Steep faint-end slopes of galaxy mass and luminosity functions at z≥ 6 and the implications for reionization

Abstract

We present the results of a numerical study comparing photometric and\nphysical properties of simulated z=6-9 galaxies to the observations taken by\nthe WFC3 instrument aboard the Hubble Space Telescope. Using cosmological\nhydrodynamical simulations we find good agreement with observations in\ncolor-color space at all studied redshifts. We also find good agreement between\nobservations and our Schechter luminosity function fit in the observable range,\nMuv<= -18, provided that a moderate dust extinction effect exists for massive\ngalaxies. However beyond what currently can be observed, simulations predict a\nvery large number of low-mass galaxies and evolving steep faint-end slopes from\nalpha_L = -2.15 at z=6 to alpha_L = -2.64 at z=9, with a dependence of\n|alpha_L| \\propto (1+z)^0.59. During the same epoch, the normalization phi*\nincreases and the characteristic magnitude Muv* becomes moderately brighter\nwith decreasing redshift. We find similar trends for galaxy stellar mass\nfunction with evolving low-mass end slope from alpha_M = - 2.26 at z=6 to\nalpha_M = -2.87 at z=9, with a dependence of |alpha_M| \\propto (1+z)^0.65.\nTogether with our recent result on the high escape fraction of ionizing photons\nfor low-mass galaxies, our results suggest that the low-mass galaxies are\nimportant contributor of ionizing photons for the reionisation of the Universe\nat z>=6.\n