line denotes
the evolution of the characteristic break (L⋆ ) in the luminosity function of AGN, based on the pure luminosity
evolution model (Eq. 21) from Aird et al. (2010), that
illustrates the effectiveness of surveys, reaching depths
similar to the E-CDF-S, to detect AGN contributing to
the faint end slope of the XLF up to z ∼ 3 (e.g., Barger et
al. 2005; Aird et al. 2008). We are now able to fill in theregion of L − z space (LX < 1044 erg s−1 ; 1.5 . z . 2.5)
previously lacking identification with objects that haveweak emission lines (either broad or narrow). Our deep
VLT exposures with the LR-blue grism effectively enable identification of X-ray sources without strong optical signs of an active nucleus, thus mitigating the strong
bias evident in most spectroscopic surveys (i.e., the ’redshift desert’; Brandt & Hasinger (See Figure 6 of 2005)).
In Figure 7, we show optical spectra of four moderateluminosity AGN that we have identified at redshifts forwhich wavelength coverage below 5000 Å facilitates object classification. In the first three panels (a: #162; b:#357; c: #484), the spectra are of very faint counterparts (R = 24.8 − 26) and mainly show evidence of faintemission lines clearly visible in the 2D images. The first
spectrum (#162) is of a counterpart to an X-ray sourcethat has been identified by Donley et al. (2010), desig-

11

nated as IRBG1, to have the highest 24 µm-to-opticalflux ratio (f24 /fR = 3961) among Spitzer sources detected in deep Chandra fields. The spectrum here wasobtained from our deep 9 hour exposure of the central
CDF-S region with the VLT/VIMOS. The spectroscopic
redshift is based not only on the emission lines of CIV
and CIII] but interstellar absorption lines of AlII and
FeII associated with the host galaxy. We classify thisobject as a NELG due to the width of the CIV line tobe ∼1200 km s−1 that supports the conclusion of Donley et al. (2010) that the IR emission from this object
is likely to be due to an obscured AGN. In the secondcase (panel b), the redshift is based on a significant detection of an emission line at 4487 Å that we identify as
CIV that may be further supported by a possible detection of CIII] that is uncertain due to a nearby night sky
emission feature thus the quality flag is set appropriately.
In this case, the spectroscopic and photometric redshifts
are not in agreement although the photometric estimate
does indicate that the X-ray source is at high redshift(zphot = 1.154). In panel c, we have high confidence inthe spectroscopic redshift (z = 2.403) based on a strong
Lyα emission line, and CIV emission. This identification
is notable given its faint magnitude (R = 25.96). Optical counterparts in panels b and c are classified as type
2 AGNs based on their narrow line widths. Lastly, weshow the spectrum associated with ECDFS #667 thatis optically brighter than the previous three cases thus
resulting in a highly secure redshift given by the strong
CIV emission, and weaker CIII] lines that are both broadin line width and typical of optically-selected QSOs.
To conclude here, we highlight that it is important
to determine how the more complete sampling of the
LX − z plane (Figure 6) impacts recent determinationsof the AGN XLF particularly the faint-end slope at
1.5 . z . 3. Such accurate assessments of the XLF
are crucial to better constrain our understanding of theevolution of the black-hole mass function (e.g., Tamura,
Ohta & Ueda 2006; Merloni & Heinz 2008). For example, Yencho et al. (2009) claim that they have a high levelof spectroscopic completeness with respect to the identification of BLAGNs thus the flatness of the observed
faint-end slope is robust. We assert that this may not be
the case since many of the optically-faint X-ray sources
identified herein do have broad emission lines thus thefaint-end slope may actually be steeper than observed inspectroscopically-selected samples. Recent evidence doessuggest that incompleteness with respect to accurate redshift determination is likely to have a significant impact
on the measurement of the faint-end slope at z > 1 (Airdet al. 2010). Therefore, it is imperative to place stringent
constraints on the faint-end of the luminosity distribution of AGNs with a sample that has secure redshifts
down to the faintest optical limits feasible, such as that
presented here. Given the complexities in properly dealing with multi-band selection and incompleteness, we reserve such an analysis to a separate study now underway.
5.1.1. Type 2 QSOs
X-ray surveys with Chandra (e.g., Stern et al. 2002;
Norman et al. 2002; Barger et al. 2003; Alexander et al.
2008) and XMM-N ewton (Dwelly & Page 2006; Mainieri
et al. 2007; Vignali et al. 2009) are now amassing samples of obscured (i.e., type 2) QSOs, a previously elu-

12

Silverman et al.

Fig. 7.— Optical spectra from the VLT/VIMOS of four moderate-luminosity (log LX ∼ 43.5) AGN at 1.5 < z < 2.5. The units are−1
flux density (erg cm−2 s−1 Å ). Spectra in panels b and c have been lightly smoothed with a boxcar of 2 pixels. The quality of theassigned spectroscopic redshift is given in each case along with its photometric redshift for the one case (panel b) with lower confidence.
Each one-dimensional spectrum is accompanied by its 2D image.

sive population thought to be prevalent based on unification models and the spectral shape of the CXRB. Thesesources are typically characterized by their high luminosity in the X-ray band (LX > 1044 erg s−1 ) and significant obscuration in both the optical, thus resulting in alack of broad (F W HM > 2000 km s−1 ) emission lines,
and X-ray, with absorbing column densities greater than
1022 cm−2 . Based upon their detection rate in X-ray
surveys, this population is, at most, equivalent in size
to the type 1 QSO population (Treister & Urry 2006;
Gilli et al. 2007; Hasinger 2008; Alexander et al. 2008),
and unlikely to match the proportions (i.e., obscuredto-unobscured ratio) seen in the less-luminous AGNs.
Although, deeply embedded (i.e., Compton-thick) QSOs
continue to be found (Alexander et al. 2008) that may
elevate the known space density of galaxies having significant obscured accretion (e.g., Polletta et al. 2006; Tozzi
et al. 2006; Daddi et al. 2007; Fiore et al. 2009; Treister
et al. 2009b). In any case, these type 2 QSOs likely represent an important piece to the puzzle in order to under-

stand the mechanisms driving the evolution of SMBHs
in general. For example, Fabian et al. (2008, 2009) argue that the dearth of obscured (log NH & 21.5) AGNs
accreting close to their Eddington rate is due to a feedback mechanism such as radiation pressure that effectively clears out the obscuring medium. Therefore, an
improvement in the statistics is warranted especially ina survey field of remarkable multi-wavelength coverage
to determine accurate black hole masses and accretion
rates.
To gauge the improvement in the numbers of type 2
QSOs in our newly compiled sample, we find that there
are 17 X-ray sources in the broad-band selected sample
that have a secure spectroscopic redshift (F lag ≥ 1.5),
L0.5−8.0keV > 1044 erg s−1 , and an optical spectrum with
only narrow emission lines. If we relax the criteria on
the quality flag for the redshift (F lag ≥ 1), the sample
increases substantially to 33. We have identified 7 outof the 17 high-quality identifications through our spectroscopic program. In Figure 8, we display our optical

Optical spectroscopy of X-ray sources

13

Fig. 8.— Newly-identified Type 2 QSO candidates in the E-CDF-S. The Lehmer et al. (2005) catalog number, optical magnitude andredshift are labelled in each panel. Black circles are shown in some image cutouts to aid in the identification of the target.

spectra and image cutouts of this group having redshifts
spanning a wide range 0.5 < z < 3.5. Two of the lower
redshift QSOs (#47, #301) have [NeIV] and [NeV] emission lines fully consistent with ionization from a nonstellar continuum (Gilli et al. 2010). The highest redshift
QSOs (#337, #660) are very similar to those previously
identified cases mentioned above (Stern et al. 2002; Norman et al. 2002). We have performed X-ray spectral
fits (Bauer et al. in preparation) to check whether thesesources have high levels of absorption, as expected given
the significant amount of extinction needed to hide thebroad line region. For the five QSOs that have sufficient X-ray counts (> 50) in the 0.5–8.0 keV band to
perform a fit with a power law model having two freeparameters (Γ, NH ), we have measured X-ray absorbing
column densities in the range of 8.25 − 24.0 × 1022 cm−2
thus indicative of values typical for X-ray selected type
2 QSOs (e.g., Tozzi et al. 2006; Mainieri et al. 2007).
Further support of their ’type 2’ nature requires deepnear-infrared spectroscopy to rule out a broad component to the Hα and Hβ emission lines (Akiyama, Ueda
& Ohta 2002). We conclude by emphasizing that the
optical magnitudes of these X-ray sources are very faint(RAB ∼ 24 − 25) thus further deep spectroscopy, particularly covering the blue end of the optical spectrum, iswarranted to make accurate estimates on the obscured
fraction of luminous QSOs at high redshifts.
5.1.2. AGN in unremarkable galaxies at z > 1.5
For almost three decades, it has been known that Xray observations are able to identify AGNs that lack optical signs, including narrow emission lines, of an active
nucleus (e.g., Elvis et al. 1981; Maiolino et al. 2003; Severgnini et al. 2003; Caccianiga et al. 2007). Their nature

has often been attributed to excess extinction (e.g., Comastri et al. 2002; Civano et al. 2007) possibly related
to their host galaxies being viewed edge-on (Rigby et al.
2006). Additional explanations include dilution by thehost galaxy (e.g., Moran et al. 2002; Georgantopoulos& Georgakakis 2005), unusually weak ionizing continua
possibly related to a radiatively-inefficient accretion flow
(Yuan & Narayan 2004), or beamed emission in the formof a relativistic jet (Caccianiga & Maccacaro 1997; Worrall et al. 1999). These AGN are typically X-ray faint,
thus the Chandra Deep fields are providing significant
samples for detailed studies. To date, most objects inthis class have been identified at z ≤ 1.2 due to thedifficulty of identifying galaxies without strong spectralfeatures at higher redshifts.
With our current spectroscopic campaign, we haveidentified, for the first time, four such systems at z > 1.5(mainly due to our deep exposures) with two of them having highly secure spectroscopic redshifts. Three ALGs
are present in the broad-band (0.5–8.0 keV) sample while
the other one is only detected in the soft-band (0.5–2.0
keV). Optical spectra are shown in Figure 9a − c withaccompanying 2D images that further demonstrate therobustness of the continuum and the reliability of absorption lines in these systems. Their faint optical magnitudes (RAB ∼ 24 − 25) have likely hampered previous
identification programs. Spectroscopic redshifts are determined by the presence of interstellar absorption lines(e.g., OII+SiIV, CII], CIV, AlII), and the continuum
shape as compared to high-redshift galaxy templates thatprovide further assurance of their redshifts.
We first highlight the spectroscopic identification ofan ALG at z = 3.208 (Figure 9a), the most distant ofsuch galaxies known to date. This X-ray source is de-14

Silverman et al.

−1

Fig. 9.— Optical spectra of high-redshift (z ∼ 2 − 3) AGN in normal galaxies. The units are flux density (erg cm−2 s−1 Å ). These
spectra were acquired with VIMOS using the LRblue grism. The data are binned by a factor of 2 here thus producing a resolution of 10.6
Å pix−1 . Spectroscopic features are labeled with the quality flag on the spectroscopic redshift and photometric redshift estimate given inparenthesis for the case (c) having low confidence. Each 2D image is shown as in the previous figure.

tected by Chandra in the soft-band (0.5–2 keV; ECDFS#247). Even though only 6 counts have been detected,
the source is reliable given the WAVDETECT threshold of 1 × 10−6 , two adjacent pixels each having two
detected photons, photon arrival times that are roughly
equally spaced, and the X-ray source is clearly coincident with an optical counterpart. We show the optical
spectrum obtained from the 9hr exposure with the VLT.
The 2D spectrum clearly illustrates the existence of absorption lines (Lyα, SiII, OII+SiIV, CII]), and a faintnarrow Lyα line clearly evident within the absorption
trough that appears to be spatially extended (∼ 2.5′′ ;19 kpc). In a second case (Fig. 9b; ECDFS #399), webase the spectroscopic redshift on an absorption line at∼4000 Å that is attributed to CII] and further supported
by a possible detection of Lyα in emission that falls close
to the end of the spectrum. Interestingly, there is no hintof CIV emission that is a typical signature for AGN activity. In panel c, we show a spectrum of an ALG that has apossible redshift of z = 2.296 although with considerable
uncertainty (flag=1). The fourth high redshift ALG (notshown) is of similar significance. We conclude that the
X-ray emission, from this class of objects at these highredshifts, is dominated by accretion onto a SMBH basedon their high X-ray luminosities (1043 < LX < 1044 erg
s−1 ).

6. TRACING LARGE-SCALE STRUCTURE WITH

AGNS AND GALAXY GROUPS
6.1. Extended X-ray sources and group/cluster
identification
The E-CDF-S not only includes unresolved X-ray
sources, but also a significant number of extended sources
clearly associated with galaxy overdensities (i.e., groups
and clusters; Giacconi et al. 2002; Rosati et al. 2002;
Lehmer et al. 2005). The identification of such structures enables us to compare their large-scale distribution
with that of AGNs. Here, we utilize both the Chandra
and XMM-N ewton observations of the E-CDF-S to construct an extended source catalog (Finoguenov et al. in
preparation) and identify potential galaxy members thatare then targeted for optical spectroscopy.
We briefly describe the XMM-N ewton observations
and subsequent analysis; we refer the reader toFinoguenov et al. (2009) for details on the extended
source detection method using combined Chandra and
XMM-N ewton data. The exposure time of the XMMN ewton observation is 541 ks; a final exposure of 309
ks was left after filtering events for periods of low background as described in Zhang et al. (2004). We follow thedetailed background removal procedure of Finoguenov etal. (2007) and monitor the appearance of atypical background pattern in MOS chips as identified in Snowden

Optical spectroscopy of X-ray sources

15

TABLE 6
Identification of extended groups/clusters
Index

XRA

XDEC

ORA

ODEC

Redshift

Type

Telescope

29
21

53.07578
53.19061-27.78844-27.68395

56
28
55
30

53.34735
53.33588
53.28957
52.96078-27.77114-27.79903-27.76900-27.82475

311052.96858
52.99811-27.83918-27.90738

53.07515
53.19500
53.18832
53.18774
53.18543
53.19051
53.34654
53.33691
53.33158
52.96048
52.95444
52.95737
52.96692
52.99954
52.99481-27.78851-27.68726-27.68261-27.68598-27.68917-27.70362-27.77106-27.79873-27.83082-27.82368-27.82728-27.82756-27.84015-27.90867-27.90663

0.73420.73280.73240.73280.72820.73290.83470.12880.68790.68120.68200.68030.67920.73810.7385

NELG
ALG
ALG
ALG
NELG
NELG
NELG
ALG
ALG
NELG
ALG
NELG
ALG
ALG
ALG

VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
VLT/VIMOS
KECK/DEIMOS
KECK/DEIMOS
KECK/DEIMOS
KECK/DEIMOS

Fig. 10.— Two examples of spectroscopically-identified groups/clusters (Table 10; #30, #21) with extended X-ray emission detected by
XMM-N ewton and Chandra as shown by the contours overlaid onto the WFI R-band image. Galaxies within a narrow redshift interval(∆z ∼ 0.01) are labelled: green triangles mark those with spectroscopic redshifts from our program while blue circles are from the GOODS
spectroscopic master catalog.

et al. (2008). The next step entails the removal of point
sources from our background-subtracted and vignettingcorrected images. We implement a source detection algorithm employing wavelets at the scales of 8 and 16 arcseconds. Subsequently, we restore the spatial distributionof the source flux based on knowledge of the PSF. After
subtracting a model image, we proceed with the analysisof the extended emission. The final catalog of extended
objects contains 52 sources, many of which have previously been identified (Giacconi et al. 2002; Lehmer et al.
2005). Newly detected sources are then compared withphotometric galaxy catalogs with most having a clear
association to an overdensity of galaxies.
We have targeted for spectroscopic observations a subset of galaxies that are potentially associated with thesegalaxy groups. In Table 6, we list galaxies that have asecure (f lag = 2) spectroscopic redshift (0.6 . z . 0.8)
thus providing a likely distance to eight extended X-ray
sources given their agreement with photometric redshiftestimates of associated group galaxies. At the time ofthe spectroscopic observations, no spectroscopic confirmation had been obtained for these groups. With therecent spectroscopic campaign from GOODS (Balestra
et al. 2010), six of the eight groups have further confirmation of their redshift. In Figure 10, we show two examples, with the galaxies that are used for the redshift determination labelled. The estimated X-ray luminosities(L0.5−2.0 keV ∼ 1041 − 1042 erg s−1 ) indicate that these
new systems are low-mass galaxy groups (Mhalo ∼ 1013
M⊙ ; Finoguenov et al., in preparation) thus improving
upon such samples at high redshift (see Leauthaud etal. 2009, for recent progress on the LX − Mhalo relation). The statistical properties of the extended X-ray
sources revealed by these observations will be presented
in Finoguenov et al. (in preparation). Below, we utilize
our new galaxy group catalog to map further the largescale features in the CDF-S that are typically traced by
AGNs.
6.2. Physical extent of large-scale structures
Deep X-ray surveys provide a unique opportunity tomeasure the clustering strength of AGNs on ∼10 Mpc
scales by utilizing the prevalence of moderate-luminosity(logLX ∼43) AGNs. Exceptional structures have been

16

Silverman et al.

observed in the 1 Ms CDF-S at z=0.67 and z=0.73 (Gilli
et al. 2003) that are narrow in redshift space (δz . 0.02)
and contain about 1/3 of the identified X-ray sources.
These sheets, equally populated with X-ray sources, arealso clearly evident in the galaxy distribution (K20;
Cimatti et al. 2002) with the z=0.73 spike having about
twice as many galaxies as the one at z=0.67, suggestingthat AGN activity may be sensitive to large-scale environment (Silverman et al. 2008a; Lehmer et al. 2009). Although, the dependency of AGN activity on host-galaxy
stellar mass (e.g., Kauffmann et al. 2004; Silverman etal. 2009a; Brusa et al. 2009; Xue et al. 2010) can mimic
signs of an environmental effect. Similar redshift features
have been found in the CDF-N, though far less prominent(1/8 of the X-ray sources) and are evidence of the large
cosmic variance between the deep fields: the correlation
length in the CDF-S (r0 =8.6±1.2 Mpc) is twice that in
the CDF-N (r0 =4.2±0.4 Mpc; Gilli et al. 2005). The
AGNs in these two narrow redshift intervals are spread
across the CDF-S field of view (17′ x17′ ) corresponding
to a linear size of > 7 Mpc; the z = 0.73 structure
is more compact (Adami et al. 2005; Salimbeni et al.
2009). These large-scale structures also include galaxygroups/clusters with extended X-ray emission, thus providing a tracer of the cosmic web which we further map
here with the extended area coverage of the field. Furthermore, we investigate the existence of higher redshift
structures identified including those at z=1.62 and 2.57,
which have been typically traced with only 4-5 objects(Gilli et al. 2003).
We show a histogram of the redshift distribution in
Figure 11 to determine whether the highly structured
features within the central 1 Ms CDF-S (Gilli et al. 2003)
are evident on larger scales. We label the spectroscopic
features discussed below with a letter designation (a−h).
Overall, the redshift distribution is clearly more evenly
populated as compared to that shown in Figure 1 of Gilli
et al. (2003), thus effectively reducing the level of cosmic
variance seen in the 1 Ms CDF-S. The well-known dominant spikes at z = 0.67 (b) and z = 0.73 (c) persist andextend beyond the central region (see below for further
discussion on their physical extent). At lower redshifts,
we confirm the significant overdensity at z = 0.53 (a),having 11 AGN in our sample, as reported by Treister
et al. (2009a); this result agrees with the galaxy distribution from the K20 survey. We note that the redshift
distribution below z < 0.5 is dissimilar to that presented
in Treister et al. (2009a) with less pronounced features inour sample. At higher redshifts (z ∼ 1), we not only confirm the previously-reported redshift spike at z = 1.04 (e)with better statistics (10 AGN) but find an equally populated overdensity at z = 0.97 (d), not seen in the K20
galaxy distribution but detected using a detailed analysisof the GOODS-MUSIC photometric catalog (Salimbeni
et al. 2009). At even higher redshifts, we confirm the enhancements previously seen at z = 1.62 (g) and 2.57 (h),and identify a new feature at z = 1.22 (f) that is not spatially concentrated on the sky. As a final note, we show
the distribution of galaxies with photometric redshifts
from GOODS-MUSIC (Fig. 11) and find that most of
the structures discussed here are smoothed out; the only
two identified features are enhancements at z = 0.67 andz ∼ 1 that are actually composed of two distinct structures as detailed above.

Fig. 11.— Spectroscopic redshift distribution of E-CDF-S X-ray
sources up to z = 3. Data have been binned with an interval of∆z = 0.02. For comparison, we plot the distribution of galaxies
with photometric redshifts from GOODS-MUSIC (red histogram).
Discrete features, based on the spectroscopic sample and describedin the text, are labeled by letter.

6.2.1. Structures at z ∼ 0.7
We show in Figure 12 the angular extent ofspectroscopically-confirmed AGN and galaxy groups
falling within the redshift spikes at z = 0.67 (panel a;∆z=0.04) and z = 0.73 (panel b; ∆z = 0.034). For theslightly lower redshift spike (z ∼ 0.67), it is evident thata significant number of AGN fall outside the area covered by the 2 Ms field. Based upon visual inspection,
there do appear to be slightly more AGNs and galaxygroups in the southwestern quadrant, although no clear
association between AGN and galaxy groups is evident.
Based on this distribution, we estimate that the structure has a physical angular extent of & 12 Mpc. The
width is shown in Figure 12 (bottom panel) and has acomoving dimension of 67.7 Mpc based on a Gaussian fit
to the redshift distribution; we describe this structure asa thick slab rather than a narrow sheet.
We find that the structure at z = 0.73 differs in bothangular extent and radial size as compared to the structure at z = 0.67. The AGNs are not spread over thefull field-of-view of the E-CDF-S and likely span a welldefined region (Fig. 12b) that is linear in shape with dimensions ∼ 10 Mpc (length) ×3 Mpc (width) and at
a position angle of roughly 45 degrees on the sky. Remarkably, the seven spectroscopically-identified galaxygroups appear to trace similar structure as the AGNs.
We also find that the width, along our line-of-sight, issignificantly narrower than the structure at z = 0.67
(Fig. 12 bottom). We measure a comoving radial extent
of 18.8 Mpc. Therefore, it appears that a narrow filament falls across the field-of-view of the E-CDF-S thatprovides an effective indication that such structures are
conducive to both accretion onto SMBHs and the formation of the potential wells of galaxy groups.
7. SUMMARY

Our program to acquire optical spectra and reliableredshifts for Chandra sources in the E-CDF-S is pre-

Optical spectroscopy of X-ray sources

17

Fig. 12.— T op : Spatial distribution of AGNs (squares) having spectroscopic redshifts within the two main redshift spikes: (a) z = 0.67(∆z = 0.04), (b) z = 0.73 (∆z = 0.034). Black circles mark the positions of spectroscopically-confirmed galaxy groups associated withextended X-ray emission with their radius indicative of r200 . The grey scale image is a smoothed version of the Chandra mosaic of the
E-CDF-S . Bottom : Co-moving radial distance over the redshift interval 0.53 < z < 1.32 for AGNs with spectroscopic redshifts. Prominent
structures are evident at z=0.67, 0.73, 0.97, 1.04, and 1.22. The well-known features at z ∼ 0.7 are fit by a gaussian function, as shown incolor, that have resulting widths (FWHM) of 67.7 (red) and 18.8 (green) Mpc.

sented. We utilize multi-slit instruments on both the
VLT and Keck to identify the faintest X-ray sources byimplementing deep exposures ranging from 2-9 hours.
These observations result in new spectroscopic redshifts
for 283 X-ray sources, including a subset belonging to thedeeper 2 Ms data; our high identification rate of X-ray
sources reaches 80% with the inclusion of photometricredshifts. Based on all available catalogs, there now exist over 500 X-ray sources in the full CDF-S area thathave a spectroscopic redshift.
We highlight a number of significant and unique contributions that this program brings to the study of the
faint X-ray source population.
• Improved coverage of the accessible luminosityredshift plane that includes AGNs, both with andwithout broad emission lines, spanning the faintend of the luminosity function at 1.5 < z < 3.
• The secure identification of seven new type 2 QSOs
(0.5 . z . 3.5) in the E-CDF-S field.
• The use of interstellar absorption (e.g., SiII,
OI+SiIV, CII, CIV]) lines to identify an elusive
population of optically-faint galaxies (Rmag ∼25) at z ∼ 2; we highlight the highest redshift
absorption-line galaxy (z = 3.208); this object has
no obvious signs of an AGN based on a deep (9hr)
optical spectrum.
• Improved AGN and galaxy groups samples thatfurther aid our ability to trace large-scale structures evident in the CDF-S. For example, the previously known redshift spikes (z = 0.67 and 0.73)
do extend beyond the original CDF-S field. The
higher redshift structure (z = 0.73) appears to be
more filamentary.
Further spectroscopic observations with VLT and the
Keck are currently being undertaken to increase the levelof completeness with respect to the identification of Xray sources in deep Chandra fields. In particular, twomore deep exposures (9hr each) in the E-CDF-S with the
VLT are currently being executed that target the optically faint population. With this remarkable data set, we
expect to further our understanding of the clustering ofmoderate-luminosity AGN, the luminosity function, andthe relation of AGN activity to their larger-scale environments that all shed light on the mechanisms responsible
for the growth of SMBHs and the galaxies in which they

18

Silverman et al.

reside.
The authors recognize support from Michael Cooper
for the use of the DEEP2 pipeline that was developed
at UC Berkeley with support from NSF grant AST0071048. WNB, BL, and YX acknowledge support fromthe Chandra X-ray Center grant SP8-9003A and NASAADP grant NNX10AC99G. GS acknowledges support of
the Polányi Fellowship of NKTH. DA is funded by theRoyal Society and Leverhulme Trust. MS and GH acknowledge support bei the Leibniz Prize of the Deutsche

Forschungsgemeinschaft, DFG (HA 1850/28-1). AC is
supported through ASI-INAF grants I/023/05/00 andI/088/06. PT acknowledges financial contribution fromcontract ASIINAF I/088/06/0. “The authors wish torecognize and acknowledge the very significant culturalrole and reverence that the summit of Mauna Kea has
always had within the indigenous Hawaiian community.
We are most fortunate to have the opportunity to conduct observations from this mountain.”
Facilities:
VLT:Melipal(VIMOS),Keck:II(DEIMOS),CXO (ACIS-I)

APPENDIXOPTICAL IDENTIFICATION OF ADDITIONAL TARGETS
X-ray sources in the 2 Ms CDF-S
During our observing campaigns with both the VLT and Keck, we placed additional slits on optical counterparts to
X-ray sources in the 1 and 2 Ms catalogs (Giacconi et al. 2002; Luo et al. 2008) that have fluxes below the limit of
the E-CDF-S survey (Lehmer et al. 2005). We use the optical counterpart as reported in Giacconi et al. (2002) basedon VLT/FORS1 images or that from the EIS survey (Arnouts et al. 2001). The optical counterparts to X-ray sources
solely detected in the 2 Ms catalog are reported in Luo et al. (2008) based on a WFI R-band image. In total, we havespectroscopically-identified 49 additional X-ray sources that are listed in Table 7.
Counterparts to radio (VLA) sources
To fully exploit the multiplex capabilities of VLT/VIMOS and KECK/DEIMOS, we have acquired spectra of opticalcounterparts to radio sources as secondary targets. The full E-CDF-S area has been imaged with the Very Large Array(VLA) at 1.4 GHz in two dedicated surveys (Kellermann et al. 2008; Miller et al. 2008). Using deep multi-wavelength
ancillary data (from the optical to the mid-IR), Mainieri et al. (2008) find reliable counterparts for 254 (∼ 95%) of the
266 radio sources in these catalogs. Here, we have acquired spectra and quality redshifts for 48 VLA sources covering abroad range in redshift 0.12 < z < 3.68. A number of these redshifts were presented in Mainieri et al. (2008). We givethe full list of identifications (Table 8) and provide optical spectra for the community. We note that the objects listed
in this table are not associated with an X-ray source in either the Lehmer et al. (2005) or Luo et al. (2008) catalogs.
We note that there are 18 newly-acquired spectroscopic identifications of X-ray sources that have radio counterparts
and are listed in either Table 4 or 7.
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20Silverman et al.
TABLE 7
Redshifts for X-ray sources solely detected in the 1-2 Msec catalogs
XIDa

RAopt (J2000)

DECopt (J2000)

Redshift

QualityClass

Program

18
31
51
53
92
116
130
135
136
143

52.958828
52.975166
53.001734
53.003757
53.033335
53.047677
53.057590
53.058665
53.059319
53.062076-27.828563-27.834931-27.874613-27.799100-27.782571-27.834926-27.757128-27.708438-27.779891-27.645422

0.845
0.744
0.631
0.975
2.619
2.326
0.643
2.024
0.840
0.328

2
2
2
2
2
1
1
2
2
1

NELG
NELG
ALG
NELG
NELG
ALG
NELG
BLAGN
NELG
ALG

15
12
10
10
12
12
5
12
15
5

Note. — An abbreviated version of the table is shown. The full table is provided online
with 49 entries.
a XID from Luo et al. (2008).

TABLE 8Spectroscopic redshifts of VLA radio sources
Radio ID

RAopt (J2000)

DECopt (J2000)

RAB

Redshift

QualityClass

Program

5
12
20
27
34
51
59
69
75
82

52.81024
52.83395
52.86917
52.8925652.93052
52.96157
52.97519
52.99545
53.01342
53.02682-27.92961-27.65033-27.82635-27.64125-27.85055-27.78431-27.83488-27.71780-27.88690-27.79132

23.4
20.3
22.8
22.9
19.1
23.6
21.9
23.2
24.9
22.7

1.591
0.526
0.846
1.040
0.310
2.259
0.744
1.043
1.396
1.021

2
2
2
2
2
2
2
2
9
2

BLAGN
NELG
ALG
ALG
NELG
BLAGN
NELG
NELG
NELG
NELG

KECK
KECK
VLT
KECK
KECK
KECK
VLT
KECK
KECK
VLT

Note. — An abbreviated version of the table is shown. The full table is provided online with 48 entries.