Role of primordial black holes in the direct collapse scenario of supermassive black hole formation at high redshifts

Abstract

In this paper, we explore the possibility of accreting primordial black holes as the source of heating for the collapsing gas in the context of the direct collapse black hole scenario for the formation of super-massive black holes (SMBHs) at high redshifts, $z\sim$ 6–7. One of the essential requirements for the direct collapse model to work is to maintain the temperature of the in-falling gas at $\approx$10$^4$ K. We show that even under the existing abundance limits, the primordial black holes of masses $\gtrsim$10$^{-2}M_\odot$, can heat the collapsing gas to an extent that the $\mathrm{H}_2$ formation is inhibited. The collapsing gas can maintain its temperature at $10^4$ K till the gas reaches a critical density $n_{{c}} \,{\approx }\, 10^3~\hbox {cm}^{-3}$, at which the roto-vibrational states of $\mathrm{H}_2$ approaches local thermodynamic equilibrium and $\mathrm{H}_2$ cooling becomes inefficient. In the absence of $\mathrm{H}_2$ cooling, the temperature of the collapsing gas stays at $\approx$10$^4$ K even as it collapses further. We discuss scenarios of subsequent angular momentum removal and the route to find collapse through either a supermassive star or a supermassive disk.