Feedback from active galactic nuclei : a study of its impact and numerical implementations

Abstract

Feedback from active galactic nuclei is now widely regarded as playing a fundamental role in modern theories of galaxy formation. Recent research has highlighted that not only may AGN feedback suppress star formation by heating, it may also promote star formation by producing rapidly cooling shocks. From both a theoretical and numerical perspective, modelling the physical processes responsible is highly challenging, as 13 orders of magnitude in both spatial and temporal scales are involved. In the literature, there are several different numerical approaches to modelling AGN feedback in galactic and cosmological contexts. The various models make different physical assumptions and are simulated using different numerical codes starting from different initial conditions. Thus, a direct comparison of the results is not possible. We present a study of six distinct approaches to modelling AGN feedback within gravitohydrodynamic simulations of major mergers of Milky Way-sized galaxies. To constrain differences to only be between AGN feedback models, all simulations are run using the Hydra code, including its associated star formation algorithm, and start from the same initial conditions. We focus on five key aspects of the AGN feedback algorithms: the black hole accretion rate, energy feedback rate and method, particle accretion algorithm, black hole advection algorithm and black hole merger algorithm. Our results yield a wide variation in the accretion behaviours of the models, which reinforces the fact that there remains much to be learnt about the evolution of galactic nuclei. Using an augmented version of Zeus-3D, we model an AGN in a system loosely resembling the Local Group. When the AGN is active, shocks are formed in the gas, and stars are born in these shocks. We track the stellar distributions over half a Hubble time to determine if triggered star formation in shocks can account for the observed stellar growth in a galaxy's outer regions. At all times, the stellar distribution can be fit with a Sersic profile, and the stars continually migrate to larger radii.