On the effect of Lyman alpha trapping during the initial collapse of massive black hole seeds
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One viable seeding mechanism for supermassive black holes is the direct gaseous collapse route in pre-galactic dark matter halos, producing objects on the order of 10^4 - 10^6 solar masses. These events occur when the gas is prevented from cooling below 10^4 K that requires a metal-free and relatively H_2-free medium. The initial collapse cools through atomic hydrogen transitions, but the gas becomes optically thick to the cooling radiation at high densities. We explore the effects of Lyman-alpha trapping in such a collapsing system with a suite of Monte Carlo radiation transport calculations in uniform density and isotropic cases that are based from a cosmological simulation. Our method includes both non-coherent scattering and two-photon line cooling. We find that Lyman-alpha radiation is marginally trapped in the parsec-scale gravitationally unstable central cloud, allowing the temperature to increase to 50,000 K at a number density of 3*10^4 cm^-3 and increasing the Jeans mass by a factor of five. The effective equation of state changes from isothermal at low densities to have an adiabatic index of 4/3 around the temperature maximum and then slowly retreats back to isothermal at higher densities. Our results suggest that Lyman-alpha trapping delays the initial collapse by raising the Jeans mass. Afterward the high density core cools back to 10^4 K that is surrounded by a warm envelope whose inward pressure may alter the fragmentation scales at high densities.