  A scenario is put forth for the formation of supermassive black holes at the
centers of galaxies. It depends upon the formation of a Mestel disk with a flat
rotation curve, where the mass is proportional to r and thickness to 1/r. Such
disks should form from the collapse of uniformly rotating, isolated, gaseous
clouds, either proto-galactic, galaxy-mass damped Lyman alpha clouds or the gas
that survives galaxy mergers. We propose that in any case the disk will be
unstable to the Rossby vortex instability (RVI). This instability grows from
any large, steep pressure gradient in an optically thick disk. Such pressure
gradients either occur adjacent to compact objects or could be triggered by
individual supernovae in and around the disk. Upon excitation, the RVI
transports angular momentum far more efficiently than the turbulence of a
viscosity-based Shakura-Sunyaev disk. The critical column density condition in
a L* Mestel disk predicts that the black hole mass is ~ 10^8 Msolar and is also
proportional to the 1/4 power of the velocity dispersion, in accordance with
observations.
