On the faint-end of the high-$z$ galaxy luminosity function

Abstract: Recent measurements of the Luminosity Function (LF) of galaxies in the Epoch of Reionization (EoR, $z\gsim6$) indicate a very steep increase of the number density of low-mass galaxies populating the LF faint-end. However, as star formation in low-mass halos can be easily depressed or even quenched by ionizing radiation, a turnover is expected at some faint UV magnitudes. Using a physically-motivated analytical model, we quantify reionization feedback effects on the LF faint-end shape. We find that if reionization feedback is neglected, the power-law Schechter parameterization characterizing the LF faint-end remains valid up to absolute UV magnitude $\sim -9$. If instead radiative feedback is strong enough that quenches star formation in halos with circular velocity smaller than 50 km s$^{-1}$, the LF starts to drop at absolute UV magnitude $\sim -15$, i.e. slightly below the detection limits of current (unlensed) surveys at $z\sim5$. The LFs may rise again at higher absolute UV magnitude, where, as a result of interplay between reionization process and galaxy formation, most of the galaxy light is from relic stars formed before the EoR. We suggest that the galaxy number counts data, particularly in lensed fields, can put strong constraints on reionization feedback. In models with stronger reionization feedback, stars in galaxies with absolute UV magnitude higher than $\sim -13$ and smaller than $\sim - 8$ are typically older. Hence, the stellar age - UV magnitude relation can be used as an alternative feedback probe.