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We present a first-look analysis of the JWST ERO data in the SMACSJ0723.3-7327 cluster field. We begin by reporting 10 new spectroscopicredshifts from $\lambda_\mathrm{obs}=1.8-5.2\mu$m NIRSpec medium-resolution($R=\lambda/\Delta\lambda = 1000$) data. These are determined via multiplehigh-SNR emission line detections, with 5 objects at $1 < z < 3$ displayingmultiple rest-frame near-infrared Hydrogen Paschen lines, and 5 objects at $5 <z < 9$ displaying rest-frame optical Oxygen and Hydrogen Balmer lines. For the5 higher-redshift galaxies we extract fluxes in 6 NIRCam bands spanning$\lambda_\mathrm{obs}=0.8-5\mu$m and perform spectral energy distributionfitting, in combination with existing HST photometry. The $7 < z < 9$ objectsexhibit a U-shaped pattern across the F277W, F356W and F444W bands, indicatinga Balmer break seen in emission (Balmer jump) and high-equivalent-width[O\,\textsc{iii}] emission. This indicates an extremely young stellarpopulation, with the bulk of the current mass having formed within the past 10Myr. We report robust stellar masses and mean stellar ages from our spectralfitting, with the four $z > 6$ galaxies exhibiting low stellar masses fromlog$_{10}(M_*/$M$_\odot)=7.1-8.2$ and correspondingly young mean stellar agesof only a few Myr. This work highlights the critical importance of combininglarge upcoming NIRCam surveys with NIRSpec follow-up to measure thespectroscopic redshifts necessary to robustly constrain physical parameters.

A first look at the SMACS0723 JWST ERO: spectroscopic redshifts, stellar masses and star-formation histories