The Faint End of the UV Luminosity Function at 0.4 &lt; <i>z</i> &lt; 0.7 from the Hubble Frontier Fields

Abstract By extending the Hubble Frontier Fields (HFF) observations to the F225W band using the Hubble Space Telescope (HST) WFC3/UVIS, we measure the rest-frame UV luminosity function (LF) of galaxies at 0.4 &lt; z &lt; 0.7, pushing into the low-luminosity galaxy regime. In this first paper of a series, we describe the HST Cycle-27 GO-15940 F225W observations and data reduction, and we present a corresponding catalog for the A2744 field, which is the most data-rich HFF cluster field. Combining deep near-UV imaging and the high magnification from the strong gravitational lensing of the foreground cluster, we identify 152 faint galaxies with −19.5 &lt; M UV &lt; –12.1 at 0.4 &lt; z &lt; 0.7 through hybrid photometric–spectroscopic redshift selection from the A2744 F225W catalog. Using a sample defined by a 50% completeness cut and applying maximum likelihood estimation, we derive the best-fit Schechter parameters for the UV LF at z ∼ 0.55 down to M UV &lt; –13.5 mag, including a faint-end slope of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>α</mml:mi> <mml:mo>=</mml:mo> <mml:mspace width="0.1em"/> <mml:mtext>–</mml:mtext> <mml:mspace width="0.1em"/> <mml:mn>1.32</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.074</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.072</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . We incorporate a curvature parameter δ in the parameter estimation to account for a possible turnover at the faint end of the UV LF, leveraging the exceedingly low luminosities probed by our sample. Our results rule out a turnover brighter than M UV = –15.5 at the 3 σ confidence level.