The collapse of atomically-cooled primordial haloes. I. High Lyman-Werner backgrounds

Pristine, atomically cooled haloes may be the sites of primordial quasar formation because atomic cooling triggers rapid baryon collapse that can create 10⁴–10⁵ M_⊙ black hole seeds. However, no numerical simulation has ever followed the collapse of these haloes for the times required to form supermassive stars and direct-collapse black holes (DCBHs). We have now modelled baryon collapse in atomically cooled haloes with a wide range of spin parameters and assembly histories for times that are sufficient for DCBH formation. Fragmentation of accretion discs after ∼500 kyr is nearly ubiquitous in these haloes and in most cases leads to the formation of binary or multiple supermassive stellar systems. They also confirm that rapid baryon collapse proceeds for the times required for these stars to form DCBHs. Our simulations suggest that binary or even multiple DCBH formation was the rule rather than the exception in the primordial Universe.