How an improved implementation of H2 self-shielding influences the formation of massive stars and black holes

TL;DR

This study improves the modeling of H2 self-shielding in 3D simulations, revealing a lower critical UV flux for direct collapse, which could significantly increase the predicted number of black hole seeds in the early universe.

Contribution

The paper introduces an advanced method for calculating H2 self-shielding in 3D simulations, accounting for gas geometry and velocity, leading to more accurate J_crit estimates.

Findings

J_crit is approximately half of previous estimates (~2000 vs. ~4000 J_21).

Lower J_crit increases the predicted number of haloes capable of forming black hole seeds.

Directional dependence of H2 column density is crucial for accurate self-shielding modeling.

Abstract

High redshift quasars at z>6 have masses up to ~$10^9$ M$_\odot$. 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 (H2) formation is inhibited by a strong external 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, H2 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 H2 against photodissociating radiation. We find a value of J_crit that is a factor of two smaller than with the Jeans approach (~2000 J_21 vs. ~4000 J_21). The main reason for this difference is the strong directional dependence of the H2 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.