  In the gravitational field of a Schwarzschild-like black hole, particles
infalling from rest at infinity, and black hole "wind" particles with
relativistic velocity leaking radially out from the nominal horizon, both have
the same magnitude of velocity at any radius from the hole. Hence when equally
massive infalling and wind particles collide at any radius, they yield
collision products with zero center of mass radial velocity, which can then
nucleate star formation at the collision radius. We suggest that this gives a
mechanism by which a central black hole can catalyze galaxy formation. For disk
galaxies, this mechanism explains the observed approximately exponential
falloff of the surface brightness with radius, and gives an estimate of the
associated scale length.
