  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.
