Supermassive Black Hole Formation at High Redshifts via Direct Collapse in a Cosmological Context

TL;DR

This study uses high-resolution simulations to explore how seed supermassive black holes form via direct collapse in the early universe, revealing a two-stage process involving atomic cooling and gas decoupling from dark matter.

Contribution

It provides a detailed, cosmological context simulation of the direct collapse process, highlighting the two-stage collapse mechanism and angular momentum transfer.

Findings

Collapse proceeds in two stages triggered by atomic cooling and gas decoupling.

Gas loses angular momentum through gravitational torques, enabling collapse.

Supersonic turbulence suppresses fragmentation during collapse.

Abstract

We study the early stage of the formation of seed supermassive black holes via direct collapse in dark matter (DM) halos, in the cosmological context. We perform high-resolution zoom-in simulations of such collapse at high-$z$. Using the adaptive mesh refinement code ENZO, we resolve the formation and growth of a DM halo, until its virial temperature reaches $\sim 10^4$K, atomic cooling turns on, and collapse ensues. We demonstrate that direct collapse proceeds in two stages, although they are not well separated. The first stage is triggered by the onset of atomic cooling, and leads to rapidly increasing accretion rate with radius, from $\dot M\sim 0.1\,M_\odot {\rm yr^{-1}}$ at the halo virial radius to few $M_\odot \,{\rm yr^{-1}}$, around the scale radius $R_{\rm s}\sim 30$pc of the NFW DM density profile. The second stage of the collapse commences when the gas density takes precedence over the DM density. This is associated with the gas decoupling from the DM gravitational potential. The ensuing collapse approximates that of an isothermal sphere with $\dot M ( r )\sim $const. We confirm that the gas loses its angular momentum through non-axisymmetric perturbations and gravitational torques, to overcome the centrifugal barrier. During the course of the collapse, this angular momentum transfer process happens on nearly all spatial scales, and the angular momentum vector of the gas varies with position and time. Collapsing gas also exhibits supersonic turbulent motions which suppress gas fragmentation, and are characterized by density PDF consisting of a lognormal part and a high-density power law tail.