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THE ASTROPHYSICAL JOURNAL, 560 : 86 õ 91, 2001 October 10
( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

V

1WGA J1226.9]3332 : A HIGH-REDSHIFT CLUSTER DISCOVERED BY CHANDRA I. CAGNONI,1,2 M. ELVIS,2 D.-W. KIM,2 P. MAZZOTTA,2,3 J.-S. HUANG,2 AND A. CELOTTI1
Received 2000 December 11 ; accepted 2001 May 31

ABSTRACT We report the detection of 1WGA J1226.9]3332 as an arcminute-scale extended X-ray source with the Chandra X-Ray Observatory. The Chandra observation and R- and K-band imaging strongly support the identiïcation of 1WGA 1226.9]3332 as a high-redshift cluster of galaxies, most probably at z \ 0.85 ^ 0.15, with an inferred temperature kT \ 10`4 keV, and an unabsorbed luminosity (in a ~3 r \ 120@@ aperture) of 1.3`0.16 ] 1045 ergs s~1 (0.5õ10 keV). This indication of redshift is also supported ~0.14 by the K- and R-band imaging and is in agreement with the spectroscopic redshift of 0.89 found by Ebeling and coworkers. The surface brightness proïle is consistent with a b model with b \ 0.770 ^ 0.025, r \ 18A ^ 0A (corresponding to 101 ^ 5 kpc at z \ 0.89), and S(0) \ 1.02 ^ 0.08 .1 .9 c counts arcsec~2. 1WGA J1226.9]3332 was selected as an extreme X-rayõloud source with F /F [ 60 ; XV this selection method, thanks to the large area sampled, seems to be a highly efficient method for ïnding luminous, high-z clusters of galaxies. Subject headings : galaxies : clusters : general õ galaxies : clusters : individual (1WGA J1226.9]3332) õ galaxies : high-redshift õ X-rays : galaxies : clusters õ X-rays : individual (1WGA J1226.9]3332) On-line material : color ïgure
1.

INTRODUCTION

Clusters of galaxies are good tracers of the large-scale structure of the matter distribution in the universe. The standard models of structure formation predict that cluster distribution and evolution are fully determined by the spectrum of primordial perturbations and cosmological parameters ) and " (e.g., Press & Schechter 1974, and later works) ;0 thus, observations of high-redshift clusters constrain these parameters (e.g., Oukbir & Blanchard 1992). Moreover, X-ray measurements of high-redshift clusters of galaxies can place strong constraints on the thermodynamic evolution of the intracluster medium (ICM). For example, the luminosity-temperature (L -T ) relation at dierent redX shifts probes the interrelated evolution of the cluster baryon mass and the total mass (e.g., Kaiser 1991 ; Evrard & Henry 1991, and later works). Furthermore, X-ray and SunyaevZeldovich eect observations of a sample of objects at different z can be used to obtain an independent estimate of H (for a review see, e.g., Birkinshaw 1999). 0 Given this potential wealth of information, in the past few years a great eort has been made to search for highredshift clusters of galaxies (e.g., Rosati et al. 1998 ; Vikhlinin et al. 1998). Among the dierent methods, X-ray surveys have played the most important role, but no method has allowed the identiïcation of more than a handful of clusters at z [ 0.8. The ïnding of clusters from X-ray data was complicated mainly by the low spatial and/or spectral resolution of the previous X-ray missions. Einstein and ROSAT marked an important step in the study of clusters, but thanks to Chandraîs subarcsecond spatial resolution, it is now pos1 International School for Advanced Studies, Via Beirut 4 -34138, Trieste, Italy ; icagnoni=cfa.harvard.edu (or ilale=sissa.it), celotti= sissa.it. 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ; melvis=cfa.harvard.edu, dkim=cfa.harvard.edu, jhuang=cfa.harvard.edu. 3 ESA fellow ; pmazzotta=cfa.harvard.edu.

sible to distinguish easily between point sources and the more diuse X-ray emission from clusters at any redshift. We present the Chandra observation of 1WGA J1226.9]3332. This source is one of 16 peculiar ROSAT PSPC sources selected for their extremely high X-rayõtoõ optical ÿux ratio (Cagnoni et al. 2001 ; I. Cagnoni et al., in preparation). 1WGA J1226.9]3332 is a bright (F [ 10~13 ergs cm~2 s~1 ) WGA Catalog 0.1h2.4keV (WGACAT ; White, Giommi, & Angelini 1994) source with blank ïelds, i.e., no optical counterparts on the Palomar Observatory Sky Survey up to O \ 21.5. The extreme F /F ratio that follows from this is incompatible with all XV major and common classes of extragalactic sources, including normal quasars, active galactic nuclei (AGNs), normal galaxies, and nearby clusters of galaxies (Maccacaro et al. 1988). Possibilities for the nature of these "" blanks îî (I. Cagnoni et al., in preparation) include (1) type 2 quasars, i.e., high-luminosity, high-redshift, heavily obscured quasars, the bright analogs of Seyfert 2 galaxies ; (2) lowmass Seyfert 2 galaxies, that is, AGNs powered by a lowmass obscured black hole (i.e., an obscured, narrow-line Seyfert 1 galaxy) ; (3) AGNs with no big blue bump, e.g., advection-dominated accretion ÿows (ADAFs) ; (4) isolated neutron stars undergoing Bondi accretion from the ISM (Madau & Blaes 1994) ; (5) c-ray burst X-ray afterglows ; (6) failed clusters, in which a large overdensity of matter has collapsed but has not formed galaxies (Tucker, Tananbaum, & Remillard 1995) ; and, most relevant to this paper, (7) high-redshift clusters of galaxies. Here we present strong evidence that 1WGA J1226.9]3332 is indeed a highredshift cluster. We use H \ 75 km s~1 Mpc~1 and 0 q \ 0.5 ; errors in the paper represent 1 p conïdence levels, 0 unless explicitly stated otherwise.
2. CHANDRA

OBSERVATIONS

1WGA J1226.9]3332 was one of two blanks for which we obtained cycle 1 Chandra (Weisskopf, OîDell, & van Speybroeck 1996) observing time. It was observed in the 86


1WGA J1226.9]3332 : HIGH-z GALAXY CLUSTER
39m00s 38m00s 37m00s 36m00s 35m00s 34m00s 33m00s 32m00s 33d31m00s

87

15s

12h27m00s

45s

FIG. 1.õChandra view of 1WGA J1226.9]3332. L eft : Cleaned data (see text) from the whole S3 chip. Right : Zoom of the cluster (smoothed and background-subtracted).

ACIS-S conïguration (G. P. Garmire et al., in with the backside-illuminated S3 chip for a exposure time of 9832.3 s. The data were described in Markevitch et al. (2000), and for and image analysis we used the latest available ground data set.4

preparation) total useful cleaned as the spectral ACIS back-

2.1. Spatial Analysis To maximize the signal-to-noise ratio, we extracted the image in the 0.5õ5 keV band (Figs. 1a and 1b). Along with a number of faint pointlike sources, 1WGA J1226.9]3332 is clearly seen as an extended source on an arcminute scale. It shows azimuthal symmetry, except for a possible excess to the northwest, D40A from the cluster center. After subtracting the background from the same regions in a normalized ACIS background map, we extracted the X-ray surface brightness proïle (Fig. 2) in concentric annular regions centered on the X-ray emission peak and
4 This information is under "" ACIS Background îî at asc.harvard.edu/cal/Links/Acis/acis/WWWacis­cal.html. http ://

chosen in order to have 20 counts per annulus. The proïle appears to be smooth, without any obvious central excess related to a cooling ÿow (Figs. 1 and 2). We ïtted the surface brightness proïle with a standard b model5 (Cavaliere & Fusco-Femiano 1976) using the SHERPA (A. Siemiginowska et al., in preparation) modeling and ïtting tool from the Chandra X-Ray Center (CXC) analysis package CIAO 2.0 (M. Elvis et al., in preparation). We obtained best-ït values of b \ 0.770 ^ 0.025, r \ 18A .1 c ^ 0A (corresponding to 101 ^ 5 kpc at z \ 0.89), and .9 S(0) \ 1.02 ^ 0.08 counts arcsec~2 with a s2 of 27.5 for 26 degrees of freedom (dof). The excess to the northwest of the cluster is also visible in the radial proïle ; a drop in the surface brightness at D40A is present in the radial proïle for the northwest sector. Similar features in the surface brightness radial proïles were detected by Chandra in nearby clusters (e.g., Markevitch et al. 1999, 2000 ; Vikhlinin, Markevitch, & Murray 2001 ; Mazzotta et al. 2001) and were interpreted as signs of subclump motion. 2.2. Spectral Analysis We extracted an overall spectrum in a circle with r \ 120@@ in the 0.5õ10 keV band in PI (pulse-height invariant) channels, corrected for the gain dierence between the dierent regions of the CCD. The spectrum (Fig. 3) contained D1100 net counts, and we binned it in order to have 100 counts per bin.6 Both the eective area ïle (ARF) and the redistribution matrix (RMF) were computed by weighting each of the position-dependent ARFs and RMFs by their X-ray brightness. We ïtted the spectrum in the 0.5õ10 keV range with an absorbed RaymondSmith model (Raymond & Smith 1977) using SHERPA. The source redshift was treated as unknown, and because of the low statistics, no iron line or other line complex features were expected to be visible.
5 S(r) \ S(0)[1 ] (r/r )2]~3b`1@2. c 6 A smaller binning, e.g., as in Fig. 3, leads to similar results.

FIG. 2.õBackground-subtracted surface brightness radial proïle of 1WGA J1226.9]3332 (error bars), together with the best-ït b model (see text).


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absorbed 0.5õ2.0 keV ÿux8 in an r \ 120@@ aperture is (3.0 ^ 0.3) ] 10~13 ergs cm~2 s~1, consistent with the Ebeling et al. (2001) PSPC measurement ; the 0.5õ10.0 keV absorbed ÿux in the same aperture (Table 1) is (8.03`0.96) ~0.88 ] 10~13 ergs cm~2 s~1. For z \ 0.89, the unabsorbed bolometric and 0.5õ2.0 keV band luminosities are L \ (2.2 ^ 0.2) ] 1045 ergs s~1 and L \ (4.4 ^ 0.5) X(0.5 h2.0) ] 1044 ergs s~1, respectively.
3

. OPTICAL, INFRARED, AND RADIO OBSERVATIONS

FIG. 3.õChandra ACIS spectrum and residuals for a Raymond-Smith model with kT \ 10.23 keV and z \ 0.89. The spectrum was binned, for display purposes, to obtain a minimum of 30 counts per bin. [See the electronic edition of the Journal for a color version of this ïgure.]

In order to get an estimate of the temperature, we ïx the equivalent hydrogen column density to the Galactic value (N \ 1.38 ] 1020 cm~2 ; Stark et al. 1992) and the metal H abundance to 0.3 times the solar value, and we draw conïdence levels for T and z (Fig. 4). We ïnd a best-ït value of kT \ 10.24 keV and z \ 0.85 (s2 is 18.68 for 20 dof)7. However, as shown in Figure 4, these values are not well constrained. While submitting this paper we found that the same object had been independently identiïed as a cluster of galaxies in the WARPS survey (Ebeling et al. 2001) and observed for a Sunyaev-Zeldovich eect measurement by Joy et al. (2001). The cluster spectroscopic redshift measured by Ebeling et al. (2001) was z \ 0.89, which is within the errors of our estimate based on Chandra and optical/IR constraints (see ° 4). Fixing the redshift to this value, we obtain a temperature of kT \ 10.47`4 keV (kT \ 12.07 ~3 keV using a normalized background), which is in good agreement with that obtained from the Sunyaev-Zeldovich measurement by Joy et al. (2001 ; kT \ 10.0`2.0 keV). The ~1.5
7 By normalizing the background map using an area of 1WGA J1226.9]3332 observation without sources (7.56% lower background), consistent results are obtained : kT \ 11.94 keV and z \ 0.87.

We obtained an R band image of the 1WGA J1226.9]3332 ïeld on 1997 February 2, using the Smithsonian Astrophysical Observatory (SAO) 1.2 m telescope on Mount Hopkins. An R \ 20.4 ^ 0.2 galaxy is detected less than 1A from the X-ray centroid (Table 1). A K-band image of the ïeld was obtained using NSFCam at the NASA Infrared Telescope Facility (IRTF) on 2000 January 31. The same galaxy is also seen, but is much brighter, at a K \ 15.5 (Fig. 5) isophotal magnitude. The K-band magnitude within the core radius of the X-ray source is K \ 15.4, implying that little large-scale IR emission is present. The R[K color of this object is 5.1, with an estimated uncertainty of 0.3 mag due to the poor image quality in both the K and R bands. Comparing the R- and K-band images, it is clear that there are many more objects detected in the K than in the R band. We detect 23 objects with K \ 19.5 mag in the 1@ .5 ] 1.5 image, using source extractor software (SExtractor ; @ Bertin & Arnouts 1996), versus only 4 at R. The star/galaxy classiïcation was carried out morphologically using the Kron radius (Bertin & Arnouts 1996), and three objects are classiïed as pointlike. These objects are also detected in the R band and have bluer colors (R [ K \ 4.2) than the extended objects ; we argue that they are stars. We should not have to worry about star contamination of the sample, since there are very few stars with K [ 16 (e.g., Glazebrook et al. 1994). The rest of the morphologically extended objects are galaxies. These, including the bright one located at the X-ray centroid, have very similar red colors (4.5 \ R [ K \ 6.5), implying that they likely have similar redshifts and belong to a cluster. A high-z cluster, CIG J0848]4453 at z \ 1.27, was detected in a near-IR ïeld
8 The main component in the ÿux error is the uncertainty of the cluster temperature (quoted in the text) ; using the ^1 p values of T , we derive an error of `11 for the ÿux. Other contributions come from the normal~12 ization chosen for the background (^9%), from the fraction of counts lost using the r \ 120@@ aperture assuming a b proïle (^5%), and from the Poisson error of the counts (^3%).

TABLE 1 1WGA J1226.9]3332 OBSERVATIONS Energy Band X-ray ........ R band ...... K band ...... Radio........ Exposure (ks) 9832.3 900 1200 ... Oseta (arcsec) 0.0 0.87 2.0 0.79 Count Rate (counts s~1) 0.107 ^ 0.006b ... ... ...

Instrument Chandra SAO 48 in IRTF FIRST

Date 2000 1997 2000 . Jul 31 Feb 2 Jan 31 ..

R.A. (J2000) 12 12 12 12 26 26 26 26 58.2 58.2 58c 58.19

decl. (J2000) ]33 ]33 ]33 ]33 32 32 32 32 48.28 48.7 48c 48.61

Flux (units) 8.03 ] 10~13 (ergs cm~2 s~1)b 20.4 ^ 0.2 (Mag) 15.5 (Mag) 3.61 ^ 0.18 (mJy at 1.4 GHz)

NOTE.õUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Oset from Chandra position. b [0.5õ10 keV] in a circle with r \ 120@@, computed from the spectral model within SHERPA. c Because of the lack of bright stars in the K-band image, we obtained an estimate of the position using the objects in common with the R-band image.


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FIG. 4.õContours of the ït with a Raymond-Smith plasma at 1, 2, and 3 p. The three blue solid lines represent the luminosity-temperature relation (Markevitch 1998) for q \ 0.5, and the three blue dashed lines show q \ 0. The red lines represent the limits on the redshift obtained from the R-K color of 0 0 the brightest object, assuming it is a ïrst-ranked elliptical (Coleman et al. 1980), while the dotted lines show the limits of the magnitude of the ïrst-ranked elliptical (see text). The ïlled circle corresponds to the best ït to the Chandra spectrum.

survey using similar color criteria in an overdense region (Stanford et al. 1997). The apparent asymmetry of the galaxy distribution may be an artifact of the longer exposure time at the center of the K-band mosaic image, which is oset from the X-ray centroid. The galaxy surface density in the K-band image is clearly high (Fig. 5). We generate K-band galaxy number counts in our ïeld in the range 15 \ K \ 19.5, and compare them with those obtained from the near-IR ïeld survey in this range (Gardner, Cowie, & Wainscoat 1993 ; Saracco et al. 1999 ; Huang et al. 1997, 2001 ; Minezaki et al. 1998), with their coverage ranging from 200 arcmin2 to 10 deg2. The number counts obtained from our image are substantially higher (greater than 10 p, a factor of 100 at K \ 19) than those from the ïeld surveys. Such a large excess cannot be due to Poisson statistics or magnitude errors.

Both the high density and the similar (extremely red) colors for these galaxies thus imply that they are likely to be members of a cluster. The Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey detected a faint 3.61 ^ 0.18 mJy source at 1.4 GHz (Becker, White, & Helfand 1995) close to the center of the X-ray emission (Table 1). The radio source is pointlike (FWHM ¹ 0A91, ¹16 kpc at z \ 0.85) and has . a luminosity L (z \ 0.85) D 6.6 ] 1024 W Hz~1, compatR ible with low-luminosity radio-loud AGNs (e.g., Zirbel & Baum 1995).
4

. DISCUSSION AND CONCLUSION

Chandra has shown that 1WGA J1226.9]3332 is an extended X-ray source, with a hard, high-temperature thermal spectrum. Optical and IR imaging has shown that


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FIG. 5.õK-band image of 1WGA J1226.9]3332. The circle shows the Chandra X-ray core radius of 18A.

1WGA J1226.9]3332 has faint optical/IR counterparts. A cluster of galaxies is the only known type of object that could ït such a description. Moreover, the K-band image shows a strong excess of galaxies compared with ïeld counts at K [ 17 (Fig. 5) around 1WGA J1226.9]3332. Physical clustering is the only possible explanation. Several lines of argument go on to suggest that it is a high-redshift cluster of galaxies. Below, we list these arguments and try to constrain the temperature and redshift of 1WGA J1226.9]3332. These results are summarized in Figure 4. (1) The Chandra X-ray proïle is well ïtted by a b model with b \ 0.77, a value in agreement with a typical relaxed cluster (e.g., Jones & Forman 1999, and references therein). Moreover, if the cluster redshift is 0.7 \ z \ 1.2, then the observed angular core radius, r \ 18A ^ 0A (101 ^ 5 kpc .1 .9 c at z \ 0.89, with the assumed cosmology), corresponds to a linear size of 90 \ r \ 150 kpc for any value of ) (130 \ r \ 220 kpc for any value of ) for H \ 50 km s~1 0

Mpc~1), which are values consistent with a typical relaxed cluster. (2) It is well known that clusters of galaxies follow a well-deïned luminosity-temperature relation (e.g., Markevitch 1998, and references therein). Recently, it has been shown that the local L -T relation does not evolve (or is X consistent with little evolution) with redshifts up to z B 0.8 (see, e.g., Wu, Xue, & Fang 1999 ; Della Ceca et al. 2000 ; Fairley et al. 2000, and references therein). Figure 4 shows that the L -T relation [L \ AT a , where T \ T /6 keV, 6 0.40, and A \ (1.71 ^ 0.21) ] 106 h~2 ergs 44 a \ 2.02 ^X s~1] from Markevitch (1998) requires a 3 p lower limit of T [ 4 keV and z [ 0.4, while the 1 p limits require T [ 7.5 keV and z [ 0.65 if no evolution is assumed. The best-ït value of 0.85 obtained from the Chandra spectrum (Fig. 4, ïlled circle) is consistent with the unevolving L -T relation. X (3) K-band imaging has shown that 1WGA J1226.9]3332 has a galaxy at K \ 15.5. If this source is a


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ïrst-ranked elliptical cluster with M \[26.7 ^ 0.5 K (Collins & Mann 1998), then assuming negligible K-band correction (q \ 0.5 and H \ 50 km s~1 Mpc~1, as in 0 0 Collins & Mann 1998), it has z \ 0.68`0.30 (Fig. 4, dotted ~0.19 lines). (4) The color of the K band galaxies is extremely red, with none bluer than R [ K D 4.5 and a maximum of R [ K \ 6.5. Only an unevolving elliptical galaxy at 0.7 \ z \ 1.5 or an Sbc galaxy at z [ 1.1 can have such a red color (Coleman, Woo, & Weedman 1980). Using the more accurate R-K D 5.1 ^ 0.3 of the ïrst-ranked elliptical, we can restrict the redshift range to 0.75 \ z \ 1.0 (Fig. 4, red lines). Using all these constraints, we conclude that 1WGA J1226.9]3332 is a distant cluster of galaxies with a most probable redshift of 0.85 ^ 0.15 and not smaller than z \ 0.65. This gives kT \ 10`4 keV and implies an X-ray lumi~3 nosity, determined in an r \ 120@@ aperture, of L \ 1.3`0.16 ] 1045 ergs s~1, corresponding, for X(0.5 h10keV) ~0.14 z \ 0.85, to a bolometric L \ (2 ^ 0.2) ] 1045 ergs s~1. Our estimated redshift is similar to the spectroscopic z \ 0.89 found by Ebeling et al. (2001). The blank-ïeld X-ray source 1WGA J1226.9]3332 is thus a highly luminous and massive high-redshift cluster and a useful source for determining the evolution of the cluster X-ray luminosity function (e.g., Rosati et al. 1998). Since models in the direction of a low-) universe (with or without cosmological constant, e.g., Henry 2000 ; Borgani & Guzzo 2001) predict a higher density of high-redshift clus-

ters compared to high-) models, ïnding hot, high-redshift clusters has a strong impact on cosmological models. Since such high-luminosity, high-redshift clusters should be rare, the relative ease with which this discovery was made is potentially of great signiïcance. The search for high F /F sources (blanks), sampling a large area of the sky, is XV an efficient method for ïnding very luminous high-redshift clusters ; typical serendipitous ÿux-limited surveys can ïnd (and have found) z [ 0.6 clusters, but they are inefficient at ïnding rare luminous clusters, since they cover relatively small areas (D100 deg2). This methodology is a useful complement to serendipitous ÿux-limited surveys. We gratefully acknowledge the work by the whole Chandra team in making Chandra a great success and the sta of CXC for the rapid reprocess of the data and the CIAO data analysis software. We are grateful to M. Markevitch for making his data available in electronic form and for his prompt answers to ACIS background related questions. We thank M. Massarotti, R. Della Ceca, M. Chiaberge, S. Borgani, and S. Andreon for useful discussion. I. C. thanks A. Fruscione, F. Nicastro, and A. Siemiginowska for a quick introduction to CIAO. We are also grateful to the stas of NASA-IRTF and FLWO. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. This work was supported by NASA grant GO 0-1086X and by the Italian MURST (IC and AC). P. M. acknowledges an ESA fellowship.

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