The Evolution of the Galaxy Stellar Mass Function at z = 4-8: A Steepening Low-mass-end Slope with Increasing Redshift

Abstract

We present galaxy stellar mass functions (GSMFs) at z = 4-8 from a rest-frame ultraviolet (UV) selected sample of ∼4500 galaxies, found via photometric redshifts over an area of ∼280 arcmin² in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)/Great Observatories Origins Deep Survey (GOODS) fields and the Hubble Ultra Deep Field. The deepest Spitzer/IRAC data to date and the relatively large volume allow us to place a better constraint at both the low- and high-mass ends of the GSMFs compared to previous space-based studies from pre-CANDELS observations. Supplemented by a stacking analysis, we find a linear correlation between the rest-frame UV absolute magnitude at 1500 Å ({M}_{{UV}}) and logarithmic stellar mass ({log}{M}_* ) that holds for galaxies with {log}({M}_* /{M}_⊙ )≲ 10. We use simulations to validate our method of measuring the slope of the {log}{M}_* -M _UV relation, finding that the bias is minimized with a hybrid technique combining photometry of individual bright galaxies with stacked photometry for faint galaxies. The resultant measured slopes do not significantly evolve over z = 4-8, while the normalization of the trend exhibits a weak evolution toward lower masses at higher redshift. We combine the {log}{M}_* -M _UV distribution with observed rest-frame UV luminosity functions at each redshift to derive the GSMFs, finding that the low-mass-end slope becomes steeper with increasing redshift from α =-{1.55}_-0.07^+0.08 at z = 4 to α =-{2.25}_-0.35^+0.72 at z = 8. The inferred stellar mass density, when integrated over {M}_* ={10}⁸-10¹³ M _⊙, increases by a factor of {10}_-2⁺³⁰ between z = 7 and z = 4 and is in good agreement with the time integral of the cosmic star formation rate density.