Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass to Light Ratios from z 7 to z 4

TL;DR

This study derives galaxy stellar mass functions from z 7 to z 4 using deep Hubble and Spitzer data, revealing a steepening of the mass function slopes and a significant growth in stellar mass density over this period.

Contribution

It provides the first detailed mass-luminosity relation at z 4 and uses it to derive more accurate galaxy mass functions and stellar mass densities from z 7 to z 4.

Findings

Steeper low-mass slopes of the mass functions (α_M -1.4 to -1.6).

A growth in stellar mass density proportional to (1+z)^{-3.4} from z 7 to z 4.

A well-defined mass-luminosity relation with luminosity-dependent M/L ratios at z 4.

Abstract

We derive stellar masses from SED fitting to rest-frame optical and UV fluxes for 401 star-forming galaxies at z 4, 5, and 6 from Hubble-WFC3/IR observations of the ERS combined with the deep GOODS-S Spitzer/IRAC data (and include a previously-published z 7 sample). A mass-luminosity relation with strongly luminosity-dependent M/Luv ratios is found for the largest sample (299 galaxies) at z 4. The relation M \propto L_{UV,1500}^(1.7+/-0.2) has a well-determined intrinsic sample variance of 0.5 dex. This relation is also consistent with the more limited samples at z 5-7. This z 4 mass-luminosity relation, and the well-established faint UV luminosity functions at z 4-7, are used to derive galaxy mass functions (MF) to masses M~10^8 at z 4-7. A bootstap approach is used to derive the MFs to account for the large scatter in the M--Luv relation and the luminosity function uncertainties, along with an analytical crosscheck. The MFs are also corrected for the effects of incompleteness. The incompleteness-corrected MFs are steeper than previously found, with slopes \alpha_M-1.4 to -1.6 at low masses. These slopes are, however, still substantially flatter than the MFs obtained from recent hydrodynamical simulations. We use these MFs to estimate the stellar mass density (SMD) of the universe to a fixed M_{UV,AB}<-18 as a function of redshift and find a SMD growth \propto(1+z)^{-3.4 +/-0.8} from z 7 to z 4. We also derive the SMD from the completeness-corrected MFs to a mass limit M~10^{8} Msun. Such completeness-corrected MFs and the derived SMDs will be particularly important for model comparisons as future MFs reach to lower masses.