Why are AGN found in high-mass galaxies?

Mon. Not. R. Astron. Soc. 391, 785–792 (2008) doi:10.1111/j.1365-2966.2008.13907.x Why are AGN found in high-mass galaxies? Lan Wang1,2 and Guinevere Kauffmann2 1 Department of Astronomy, Peking University, Beijing 100871, China 2 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany Accepted 2008 September 2. Received 2008 August 12; in original form 2008 January 23 ABSTRACT Key words: galaxies: haloes – galaxies: interactions – galaxies: nuclei. 1 I N T RO D U C T I O N By studying active galactic nuclei (AGN), we learn about the physical mechanisms that trigger accretion on to the central supermassive black holes of galaxies. When a black hole accretes, it increases in mass. By studying populations of AGN at low and at high redshifts, we hope to infer the history of how black holes build up their mass. It has been established that supermassive black holes most occur in galaxies with bulges (Kormendy & Richstone 1995), and that the mass of the black hole correlates with the luminosity and the stellar velocity dispersion of the host bulge (Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000). This indicates that the formation of galaxies and supermassive black holes are likely to be closely linked. In the local Universe, the fraction of bulge-dominated galaxies hosting AGN decreases at lower stellar masses (Ho, Filippenko & Sargent 1997; Kauffmann et al. 2003). In order to form a black hole, it is necessary for gas to lose angular momentum and sink to the centre of the galaxy (Haehnelt & Rees 1993; Volonteri, Haardt & Madau 2003). The gravitational torques that operate during galaxy–galaxy mergers are known to be a very effective mechanism for concentrating gas at the centres of galaxies  E-mail:  C 2008 The Authors. Journal compilation  C 2008 RAS (Mihos & Hernquist 1996). Models for AGN evolution have often assumed that black holes are formed and fuelled, and AGN activity is triggered during major mergers of galaxies (Kauffmann & Haehnelt 2000; Wyithe & Loeb 2003; Croton et al. 2006). At low and moderate redshifts, there is no conclusive observational evidence that mergers play a significant role in triggering AGN activity in galaxies. In the local Universe, Li et al. (2006) have shown that narrow line AGN do not have more close companions than matched samples of inactive galaxies. Even at intermediate redshifts (z ∼ 0.4–1.3), moderate luminosity AGN hosts do not have morphologies indicative of an ongoing merger or interaction (Hasan 2007). The conclusion seems to be that although major mergers may be responsible for AGN activity in some galaxies, other fuelling mechanisms are likely to be most important in the low-redshift Universe. It has also been established that high-mass black holes have largely stopped growing at early cosmic epochs, whereas low-mass black holes are still accreting at significant rates today (Heckman et al. 2004). X-ray observations show that very high luminosity AGN activity peaked at early cosmic epochs (z ∼ 2), while low-luminosity AGN activity peaks at lower redshifts (Steffen et al. 2003; Barger et al. 2005; Hasinger, Miyaji & Schmidt 2005). It has been postulated that this so-called ‘antihierarchical’ growth of supermassive black holes can be explained if there are two modes There is a strong observed mass dependence of the fraction of nearby galaxies that contain either low-luminosity [low-ionization nuclear emission-line region (LINER) type] or higher luminosity (Seyfert or composite type) active galactic nuclei (AGN). This implies that either only a small fraction of low-mass galaxies contain black holes, or that the black holes in these systems only accrete rarely or at very low rates, and hence are generally not detectable as AGN. In this paper, we use semi-analytic models implemented in the Millennium Simulation to analyse the mass dependence of the merging histories of dark matter haloes and of the galaxies that reside in them. Only a few per cent of galaxies with stellar masses less than M ∗ < 1010 M are predicted to have experienced a major merger. The fraction of galaxies that have experienced major mergers increases steeply at larger stellar masses. We argue that if a major merger is required to form the initial seed black hole, the mass dependence of AGN activity in local galaxies can be understood quite naturally. We then investigate when the major mergers that first create these black holes are predicted to occur. High-mass galaxies are predicted to have formed their first black holes at very early epochs. The majority of low-mass galaxies never experience a major merger and hence may not contain a black hole, but a significant fraction of the supermassive black holes that do exist in low-mass galaxies are predicted to have formed recently. 786 L. Wang and G. Kauffmann 2 S I M U L AT I O N A N D M E R G E R T R E E S The Millennium Simulation (Springel et al. 2005) is used in this work to study the merging histories of dark matter haloes. The merging histories of galaxies can be inferred when the simulation is combined with semi-analytic models that follow gas cooling, star formation, supernova and AGN feedback and other physical processes that regulate how the baryons condense into galaxies. The Millennium Simulation follows N = 21603 particles of mass 8.6 × 108 h−1 M from redshift z = 127 to the present day, within a comoving box of 500 h−1 Mpc on a side. The cosmological parameters values in the simulation are consistent with the determinations from a combined analysis of the 2dF Galaxy Redshift Survey (2dFGRS; Colless et al. 2001) and first year Wilkinson Microwave Anisotropy Probe (WMAP) data (Spergel et al. 2003). A flat  cold dark matter (CDM) cosmology is assumed with m = 0.25, b = 0.045, h = 0.73,  = 0.75, n = 1 and σ 8 = 0.9. Full particle data are stored at 64 output times. For each output, haloes are identified using a friends-of-friends (FOF) group finder. Substructures (or subhaloes) within a FOF halo are located using the SUBFIND algorithm of Springel et al. (2001). The self-bound part of the FOF group itself also appears in the substructure list. This main subhalo typically contains 90 per cent of the mass of the FOF group. After finding all substructures in all the output snapshots, subhalo merging trees are built that describe in detail how these systems merge and grow as the universe evolves. Since structures merge hierarchically in CDM universes, for any given subhalo, there can be several progenitors, but in general each subhalo only has one descendant. Merger trees are thus constructed by defining a unique descendant for each subhalo. We refer below halo to the main substructure that can represent the FOF halo, while subhalo refers to substructure other than the main one. Halo merger happens when two FOF group merge into one group and one of the haloes becomes a subhalo of the larger structure. The substructure merger (...truncated)