A Glimpse of the New Redshift Frontier Through Abell S1063

Abstract

We report the discovery of two galaxy candidates at redshifts between $15.7<z<16.4$ in JWST observations from the GLIMPSE survey. These robust sources were identified using a combination of Lyman-break selection and photometric redshift estimates. The ultra-deep NIRCam imaging from GLIMPSE, combined with the strong gravitational lensing of the Abell S1063 galaxy cluster, allows us to probe an intrinsically fainter population (down to $M_{\mathrm{UV}}=-17.0$ mag) than previously achievable. These galaxies have absolute magnitudes ranging from $M_{\mathrm{UV}}=-17.0$ to $-17.2$ mag, with blue (${\hat{I}}^2\simeq -2.87$) UV continuum slopes, consistent with young, dust-free stellar populations. The number density of these objects, log${}_{10}$($\ddot{I}•$/[Mpc${}^{-3}$ mag${}^{-1}$])=$-3.47_{-0.10}^{+0.13}$ at $M_{\mathrm{UV}}=-17$ is in clear tension with pre-JWST theoretical predictions, extending the over-abundance of galaxies from $z\sim 10$ to $z\sim 17$. 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 $z\sim 16$, 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 towards fainter luminosities, implying that future surveys of cosmic dawn will need to explore this faint luminosity regime.

16 pages, 7 figures, 2 tables. ApJL in press