The impact of angular momentum on black hole accretion rates in simulations of galaxy formation

Feedback from energy liberated by gas accretion on to black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function. Most previous implementations of BH accretion in hydrodynamical simulations of galaxy formation have assumed that BHs grow at an accretion rate that is proportion to the Bondi rate. A major concern is that the Bondi accretion rate is inappropriate when the accreting material has significant angular momentum. We present an improved accretion model that takes into account the circularization and subsequent viscous transport of infalling material, and implemented as a 'subgrid' model in hydrodynamic simulations. The resulting accretion rates are generally low in low mass (less than or similar to 10(11.5) M-circle dot) haloes, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion peaks at similar to 10 per cent of the Eddington rate, which is thought to be conducive to the formation of radio jets. The resulting accretion rate depends strongly on the effective pressure of the gas surrounding the BH, which in turn depends strongly on halo mass. This induces a sharp transition in the importance of BH feedback. In small haloes, the growth of galaxies is regulated by star formation and supernova feedback, but above a halo mass of 10(11.5) M-circle dot, rapid BH growth leads to the suppression of star formation and reduced growth of stellar mass with increasing halo mass.