A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization
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2026
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| author | Jecmen, Michelle C. Chisholm, John Atek, Hakim Kokorev, Vasily Endsley, Ryan Chemerynska, Iryna Furtak, Lukas J. Pan, Richard Fujimoto, Seiji Naidu, Rohan P. Muñoz, Julian B. Adamo, Angela Asada, Yoshihisa Basu, Arghyadeep Berg, Danielle A. Blaizot, Jeremy Dessauges-Zavadsky, Miroslava Giovinazzo, Emma Hsiao, Tiger Yu-Yang Katz, Harley Korber, Damien McKinney, Jed McQuinn, Kristen. B. W. Oesch, Pascal A. Schaerer, Daniel |
| author_facet | Jecmen, Michelle C. Chisholm, John Atek, Hakim Kokorev, Vasily Endsley, Ryan Chemerynska, Iryna Furtak, Lukas J. Pan, Richard Fujimoto, Seiji Naidu, Rohan P. Muñoz, Julian B. Adamo, Angela Asada, Yoshihisa Basu, Arghyadeep Berg, Danielle A. Blaizot, Jeremy Dessauges-Zavadsky, Miroslava Giovinazzo, Emma Hsiao, Tiger Yu-Yang Katz, Harley Korber, Damien McKinney, Jed McQuinn, Kristen. B. W. Oesch, Pascal A. Schaerer, Daniel |
| contents | As observations have yet to constrain the ionizing properties of the faintest (M$_{\rm UV}$ > -16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ($β$) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (f$_{\rm esc}$) for such faint galaxies at high-redshift. Leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of strong lensing field Abell S1063, we estimate UV continuum slopes of 555 galaxies at z $>$ 6 with absolute magnitudes down to M$_{\rm UV}$ $\simeq -$12.5. We find a modest evolution of $β$ with redshift and a flattening in the $β$-M$_{\rm UV}$ relation such that galaxies fainter than M$_{\rm UV}$ $\sim -$16.5 no longer exhibit the bluest UV slopes. The 138 ultra-faint galaxies with M$_{\rm UV}$ $> -$16 are a diverse population encompassing dusty (30\%), old (15\%), and low-mass (50\%) galaxies. We apply the empirical $β$-f$_{\rm esc}$ relation from local Lyman continuum leakers, finding the mean f$_{\rm esc}$ peaks at $\sim 20\%$ at M$_{\rm UV}=-$16.5 and declines towards fainter galaxies, while remaining consistent with f$_{\rm esc}$ = 14\% within uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with M$_{\rm UV}$ between $-18$ and $-14$ supplied $\sim 60\%$ of the ionizing photons to cosmic reionization, while the lower f$_{\rm esc}$ of fainter galaxies produces a natural cutoff in the ionizing photon production rate density. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_19995 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization Jecmen, Michelle C. Chisholm, John Atek, Hakim Kokorev, Vasily Endsley, Ryan Chemerynska, Iryna Furtak, Lukas J. Pan, Richard Fujimoto, Seiji Naidu, Rohan P. Muñoz, Julian B. Adamo, Angela Asada, Yoshihisa Basu, Arghyadeep Berg, Danielle A. Blaizot, Jeremy Dessauges-Zavadsky, Miroslava Giovinazzo, Emma Hsiao, Tiger Yu-Yang Katz, Harley Korber, Damien McKinney, Jed McQuinn, Kristen. B. W. Oesch, Pascal A. Schaerer, Daniel Astrophysics of Galaxies As observations have yet to constrain the ionizing properties of the faintest (M$_{\rm UV}$ > -16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope ($β$) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (f$_{\rm esc}$) for such faint galaxies at high-redshift. Leveraging ultra-deep JWST/NIRCam GLIMPSE imaging of strong lensing field Abell S1063, we estimate UV continuum slopes of 555 galaxies at z $>$ 6 with absolute magnitudes down to M$_{\rm UV}$ $\simeq -$12.5. We find a modest evolution of $β$ with redshift and a flattening in the $β$-M$_{\rm UV}$ relation such that galaxies fainter than M$_{\rm UV}$ $\sim -$16.5 no longer exhibit the bluest UV slopes. The 138 ultra-faint galaxies with M$_{\rm UV}$ $> -$16 are a diverse population encompassing dusty (30\%), old (15\%), and low-mass (50\%) galaxies. We apply the empirical $β$-f$_{\rm esc}$ relation from local Lyman continuum leakers, finding the mean f$_{\rm esc}$ peaks at $\sim 20\%$ at M$_{\rm UV}=-$16.5 and declines towards fainter galaxies, while remaining consistent with f$_{\rm esc}$ = 14\% within uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with M$_{\rm UV}$ between $-18$ and $-14$ supplied $\sim 60\%$ of the ionizing photons to cosmic reionization, while the lower f$_{\rm esc}$ of fainter galaxies produces a natural cutoff in the ionizing photon production rate density. |
| title | A GLIMPSE into the UV Continuum Slopes of the Faintest Galaxies in the Epoch of Reionization |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2601.19995 |