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

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.