The Effects of AGN Feedback on Present-Day Galaxy Properties in Cosmological Simulations

Abstract: We show that feedback from active galactic nuclei (AGN) plays an essential role in reproducing the down-sizing phenomena, namely: the colour-magnitude relation; specific star formation rates; and the $\alpha$ enhancement of early type galaxies. In our AGN model, black holes originate from Population III stars, in contrast to the merging scenario of previous works. In this paper, we show how the properties of present-day galaxies in cosmological chemo-hydrodynamical simulations change when we include our model for AGN feedback. Massive galaxies become redder, older, less massive, less compact, and show greater $\alpha$ enhancement than their counterparts without AGN. Since we reproduce the black hole mass and galaxy mass relation, smaller galaxies do not host a supermassive black hole and their star formation history is affected very little, but they can get external enrichment from nearby AGN depending on their environment. Nonetheless, the metallicity change is negligible, and the mass--metallicity relations, which are mainly generated by supernova feedback at the first star burst, are preserved.