A Glimpse of the New Redshift Frontier through AS1063

Abstract

We report the discovery of two galaxy candidates at redshifts between 15.7 < z < 16.4 in James Webb Space Telescope (JWST) observations from the GLIMPSE survey. These robust sources were identified using a combination of Lyman break selection and photometric redshift estimates. The ultradeep NIRCam imaging from GLIMPSE, combined with the strong gravitational lensing of the AS1063 galaxy cluster, allows us to probe an intrinsically fainter population (down to M UV = −17.0 mag) than previously achievable. These galaxies have absolute magnitudes ranging from M UV = −17.0 to −17.2 mag, with blue ( β ≃ −2.87) ultraviolet (UV) continuum slopes, consistent with young, dust-free stellar populations. The number density of these objects, log 10 ( ϕ /[Mpc −3 mag −1 ]) = − 3.4 7 − 0.10 + 0.13 at M UV = −17, is in clear tension with pre-JWST theoretical predictions, extending the overabundance of galaxies from z ∼ 10 to z ∼ 17. These results, together with the scarcity of brighter galaxies in other public surveys, suggest a steep decline in the bright end of the UV luminosity function at z ∼ 16, implying efficient star formation and possibly a close connection to the halo mass function at these redshifts. Testing a variety of star formation histories suggests that these sources are plausible progenitors of the unusually UV-bright galaxies that JWST now routinely uncovers at z = 10–14. Overall, our results indicate that the luminosity distribution of the earliest star-forming galaxies could be shifting toward fainter luminosities, implying that future surveys of cosmic dawn will need to explore this faint luminosity regime.