Effect of molecular hydrogen self-shielding modeling on early Reionization Era galaxies in radiative hydrodynamic cosmological simulations

Accurately modeling molecular hydrogen (H2) is an important task in cosmological simulations because it regulates star formation. One fundamental property of H2 is the ability to self-shield, a phenomenon in which the H2 in the outer layer of a molecular cloud absorbs the photodissociating Lyman-Werner UV radiation and shields the inner H2. Historically, numerical approximations have been utilized to avoid intensive ray-tracing calculations. This paper evaluates the use of the Sobolev-like density-gradient approximation in H2 self-shielding modeling and tests its agreement with a more rigorous adaptive ray-tracing method in cosmological simulations. We ran four high-resolution zoom-in cosmological simulations to investigate the models’ effects in the early Reionization Era (z ≥ 12). We find that the approximation model returns a higher H2 photodissociation rate in low gas density environments but a lower rate when gas density is high, resulting in low-mass halos having less H2 while high-mass halos having more H2. The approximation also hinders star formation in small halos, but it less affects the stellar mass of larger halos. Inside a halo, the discrepancies between the two models regarding H2 fraction, temperature, and stellar mass are radially dependent. On a large scale, the simulations using the approximation have less H2 in the intergalactic medium and may experience a slower reionization process. These results show that the Sobolev-like approximation alters properties of galaxies and the large-scale universe when compared to the ray-tracing treatment, emphasizing a need for caution when interpreting results from these two techniques in cosmological simulations.