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 ∼105 M⊙ 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 ∼105 M⊙ 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.