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The Astrophysical Journal, 645:955 - 976, 2006 July 10
# 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.

ChaMP SERENDIPITOUS GALAXY CLUSTER SURVEY
W. A. Barkhouse, P. J. Green,1,2 A. Vikhlinin,2 D.-W. Kim,2 D. Perley,3 R. Cameron,4 J. Silverman,1,5 A. Mossman, R. Burenin,6 B. T. Jannuzi,7 M. Kim,2 M. G. Smith,8 R. C. Smith,8 H. Tananbaum,2 and B. J. Wilkes2
Received 2005 June 21; accepted 2006 March 19
1,2 2

ABSTRACT We present a survey of serendipitous extended X-ray sources and optical cluster candidates from the Chandra Multiwavelength Project (ChaMP). Our main goal is to make an unbiased comparison of X-ray and optical cluster detection methods. In 130 archival Chandra pointings covering 13 deg 2, we use a wavelet decomposition technique to detect 55 extended sources, of which 6 are nearby single galaxies. Our X-ray cluster catalog reaches a typical flux limit of about $10À14 ergs cmÀ2 sÀ1, with a median cluster core radius of 21 00 . For 56 of the 130 X-ray fields, we use the ChaMP's deep NOAO 4 m MOSAIC g 0 , r 0 , and i 0 imaging to independently detect cluster candidates using a Voronoi tessellation and percolation ( VTP) method. Red-sequence filtering decreases the galaxy fore- and background contamination and provides photometric redshifts to z $ 0:7. From the overlapping 6.1 deg 2 X-ray/optical imaging, we find 115 optical clusters (of which 11% are in the X-ray catalog) and 28 X-ray clusters (of which 46% are in the optical VTP catalog). The median redshift of the 13 X-ray/optical clusters is 0.41, and their median X-ray luminosity (0.5 - 2 keV ) is LX ? ð2:65 Æ 0:19Þ ; 1043 ergs sÀ1. The clusters in our sample that are only detected in our optical data are poorer on average ($4 ) than the X-ray/optically matched clusters, which may partially explain the difference in the detection fractions. Subject headings: galaxies: clusters: general -- surveys -- X-rays: galaxies: clusters g

1. INTRODUCTION A primary goal of modern astronomy is to study the formation and evolution of galaxies. Clusters of galaxies provide us with laboratories in which galaxy evolution can be studied over a large range in cosmic look-back time. The high-density cluster environment probes the impact of high galaxy density on the fate of the cluster galaxy population. Interactions, mergers, and dynamical effects (e.g., tidal forces and ram pressure stripping) may play significant roles in shaping galaxy evolution in these type of locales (e.g., Dubinski 1998; Moore et al. 1999). Galaxy clusters are also the most massive, mainly virialized, concentrations of matter in the universe and act as tracers of the underlying dark matter. Clusters thus also play a key role constraining fundamental cosmological parameters such as m (the matter-density parameter) and 8 (the rms density fluctuation on a scale of 8 hÀ1 Mpc). The number density of clusters as a function of mass and redshift strongly depends on m and 8 (see Rosati et al. 2002 and references therein). This remarkable fea1 Visiting Astronomer, Kitt Peak National Observatory ( KPNO) and Cerro Tololo Inter-American Observatory (CTIO), National Optical Astronomy Observatory ( NOAO), operated by the Association for Universities for Research in Astronomy (AURA), Inc., under contract to the National Science Foundation ( NSF ). 2 Harvard-Smithsonian, Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. 3 Department of Astronomy, 601 Campbell Hall, University of California, Berkeley, CA 94720. 4 Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, CA 94025. 5 Max-Plank-Institut f ur extraterrestrische Physik, Giessenbachstrasse, Å 85741 Garching, Germany. 6 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya Street 84/32, Moscow 117997, Russia. 7 National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726. 8 Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile.

ture of hierarchical cluster formation, via the Press-Schechter formalism ( Press & Schechter 1974), affords the opportunity to provide an independent confirmation of various cosmological quantities obtained recently by other techniques (e.g., the Wilkinson Microwave Anisotropy Probe [WMAP]; Bennett et al. 2003). To facilitate the investigation of galaxy cluster evolution and provide constraints on cosmological parameters, a large sample of galaxy clusters spanning a redshift range from 0 < z < 1:5is required. The search for galaxy clusters has been conducted mainly using optical and X-ray techniques. Although these methods overlap in their ability to distinguish galaxy clusters from the general background, they sample different regions of parameter space that encompass nearly the complete range in physical attributes of clusters. For example, X-ray techniques detect clusters via extended emission from the hot gas that makes up the intracluster medium ( ICM; e.g., Vikhlinin et al. 1998b). X-ray detection suffers little from source confusion (e.g., Basilakos et al. 2004; Popesso et al. 2004) but tends to select more massive, virialized clusters. By contrast, optical methods rely on the detection of an overdensity of galaxies or a population of early-type galaxies with a narrow range of colors (red sequence; Gladders & Yee 2000). Optical methods are generally more sensitive to nonvirialized (or young) systems than X-ray searches, but they are also susceptible to projection effects and bias toward more evolved galaxy populations. Multiwavelength cluster detection schemes help to ensure a higher degree of completeness and reliability in any cluster compilation ( Postman 2002). Several recent studies have compared cluster samples compiled from independent techniques using optical and X-ray data. Donahue et al. (2002), for example, applied a matched-filter method to optical data and found that 60% (26 out of 43) of ROSAT X-ray clusters had optical matches. They also determined that optical clusters/groups outnumbered X-ray extended sources by a factor of 3. Gilbank et al. (2004), applying a matched-filter algorithm to optical data, found 75% matches (9/12) to a sample 955


956

BARKHOUSE ET AL. taken from Stark et al. (1992) and are tabulated in Table 1 for fields containing at least one extended X-ray source. The optical data for this study consists of ChaMP mosaic images acquired from NOAO 4 m telescopes in the g 0 , r 0 , and i 0 bandpasses (see Table 2). The optical and X-ray imaging overlap by 6.1 deg 2. These data are used for source identification and to compare optical cluster detection methods against X-ray techniques for the area in common (see x 3). Details of image reduction and analysis for the initial sample of six ChaMP mosaic fields are presented in Green et al. (2004), and an overview of our complete sample of 56 fields is in W. A. Barkhouse et al. (2006, in preparation). In summary, our optical exposure times were scaled to the X-ray exposures to probe a constant X-ray/optical flux ratio. The optical follow-up was optimized to probe AGN counterparts and not faint galaxies at a similar redshift for a given X-ray luminosity. The image reduction was performed using the mscred package within the IRAF10 environment. Object detection and photometry was conducted using SExtractor ( Bertin & Arnouts 1996). Photometric calibrations were done using standard stars from Landolt (1992), which were converted to the SDSS photometric system using the transformation equations from Fukugita et al. (1996). Table 2 summarizes the optical properties of the 36 mosaic fields that overlap X-ray fields containing X-ray-detected extended sources or optical cluster candidates. 3. GALAXY CLUSTER DETECTION METHODS We have used the X-ray and optical data sets from ChaMP to search for and contrast galaxy cluster samples compiled independently from X-ray and optical cluster search techniques for the overlapping 6.1 deg 2 sky coverage. In the following sections we describe each detection method, with an emphasis on the description of the optical technique (see Vikhlinin et al. 1998b for a detailed description of the extended X-ray source detection algorithm). 3.1. Extended X-Ray Source Detection The extended X-ray source detection is based on a wavelet decomposition technique--plus a maximum likelihood method to determine the significance of each detected extended source-- that is similar to the method described in Vikhlinin et al. (1998b). In brief, each extended source was detected in the 0.7 - 2 keV energy band to maximize the contrast of the cluster ICM against the X-ray background. A Gaussian kernel was fit to each wavelet source and its best-fit radius was compared with the point-spread function ( PSF ) size appropriate for the measured off-axis angle. Those objects determined to be ``pointlike'' were then subtracted, and the detection process applied to the resultant image. The sample of X-ray sources deemed ``extended'' was then fit on the original image to a standard  model, I ðr; rc Þ ? I0 ?1 þ ðr/rc Þ2 À3þ0:5 (Cavaliere & Fusco-Femiano 1976), with point sources masked out. Since a free fit was not possible due to the small number of photons expected for most sources, we fixed the value for  at 0.67 (e.g., Vikhlinin et al. 1998b; Donahue et al. 2002; Moretti et al. 2004).
3.1.1. Final X-Ray-Selected Cluster Catalog

of ROSAT X-ray clusters. Using a cluster detection method based on the red sequence of early-type cluster galaxies, Gilbank et al. (2004) was able to achieve a matched fraction of 100% (10/10) using the same X-ray data set. In addition, Basilakos et al. (2004) and Kolokotronis et al. (2006) -- using a smoothing percolation technique on optical data--found matched fractions of 75% (3/4) and 68% (13/19), respectively, for extended X-ray sources compiled from archival XMM-Newton observations. In all such studies, the matched fractions depend on the relative optical / X-ray flux limits and the sensitivity of the detection algorithms. The aims of this paper are to present a new serendipitous X-ray cluster sample based on data from the Chandra Multiwavelength Project (ChaMP; Kim et al. 2004a; Green et al. 2004)9 and to make an unbiased comparison of X-ray and optical cluster detection methods. We thus explore a variety of questions: Are there massive, X-ray-luminous clusters that are optically poor? Do all massive clusters emit X-rays? What types of optical clusters retain hot gas, and why? In addition, we present the X-ray and optical properties of our sample of serendipitously detected clusters/ groups, including a comparison of X-ray luminosity with optical cluster richness. Finally, we provide the community with a compilation of newly discovered clusters/groups that can be used in conjunction with other samples to constrain cosmological parameters. This sample should also help to address how the more numerous ( but less well studied) low-luminosity clusters and groups evolve. This paper is organized as follows. In x 2 we describe the sample selection and X-ray and optical observations. In x 3 we discuss our X-ray and optical cluster detection methods. The properties of our X-ray and optical cluster candidates are presented in x 4, along with a comparison of the two compilations. Finally, in x 5 we compare our results with previous studies and discuss possible bias inherit in our X-ray and optical cluster detection schemes. Unless otherwise indicated, we use m ? 0:3, k ? 0:7, and H0 ? 70 km sÀ1 MpcÀ1 throughout. 2. SAMPLE SELECTION AND OBSERVATIONS This study makes use of the data provided by ChaMP. ChaMP is a $13 deg 2 ( based on Cycles 1 and 2 Chandra archival data) survey of serendipitous Chandra X-ray sources at flux levels ( fX $ 10À15 to 10À14 ergs sÀ1 cmÀ2), intermediate between the Chandra deep surveys and previous X-ray missions. Optical follow-up of ChaMP fields was conducted using the MOSAIC camera on the KPNO and CTIO 4 m telescopes. The mosaic imaging of ChaMP was designed to search for optical counterparts to active galactic nuclei (AGNs) in part to drive our spectroscopy identification program. At present, 56 mosaic fields in g 0 , r 0 , and i 0 to r 0 P 25 (5 detection) have been acquired ( W. A. Barkhouse et al. 2006, in preparation). For a description of ChaMP methods, analysis, and early science results, see Kim et al. (2004a, 2004b, 2006), Green et al. (2004), Silverman (2004), and Silverman et al. (2005a, 2005b). The X-ray data for this study are drawn from 130 fields selected from Chandra AO1 and AO2 observing periods. The fields were selected based on the following criteria: (1) include only ACIS imaging fields (excluding the ACIS-S4 chip); (2) include only fields more than 20 from the Galactic plane to minimize extinction; (3) exclude fields dominated by large extended sources; (4) include no planetary observations; (5) include no survey observations by PI; and (6) include no fields close to the LMC, SMC, and M31 (see Kim et al. [2004a] for a detailed discussion of selection criteria and X-ray data reductions). Galactic NH values are
9

The initial sample of extended X-ray sources is comprised of PI target clusters, serendipitous clusters, nearby bright galaxies, and spurious detections caused by chip gaps, edge effects, etc. Visual inspection and cross-correlation to Chandra PI targets
IRAF is distributed by NOAO, which is operated by AURA, Inc., under the cooperative agreement with the NSF.
10

See http://hea-www.harvard.edu/CHAMP.


TABLE 1 X-Ray-detected Ex tended S ources Source Name CXOMP J002650.2+171935 ........................ J005848.1À280035 ....................... J010214.1+314915 ........................ J010607.0+004943 ........................ J010610.3+005126 ........................ J013642.6+204843 ........................ J033639.4À045515 ....................... J033722.6À045906 ....................... J033755.1À050733 ....................... J033757.8À050001 ....................... J040351.2À170823 ....................... J054152.7À410702 ....................... J054240.1À405503 ....................... J063057.7+820701 ........................ J090634.4+340055 ........................ J091008.4+541852 ........................ J091126.6+055012 ........................ J091301.4+054814 ........................ J093102.2+791320 ........................ J093352.9+552619 ........................ J095012.8+142351 ........................ J101008.7À124013 ....................... J101115.3À124147 ....................... J105624.6À033517 ....................... J111405.8+403157 ........................ J111726.1+074335 ........................ J111730.2+074618 ........................ J114008.2À263132 ....................... J114118.8+660209 ........................ J122927.1+752037 ........................ J122940.6+752106 ........................ J131709.9+285513 ........................ J131722.0+285353 ........................ J134507.8+000359 ........................ J134514.6À000846 ....................... J141152.6+520937 ........................ J141556.8+230727 ........................ J141602.1+230647 ........................ J153259.2À004414 ....................... J153415.0+232459 ........................ J154932.0+213300 ........................ J160847.1+654139 ........................ J160948.4+660056 ........................ J165514.4À082944 ....................... J205537.4À043334 ....................... J205617.2À044154 ....................... J220455.8À181524 ....................... J221326.2À220532 ....................... J223538.4+340609 ........................ J223614.5+335648 ........................ J230150.7+084352 ........................ J230227.7+083901 ........................ J230252.0+084137 ........................ J230311.1+085131 ........................ J234817.8+010617 ........................ R.A. (J2000.0) 00 00 01 01 01 01 03 03 03 03 04 05 05 06 09 09 09 09 09 09 09 10 10 10 11 11 11 11 11 12 12 13 13 13 13 14 14 14 15 15 15 16 16 16 20 20 22 22 22 22 23 23 23 23 23 26 58 02 06 06 36 36 37 37 37 03 41 42 30 06 10 11 13 31 33 50 10 11 56 14 17 17 40 41 29 29 17 17 45 45 11 15 16 32 34 49 08 09 55 55 56 04 13 35 36 01 02 02 03 48 50.2 48.1 14.1 07.0 10.3 42.6 39.4 22.7 55.1 57.8 51.2 52.7 40.1 57.7 34.4 08.4 26.6 01.4 02.2 52.9 12.8 08.7 15.3 24.6 05.8 26.1 30.2 08.2 18.8 27.1 40.6 09.9 22.0 07.8 14.6 52.6 56.8 02.1 59.2 15.0 32.0 47.1 48.4 14.4 37.4 17.2 55.8 26.2 38.4 14.5 50.7 27.7 52.0 11.1 17.8 Decl. (J2000.0) +17 À28 +31 +00 +00 +20 À04 À04 À05 À05 À17 À41 À40 +82 +34 +54 +05 +05 +79 +55 +14 À12 À12 À03 +40 +07 +07 À26 +66 +75 +75 +28 +28 +00 À00 +52 +23 +23 À00 +23 +21 +65 +66 À08 À04 À04 À18 À22 +34 +33 +08 +08 +08 +08 +01 19 00 49 49 51 48 55 59 07 00 08 07 55 07 00 18 50 48 13 26 23 40 41 35 31 43 46 31 02 20 21 55 53 03 08 09 07 06 44 24 33 41 00 29 33 41 15 05 06 56 43 39 41 51 06 35.7 35.7 15.6 43.7 26.1 43.7 15.4 05.8 33.6 00.9 23.2 02.7 03.3 01.2 55.6 52.3 12.5 14.0 20.9 19.6 51.7 13.1 47.1 17.4 57.4 35.3 18.7 32.6 09.4 37.2 06.6 13.7 53.0 59.0 46.5 37.2 27.1 47.8 14.7 59.7 00.7 39.2 56.9 44.0 34.8 54.8 24.3 32.4 09.3 48.4 52.5 01.4 37.0 31.2 17.2 Exposurea (s) 40,346 12,138 54,166 3757 3757 45,094 60,512 60,512 60,512 60,512 3891 51,050 51,050 47,933 9907 107,136 29,165 29,162 19,165 41,296 13,962 44,730 44,733 90,211 30,054 26,832 26,832 39,978 119,222 48,010 48,010 112,806 112,806 9760 9760 92,102 14,755 14,755 5152 57,181 42,688 44,648 44,648 9152 44,880 44,880 5146 20,774 19,955 19,955 109,955 109,955 109,955 109,955 37,322 Galactic NHb (1020 cmÀ2) 4.19 1.55 5.50 3.15 3.15 5.71 4.98 4.98 4.98 4.98 2.30 3.59 3.59 5.27 2.28 1.98 3.70 3.70 1.90 1.99 3.13 6.74 6.74 3.67 1.91 4.01 4.01 4.96 1.18 2.69 2.73 1.04 1.04 1.93 1.93 1.34 1.91 1.91 6.25 4.28 4.30 2.83 2.83 13.40 4.96 4.96 2.79 2.49 7.74 7.74 5.05 5.05 5.05 5.05 3.81

ObsID 929 2248 521 2180 2180 2129 796 796 796 796 2182 914 914 1602 1596 2227 419 419 839 805 2095 926 926 512 2209 363 363 898 536 2253 2253 2228 2228 2251 2251 2254 2024 2024 2085 869 326 2127 2127 615 551 551 2114 1479 789 789 918 918 918 918 861

Note.-- Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Vignetting-corrected exposure time. b Galactic NH values are taken from Stark et al. (1992).


TABLE 2 Optical Mosaic F ields Total Exposure (s) 2100 2000 4500 1200 1500 1500 900 600 600 1000 500 360 900 600 600 4500 2400 2000 3000 3000 3000 1000 500 500 1400 500 500 1800 1440 1260 2700 2400 1200 2400 2100 2520 180 120 180 180 180 180 1260 1080 1170 1800 1200 900 3500 3000 2000 990 810 900 1800 1200 900 4500 3000 1500 1800 1200 900 900 720 Air Mass (mean) 1.05 1.15 1.18 2.26 1.94 1.46 1.50 1.40 1.32 1.36 1.28 1.25 1.06 1.08 1.09 1.24 1.24 1.32 1.15 1.04 1.00 1.33 1.41 1.50 1.04 1.02 1.02 1.27 1.40 1.52 1.38 1.61 1.30 1.11 1.28 1.48 1.07 1.09 1.11 1.06 1.06 1.06 1.24 1.17 1.17 1.02 1.06 1.10 1.46 1.44 1.52 1.04 1.06 1.08 1.02 1.00 1.00 1.13 1.09 1.14 1.15 1.10 1.07 1.01 1.02 FWHM b (arcsec) 1.1 1.3 1.1 1.8 1.6 1.3 1.3 1.7 1.1 1.6 1.6 1.2 1.0 1.0 1.1 1.3 1.1 1.3 1.5 1.2 1.1 1.9 2.0 1.7 1.5 1.5 1.2 1.4 1.1 1.4 1.1 1.2 1.1 1.8 1.6 1.3 1.4 1.3 1.2 1.1 1.0 1.0 1.4 1.4 1.1 1.0 1.0 0.8 1.1 1.0 1.1 1.2 1.2 1.2 1.0 1.0 0.8 1.2 1.3 1.2 1.1 1.2 0.9 1.6 1.3 MTOc (mag) 24.88 24.38 24.62 23.62 23.88 23.12 24.38 23.62 23.88 24.12 23.38 22.88 24.88 24.38 23.62 24.88 24.38 23.62 24.62 24.38 23.88 22.88 22.12 22.12 23.62 22.88 23.12 24.88 24.62 23.62 25.12 24.62 23.62 24.62 23.88 23.62 23.88 23.62 22.88 23.62 23.62 22.62 24.62 24.38 23.62 25.38 24.62 23.88 25.12 24.62 23.62 25.12 24.38 23.62 25.38 24.62 23.88 24.88 24.38 23.38 24.88 24.38 23.88 24.38 24.12

ObsID 326.............................................

E(B À V ) 0.046

a

Telescope KPNO 4 m

UT Date 2001 Jun 13 2001 Jun 12 2001 Jun 13

Filter g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Dithers 3 5 15 2 5 5 3 3 3 2 1 1 3 3 3 5 3 5 5 5 5 2 1 1 2 1 1 2 3 3 3 3 3 3 3 7 3 2 3 3 3 3 3 3 3 3 3 3 5 5 5 3 3 3 3 3 3 5 5 5 3 3 3 3 3

363.............................................

0.041

KPNO 4 m

367.............................................

0.047

KPNO 4 m

2004 Jun 19

431.............................................

0.071

KPNO 4 m

2000 Jun 11

507.............................................

0.061

CTIO 4 m

2003 Apr 7

512.............................................

0.034

KPNO 4 m

2001 Feb 22

521.............................................

0.061

KPNO 4 m

2001 Oct 24

541.............................................

0.007

KPNO 4 m

2000 Jun 12

546.............................................

0.035

KPNO 4 m

2000 Jun 11

551.............................................

0.079

KPNO 4 m

2000 Oct 17

796.............................................

0.046

KPNO 4 m

2001 Oct 24

800.............................................

0.019

KPNO 4 m

2001 Jun 14

813.............................................

0.015

CTIO 4 m

2000 Sep 30

842.............................................

0.058

CTIO 4 m

2000 Sep 30

861.............................................

0.025

CTIO 4 m

2000 Sep 29

898.............................................

0.038

CTIO 4 m

2003 Apr 7

913.............................................

0.014

KPNO 4 m

2001 Oct 23

914.............................................

0.036

CTIO 4 m

2000 Sep 29

915.............................................

0.051

CTIO 4 m

2003 Apr 7

918.............................................

0.081

KPNO 4 m

2001 Oct 23

926.............................................

0.071

CTIO 4 m

928.............................................

0.052

CTIO 4 m

2003 2003 2003 2000

Apr Apr Apr Sep

6 7 7 29


ChaMP GALAXY CLUSTER SURVEY
TABLE 2-- Continued Total Exposure (s) 810 2700 2550 2080 1200 900 720 800 750 400 1805 810 570 1800 1200 900 1800 1200 900 1800 1200 900 900 600 600 100 90 85 400 150 120 400 150 120 1800 1200 900 1800 1200 900 1800 1200 900 1800 1200 900 Air Mass (mean) 1.04 1.11 1.07 1.08 1.21 1.39 1.44 1.56 1.56 1.56 1.08 1.09 1.10 1.08 1.16 1.24 1.54 1.36 1.28 1.03 1.03 1.04 1.17 1.24 1.30 1.21 1.21 1.21 1.06 1.07 1.08 1.30 1.34 1.28 1.36 1.43 1.54 1.15 1.10 1.07 1.20 1.15 1.12 1.07 1.13 1.20 FWHM b (arcsec) 1.0 1.2 1.3 1.2 1.0 1.0 1.1 1.4 1.1 1.0 1.0 1.0 0.9 1.1 1.0 1.2 1.1 1.1 0.9 1.0 1.0 0.8 0.9 1.0 0.9 2.1 1.6 1.3 1.0 0.9 0.9 1.1 1.1 0.9 1.4 1.1 1.4 1.7 1.4 1.2 1.4 1.4 1.2 0.9 0.8 0.9

959

ObsID

E(B À V )a

Telescope

UT Date

Filter i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i g r i
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Dithers 3 3 3 4 2 2 2 1 1 1 5 3 3 3 3 3 3 3 3 3 3 3 3 3 3 1 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3

MTOc (mag) 23.62 25.62 24.62 24.38 24.88 24.38 23.62 24.38 23.88 23.12 24.38 23.88 23.62 24.88 24.38 23.62 24.62 24.38 23.62 25.12 24.62 24.12 24.88 24.38 23.88 22.88 22.38 21.38 23.62 23.12 22.12 23.12 22.88 22.12 24.88 24.62 23.62 24.38 24.12 23.62 25.12 24.38 23.88 25.12 24.88 24.12

930.............................................

0.021

KPNO 4 m

2001 Feb 22

1479...........................................

0.033

CTIO 4 m

2001 Aug 22

1602...........................................

0.080

KPNO 4 m

2001 Oct 23

1644...........................................

0.030

CTIO 4 m

2001 Aug 9

1657...........................................

0.027

KPNO 4 m

2004 Jun 17

1694...........................................

0.065

KPNO 4 m

2004 Jun 17

1899...........................................

0.041

KPNO 4 m

2004 Jun 17

2024...........................................

0.024

KPNO 4 m

2004 Jun 19

2099...........................................

0.044

KPNO 4 m

2001 Dec 14

2113...........................................

0.026

CTIO 4 m

2001 Aug 9

2114...........................................

0.030

CTIO 4 m

2001 Aug 9

2127...........................................

0.034

KPNO 4 m

2004 Jun 19

2210...........................................

0.014

KPNO 4 m

2004 Jun 18

2221...........................................

0.020

KPNO 4 m

2004 Jun 18

2228...........................................

0.009

KPNO 4 m

2004 Jun 19

a b c

Galactic extinction values are calculated from the maps of Schlegel et al. (1998). FWHM of point sources in final stacked image. Turnover magnitude of galaxy counts using 0.25 mag bins prior to extinction correction.

was used to assemble a final list consisting of 55 high-confidence serendipitously detected extended sources (see Table 1 and Fig. 1). From the sample of 55 extended X-ray sources, 6 were found to be associated with low-redshift galaxies (3 ellipticals, 2 spirals, and 1 S0/Sa galaxy). The X-ray flux for each source was computed from the total number of counts by extrapolating the  model fit to infinity. Also assumed was a Raymond-Smith thermal spectrum with a temperature of TX ? 2 keV, a solar abundance of Z ? 0:3, and Galactic extinction appropriate for each field. We use TX ? 2 keV since it is appropriate based on the median LX of our cluster sample (LX $ 1043 ergs sÀ1) and the TX -LX relation (e.g.,

White et al. 1997). Using TX ? 5 keV, for example, will change fX by $8%. X-ray flux values were converted to the 0.5 - 2 keV energy band and uncertainties derived from Poisson statistics. X-ray luminosities were calculated from measured fluxes using redshift estimates derived from (in order of preference): (1) the ChaMP spectroscopic program (Green et al. 2004) or (2) published spectroscopic redshifts or (3) were estimated from our red-sequencefiltered VTP optical cluster detection method (see x 3.2). In Table 3 the X-ray properties of our extended source catalog are tabulated. Figure 2 shows the all-sky distribution of our final sample of 55 extended X-ray sources.


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BARKHOUSE ET AL. cells--the so-called Kiang distribution ( Kiang 1966). Overdense regions composed of adjacent Voronoi cells are flagged as potential clusters if their density is greater than a specified threshold. A random fluctuation in the background can potentially exceed the imposed threshold constraint and thus be counted as a real cluster. This contamination is minimized by computing the probability ( based on simulations) that a given detection is a random background fluctuation and then only including regions above an acceptable level (see x 3.2.2). To improve the contrast of cluster galaxies with respect to the background field population, we have implemented a refined version of the VTP method that takes advantage of the existence of the red sequence in the color-magnitude relation of early-type cluster galaxies (e.g., Baum 1959; Sandage & Visvanathan 1978; ? Lopez-Cruz et al. 2004). The red sequence for early-type cluster galaxies shifts to progressively redder observed colors as the 4000 8 break moves through the filter bandpasses with increasing redshift (i.e., the K -correction; Humason et al. 1956; Oke & Sandage 1968). The position of the cluster red sequence in the colormagnitude plane can be used as an estimator of redshift (Gladders ? & Yee 2000; Lopez-Cruz et al. 2004). Thus, choosing appropriate filters enables foreground and background galaxies to be culled to minimize the contamination from the field galaxy population. As an example, Kim et al. (2002) applied the VTP algorithm to SDSS galaxy catalogs constructed by selecting galaxies relative to the expected red sequence in the gà À rà versus rà color-magnitude plane for clusters at various redshifts. The color width blueward of the red sequence for each redshift slice was chosen to be relatively broad [Áð gà À rÃ Þ $ 0:6 mag; see their Fig. 2]. The ChaMP optical data consist of magnitudes measured in the g 0 , r 0 , and i 0 bands. Since we are interested in assembling a cluster sample that extends to high redshift (z > 0:5), we have elected to use the r 0 À i 0 color to select galaxies, since g 0 À r 0 becomes degenerate at z P 0:4 ( T. Kodama 2004, private communication). The r 0 À i 0 color allows us to sample cluster red sequences out to z $ 0:7 (see Fig. 4). The basic procedure is to construct catalogs containing galaxies with a r 0 À i 0 color distribution that matches a particular red sequence for a given redshift. Catalogs are produced for red sequences that sample the redshift range from z ? 0:05 - 0.70. VTP is then applied to each galaxy catalog, and the most significant detections (as flagged by VTP) are included in the final cluster compilation for a given field. The advantage of this technique over the standard VTP method (e.g., Ramella et al. 2001) is that the ``noise'' from field galaxies is reduced and also that the redshift of the detected cluster can be estimated from the catalog yielding the greatest detection significance. Several recent studies have been successful in using color cuts relative to the red sequence to search for clusters using optical data (e.g., Gladders & Yee 2000; Goto et al. 2002; Nichol 2004; Hsieh et al. 2005). For the location of the red sequence in the color-magnitude plane, we adopt the models of Kodama & Arimoto (1997) transformed to the SDSS filters ( T. Kodama 2004, private communication). Each galaxy catalog is generated for a specific red sequence by selecting galaxies with a r 0 À i 0 color within Æ0.1 mag of the red-sequence line (all galaxies are corrected for galactic extinction prior to the selection process; see Table 2). We choose a color width of 0.1 mag either side of the red sequence, since the measured dispersion of cluster galaxies along the red sequence is ? $0.07 mag (e.g., Bower et al. 1992; Ellis et al. 1997; LopezCruz et al. 2004). To ensure that our galaxy catalogs sample the complete range in color for our expected cluster redshift distribution, we construct galaxy samples for 27 red sequences from z ? 0:05 - 0.70 ( Fig. 4). The density of these model red sequences in

Fig. 1.-- Part of a smoothed Chandra image, ObsID 796, showing the location of three serendipitously detected extended X-ray sources on three ACIS-I chips (circles;40 00 in radius). The PI target, the blue compact dwarf galaxy SBS 0335À052, is located near the center of the image.

3.2. Optical Cluster Detection The detection of galaxy clusters from optical data has had a