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