Direct collapse black hole formation via high-velocity collisions of protogalaxies

Abstract

We propose high-velocity collisions of protogalaxies as a new pathway to form supermassive stars (SMSs) with masses of ~105M⊙ at high redshift (z > 10). When protogalaxies hosted by dark matter haloes with a virial temperature of ~104 K collide with a relative velocity ≳200 km s-1, the gas is shock-heated to ~106 K and subsequently cools isobarically via free- free emission and He+, He, and H line emission. Since the gas density (≳104 cm-3) is high enough to destroy H2 molecules by collisional dissociation, the shocked gas never cools below ~104 K. Once a gas cloud of ~105M⊙ 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 nDCBH ~ 10-9 Mpc-3 (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.