The Evolution of the Ultraviolet Luminosity Function from z ~ 0.75 to z ~ 2.5 Using HST ERS WFC3/UVIS Observations

We present UV luminosity functions (LFs) at 1500 Å derived from the Hubble Space Telescope Early Release Science WFC3/UVIS data acquired over ~50 arcmin² of the GOODS-South field. The LFs are determined over the entire redshift range z = 0.75-2.5 using two methods, similar to those used at higher redshifts for Lyman break galaxies (LBGs): (1) 13 band UV+optical+NIR photometric redshifts to study galaxies in the range z = 0.5-2 in three bins of dz = 0.5 and (2) dropout samples in three redshift windows centered at z ~ 1.5, z ~ 1.9, and z ~ 2.5. The characteristic luminosity dims by 1.5 mag from z = 2.5 to z = 0.75, consistent with earlier work. However, the other Schechter function parameters, the faint-end slope and the number density, are found to be remarkably constant over the range z = 0.75-2.5. Using these LF determinations, we find the UV luminosity density to increase by ~1.4 dex according to (1 + z)^2.58±0.15 from z ~ 0 to its peak at z ~ 2.5. Strikingly, the inferred faint-end slopes for our LFs are all steeper than α = -1.5, in agreement with higher-redshift LBG studies. Since the faint-end slope in the local universe is found to be much flatter with α ~= -1.2, this poses the question as to when and how the expected flattening occurs. Despite relatively large uncertainties, our data suggest α ~= -1.7 at least down to z ~ 1. These new results from such a shallow early data set demonstrate very clearly the remarkable potential of WFC3/UVIS for the thorough characterization of galaxy evolution over the full redshift range z ~ 0.5 to z ~ 3.

Based on data obtained with the Hubble Space Telescope operated by AURA, Inc., for NASA under contract NAS5-26555.