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

Abstract

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 $z\simeq 6$--11. Rather than treating the UVLF turnover ($M_{\mathrm{t}}$) 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 $M_{\mathrm{t}}$. While we find no direct evidence for a turnover down to $M_{\mathrm{UV}}=-13.5$ at $z=6$, our analysis can only confidently exclude weak, medium, and strong turnover models down to $M_{\mathrm{t}}=-15.9$, $-15.1$, and $-14.8$, respectively. Across these models, we infer lower limits of the UV luminosity, star formation density, and the ionization rate as: $\ddot{I}{}_{\mathrm{UV}}\ge 22\times 10^{25}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{H}{\mathrm{z}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-3}$, $\mathrm{SFRD}\ge 25\times 10^{-3}{M}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-3}$, and ${\log }_{10}({\dot{n}}_{\mathrm{ion}}/{\mathrm{s}}^{-1}\mathrm{Mp}{\mathrm{c}}^{-3})\ge 51.02$. We find that galaxies fainter than the conventional $M_{\mathrm{UV}}=-17$ limit contribute more than half of the UV luminosity density and at least $\sim 64\%$ 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 $M_{\mathrm{t}}$, sources fainter than the turnover remain contributors to both $\ddot{I}{}_{\mathrm{UV}}$ and ${\dot{n}}_{\mathrm{ion}}$, emphasizing the need to consider the turnover and its shape during reionization.

25 pages, 17 Figures, Submitting to MNRAS