The halo occupation distribution of black holes

Abstract

We investigate the halo occupation distribution (HOD) of black holes within a hydrodynamic cosmological simulation that directly follows black hole growth. Similar to the HOD of galaxies/subhalos, we find that the black hole occupation number can be described by the form N_BH proportional to 1+ (M_Host)^alpha where alpha evolves mildly with redshift indicating that a given mass halo (M_Host) at low redshift tends to host fewer BHs than at high redshift (as expected as a result of galaxy and BH mergers). We further divide the occupation number into contributions from black holes residing in central and satellite galaxies within a halo. The distribution of M_BH within halos tends to consist of a single massive BH (distributed about a peak mass strongly correlated with M_Host), and a collection of relatively low-mass secondary BHs, with weaker correlation with M_Host. We also examine the spatial distribution of BHs within their host halos, and find they typically follow a power-law radial distribution (i.e. much more centrally concentrated than the subhalo distribution). Finally, we characterize the host mass for which BH growth is feedback dominated (e.g. star formation quenched). We show that halos with M_Host > 3 * 10^12 M_sun have primary BHs that are feedback dominated by z~3 with lower mass halos becoming increasingly more affected at lower redshift.