Discovery and properties of ultra-high redshift galaxies (9 < z < 12) in the JWST ERO SMACS 0723 Field

We present a reduction and analysis of the James Webb Space Telescope (JWST) SMACS 0723 field using new post-launch calibrations to conduct a search for ultra-high-redshift galaxies (z > 9) present within the Epoch of Reionization. We conduct this search by modelling photometric redshifts in several ways for all sources and by applying conservative magnitude cuts (m(F200W) < 28) to identify strong Lyman breaks greater than 1 mag. We find four z > 9 candidate galaxies which have not previously been identified, with one object at z = 11.5, and another which is possibly a close pair of galaxies. We measure redshifts for candidate galaxies from other studies and find the recovery rate to be only 23 per cent, with many being assigned lower redshift, dusty solutions in our work. Most of our z > 9 sample show evidence for Balmer-breaks, or extreme emission lines from H beta and [O iii], demonstrating that the stellar populations could be advanced in age or very young depending on the cause of the F444W excess. We discuss the resolved structures of these early galaxies and find that the Sersic indices reveal a mixture of light concentration levels, but that the sizes of all our systems are exceptionally small (<0.5 kpc). These systems have stellar masses M-* similar to 10(9.0) M-circle dot, with our z similar to 11.5 candidate a dwarf galaxy with a stellar mass M-* similar to 10(7.8)-10(8.2) M-circle dot. These candidate ultra high-redshift galaxies are excellent targets for future NIRSpec observations aimed to better understand their physical nature.

Automated summary

We conduct a search for ultra-high-redshift galaxies (z > 9) using new post-launch calibrations and identify four candidate galaxies, including one at z = 11.5 and a possible pair of galaxies. However, the recovery rate for these candidates is low, with many being assigned lower redshifts in our work. Our analysis reveals evidence of Balmer-breaks and extreme emission lines, indicating advanced or very young stellar populations depending on the cause of the excess. The resolved structures of these early galaxies show a mixture of light concentration levels, with exceptionally small sizes (<0.5 kpc).