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

TL;DR

This paper proposes a new pathway for forming supermassive stars and black holes at high redshift through high-velocity protogalaxy collisions, involving shock heating and suppression of molecular cooling.

Contribution

It introduces a novel mechanism for direct-collapse black hole formation via high-velocity galaxy collisions, supported by analytic estimates and cosmological simulations.

Findings

Estimated number density of DCBHs at z=10 is ~10^{-9} Mpc^{-3}.

Shock heating prevents cooling below 10^4 K, enabling supermassive star formation.

Potential explanation for the abundance of high-redshift quasars.

Abstract

We propose high-velocity collisions of protogalaxies as a new pathway to form supermassive stars (SMSs) with masses of ~ 10^5 Msun at high redshift (z > 10). When protogalaxies hosted by dark matter halos with a virial temperature of ~ 10^4 K collide with a relative velocity > 200 km/s, the gas is shock-heated to ~ 10^6 K and subsequently cools isobarically via free-free emission and He^+, He, and H line emission. Since the gas density (> 10^4 cm^{-3}) is high enough to destroy H_2 molecules by collisional dissociation, the shocked gas never cools below ~ 10^4 K. Once a gas cloud of ~ 10^5 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 n_{DCBH} ~ 10^{-9} Mpc^{-3} (comoving) by z = 10. This could potentially explain the abundance of bright high-z quasars.