Radiation Hydrodynamical Simulations of the Birth of Intermediate-Mass Black Holes in the First Galaxies

TL;DR

This study uses radiation hydrodynamical simulations to explore how moderate Lyman-Werner UV backgrounds in early galaxies could lead to the formation of numerous intermediate-mass black holes, rather than the rarer direct-collapse black holes.

Contribution

It demonstrates that moderate UV backgrounds can produce more intermediate-mass black holes in the early universe than previously thought, challenging the focus on extreme conditions for black hole seed formation.

Findings

Moderate UV backgrounds enable gas collapse and star formation in early halos.

Pop III stars formed under these conditions reach masses of 1800-2800 solar masses.

More IMBHs are produced than DCBHs in early universe scenarios.

Abstract

The leading contenders for the seeds of $z > 6$ quasars are direct-collapse black holes (DCBHs) forming in atomically-cooled halos at $z \sim$ 20. However, the Lyman-Werner (LW) UV background required to form DCBHs of 10$^5$ \Ms\ are extreme, about 10$^4$ J$_{21}$, and may have been rare in the early universe. Here we investigate the formation of intermediate-mass black holes (IMBHs) under moderate LW backgrounds of 100 and 500 J$_{21}$ that were much more common at early times. These backgrounds allow halos to grow to a few 10$^6$ - 10$^7$ \Ms\ and virial temperatures of nearly 10$^4$ K before collapsing but do not completely sterilize them of H$_2$. Gas collapse then proceeds via Ly$\alpha$ and rapid H$_2$ cooling at rates that are 10 - 50 times those in normal Pop III star-forming haloes but less than those in purely atomically-cooled haloes. Pop III stars accreting at such rates become blue and hot, and we find that their ionizing UV radiation limits their final masses to 1800 - 2800 \Ms\, at which they later collapse to IMBHs. Moderate LW backgrounds thus produced IMBHs in far greater numbers than DCBHs in the early universe.