Tilting at the Turnover: Modeling the Faint-End of the UV Luminosity Function Behind Abell s1063 with JWST

We leverage the strong gravitational field of Abell S1063 to identify faint, highly magnified galaxies using ultra-deep James Webb Space Telescope (JWST)/NIRCam imaging from the GLIMPSE survey and ancillary Hubble Space Telescope (HST)/ACS imaging from the Hubble Frontier Fields program. We construct a photometric catalogue of lensed high-redshift candidates and use these sources to constrain the faint end of the rest-frame UV luminosity function (UVLF) over z 6 --11. Rather than treating the UVLF turnover (M t) as a hard cutoff, we model it as a gradual quadratic suppression and explicitly account for the potential continued contribution of galaxies beyond the turnover. In a shallow-turnover scenario, up to one-third of the UV luminosity density can arise from sources fainter than M t. While we find no direct evidence for a turnover down to M UV=-13.5 at z=6, our analysis can only confidently exclude weak, medium, and strong turnover models down to M t=-15.9, -15.1, and-14.8, respectively. Across these models, we infer lower limits of the UV luminosity, star formation density, and the ionization rate as: rho UV at least22 times10 25, erg,s-1,Hz-1,Mpc-3, SFRD at least25 times10-3,M, yr-1,Mpc-3, and 10(ion/ s-1,Mpc-3) at least51.02. We find that galaxies fainter than the conventional M UV=-17 limit contribute more than half of the UV luminosity density and at least about64% of the ionizing photons produced by star-forming galaxies at z=6. Because our turnover model permits a suppressed, but non-zero, galaxy population beyond M t, sources fainter than the turnover remain contributors to both rho UV and ion, emphasizing the need to consider the turnover and its shape during reionization.