Publication Date:
2015-07-16
Description:
High-redshift quasars at z 〉 6 have masses up to ~10 9 M . One of the pathways to their formation includes direct collapse of gas, forming a supermassive star, precursor of the black hole seed. The conditions for direct collapse are more easily achievable in metal-free haloes, where atomic hydrogen cooling operates and molecular hydrogen (H 2 ) formation is inhibited by a strong external (ultraviolet) UV flux. Above a certain value of UV flux ( J crit ), the gas in a halo collapses isothermally at ~10 4 K and provides the conditions for supermassive star formation. However, H 2 can self-shield, reducing the effect of photodissociation. So far, most numerical studies used the local Jeans length to calculate the column densities for self-shielding. We implement an improved method for the determination of column densities in 3D simulations and analyse its effect on the value of J crit . This new method captures the gas geometry and velocity field and enables us to properly determine the direction-dependent self-shielding factor of H 2 against photodissociating radiation. We find a value of J crit that is a factor of 2 smaller than with the Jeans approach (~2000 J 21 versus ~4000 J 21 ). The main reason for this difference is the strong directional dependence of the H 2 column density. With this lower value of J crit , the number of haloes exposed to a flux 〉 J crit is larger by more than an order of magnitude compared to previous studies. This may translate into a similar enhancement in the predicted number density of black hole seeds.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
