Supermassive Black Hole Formation at High Redshifts via Direct Collapse in a Cosmological Context [GA]

We study the early stage of the formation of seed SMBHs via direct collapse in DM halos, in the cosmological context. We have performed high-resolution zoom-in simulations of such collapse at high redshifts, and have compared it with gas collapse within the isolated DM halo model of Choi et al. Using the AMR code ENZO, we have resolved the formation and growth of a DM halo via cold accretion of the filamentary and diffuse gas, until its virial temperature has reached $\sim 10^4$K, atomic cooling has turned on, and collapse has ensued. We confirm our previous result that direct collapse proceeds in two stages, although, as expected, they are not as well separated. The first stage is triggered by the onset of atomic cooling, and leads to rapidly increasing accretion rate with radius, from $\dot M\sim 0.1\,M_\odot\,{\rm yr^{-1}}$ at the halo virial radius to a few $M_\odot\,{\rm yr^{-1}}$, just inside the scale radius $R_{\rm s}\sim 30$pc of the NFW DM density profile. The second stage of the runaway collapse commences when the gas density takes precedence over the DM density. We find that this is associated with the gas decoupling from the background gravitational potential of the DM, at $\sim R_{\rm s}$, and that the ensuing collapse approximates that of isothermal sphere with $\dot M\sim $ const. within this radius. We confirm that the gas loses its angular momentum through non-axisymmetric perturbations to overcome the centrifugal barrier. During the course of the collapse, this angular momentum transfer process happens on nearly all spatial scales, and the angular momentum vector of the gas varies with position and time. Collapsing gas also exhibits supersonic turbulent motions which suppress gas fragmentation, and which are characterized by a density probability distribution consisting of a lognormal part and a high-density power law tail.

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J. Choi, I. Shlosman and M. Begelman
Wed, 10 Dec 14

Comments: 14 pages and 11 figures submitted to MNRAS