Abstract
We present the highest-resolution three-dimensional simulation to date of the collapse of an atomic cooling halo in the early Universe. We use the moving-mesh code arepo with the primordial chemistry module introduced in Greif (2014), which evolves the chemical and thermal rate equations for over more than 20 orders of magnitude in density. Molecular hydrogen cooling is suppressed by a strong Lyman-Werner background, which facilitates the near-isothermal collapse of the gas at a temperature of about $10^4\,$K. Once the central gas cloud becomes optically thick to continuum emission, it settles into a Keplerian disc around the primary protostar. The initial mass of the protostar is about $0.1\,{\rm M}_\odot$, which is an order of magnitude higher than in minihaloes that cool via molecular hydrogen. The high accretion rate and efficient cooling of the gas catalyse the fragmentation of the disc into a small protostellar system with 5-10 members. After about 12 yr, strong gravitational interactions disrupt the disc and temporarily eject the primary protostar from the centre of the cloud. By the end of the simulation, a secondary clump has collapsed at a distance of $\simeq 150\,$au from the primary clump. If this clump undergoes a similar evolution as the first, the central gas cloud may evolve into a wide binary system. High accretion rates of both the primary and secondary clumps suggest that fragmentation is not a significant barrier for forming at least one massive black hole seed.
Highlights
Black holes (BHs) are a key ingredient in the formation and evolution of galaxies
Since the accretion rate in a Jeans-unstable cloud scales as M ∝ T 3/2, molecular hydrogen cooling must be suppressed until the virial temperature of the halo is high enough that Lyα cooling becomes important
The six panels show a zoom-in on the central gas cloud, ranging from 10 pc down to scales of 10 au
Summary
Black holes (BHs) are a key ingredient in the formation and evolution of galaxies. In the local Universe, the stellar velocity dispersion in galaxy bulges is correlated with the mass of the BH at their centre (Ferrarese & Merritt 2000; Gebhardt et al 2000). Since the accretion rate in a Jeans-unstable cloud scales as M ∝ T 3/2, molecular hydrogen cooling must be suppressed until the virial temperature of the halo is high enough that Lyα cooling becomes important This may be achieved by a Lyman-Werner (LW) radiation background (Omukai 2001; Bromm & Loeb 2003; Volonteri & Rees 2005; Spaans & Silk 2006; Schleicher, Spaans & Glover 2010; Johnson et al 2013). The simulations of Regan, Johansson & Haehnelt (2014) displayed the formation of a disc-like object at the centre of the halo, which in some cases fragmented on a scale of 100 au These simulations suffered from limited resolution, and did not include the relevant H2 cooling and chemistry. All distances are quoted in proper units, unless noted otherwise
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