Cosmological Simulations with AGN Feedback

Abstract

We implement a model for, and study the effects of, AGN feedback in cosmological hydrodynamical simulations. In our model, black holes form high-density, primordial gas, to imitate the likely channels of black hole formation in the early Universe. We find that a black hole seed mass of 102−3h−1M⊙ is required to produce simulations that match the cosmic star formation rate density, and present-day black hole mass – velocity dispersion and galaxy size – velocity dispersion relations. We therefore suggest that Population iii stars can be the progenitors of the super-massive black holes seen today. Using our fiducial model, we run two large simulations ((25h−1 Mpc)3), one with and one without AGN feedback. With these, we follow the population of galaxies that forms across cosmic time, and find that the inclusion of AGN feedback improves the agreement of simulated and observed galaxy properties, such as the mass and luminosity functions. This agreement is best at z = 0, and fairly good out to z = 2−3. Evidence for downsizing in the evolution of galaxies is found, both in the present-day colour–magnitude and [α/Fe]–velocity dispersion relations, and by the fact that high-mass galaxies attain their present-day metallicity earlier and faster than do low-mass ones. With our hydrodynamical simulations, we can also investigate the internal structure of galaxies, and look at the effects of galaxy mergers and AGN feedback on the stellar and gas-phase metallicity gradients of galaxies. Stellar metallicity gradients are found to be sensitive to galaxy mergers, while gas-phase metallicity gradients are more affected by AGN activity. This suggests that simultaneous measurements of these two quantities can help disentangle the actions of mergers and AGN feedback on a galaxy’s history. Finally, we develop a new method to identify massive AGN-driven outflows from the most massive simulated galaxy. These events cause the intra-cluster medium to be hotter and more chemically enriched compared to the simulation without AGN feedback, and therefore AGN feedback may be required in order to attain the metallicities observed in clusters.