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

2025/12/14 by Hudson, Nadara, Endsley, Ryan, Chisholm, John
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences

paper · doi:10.48550/arxiv.2512.12813

Abstract

JWST continues to reveal an astonishing number of massive quiescent galaxies at z>4, with number densities \gtrsim10× 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 (β=-0.6) yet UV-bright (F115W = 26.0 mag) z∼6 star-forming system selected as a Lyman-break galaxy (LBG) over ≈500 arcmin² of deep NIRCam imaging. This galaxy (UDS₄3065) shows photometric colors implying a prominent Balmer break and strong Hα emission, consistent with a dramatic burst of star formation (SFR ≈ 500-1000 M_\odot/yr) occurring 5-10 Myr ago that formed 20-40% of its total stellar mass (≈1.5×10¹⁰M_\odot) with little activity since. This galaxy is one of only two objects with M_∗>10¹⁰M_\odot across our full sample of 813 z∼6 star-forming LBGs, and the only galaxy with a confident extremely-red UV slope (β>-1). UDS₄3065 is clearly resolved yet compact in F444W (r_e=400±10 pc) indicating a very high stellar mass surface density of log(Σ_eff/(M_\odot kpc^-2))=10.25±0.13 comparable to quenched z∼2-7 galaxies. If the inferred star formation history (SFH) of UDS₄3065 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.

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