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 similar to 10(5)M(circle dot) at high redshift (z > 10). When protogalaxies hosted by dark matter haloes with a virial temperature of similar to 10(4) K collide with a relative velocity greater than or similar to 200 km s(-1), the gas is shock-heated to similar to 10(6) K and subsequently cools isobarically via freefree emission and He+, He, and H line emission. Since the gas density (greater than or similar to 10(4) cm(-3)) is high enough to destroy H-2 molecules by collisional dissociation, the shocked gas never cools below similar to 10(4) K. Once a gas cloud of similar to 10(5)M(circle dot) reaches this temperature, it becomes gravitationally unstable and forms an SMS which will rapidly collapse into a supermassive black hole 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) similar to 10(-9) Mpc(-3) (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.