Can supermassive black hole seeds form in galaxy mergers?

TL;DR

This paper challenges previous claims that galaxy mergers can rapidly form supermassive black hole seeds through high accretion rates, showing that realistic cooling processes prevent such rapid growth.

Contribution

It demonstrates that more realistic gas cooling significantly reduces accretion rates, questioning the viability of merger-driven supermassive black hole seed formation.

Findings

Gas cools rapidly (< 1 yr), reducing accretion rates.

Disk becomes gravitationally unstable within 100 yr.

Supermassive black hole seeds cannot form via this mechanism under realistic conditions.

Abstract

It has been recently suggested that supermassive black holes at z = 5-6 might form from super-fast (\dot M > 10^4 Msun/yr) accretion occurring in unstable, massive nuclear gas disks produced by mergers of Milky-Way size galaxies. Interestingly, such 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 disk rapidly (< 1 yr) cools, the accretion rate drops, and the central core can grow only to \approx 100 Msun. In addition, most of the disk becomes gravitationally unstable in about 100 yr, further quenching the accretion. We conclude that this scenario encounters a number of difficulties that possibly make it untenable.