Abstract
One of the main goals of the JWST is to study the first galaxies in the
Universe. We present a systematic photometric analysis of very distant galaxies
in the first JWST deep field towards the massive lensing cluster SMACS0723. As
a result, we report the discovery of two galaxy candidates at $z\sim16$, only
$250$ million years after the Big Bang. We also identify two candidates at
$z12$ and 11 candidates at $z10-11$. Our search extended out to
$złesssim21$ by combining color information across seven NIRCam and NIRISS
filters. By modelling the Spectral Energy Distributions (SEDs) with EAZY and
BEAGLE, we test the robustness of the photometric redshift estimates. While
their intrinsic (un-lensed) luminosity is typical of the characteristic
luminosity L$^*$ at $z>10$, our high-redshift galaxies typically show small
sizes and their morphologies are consistent with disks in some cases. The
highest-redshift candidates have extremely blue UV-continuum slopes $-3 < \beta
<-2.5$, young ages $10-100$\,Myr, and stellar masses
$łog(M_\star/M_ødot)=8.4-8.8$ inferred from their SED modeling
which indicate a rapid build-up of their stellar mass. Our search clearly
demonstrates the capabilities of JWST to uncover robust photometric candidates
up to very high redshifts, and peer into the formation epoch of the first
galaxies.
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