Co-evolution of Supermassive Black Holes and their Host L* galaxies: implications for Milky Way and M31

Abstract

We investigate the origin of the scatter in the supermassive black hole (BH) masses for galaxies in the L* regime, using the ARTEMIS and EAGLE simulations. By classifying galaxies based on their central BH / galaxy stellar masses ratios, we follow the evolution of galaxies with the lowest and highest such ratios (denoted LBH and HBH, respectively). We find that the properties of these two galaxy samples are comparable at z ~ 2 but diverge significantly towards lower redshifts. Galaxies with less massive BHs were able to maintain higher gas fractions and sustained star formation during their evolution, whereas those with more massive BHs formed stars earlier, grew BHs faster and experienced more efficient feedback and subsequent quenching. The simulations broadly match the observed scatter in the BH masses and galaxy morphologies in the L* regime and explain the origin of this scatter in terms of differences between merger histories. Galaxies with more active merger histories contain more massive BHs at present time and tend to be elliptical, while galaxies with more quiescent histories have less massive BHs and tend to be disc-like. Mergers enhance BH growth through higher gas accretion rates onto central regions and direct BH-BH mergers. However, these channels operate differently: in HBH galaxies, BHs grow primarily (~ 90%) by gas accretion, whereas in LBH they grow both through gas accretion (~ 60%) and BH-BH mergers (~40%). Our results suggest that the different BH masses in MW and M31 could be explained by differences in merger histories.