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 n_DCBH ∼ 10^{- 9} Mpc^{- 3} (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.