Impact of H_2-based star formation model on the z>=6 luminosity function and the ionizing photon budget for reionization

TL;DR

This study uses cosmological simulations with an H_2-based star formation model to analyze its effects on the galaxy luminosity function at high redshift and the implications for cosmic reionization, revealing fewer low-luminosity galaxies but sustained reionization capability.

Contribution

It introduces an H_2-based star formation model into cosmological simulations and examines its impact on the UV luminosity function and reionization at z>=6.

Findings

Fewer low-luminosity galaxies at Muv>-18 with the H_2 model.

The turnover point in the luminosity function evolves from Muv=-17.33 to -15.38 between z=8 and z=6.

Total star formation rate density remains sufficient for reionization despite reduced low-luminosity galaxy numbers.

Abstract

We present the results of a numerical study examining the effect of H_2-based star formation (SF) model on the rest-frame UV luminosity function and star formation rate function (SFRF) of z>=6 galaxies, and the implications for reionization. Using cosmological hydrodynamical simulations outfitted with an H_2-SF model, we find good agreement with our previous results (non-H_2 SF model) and observations at Muv<=-18. However at Muv>-18, we find that the LF deviates from both our previous work and current observational extrapolations, producing significantly fewer low-luminosity galaxies and exhibiting additional turnover at the faint end. We constrain the redshift evolution of this turnover point using a modified Schechter function that includes additional terms to quantify the turnover magnitude (Muv^t) and subsequent slope (Beta). We find that Muv^t evolves from Muv^t=-17.33 (at z=8) to -15.38 (z=6), while Beta becomes shallower by \Delta Beta=0.22 during the same epoch. This occurs in an Muv range which will be observable by JWST. By integrating the SFRF, we determine that even though H_2-SF model significantly reduces the number density of low luminosity galaxies at Muv>-18, it does not suppress the total SFR density enough to affect the capability of SF to maintain reionization.