Angular Momentum Transport in Early-formed Objects by Cosmic Background Radiation: Radiation-hydrodynamical Approach
The effects of radiation force by the cosmic background radiation (CBR) on early-formed objects shortly after the cosmological recombination are explored. In particular, we are interested in the angular momentum transport in massive disks that originate from tidally spun-up density fluctuations. The radiation force by the CBR is calculated by solving the radiative transfer equation and including Thomson scattering, under the assumption that a disk is locally approximated to be a plane-parallel medium in longitudinal motion. As a numerical technique, we employ a variable Eddington factor method. The results show that the efficiency of angular momentum extraction by the CBR decreases exponentially with optical depth even if the radiative diffusion is effective. This implies that the photon-scattering process in moving media proceeds just like the pure absorption process as far as momentum or angular momentum transport is concerned. Because of the present effects, the distributions of the spin probability of early-formed disks could be significantly modified in the presence of the strong CBR. An optically thin disk originating from a large spin parameter (λ > 0.05) would shed angular momentum until it becomes optically thick, resulting in λ ~ 0.05, almost regardless of mass scales of objects and the initial power spectrum. Also, the CBR force likely helps to enhance the effects of shear viscosity, thereby enabling a seed black hole form at z > 10 to account for quasar formation.