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The Distribution of Active Galactic Nuclei in a Large Sample of Galaxy Clusters

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The Distribution of Active Galactic Nuclei in a Large Sample of Galaxy Clusters Mon. Not. R. Astron. Soc. 392, 1509–1531 (2009) doi:10.1111/j.1365-2966.2008.14161.x The distribution of active galactic nuclei in a large sample of galaxy clusters R. Gilmour,1,2 P. Best 2 and O. Almaini3 1European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile 2Scottish Universities Physics Alliance, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ 3School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD Accepted 2008 October 28. Received 2008 October 3; in original form 2008 June 13 ABSTRACT We present an analysis of the X-ray point source populations in 182 Chandra images of galaxy clusters at z > 0.1 with exposure time >10 ks, as well as 44 non-cluster fields. The analysis of the number and flux of these sources, using a detailed pipeline to predict the distribution of non-cluster sources in each field, reveals an excess of X-ray point sources associated with the galaxy clusters. A sample of 148 galaxy clusters at 0.1 < z < 0.9, with no other nearby clusters, shows an excess of 230 cluster sources in total, an average of ∼1.5 sources per cluster. The lack of optical data for these clusters limits the physical interpretation of this result, as we cannot calculate the fraction of cluster galaxies hosting X-ray sources. However, the fluxes of the excess sources indicate that over half of them are very likely to be active galactic nuclei (AGN), and the radial distribution shows that they are quite evenly distributed over the central 1 Mpc of the cluster, with almost no sources found beyond this radius. We also use this pipeline to successfully reproduce the results of previous studies, particularly the higher density of sources in the central 0.5 Mpc of a few cluster fields, but show that these conclusions are not generally valid for this larger sample of clusters. We conclude that some of these differences may be due to the sample properties, such as the size and redshift of the clusters studied, or a lack of publications for cluster fields with no excess sources. This paper also presents the basic X-ray properties of the galaxy clusters, and in subsequent papers in this series the dependence of the AGN population on these cluster properties will be evaluated. In addition the properties of over 9500 X-ray point sources in the fields of galaxy clusters are tabulated in a separate catalogue available online or at www.sc.eso.org∼rgilmour. Key words: galaxies: active – galaxies: clusters: general – X-rays: galaxies – X-rays: galaxies: clusters. AGN would also differ between galaxy clusters and the field, and 1 INTRODUCTION within the cluster itself. Studies of active galactic nuclei (AGN) host galaxies are key to The first evidence of a link between AGN activity and environ- understanding the physical mechanisms which trigger AGN ac- ment came in 1978, when Gisler (1978) found a lack of emission- tivity, and govern the fuelling rate of the central black hole. An line galaxies in galaxy clusters relative to the field, which was important part of these studies is the external environment of the confirmed by Dressler, Thompson & Shectman (1984). More re- galaxies, which has long been known to have a significant link cently, large optical surveys have been used to identify the properties with the galaxy properties (e.g. Hubble & Humason 1931). Corre- of a significant number of host galaxies, and hence compare AGN lations such as the morphology–density (Dressler 1980) and star- activity in the same type of host galaxy but different environment. formation–density (e.g. Gomez´ et al. 2003) relations are evidence of Miller et al. (2003) find that optical AGN activity is independent the significant transformations that are associated with galaxy clus- of environment, but Wake et al. (2004) show that the level of clus- ters. If AGN activity is influenced by, for example, galaxy mergers tering depends on AGN luminosity. Kauffmann et al. (2003) find or gravitational disruption of the host galaxy, then the number of that AGN with strong [O III] emission avoid areas of high galaxy density. Best (2004) investigate the radio properties of these AGN, and find that the fraction of galaxies with radio-loud AGN increases dramatically with local galaxy density, but that all of these AGN E-mail: [email protected] have low [O III] emission and so may not be seen in optical surveys. C 2008 The Authors. Journal compilation C 2008 RAS 1510 R. Gilmour, P. Best and O. Almaini Arguably, the least biased method of detecting AGN currently is find between three and five AGN in a cluster at z = 0.08, giving a to use X-ray images, which have the added advantage that the vast fraction of 4 per cent in agreement with the mean value of Martini majority of point sources are AGN. Not long after optical surveys et al. identified a lack of emission-line galaxies in clusters, X-ray surveys Martini et al. also find tentative evidence that the clusters with began to find a surprisingly high number of point sources in fields higher AGN fraction have lower redshift, lower velocity dispersion, with galaxy clusters (Bechtold et al. 1983; Henry & Briel 1991; higher substructure and lower Butcher–Oemler (Butcher & Oemler Lazzati et al. 1998). With the advent of the Chandra X-ray telescope, 1984) fraction, but due to the small size of the sample (eight clusters) with sub-arcsecond point sources, such studies were repeated for it is not clear from this sample which, if any, of these factors is other clusters, with a range of results. Significant overdensities of affecting the AGN activity. A higher AGN fraction at high redshift point sources have been found in a number of fields with galaxy would be expected from the field evolution of AGN, but no strong clusters at moderate redshifts (Cappi et al. 2001, z = 0.5; Martini evolution is found by Branchesi et al. (2007) or Ruderman & Ebeling et al. 2002, z = 0.15; Molnar et al. 2002, z = 0.32; Johnson, Best (2005). In contrast, Cappelluti et al. (2005) find some evidence for an & Almaini 2003, z = 0.83; Martini et al. 2006, 0.05 < z < 0.31; increase in AGN with redshift, and Eastman et al. (2007) conclude D’Elia et al. 2004, z = 0.5) and groups (Jeltema et al. 2001; 0.2 < that the increase of bright AGN in clusters is up to 20 times greater z < 0.6), but Molnar et al. also found a z = 0.5 cluster without a than in the field between z ∼ 0.2 and ∼0.6. significant overdensity. More recently, studies of significant samples IfAGNaretriggeredbygalaxymergers,thenclusterswithlower of galaxy clusters by Cappelluti et al. (2005) (10 clusters, 0.24 < velocity dispersions would be expected to have higher AGN frac- z < 1.2), Ruderman & Ebeling (2005) (51 clusters, 0.3 < z < 0.7) tions, as they have a higher merger rate. Popesso & Biviano (2006) and Branchesi et al. (2007) (18 clusters, 0.25 < z < 1.01) have show that this is indeed the case for optically detected AGN, with all found significant overdensities of point sources over the full clusters with high velocity dispersions having lower AGN fractions. sample, but not necessarily in all individual fields. However, the However Martini et al. (2007) find that the velocity distribution of ChaMP project (Kim et al. 2004) found no difference in the number AGN in eight clusters is not significantly different from the velocity density of sources in fields with z > 0.3 clusters compared to those distribution of the non-active cluster galaxies. without clusters. Galaxy mergers are also more common in the outskirts of clusters, The calculated number of AGN per cluster varies significantly so the projected radial distribution of AGN should be different from in these samples, even taking into account the different depths of the host galaxies if mergers cause AGN activity. Martini et al. find the observations and the statistical variance in the number of back- no evidence for AGN to lie in galaxies in the outskirts of the cluster ground sources in each image. This is to be expected as the number compared to bright cluster galaxies; in fact on the contrary, they of AGN per cluster is, of course, related to the cluster properties, find evidence that galaxies with luminous AGN (>1042 erg s−1) such as the number of possible AGN host galaxies. However, due are more centrally clustered than the general population. Branchesi to the lack of optical data, it is hard to draw any conclusions from et al. (2007) and Ruderman & Ebeling (2005) also present evidence these samples as to how, if at all, the cluster environment affects the that most AGN are found within the central 0.5 Mpc of the clusters, AGN population. Optical imaging and spectroscopy can identify which Ruderman & Ebeling (2005) attribute to tidal encounters the X-ray detected AGN in the cluster, rather than relying on sta- with the central galaxy. In contrast, Johnson et al. (2003) find the tistical background subtraction, and also reveal the distribution and excess AGN in a cluster at z = 0.83 lie between 1 and 2 Mpc from number of normal cluster galaxies. This would allow the calculation the cluster centre. Gilmour et al. (2007) also find that AGN avoid of the fraction of cluster galaxies which host AGN, as a function the densest areas of a supercluster at z = 0.17. of cluster radius or cluster size for example, shedding light on the The wide range of results from the current studies may partly physical mechanisms affecting AGN.
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