Supermassive Black Hole Formation at High Redshifts via Direct Collapse: Physical Processes in the Early Stage

TL;DR

This study uses detailed numerical simulations to investigate how direct gas collapse in early universe dark matter halos can lead to the formation of supermassive black hole seeds, highlighting the physical processes that enable or hinder collapse.

Contribution

It provides new insights into the physical conditions and dynamics that promote SMBH seed formation via direct collapse at high redshifts, including the role of turbulence, angular momentum transfer, and fragmentation.

Findings

Collapse can lead to either central runaway or off-center fragmentation.

A disk-like structure forms inside the centrifugal barrier during collapse.

SMBH seed masses range from 2 x 10^4 to 2 x 10^6 solar masses.

Abstract

We use numerical simulations to explore whether direct collapse can lead to the formation of SMBH seeds at high-z. We follow the evolution of gas within DM halos of 2 x 10^8 Mo and 1 kpc. We adopt cosmological density profiles and j-distributions. Our goal is to understand how the collapsing flow overcomes the centrifugal barrier and whether it is subject to fragmentation. We find that the collapse leads either to a central runaway or to off-center fragmentation. A disk-like configuration is formed inside the centrifugal barrier. For more cuspy DM distribution, the gas collapses more and experiences a bar-like perturbation and a central runaway. We have followed this inflow down to ~10^{-4} pc. The flow remains isothermal and the specific angular momentum is efficiently transferred by gravitational torques in a cascade of nested bars. This cascade supports a self-similar, disk-like collapse. In the collapsing phase, virial supersonic turbulence develops and fragmentation is damped. For larger initial DM cores the timescales become longer. In models with more organized initial rotation, a torus forms and appears to be supported by turbulent motions. The evolution depends on the competition between two timescales, corresponding to the onset of the central runaway and off-center fragmentation. For less organized rotation, the torus is greatly weakened, the central accretion timescale is shortened, and off-center fragmentation is suppressed --- triggering the central runaway even in previously `stable' models. The resulting SMBH masses lie in the range 2 x 10^4 Mo - 2 x 10^6 Mo, much higher than for Population III remnants. We argue that the above upper limit appears to be more realistic mass. Corollaries of this model include a possible correlation between SMBH and DM halo masses, and similarity between the SMBH and halo mass functions, at time of formation.