We present a first-look analysis of the JWST ERO data in the SMACS J0723.3-7327 cluster field. We begin by reporting 10 new spectroscopic redshifts from 𝜆 obs = 1.8 − 5.2𝜇m NIRSpec medium-resolution (𝑅 = 𝜆/Δ𝜆 = 1000) data. These are all determined via multiple high-SNR emission line detections, with 5 objects at 1 < 𝑧 < 3 displaying multiple rest-frame near-infrared Hydrogen Paschen lines, and 5 objects at 5 < 𝑧 < 9 displaying rest-frame optical Oxygen and Hydrogen Balmer lines. For the 5 higher-redshift galaxies we extract fluxes in 6 NIRCam bands spanning 𝜆 obs = 0.8 − 5𝜇m and perform spectral energy distribution fitting using these data in combination with existing HST photometry. The 7 < 𝑧 < 9 objects exhibit a U-shaped pattern across the F277W, F356W and F444W bands, indicating the presence of a Balmer break seen in emission (Balmer jump) and high-equivalent-width [O ] emission. This is indicative of an extremely young stellar population, with the bulk of the current mass having formed within the past 10 Myr. We report robust stellar masses and mean stellar ages from our spectral fitting, with the four 𝑧 > 6 galaxies exhibiting low stellar masses from log 10 (𝑀 * /M ) = 7.4 − 8.6 and correspondingly young mean stellar ages of only a few Myr. This work highlights the critical importance of combining large upcoming NIRCam surveys with NIRSpec follow-up to measure the spectroscopic redshifts necessary to robustly constrain physical parameters.

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