Abstract
We present a robust measurement of the rest-frame UV luminosity function (LF)
and its evolution during the peak epoch of cosmic star formation at 1<z<3. We
use our deep near ultraviolet imaging from WFC3/UVIS on the Hubble Space
Telescope (HST) and existing ACS/WFC and WFC3/IR imaging of three lensing
galaxy clusters, Abell 2744 and MACSJ0717 from the Hubble Frontier Field survey
and Abell 1689. Combining deep UV imaging and high magnification from strong
gravitational lensing, we use photometric redshifts to identify 780 ultra-faint
galaxies with M_UV<-12.5 AB mag at 1<z<3. From these samples, we identified 5
new, faint multiply imaged systems in A1689. We run a Monte Carlo simulation to
estimate the completeness correction and effective volume for each cluster
using the latest published lensing models. We compute the rest-frame UV LF and
find the best-fit faint-end slopes of alpha=-1.56\pm0.04, alpha=-1.72\pm0.04
and alpha=-1.94\pm0.06 at 1.0<z<1.6, 1.6<z<2.2 and 2.2<z<3.0, respectively. Our
results demonstrate that the UV LF becomes steeper from z~1.3 to z~2.6 with no
sign of a turnover down to M_UV=-12.5 AB mag. We further derive the UV LFs
using the Lyman break "dropout" selection and confirm the robustness of our
conclusions against different selection methodologies. Because the sample sizes
are so large, and extend to such faint luminosities, the statistical
uncertainties are quite small, and systematic uncertainties (due to the assumed
size distribution, for example), likely dominate. Finally, we conclude that the
faint star-forming galaxies with UV magnitudes of -18.5<M_UV<-12.5 covered in
this study, produce the majority (55%-60%) of the unobscured UV luminosity
density at 1<z<3.
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