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 angstrom derived from the Hubble Space Telescope Early Release Science WFC3/UVIS data acquired over similar to 50 arcmin(2) 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 similar to 1.5, z similar to 1.9, and z similar to 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 similar to 1.4 dex according to (1 + z)(2.58 +/- 0.15) from z similar to 0 to its peak at z similar to 2.5. Strikingly, the inferred faint-end slopes for our LFs are all steeper than alpha = -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 alpha similar or equal to -1.2, this poses the question as to when and how the expected flattening occurs. Despite relatively large uncertainties, our data suggest alpha similar or equal to -1.7 at least down to z similar to 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 similar to 0.5 to z similar to 3.