Radiation hydrodynamical simulations of the birth of intermediate-mass black holes in the first galaxies

The leading contenders for the seeds of z > 6 quasars are direct-collapse black holes (DCBHs) forming in atomically cooled haloes at z ∼20. However, the Lyman-Werner (LW) UV background required to form DCBHs of 105 M⊙ are extreme, about 104 J21, 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 J21, which were much more common at early times. These backgrounds allow haloes to grow to a few 106-107 M⊙ and virial temperatures of nearly 104 K before collapsing, but do not completely sterilize them of H2. Gas collapse then proceeds via Lyα and rapid H2 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 M⊙ at which they later collapse to IMBHs. Moderate LW backgrounds thus produced IMBHs in far greater numbers than DCBHs in the early universe.