Radiative and Mechanical AGN Feedback in Galaxy Evolution

Abstract

Accreting black holes are thought to inject energy into surrounding gas reservoirs via jets, outflows and radiation, inhibiting the build-up of massive galaxies and suppressing star formation. Active Galactic Nuclei (AGN) feedback can potentially starve the black hole, giving rise to a relation between the black hole mass and the stellar mass of galaxies. Many previous AGN feedback models, however, do not include all known and observed feedback processes. Since the importance of AGN-driven mass and momentum outflows in limiting the infall onto the black hole has been emphasized, we develop a numerical algorithm of AGN mechanical feedback via broad absorption line winds in a three-dimensional smoothed particle hydrodynamics code, modified with a pressure-entropy formulation, that better allows for contact discontinuities and implements improved fluid mixing. We also include the detailed treatment of radiative heating, radiation pressure, and the Eddington force and propose a unified model of AGN feedback. We investigate feedback effects in simulations of a single disk galaxy, major and minor mergers of galaxies, and the formation of elliptical galaxies in a cosmological context. We show that massive, non-relativistic outflows and X-ray heating are indeed a viable mechanism to regulate the black hole growth. While the thermal feedback model, where all the feedback energy is distributed as thermal heating, produces a factor of ~10<super>2</super>-10<super>3</super> higher X-ray luminosity than expected for given stellar mass of the galaxy, our model can successfully reproduce both the observed LX-&sigma;* and MBH-&sigma;* relations. In our model, the AGN-induced outbursts result in strong galactic outflows with vw~2,000 km/s consistent with observed quasar properties. They also effectively quench star formation making ellipticals red and dead consistent with the observations. Our model shows large fluctuations in both radiant and wind outputs, naturally reproducing the two modes of AGN feedback: `wind' mode, where black holes grow rapidly near the Eddington limit and expel gas via high velocity winds and powerful radiation pressure; and a `maintenance' mode when the electromagnetic luminosity is considerably below the Eddington limit.