2026ApJ..1000...25S

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 < z < 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 < M_UV < -12.1 at 0.4 < z < 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 < -13.5 mag, including a faint-end slope of $\alpha =\,\unicode{x02013}\,1.32{4}_{-0.074}^{+0.072}$. 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 3σ confidence level.