Can supermassive black hole seeds form in galaxy mergers?

It has been recently suggested that supermassive black holes at z ≈ 5–6 might form from superfast (Ṁ ≳ 104 M⊙ yr−1) accretion occurring in unstable, massive nuclear gas discs produced by mergers of Milky Way-sized galaxies. Interestingly, such a mechanism is claimed to work also for gas enriched to solar metallicity. These results are based on an idealized polytropic equation-of-state assumption, essentially preventing the gas from cooling. We show that under more realistic conditions, the disc rapidly (<1 yr) cools, the accretion rate drops and the central core can grow only to ≈100 M⊙. In addition, most of the disc becomes gravitationally unstable in ≈100 yr, further quenching the accretion. We conclude that this scenario encounters a number of difficulties that possibly make it untenable.