Massive black hole seeds born via direct gas collapse in galaxy mergers: Their properties, statistics and environment

Abstract

We study the statistics and cosmic evolution of massive black hole seeds formed during majormergers of gas-rich late-type galaxies. Generalizing the results of the hydrosimulations fromMayer et al., we envision a scenario in which a supermassive star can form at the centreof galaxies that just experienced a major merger owing to a multiscale powerful gas inflow,provided that such galaxies live in haloes with masses above 1011M⊙, are gas rich and discdominated, and do not already host a massive black hole.We assume that the ultimate collapseof the supermassive star leads to the rapid formation of a black hole of 105M⊙ following aquasi-star stage. Using a model for galaxy formation applied to the outputs of the Millennium Simulation, we show that the conditions required for this massive black hole formation route totake place in the concordance ⊙ cold dark matter model are actually common at high redshiftand can be realized even at low redshift. Most major mergers above z~4 in haloes with mass>1011M⊙ can lead to the formation of a massive seed and, at z ~ 2, the fraction of favourablemergers decreases to about half. Interestingly, we find that even in the local universe a fraction(~20 per cent) of major mergers in massive haloes still satisfies the conditions for our massiveblack hole formation route. Those late events take place in galaxies with a markedly lowclustering amplitude, that have lived in isolation for most of their life and that are experiencinga major merger for the first time.We predict that massive black hole seeds from galaxy mergerscan dominate the massive end of the mass function at high (z > 4) and intermediate (z ~ 2)redshifts relative to lighter seeds formed at higher redshift, for example, by the collapse ofPop III stars. Finally, a fraction of these massive seeds could lie, soon after formation, abovethe MBH-MBulge relation. © 2013 The Authors.