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 ~ 105 Msun at high redshift (z > 10). When protogalaxies hosted by dark matter halos with a virial temperature of ~ 104 K collide with a relative velocity > 200 km/s, 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 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 nDCBH ~ 10-9 Mpc-3 (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.
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