Surprisingly faint first galaxies

A puzzling feature of cosmic dawn is that bright galaxies in the redshift range 10 < z < 15 are overabundant relative to HST-based expectations but have so far evaded detection at higher redshifts, despite JWST’s technical capacity to do so. Vasily Kokorev and colleagues take advantage of the lensing magnification of the galaxy cluster Abell S1063 and the ultra-deep NIRCam survey GLIMPSE to identify two robust candidates at z > 16.

Combining a Lyman-alpha dropout search with photometric redshift estimates to rule out star-bursting interlopers, the authors narrow down an initial catalogue of lensed background sources to these two final objects, with putative redshifts in the range z ~ 15.8−16.4. Though their ~200 pc effective radii and low dust content are consistent with expectations for very newly formed galaxies, photometric modelling indicates low star formation rate surface densities relative to extrapolations from lower redshifts. Equally surprising is that the volume searched behind the cluster implies a significantly higher number density than expected based on upper limits from previous JWST surveys, in which no strong candidates have yet been found at these redshifts.