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

Abstract

Feedback from energy liberated by gas accretion onto black holes (BHs) is an attrac- tive mechanism to explain the exponential cut-of at the massive end of the galaxy stellar mass function (SMF). Most previous implementations of BH accretion in hy- drodynamical 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 ac- count the circularisation and subsequent viscous transport of infalling material, and implemented as a\subgrid"model in hydrodynamic simulations. The resulting accre-tion rates are generally low in low mass (<1011:5M) halos, 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 10% 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 sur-rounding 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 1011:5M, rapid BH growth leads to the suppression of star formation and reduced growth of stellar mass with increasing halo mass.