  We present UV luminosity functions (LFs) at 1500 Angstrom derived from the
HST Early Release Science WFC3/UVIS data acquired over ~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~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 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~=-1.2, this poses the question as to when and how the expected
flattening occurs. Despite relatively large uncertainties, our data suggest
alpha~=-1.7 at least down to z~1. These new results from such a shallow early
dataset 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.
