Revealing Galaxy Candidates out to $z\sim 16$ with JWST Observations of the Lensing Cluster SMACS0723

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 $z\sim 16$, only $250$ million years after the Big Bang. We also identify two candidates at $z\sim 12$ and 6 candidates at $z\sim 9-11$. Our search extended out to $z\lesssim 21$ by combining color information across seven NIRCam and NIRISS filters. By modelling the Spectral Energy Distributions (SEDs) with $\mathtt{\text{EAZY}}$ and $\mathtt{\text{BEAGLE}}$, we test the robustness of the photometric redshift estimates. While their intrinsic (un-lensed) luminosity is typical of the characteristic luminosity L${}^{\ast }$ 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 $-3<\beta <-2.4$, young ages $\sim 10-100$,Myr, and stellar masses around $\log (M_{\star }\mathrm{/}{\mathrm{M}}_{\odot })=8.8$ 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.