The first GLIMPSE of the faint galaxy population at Cosmic Dawn with JWST: The evolution of the ultraviolet luminosity function across z ∼ 9−15

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 ~ 9-15. We identify 105 galaxy candidates spanning absolute ultraviolet (UV) magnitudes M_UV∼ −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 α = −2.01 ± 0.20 at z = 9 to α = −2.10 ± 0.19 at z = 13, in contrast to the rapid evolution observed at z ~ 0.9. Integrating the UVLF down to M_UV = −16, we find the cosmic star formation rate density, P_SFR, evolves as α(1 + z)^−2.94+0.06_-0.10, over z = 9-13, which is significantly shallower than most theoretical predictions. Extending the integration limit to M_UV = −13 reveals galaxies fainter than M_UV = −16 contribute over 50 percent of the total cosmic star formation rate density at z ~ 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 P_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.