Evolution of the UV LF from z similar to 15 to z similar to 8 using new JWST NIRCam medium-band observations over the HUDF/XDF

We present the first constraints on the prevalence of z > 10 galaxies in the Hubble Ultra Deep Field (HUDF) leveraging new NIRCam observations from JEMS (JWST Extragalactic Medium-band Survey). These NIRCam observations probe redward of 1.6 mu m, beyond the wavelength limit of HST , allowing us to search for galaxies to z > 10. These observations indicate that the highest redshift candidate identified in the HUDF09 data with HST , UDFj-39546284, has a redshift of z > 11.5, as had been suggested in analyses of the HUDF12/XDF data. This has now been confirmed with JWST NIRSpec. This source is thus the most distant galaxy discovered by HST in its > 30 yr of operation. Additionally, we identify nine other z similar to 8-13 candidate galaxies over the HUDF, two of which are new discoveries that appear to lie at z similar to 11-12. We use these results to characterize the evolution of the UV luminosity function (LF) from z similar to 15 to z similar to 8.7. While our LF results at z similar to 8.7 and z similar to 10.5 are consistent with previous findings over the HUDF, our new LF estimates at z similar to 12.6 are higher than other results in the literature, potentially pointing to a milder evolution in the UV luminosity density from z similar to 12.6. We emphasize that our LF results are uncertain given the small number of z similar to 12.6 sources and limited volume probed. The new NIRCam data also indicate that the faint z similar to 8-13 galaxies in the HUDF/XDF show blue UV-continuum slopes beta similar to -2.7, high specific star formation rates similar to 24.5 Gyr(-1), and high EW (similar to 1300 angstrom) [O III ] + H beta emission, with two z similar to 8.5 sources showing [O III ] + H beta EWs of similar to 2300 angstrom.