James Webb Space Telescope Feed Post

Harvard ADS: A Glimpse at the New Redshift Frontier Through Abell S1063

Paper abstract: We report the discovery of five galaxy candidates at redshifts between 15.9 < z < 18.6 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_{\rm UV}=-17.5 mag) than previously achievable. These galaxies have absolute magnitudes ranging from M_{\rm UV}= -17.7 to -18.0 mag, with UV continuum slopes between \beta ~eq -2.3 and \beta ~eq -3.0, consistent with young, dust-free stellar populations. The number density of these objects, log_{\rm 10} (\phi/[Mpc^{-3} mag^{-1}])=-3.43^{+0.28}_{-0.64} at M_{\rm UV}=-18 is in clear tension with pre-JWST theoretical predictions, extending the over-abundance of galaxies from z~10 to z~ 18.6. 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 ~ 17, 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.