The first GLIMPSE of the faint galaxy population at Cosmic Dawn with <i>JWST</i> : The evolution of the ultraviolet luminosity function across <i>z</i> ∼ 9−15
Iryna Chemerynska, Hakim Atek, Lukas J. Furtak, John Chisholm, Ryan Endsley, Vasily Kokorev, Joki Rosdahl, J. Blaizot, Angela Adamo, Rychard Bouwens, Seiji Fujimoto, Damien Korber, Charlotte Mason, Kristen B. W. McQuinn, Julián B. Muñoz, Priyamvada Natarajan, Erica Nelson, Pascal A. Oesch, Richard Pan, Johan Richard, Alberto Saldana-Lopez, Daniel Schaerer, Marta Volonteri, Adi Zitrin, Danielle A. Berg, Adélaïde Claeyssens, M. Dessauges‐Zavadsky, Michelle Jecmen, Ivo Labbé, Rohan P. Naidu, Maxime Trebitsch
Abstract
ABSTRACT Using ultra-deep James Webb Space Telescope (JWST)/NIRCam imaging from the GLIMPSE survey, enhanced by gravitational lensing from the Abell S1063 cluster, we investigate the faintest galaxies ever observed at redshifts $z\sim 9{\text{-}}15$. We identify 105 galaxy candidates spanning absolute ultraviolet (UV) magnitudes $M_{\mathrm{UV}}$$\sim -18$ to $-13$, about 3 mag fainter, on average, than prior JWST studies. We place strong constraints on the ultra-faint end of the UV luminosity function (UVLF), finding minimal evolution in the faint-end slope, from $\alpha =-2.01\pm 0.20$ at $z=9$ to $\alpha =-2.10\pm 0.19$ at $z=13$, in contrast to the rapid evolution observed at $z\sim 0{\text{-}}9$. Integrating the UVLF down to $M_{\mathrm{UV}}$$=-16$, we find the cosmic star formation rate density, $\rho _{\rm SFR}$, evolves as $\propto (1+z)^{-2.94^{+0.06}_{-0.10}}$, over $z=9{\text{-}}13$, which is significantly shallower than most theoretical predictions. Extending the integration limit to $M_{\mathrm{UV}}$$=-13$ reveals galaxies fainter than $M_{\mathrm{UV}}$$=-16$ contribute over 50 per cent of the total cosmic star formation rate density at $z\sim 12$. This excess may indicate enhanced star formation efficiency during the earliest phases of galaxy formation. Alternatively, it could arise from bursty star formation histories; minimal dust attenuation; or an evolving initial mass function. However, existing models incorporating these effects fail to fully reproduce the observed redshift evolution of $\rho _{\rm SFR}$. We note that low-redshift contamination and cosmic variance may affect our results, as the limited survey volume may not be representative of the broader galaxy population. Similar observations and spectroscopic confirmation are required to validate these findings.