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 $\sim$4500 galaxies, found via photometric redshifts over an area of $\sim$280 arcmin${}^2$ in the CANDELS/GOODS fields and the Hubble Ultra Deep Field. The deepest Spitzer/IRAC data yet-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 Ã ($M_{\mathrm{UV}}$) and logarithmic stellar mass ($\log M_{\ast }$) that holds for galaxies with $\log (M_{\ast }/M_{\odot })\lesssim 10$. We use simulations to validate our method of measuring the slope of the $\log M_{\ast }$-$M_{\mathrm{UV}}$ 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 $\log M_{\ast }$-$M_{\mathrm{UV}}$ 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 $\hat{I}\pm =-1.55_{-0.07}^{+0.08}$ at $z=4$ to $\hat{I}\pm =-2.25_{-0.35}^{+0.72}$ at $z=8$. The inferred stellar mass density, when integrated over $M_{\ast }=10^8$-$10^{13}{M}_{\odot }$, 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.

27 pages, 17 figures, ApJ, in press