A unified model for AGN feedback in cosmological simulations of structure formation

Debora Sijacki 2 Volker Springel 2 Tiziana Di Matteo 1 Lars Hernquist 0 0 Harvard-Smithsonian Center for Astrophysics , 60 Garden Street, Cambridge, MA 02138, USA 1 Department of Physics, Carnegie-Mellon University , 5000 Forbes Ave., Pittsburgh, PA 15213, USA 2 Max-Planck-Institut fur Astrophysik , Karl-Schwarzschild-Strae 1, 85740 Garching bei Munchen, Germany A B S T R A C T We discuss a numerical model for black hole growth and its associated feedback processes that for the first time allows cosmological simulations of structure formation to self-consistently follow the build up of the cosmic population of galaxies and active galactic nuclei (AGNs). Our model assumes that seed black holes are present at early cosmic epochs at the centres of forming haloes. We then track their growth from gas accretion and mergers with other black holes in the course of cosmic time. For black holes that are active, we distinguish between two distinct modes of feedback, depending on the black hole accretion rate itself. Black holes that accrete at high rates are assumed to be in a 'quasar regime', where we model their feedback by thermally coupling a small fraction of their bolometric luminosity to the surrounding gas. The quasar activity requires high densities of relatively cold gas around the black hole, as it is achieved through large-scale inflows triggered by galaxy mergers. For black holes with low accretion rates, we conjecture that most of their feedback occurs in mechanical form, where AGN-driven bubbles are injected into a gaseous environment. This regime of activity, which is subdominant in terms of total black hole mass growth, can be identified with radio galaxies in clusters of galaxies, and can suppress cluster cooling flows without the requirement of a triggering by mergers. Using our new model, we carry out TREESPH cosmological simulations on the scales of individual galaxies to those of massive galaxy clusters, both for isolated systems and for cosmological boxes. We demonstrate that our model produces results for the black hole and stellar mass densities in broad agreement with observational constraints. We find that the black holes significantly influence the evolution of their host galaxies, changing their star formation history, their amount of cold gas and their colours. Furthermore, the properties of intracluster gas are affected strongly by the presence of massive black holes in the cores of galaxy clusters, leading to shallower metallicity and entropy profiles, and to a suppression of strong cooling flows. Our results support the notion that AGNs are a key ingredient in cosmological structure formation. They lead to a self-regulated growth of black holes and bring the simulated properties of their host galaxies into much better agreement with observations. - It is now widely believed that most if not all galaxies with an spheroidal component harbour a supermassive black hole (BH) in their centres. Interestingly, the masses of these central BHs are found to be tightly linked with the stellar properties of their host galaxies, as expressed e.g. in the correlation of bulge velocity dispersion with BH mass (Ferrarese & Merritt 2000; Gebhardt et al. 2000; Tremaine et al. 2002), or in the relation of bulge stellar mass (Kormendy & Richstone 1995; Magorrian et al. 1998; Marconi & Hunt 2003; Haring & Rix 2004) with BH mass. The existence of these relationships indicates that the formation and evolution of galaxies is fundamentally influenced by the presence of BHs and vice versa. We thus also expect that the environment and the cosmological evolution of galaxies will affect the way BHs grow. In fact, there is a plethora of observational and theoretical studies that suggest that several different channels for interaction of BHs with their surroundings exist. At high redshift, mergers of gas-rich galaxies occur frequently and funnel copious amounts of cold gas towards the central regions of galaxies, such that the embedded BHs can reach high gas accretion rates. The radiation energy associated with the accretion can support the enormous luminosities of powerful quasars. Theoretically it has been hypothesized (Silk & Rees 1998; Fabian & Iwasawa 1999; King 2003) that quasars produce high velocity winds, which are expected to affect the properties of the host galaxy. The presence of quasar induced outflows has been observationally confirmed in a number of cases (e.g. Chartas, Brandt & Gallagher 2003; Crenshaw, Kraemer & George 2003; Pounds et al. 2003), and has first been demonstrated in simulations of merging galaxy pairs by Di Matteo, Springel & Hernquist (2005). Several numerical studies dealing with BH microphysics (e.g. Proga 2003; McKinney 2006) also predict existence of quasar outflows. Moreover, it appears that tidally disrupted galaxies are preferentially associated with active galactic nucleus (AGN) activity (for a review see Barnes & Hernquist 1992). Quasar activity hence appears to be directly linked to mergers of galaxies, and should represent the dominant mode of mass growth in the BH population. Indeed, using semi-analytic models of galaxy formation, Kauffmann & Haehnelt (2000) have demonstrated that BH growth associated with mergers in cold dark matter (CDM) models can reproduce many properties of the observed quasar population as well as the inferred BH mass density today (see also Volonteri, Haardt & Madau 2003). Based on the detailed hydrodynamical simulations of BH growth in galaxy mergers (Di Matteo et al. 2005; Springel, Di Matteo & Hernquist 2005a; Cox et al. 2006a,b; Robertson et al. 2006a,b) that have recently become available, Hopkins et al. (2005, 2006a,c) have proposed a comprehensive picture of a unified, merger-driven model for the origin of quasars and their relation to spheroid formation, which also implies the existence of a black hole fundamental plane (Hopkins et al. 2007b). Furthermore, rapid merging of the gas-rich progenitor systems of rare, massive galaxy clusters has been shown (Li et al. 2006) to be a viable formation path for supermassive BHs that are as massive at z 6 as those seen in luminous high-redshift Sloan Digital Sky Survey (SDSS) quasars (Fan et al. 2001). However, there also appears to exist another channel of BH interaction with host galaxies, which is neither related to powerful quasar activity nor associated with galaxy mergers. Evidence for this interaction can be seen in a number of local elliptical galaxies and central cluster galaxies, which contain X-ray cavities filled with relativistic plasma (Brzan et al. 2004; McNamara et al. 2005; Fabian et al. 2006; Forman et al. 2006) while harbouring seemingly dormant BHs. These X-ray depressions, often referred to as bubbles, are thought to be inflated by relativistic jets launched from the central BH. Even though the radiative output from the central BH is not significant, the associated mechanical luminosity can be very important in these systems. There has (...truncated)