The rise and fall of galaxy activity in dark matter haloes

We use the catalogue of galaxy groups constructed from the Sloan Digital Sky Survey (SDSS DR4) by Yang et al. to study the dependence of galaxy activity on stellar mass, M*, halo mass, Mh, and group hierarchy (central versus satellite galaxies). The wealth of data provided by the SDSS allows us to split the sample on the basis of galaxy activity in star-forming galaxies, galaxies with optical active galactic nuclei (AGN), composite galaxies (both star formation and optical AGN activity) and radio sources. We find a smooth transition in halo mass as the activity of central galaxies changes from star formation to optical AGN activity to radio emission. Star-forming centrals preferentially reside in haloes with Mh < 1012h-1Msolar, central galaxies with optical AGN activity typically inhabit haloes with Mh ~ 1013h-1Msolar and centrals emitting in the radio mainly reside in haloes more massive than 1014h-1Msolar. Although this seems to suggest that the environment (halo mass) determines the type of activity of its central galaxy, we find a similar trend with stellar mass: central star formers typically have stellar masses less than 1010h-2Msolar, while optical AGN hosts and central radio sources have characteristic stellar masses of ~1010.8 and ~1011.6h-2Msolar, respectively. Since more massive haloes typically host more massive centrals, it is unclear whether the activity of a central galaxy is causally connected to its stellar mass or to its halo mass. In general, satellite galaxies have their activity suppressed with respect to central galaxies of the same stellar mass. This holds not only for star formation activity, but also for AGN activity in the optical and the radio. At fixed stellar mass, we find that the activity of satellite galaxies depends only weakly on halo mass. In fact, using a set of reduced conditional probability functions, we find that for satellite galaxies the dependence of galaxy activity on halo mass is more than four times weaker than the dependence on stellar mass. All these results are consistent with a picture in which low-mass haloes accrete cold gas, while massive haloes have coronae of hot gas that promote radio activity of their central galaxies