A GLIMPSE OF THE 99%: A CENSUS OF THE FAINTEST GALAXIES DURING THE EPOCH OF REIONIZATION AND ITS IMPLICATIONS FOR GALAXY FORMATION MODELS
Abstract
We present a comprehensive study of the galaxy UV luminosity function (UVLF) at z = 6 -9 and the resulting cosmic ionizing photon budget during the Epoch of Reionization, leveraging deep JWST observations from the GLIMPSE survey. Thanks to gravitational lensing, we probe the UVLF to an unprecedented depth of MUV = -12, approximately three magnitudes deeper than previous robust constraints. Our UVLF determination incorporates a rigorous end-to-end uncertainty framework, including statistical and systematic lensing uncertainties. We find that the z ∼ 7 UVLF continues to rise steeply with a faint-end slope of α = -1.980.060.05. Crucially, our data show no clear evidence of a turnover down to MUV = -12.3, with any potential curvature being significantly shallower than tentative estimates reported in previous HST studies. The persistence of this faint population provides stringent constraints on galaxy formation models and cosmological simulations that predict an early flattening of the luminosity function due to radiative feedback or star-formation thresholds. Furthermore, post-JWST models specifically calibrated to match the UV-bright excess at z > 10 generally fail to reproduce the observed evolution toward lower redshifts and fainter magnitudes, highlighting a significant tension in our current understanding of early galaxy assembly. Integrating this UVLF with empirical constraints on the ionizing efficiency and escape fractions derived from GLIMPSE observations, we derive a comoving ionizing emissivity at z = 7 of log( ṅion/s⁻¹ Mpc⁻³) ≈ 50.85. This high emissivity suggests that faint galaxies dominate the ionizing budget, providing enough photons to maintain reionization even in a highly clumped IGM (C HII = 5). However, adopting higher ionizing efficiencies or escape fractions from the literature results in an early reionization history that completes by z ≈ 8, which stands in tension with Planck CMB optical depth measurements and IGM opacity data. As our detection of faint galaxies effectively rules out a luminosity function truncation at MUV ≥ -15, these results emphasize the need to either accurately characterize the ionizing properties of the global, low-mass galaxy population at z > 6, or to refine physical models of intergalactic medium clumping and its redshift evolution to maintain consistency with the observed reionization timeline.
Related
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BibTeX
@article{atek2026glimpse,
title = {{A GLIMPSE OF THE 99%: A CENSUS OF THE FAINTEST GALAXIES DURING THE EPOCH OF REIONIZATION AND ITS IMPLICATIONS FOR GALAXY FORMATION MODELS}},
author = {Hakim Atek and Iryna Chemerynska and Lukas Furtak and Johan Richard and John Chisholm and Vasily Kokorev and Michelle Jecmen and Damien Korber and Ryan Endsley and Richard Pan and Arghyadeep Basu and Jeremy Blaizot and Rychard Bouwens and Meriam Ezziati and Sylvain Heurtier and Kristen Mcquinn and Marcie Mun and Julian Muñoz and Pascal Oesch and Joakim Rosdahl and Alberto Saldana-Lopez and Seiji Fujimoto and Daniel Schaerer},
journal = {Open Journal of Astrophysics},
year = {2026},
month = aug,
doi = {10.33232/001c.166816},
eprint = {2604.23823},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05740231v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A GLIMPSE OF THE 99%: A CENSUS OF THE FAINTEST GALAXIES DURING THE EPOCH OF REIONIZATION AND ITS IMPLICATIONS FOR GALAXY FORMATION MODELS
AU - Hakim Atek
AU - Iryna Chemerynska
AU - Lukas Furtak
AU - Johan Richard
AU - John Chisholm
AU - Vasily Kokorev
AU - Michelle Jecmen
AU - Damien Korber
AU - Ryan Endsley
AU - Richard Pan
AU - Arghyadeep Basu
AU - Jeremy Blaizot
AU - Rychard Bouwens
AU - Meriam Ezziati
AU - Sylvain Heurtier
AU - Kristen Mcquinn
AU - Marcie Mun
AU - Julian Muñoz
AU - Pascal Oesch
AU - Joakim Rosdahl
AU - Alberto Saldana-Lopez
AU - Seiji Fujimoto
AU - Daniel Schaerer
PY - 2026
DA - 2026/08/11
JO - Open Journal of Astrophysics
DO - 10.33232/001c.166816
UR - https://hal.science/hal-05740231v1
AB - We present a comprehensive study of the galaxy UV luminosity function (UVLF) at z = 6 -9 and the resulting cosmic ionizing photon budget during the Epoch of Reionization, leveraging deep JWST observations from the GLIMPSE survey. Thanks to gravitational lensing, we probe the UVLF to an unprecedented depth of MUV = -12, approximately three magnitudes deeper than previous robust constraints. Our UVLF determination incorporates a rigorous end-to-end uncertainty framework, including statistical and systematic lensing uncertainties. We find that the z ∼ 7 UVLF continues to rise steeply with a faint-end slope of α = -1.98^0.06_0.05. Crucially, our data show no clear evidence of a turnover down to MUV = -12.3, with any potential curvature being significantly shallower than tentative estimates reported in previous HST studies. The persistence of this faint population provides stringent constraints on galaxy formation models and cosmological simulations that predict an early flattening of the luminosity function due to radiative feedback or star-formation thresholds. Furthermore, post-JWST models specifically calibrated to match the UV-bright excess at z > 10 generally fail to reproduce the observed evolution toward lower redshifts and fainter magnitudes, highlighting a significant tension in our current understanding of early galaxy assembly. Integrating this UVLF with empirical constraints on the ionizing efficiency and escape fractions derived from GLIMPSE observations, we derive a comoving ionizing emissivity at z = 7 of log( ṅion/s⁻¹ Mpc⁻³) ≈ 50.85. This high emissivity suggests that faint galaxies dominate the ionizing budget, providing enough photons to maintain reionization even in a highly clumped IGM (C HII = 5). However, adopting higher ionizing efficiencies or escape fractions from the literature results in an early reionization history that completes by z ≈ 8, which stands in tension with Planck CMB optical depth measurements and IGM opacity data. As our detection of faint galaxies effectively rules out a luminosity function truncation at MUV ≥ -15, these results emphasize the need to either accurately characterize the ionizing properties of the global, low-mass galaxy population at z > 6, or to refine physical models of intergalactic medium clumping and its redshift evolution to maintain consistency with the observed reionization timeline.
SN - 2565-6120
ER -