CORE Metadata, citation and similar papers at core.ac.uk Provided by CERN Document Server Sup erclustering at Redshift z = 0:54 1 1;2 A. J. Connolly and A. S. Szalay Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 1 D. Ko o University of California Observatories, Lick Observatory, Board of Astronomy and Astrophysics, University of California Santa Cruz, CA 95064 A. K. Romer Dept. of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, Il-60208, USA. B. Holden and R.C. Nichol Department of Astronomy and Astrophysics, University of Chicago, 5460 S. Ellis Ave, Chicago, IL 60637 T. Miyaji Max-Planck-Institut fur Extraterrestrische Physik Postf. 1603, D-85740, Garching, Germany 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, op erated by the Asso ciation of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 2 DepartmentofPhysics, Eotvos University, Budap est, Hungary, H-1088 { 2{ ABSTRACT We present strong evidence for the existence of a sup ercluster at a redshift of z =0:54 in the direction of Selected Area 68. From the distribution of galaxies with sp ectroscopic redshifts we nd that there is a large over-density of galaxies (a factor of four over the number exp ected in an unclustered universe) within the redshift range 0:530 < z < 0:555. By considering the spatial distribution of galaxies within this redshift range (using sp ectroscopic and photometric redshifts) we show that the galaxies in SA68 form a linear structure passing from the South-West of the survey eld through to the North-East (with a p osition angle of approximately 35 East of North). This p osition angle is coincident with the p ositions of the X-ray clusters CL0016+16, RX J0018.3+1618 and a new X-ray cluster, RX J0018.8+1602, centered near the radio source 54W084. All three of these sources are at a redshift of z 0:54 and have p osition angles, derived from their X-ray photon distributions, consistent with that measured for the sup ercluster. Assuming a redshift of 0.54 for the distribution of galaxies and a FWHM disp ersion in redshift of 0.020 this represents a coherent 1 1 structure with a radial extentof31h Mp c, transverse dimension of 12 h Mp c, 1 and a thickness of 4 h Mp c. The detection of this p ossible sup ercluster demonstrates the power of using X-ray observations, combined with multicolor observations, to map the large scale distribution of galaxies at intermediate redshifts. Subject headings: galaxies: distances and redshifts | large-scale structure of universe | techniques: photometric 1. Intro duction By mapping the spatial distribution of galaxies, we can determine the intrinsic scales on which galaxies cluster, from poor groups through rich clusters to sup erclusters. Quantifying the abundance of these clusterings and their evolution with time should provide imp ortant constraints on the multifarious cosmological mo dels (Liddle et al. 1996). In the lo cal universe, extensive redshift surveys have b een undertaken to map the distribution of galaxies to B 15:5 that reach to a redshift of z 0:05. Such surveys have uncovered coherent structures that extend over many tens of megaparsecs, such as the Perseus-Pisces lament (Giovanelli and Haynes 1993) or the sheet-like \Great Wall" (Geller and Huchra { 3{ 1989). These structures app ear to be common in other and even deep er (B < 20) surveys that extend beyond redshifts z 0:1 (Landy et al. 1996, Willmer et al. 1996). Ateven fainter magnitudes, redshift surveys have b een undertaken to a limit of B ' 24 (Cowie et al. 1996, Glazebro ok et al. 1995, Lilly et al. 1995). Because of the long exp osures required to reach such faint limits, these surveys have b een restricted to a few p ointings that cover tiny regions of sky, typically 10 arcmin in diameter. Consequently, while large over-densities of galaxies are visible in the deep redshift surveys (Broadhurst et al. 1990, Le Fevre et al. 1994, Cohen et al. 1996), the angular distribution of these structures and, therefore, their transverse spatial extent, have yet to b e well studied. In this letter we consider the distribution of galaxies in the direction of Selected Area 68 (Kron 1980). We identify this region as a p otential site of an intermediate redshift sup ercluster b ecause of the coherent distribution of very red galaxies, estimated to be at z 0:5, found by Ko o (1985) and the presence of two X-ray clusters at the same redshift (Hughes et al. 1995). The over-density of galaxies is quanti ed using a small sample of sp ectroscopic redshifts in x2 and then improved using a larger sample of photometric redshifts in x3. The coincidence of this structure with the p osition and orientation of three X-ray clusters is discussed in x4. Finally we show that the galaxies and clusters are consistent with a \Great Wall"-like sup ercluster, at z =0:54, that is almost edge-on with 1 1 resp ect to the line of sight. For this letter we assume an H = 100 km s Mp c and = 1 o cosmology; at redshift z 0:54, the lo okback time is nearly half the age of the universe and 1 one degree spans 13:5h Mp c. 2. The Distribution of Sp ectroscopic Redshifts in the Selected Area SA68 To determine if there exist intermediate redshift counterparts to the lo cally detected sup erclusters, we consider the sp ectroscopic and photometric galaxy catalog of Ko o and hr m Kron (Kron 1980, Ko o 1986, Munn et al. 1996) in the direction of Selected Area 68 (00 14 m +15 30 ). These data comprise a total of 1750 galaxies, B < 23:0, with multicolor J photometry in the U; B ;R and I passbands. Of these galaxies 286 have sp ectroscopically J F N measured redshifts. In Fig 1 we show the redshift distribution of the sp ectroscopic sample of galaxies. The most striking features of this redshift distribution are the regular set of p eaks. Such over-densities have b een interpreted as the intersection of the narrow p encil b eam surveys with large-scale coherent structures in the galaxy distribution (Broadhurst et al. 1990, Szalay et al. 1991, Le Fevre et al. 1994, Cohen et al. 1996). For later discussions, we consider { 4{ only the spatial and redshift distribution of those galaxies lying b etween 0:530 <z <0:555. To test whether this feature, at z = 0:55, represents a signi cant p erturbation from number of galaxies exp ected from an unclustered distribution of galaxies, we calculate the selection function for the sp ectroscopic sample of galaxies. As the sp ectroscopic sample is not formally magnitude limited we must calculate the exp ected redshift distribution using the mo dels of Gronwall and Ko o (1995). These mo dels were derived to match the observed redshift distribution of SA68 to a limit of B< 24. Taking the observed B magnitude J distribution of the sp ectroscopic galaxy sample we construct the exp ected dn/dz (the solid line in Fig 1). The exp ectation value for the number of galaxies with 0:530 <z <0:555 is 3.54. To account for a clustered distribution we calculate the variance of the exp ectation value by integrating the lo cal spatial correlation function, (r ), over this redshift volume. 2 Assuming a pro jected area of 706 arcmin for SA68 we estimate the variance in the exp ectation value to be 3.7. It should be noted that this is a conservative value (an upp er limit) as we assume no evolution of the spatial correlation function, with redshift. We would, therefore, exp ect to detect <N >=3:54 3:7 (1) galaxies with sp ectroscopic redshifts within the redshift range 0:530 < z < 0:555. The observed number of galaxies in this redshift p eak is 14, representing a factor of 4 over the unclustered exp ectation value. Fitting a Gaussian to the sp ectroscopic redshifts we determine the disp ersion of the redshift distribution to be 0.0083. If the disp ersion in redshift were due to the internal velo city of a cluster this would equate to a velo city 1 disp ersion of 2500 km s far in excess of lo cal observations (Zabludo et al. 1993, Collins et al. 1995). The radial distance corresp onding to the FWHM of the redshift distribution is 1 then 31 h Mp c. The pro jected angular distribution of the sp ectroscopic sample of galaxies within this redshift range is shown in Fig 2a. They form a linear structure passing from the South-West of the SA68 eld through to the North-East. The angular distribution extends to the limits of the survey eld. From the second moments of the galaxy distribution we nd that this structure can be represented by an ellipse with a p osition angle oriented 29 East of North hr m m and centered on 00 14 40+15 33 58 (B1950). While the number of galaxies with observed sp ectroscopic redshifts is small we can estimate whether they are consistent with a uniform distribution of galaxies across the SA68 survey eld. We do this by applying a 2 dimensional Kolmogorov-Smirnov test (Peaco ck 1983, Fasano and Franceschini 1987). For a sample size of 14, the probability that the { 5{ galaxies with sp ectroscopic redshifts are consistent with a uniform distribution is only 15%.