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 $\gtrsim 10\times$ higher than model predictions. NIRSpec spectra imply that many of these systems underwent intense starburst episodes (SFR$\gtrsim 300M_{\odot }$/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) $z\sim 6$ 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$\approx 500-1000M_{\odot }$/yr) occurring 5-10 Myr ago that formed 20-40% of its total stellar mass ($\approx 1.5\times 10^{10}{M}_{\odot }$) 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 $z\sim 6$ 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 ($r_e=400\pm 10$ pc) indicating a very high stellar mass surface density of log$({\Sigma }_{\mathrm{eff}}/(M_{\odot }{\mathrm{kpc}}^{-2}))=10.25\pm 0.13$ comparable to quenched $z\sim 2-7$ 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.

10 pages, 5 figures, submitted to ApJ