Here we present the first constraints on the prevalence of z>10 galaxies in
the Hubble Ultra Deep Field (HUDF) leveraging new NIRCam medium-band
observations taken with JWST. These NIRCam observations probe redward of
1.6microns, beyond the wavelength limit of HST, allowing us to search for
galaxies to z>10. These observations indicate that the highest redshift
candidate identified over the HUDF with HST, UDFj-39546284, has a redshift of
z=12.0+/-0.1, as had been suggested in multiple analyses of the HUDF12/XDF
data. This source thus appears to be the most distant galaxy discovered by HST
in its more than 30 years of operation. Additionally, we identify nine other
z~8-13 candidate galaxies over the HUDF, two of which are entirely new
discoveries and appear to lie at z~11 and z~12. We use these results to
characterize the evolution of the UV luminosity function (LF) from z~15 to
z~8.7. While our LF results at z~8.7 and z~10.5 are consistent with previous
findings over the HUDF, our new LF estimates at z~12.6 are substantially higher
than other results in the literature, potentially pointing to a milder
evolution in the UV luminosity density from z~12.6. We emphasize that our LF
results are uncertain given the small number of sources in our z~12.6 selection
and limited volume probed. The new NIRCam data also indicate that the faint
z~8-13 galaxies in the HUDF/XDF show very blue UV-continuum slopes beta~-2.7,
high specific star formation rates ~24.5 Gyr−1, and high EW (~1300A)
[OIII]+Hbeta emission, with two z~8.5 sources showing [OIII]+Hbeta EWs of ~2300
Angstroms.Comment: 19 pages, 14 figures, 7 tables, submitted to MNRAS, with some minor
typos correcte
