X-Ray Bright Active Galactic Nuclei in Massive Galaxy Clusters – III. New Insights Into the Triggering Mechanisms of Cluster AGN
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MNRAS 446, 2709–2729 (2015) doi:10.1093/mnras/stu2091 X-ray bright active galactic nuclei in massive galaxy clusters – III. New insights into the triggering mechanisms of cluster AGN S. Ehlert,1,2,3,4‹ S. W. Allen,2,3,4 W. N. Brandt,5,6 R. E. A. Canning,2,3,4 B. Luo,5,6 A. Mantz,7,8 R. G. Morris,2,3,4 A. von der Linden2,9 andY.Q.Xue10 1Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA 2SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA 3Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305-4060, USA 4Kavli Institute for Particle Astrophysics and Cosmology, 452 Lomita Mall, Stanford, CA 94305-4085, USA 5Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA 6 Institute for Gravitation and the Cosmos, Department of Physics, Pennsylvania State University, University Park, PA 16802, USA Downloaded from 7Kavli Institute for Cosmological Physics, 5640 S. Ellis Avenue, Chicago, IL 60637, USA 8Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA 9Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark 10Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, Anhui 230026, China http://mnras.oxfordjournals.org/ Accepted 2014 September 28. Received 2014 September 3; in original form 2014 July 22 ABSTRACT We present the results of a new analysis of the X-ray selected active galactic nuclei (AGN) population residing in and behind 135 of the most massive galaxy clusters in the redshift range of 0.2 <z<0.9 observed with Chandra. With a sample of more than 11 000 X- ray point sources, we are able to measure, for the first time, evidence for evolution in the at University of California, San Diego on January 17, 2015 cluster AGN population beyond the expected evolution of field AGN. Our analysis shows ∼M−1.2 that overall number density of cluster AGN scales with the cluster mass as 500 . There is no evidence for the overall number density of cluster member X-ray AGN depending on the cluster redshift in a manner different than field AGN, nor is there any evidence that the spatial distribution of cluster AGN (given in units of the cluster overdensity radius r500) strongly depends on the cluster mass or redshift. The M−1.2 ± 0.7 scaling relation we measure is consistent with theoretical predictions of the galaxy merger rate in clusters, which is expected to scale with the cluster velocity dispersion, σ ,as∼σ −3 or ∼M−1. This consistency suggests that galaxy mergers may be an important contributor to the cluster AGN population, a result that is further corroborated by visual inspection of Hubble images for 23 spectroscopically confirmed cluster member AGN in our sample. A merger-driven scenario for the triggering of X-ray AGN is not strongly favoured by studies of field galaxies, however, suggesting that different mechanisms may be primarily responsible for the triggering of cluster and field X-ray AGN. Key words: galaxies: active – galaxies: clusters: general – X-rays: galaxies – X-rays: galaxies: clusters. be an especially important component of modern galaxy formation 1 INTRODUCTION models, but many aspects of the connection between galaxies and Modern studies of galaxy evolution have demonstrated the essential their AGN remain largely uncertain. role central supermassive black holes can play in establishing the Luminous AGN emit across the entire electromagnetic spectrum, observed properties of galaxies on both small (∼1 pc) and large but point-like X-ray emission is one of the cleanest signatures of (∼1 kpc) scales (e.g. Silk & Rees 1998; Ferrarese & Merritt 2000). AGN (Brandt & Hasinger 2005) making X-ray observations partic- The powerful outbursts of active galactic nuclei (AGN) appear to ularly well suited for AGN population studies. Deep field surveys of X-ray AGN have already made great progress in understand- ing the typical host galaxies and environments of AGN. X-ray se- lected AGN are predominantly hosted in the most massive galax- 10 E-mail: [email protected] ies, with stellar masses (M)ofM 10 M (Xue et al. 2010). C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 2710 S. Ehlert et al. In colour–magnitude space, the host galaxies of X-ray AGN occupy as the lower star formation rates. Comparisons between the cluster the same locus of this phase space as normal galaxies with simi- and field populations of AGN using X-ray observations have led to lar stellar masses, suggesting that the host galaxy mass plays the differing conclusions, and have primarily investigated whether or key role in determining the likelihood of it hosting an AGN. This not X-ray point sources are more frequently observed in the vicin- is further supported by investigations into the clustering of X-ray ity of galaxy clusters. Other studies using larger samples of galaxy AGN in the field (see Cappelluti, Allevato & Finoguenov 2012,for clusters have argued for a statistical expectation of approximately a review), which indicate that the majority of X-ray AGN are hosted ∼1 X-ray AGN per galaxy cluster, and have shown that is usually in dark matter haloes with masses of M ∼ 1013 M h−1, a value that located within the central 1–2 Mpc of each cluster (e.g. Ruderman is consistently measured out to redshifts of z ∼ 2. & Ebeling 2005; Gilmour, Best & Almaini 2009). Multiwavelength Although it is clear from these studies that the typical host galaxy studies of cluster galaxies incorporating optical spectroscopy along environment for an AGN is reasonably well constrained to be self- with X-ray analysis have suggested that the fraction of galaxies similar out to redshifts of z ∼ 2, the triggering mechanism or mech- hosting X-ray bright AGN in clusters may increase substantially anisms that cause these luminous AGN outbursts remain largely un- with redshift, by a factor of ∼8 (e.g. Martini, Mulchaey & Kelson certain. Deep field, multiwavelength surveys of X-ray AGN suggest 2007; Martini, Sivakoff & Mulchaey 2009; Haines et al. 2012), an that different mechanisms may dominate for galaxies at different effect that was also observed in the large Bootes¨ field (Galametz Downloaded from redshifts (see Cappelluti et al. 2012, for a review). Major galaxy et al. 2009). This redshift-dependent AGN fraction is particularly mergers are likely the most important mechanisms for fuelling interesting, as it suggests that higher redshift clusters appear to have quasars at the highest luminosities and redshifts (L 1044 erg s−1; higher rates of AGN than comparable while lower redshift clusters e.g. Hopkins & Beacom 2006; Hasinger 2008; Hopkins et al. 2008). have lower rates of AGN (e.g. von der Linden et al. 2010; Pimbblet At lower redshifts (z 1), bar instabilities and less extreme galaxy– et al. 2013;Ehlertetal.2014). How the AGN fraction transitions http://mnras.oxfordjournals.org/ galaxy interactions are inferred to be more efficient at producing between these two extreme values relative to the field remains an AGN (e.g. Georgakakis et al. 2009). Investigations into the proper- open question. ties of the galaxies hosting AGN indicate that their morphologies are Previous studies of the X-ray AGN population in galaxy clus- similar to comparable galaxies that do not host AGN (e.g. Reichard ters, however, have always suffered from limited source statistics. et al. 2009; Tal et al. 2009). Because the fraction of galaxies hosting X-ray AGN are typically One useful way to explore these triggering mechanisms is to of order ∼0.1–1 per cent (e.g. Haggard et al. 2010), large samples observe the AGN populations in massive galaxy clusters. Galaxy of galaxy clusters are required to measure the cluster-specific AGN clusters are not only sites of large numbers of galaxies in close population with high precision. Understanding how the AGN popu- proximity to one another but also host a hot, diffuse, X-ray bright lation varies with cluster mass and redshift additionally requires de- intracluster medium (ICM; e.g. Sarazin 1988). Both factors are ex- tailed spectroscopy and mass proxy information that is only just be- at University of California, San Diego on January 17, 2015 pected to play a role in transforming galaxies in clusters, through coming available (Mantz et al. 2010a,b; Applegate et al. 2014; Kelly tidal encounters, mergers between neighbouring galaxies (Mamon et al. 2014; von der Linden et al. 2014). Finally, any attempt to mea- 1992; Moore, Lake & Katz 1998), or by galaxy–ICM interactions sure the cluster-specific influences on their constituent AGN popula- such as ram pressure stripping (e.g. Gunn & Gott 1972). Studying tion must also account for the cosmic evolution of X-ray AGN in the how the AGN population in clusters is related to the host cluster field (also known as the X-ray luminosity function or XLF) which properties allows us to understand more completely how the varia- has already been measured to have a strong redshift dependence tions in the merger frequency or density of the ICM may influence (e.g. Ueda et al. 2003, 2014; Hasinger, Miyaji & Schmidt 2005). a galaxy’s ability to host an AGN outburst. In this paper, we expand the analysis of Ehlert et al. (2013, 2014, Previous studies have established that galaxies in local clusters hereafter Paper I and Paper II, respectively) to a larger sample of have lower average star formation rates and higher fractions of el- galaxy clusters to test for the presence of a cluster mass and/or liptical galaxies than the field (e.g.