Direct collapse black hole formation via high-velocity collisions of
  protogalaxies
  We propose high-velocity collisions of protogalaxies as a new pathway to form
supermassive stars (SMSs) with masses of ~ 10^5 Msun at high redshift (z > 10).
When protogalaxies hosted by dark matter halos with a virial temperature of ~
10^4 K collide with a relative velocity > 200 km/s, the gas is shock-heated to
~ 10^6 K and subsequently cools isobarically via free-free emission and He^+,
He, and H line emission. Since the gas density (> 10^4 cm^{-3}) is high enough
to destroy H_2 molecules by collisional dissociation, the shocked gas never
cools below ~ 10^4 K. Once a gas cloud of ~ 10^5 Msun reaches this temperature,
it becomes gravitationally unstable and forms a SMS which will rapidly collapse
into a super massive black hole (SMBH) via general relativistic instability. We
perform a simple analytic estimate of the number density of direct-collapse
black holes (DCBHs) formed through this scenario (calibrated with cosmological
N-body simulations) and find n_{DCBH} ~ 10^{-9} Mpc^{-3} (comoving) by z = 10.
This could potentially explain the abundance of bright high-z quasars.
