Fulfilling the UV Legacy of the Hubble and Webb Deep Public Frontier Field

Hubble Frontier Field Abell 2744 is one of the most powerful known gravitational lensing clusters, with the largest high magnification area among HST observed clusters. In Cycles 1-3, JWST programs are obtaining >300 hours of imaging and spectroscopy over Abell 2744, mapping the entire high magnification area, constituting the mission's deepest, and crucially, fully public extragalactic dataset. With JWST imaging reaching 29-30AB in 20 filters at 1-5 micron (not counting the extra lensing boost), ultradeep JWST NIRISS+NIRSpec+WFSS spectroscopy, plus deep ALMA dust continuum, MUSE datacubes, and Chandra X-ray observations, Abell 2744 is a peerless legacy field.

This treasure trove of data presents an unprecedented opportunity to answer high-profile reionization questions only addressable with HST UV imaging. Here, we propose an immediately-public HST program to supplement currently-inadequate HST coverage by providing ultra-deep WFC3/UVIS (F336W) imaging over a 4x wider area, matching the full coverage of the JWST spectroscopy. The proposed F336W data enables measurements of Lyman continuum and ionizing photon escape fractions in z=3-4 sources (analogs to epoch of reionization galaxies) to unambiguously determine the role of galaxies in reionization. This data will also improve constraints on the UV luminosity function, star formation in quiescent galaxies, the metallicity of globular clusters within A2744, and the origin of the UV upturn in foreground cluster galaxies.

These observations must be taken now to leverage the unique and time-limited UV capabilities of HST and maximize the potential of a remarkable suite of observations from HST, JWST, ALMA, and Chandra.