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 percent, 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 [OIII], 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 conducted an analysis of the James Webb Space Telescope's SMACS 0723 field to search for ultra-high-redshift galaxies. We found four previously unidentified candidate galaxies, with one object at z = 11.5 and another possibly being a pair of galaxies. Most of the candidate galaxies showed evidence of color breaks and emission lines related to gas. These galaxies were exceptionally small in size and had relatively low masses. These candidates will be important targets for future observations to better understand their physical nature.