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

We study the statistics and cosmic evolution of massive black hole seeds formed during major mergers of gas-rich late-type galaxies. Generalizing the results of the hydrosimulations from Mayer et al., we envision a scenario in which a supermassive star can form at the centre of galaxies that just experienced a major merger owing to a multiscale powerful gas inflow, provided that such galaxies live in haloes with masses above 10(11) M-circle dot, are gas rich and disc dominated, and do not already host a massive black hole. We assume that the ultimate collapse of the supermassive star leads to the rapid formation of a black hole of 10(5) M-circle dot following a quasi-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 to take place in the concordance Lambda cold dark matter model are actually common at high redshift and can be realized even at low redshift. Most major mergers above z similar to 4 in haloes with mass >10(11) M-circle dot can lead to the formation of a massive seed and, at z similar to 2, the fraction of favourable mergers decreases to about half. Interestingly, we find that even in the local universe a fraction (similar to 20 per cent) of major mergers in massive haloes still satisfies the conditions for our massive black hole formation route. Those late events take place in galaxies with a markedly low clustering amplitude, that have lived in isolation for most of their life and that are experiencing a major merger for the first time. We predict that massive black hole seeds from galaxy mergers can dominate the massive end of the mass function at high (z > 4) and intermediate (z similar to 2) redshifts relative to lighter seeds formed at higher redshift, for example, by the collapse of Pop III stars. Finally, a fraction of these massive seeds could lie, soon after formation, above the M-BH-M-Bulge relation.