UVUDF: UV Luminosity Functions at the cosmic high-noon

Abstract: We present the rest-1500\AA\ UV luminosity functions (LF) for star-forming galaxies during the cosmic \textit{high noon} -- the peak of cosmic star formation rate at $1.5<z<3$. We use deep NUV imaging data obtained as part of the \textit{Hubble} Ultra-Violet Ultra Deep Field (UVUDF) program, along with existing deep optical and NIR coverage on the HUDF. We select F225W, F275W and F336W dropout samples using the Lyman break technique, along with samples in the corresponding redshift ranges selected using photometric redshifts and measure the rest-frame UV LF at $z\sim1.7,2.2,3.0$ respectively, using the modified maximum likelihood estimator. We perform simulations to quantify the survey and sample incompleteness for the UVUDF samples to correct the effective volume calculations for the LF. We select galaxies down to $M_{UV}=-15.9,-16.3,-16.8$ and fit a faint-end slope of $\alpha=-1.20^{+0.10}_{-0.13}, -1.32^{+0.10}_{-0.14}, -1.39^{+0.08}_{-0.12}$ at $1.4<z<1.9$, $1.8<z<2.6$, and $2.4<z<3.6$, respectively. We compare the star formation properties of $z\sim2$ galaxies from these UV observations with results from H\alpha\ and UV$+$IR observations. We find a lack of high SFR sources in the UV LF compared to the H\alpha\ and UV$+$IR, likely due to dusty SFGs not being properly accounted for by the generic $IRX-\beta$ relation used to correct for dust. We compute a volume-averaged UV-to-H\alpha\ ratio by \textit{abundance matching} the rest-frame UV LF and H\alpha\ LF. We find an increasing UV-to-H\alpha\ ratio towards low mass galaxies ($M_\star \lesssim 5\times10^9$ M$_\odot$). We conclude that this could be due to a larger contribution from starbursting galaxies compared to the high-mass end.