An Extremely-red, UV-bright, and Extended Galaxy at z~6 in PRIMER/UDS: An Early Massive Galaxy Caught Quenching after an Obscured Starburst?

Abstract

JWST continues to reveal an astonishing number of massive quiescent galaxies at $z>4$, with number densities $\gtrsim10\times$ higher than model predictions. NIRSpec spectra imply that many of these systems underwent intense starburst episodes (SFR$\,\gtrsim300M_\odot$/yr), though direct evidence of such starbursts in the Gyr largely comes from exceptionally rare dusty star-forming galaxies (DSFGs) selected in the far-infrared. Here, we report the discovery of an extremely red ($\beta=-0.6$) yet UV-bright (F115W = 26.0 mag) $z\sim6$ star-forming system selected as a Lyman-break galaxy (LBG) over $\approx$500 arcmin$^2$ of deep NIRCam imaging. This galaxy (UDS_43065) shows photometric colors implying a prominent Balmer break and strong H$\alpha$ emission, consistent with a dramatic burst of star formation (SFR$\,\approx\,500-1000\,M_\odot$/yr) occurring 5-10 Myr ago that formed 20-40% of its total stellar mass ($\approx1.5\times10^{10}M_\odot$) with little activity since. This galaxy is one of only two objects with $M_\ast>10^{10}M_\odot$ across our full sample of 813 $z\sim6$ star-forming LBGs, and the only galaxy with a confident extremely-red UV slope ($\beta>-1$). UDS_43065 is clearly resolved yet compact in F444W ($r_e=400\pm10$ pc) indicating a very high stellar mass surface density of log$(\Sigma_\mathrm{eff}/(M_\odot\,\mathrm{kpc}^{-2}))=10.25\pm0.13$ comparable to quenched $z\sim2-7$ galaxies. If the inferred star formation history (SFH) of UDS_43065 is corroborated with further observations, this object would seemingly represent a rarely-seen transitional phase between massive DSFGs and passive systems in the first Gyr, helping resolve the puzzling abundance of early massive quenched galaxies.