It has been recently suggested that supermassive black holes at z \approx 5 - 6 might form from super-fast ( \dot { M } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { 4 } M _ { \odot } yr ^ { -1 } ) 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 M _ { \odot } .
In addition , most of the disk becomes gravitationally unstable in \approx 100 yr , further quenching the accretion .
We conclude that this scenario encounters a number of difficulties that possibly make it untenable .