Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass to Light Ratios from z~7 to z~4
Gonzalez, Valentino; Labbe, Ivo; Bouwens, Rychard; Illingworth, Garth; Franx, Marijn; Kriek, Mariska
Abstract
We derive stellar masses from SED fitting for ~400 Lyman Break Galaxies at z~4, 5, 6, and 7 from Hubble-WFC3/IR and Spitzer IRAC observations of the ERS field. Stellar Mass Functions (MFs) are determined from our new stellar mass M - UV luminosity relation and recent, deep UV Luminosity Functions (LFs). For the 299 z~4 galaxies we find that the M-Luv relation is a log-linear relation with a slope of -1.7 +/- 0.2, which results in luminosity-dependent M/Luv ratios, and large intrinsic scatter of ~0.5 dex. There is no evidence for M/L evolution from z~7 to z~4, suggesting that the specific SFR at a given mass does not evolve rapidly. Combining the observed M-Luv relation with the UV LFs results in steep MFs with slopes alpha_M ~-1.4 to -1.6 at low masses. This slope, however, is flatter than the MFs obtained from recent hydrodynamical simulations. We use these MFs to estimate the Stellar Mass Density (SMD) to a fixed Muv < -18 as a function of redshift and find a SMD growth proportional to (1+z)^(-3.4 +/- 0.8) from z~7 to z~4. We also compare the SMD growth to that from the observed SFR density, accounting for the LF evolution by using an evolving limit of 0.2L*(z). The resulting SMD growth proportional to (1+z)^(-2.8 +/- 0.9) shows a similar increasing trend to the integral of the SFR density proportional to (1+z)^(-6.3 +/- 0.6), but significant differences remain. These could result from too large dust corrections to the SFR at later times, stellar masses being overestimated at early times, or from non-monotonic star formation histories.