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

TL;DR

This study reports the discovery of UDS_43065, an extremely red yet UV-bright galaxy at $z_{phot}\approx5.6$ with a strong Balmer break and significant H$\alpha$ emission, indicating a recent starburst with SFR~$5\times10^2$–$10^3$ $M_\odot$ yr$^{-1}$ about 5–10 Myr ago that formed ~20–40% of its stellar mass ($M_*\approx1.5\times10^{10}M_\odot$). Using ~813 $z\sim6$ LBGs over ~500 arcmin$^2$ and non-parametric SFHs modeled with beagle and prospector, the authors derive consistent stellar masses and rapid, recent SFHs, finding a very high stellar mass surface density with $\log_{10}(\Sigma_{eff}/(M_\odot\!\,\mathrm{kpc}^{-2}))\approx10.25$ and a compact size of $r_e\approx400$ pc. If confirmed, UDS_43065 represents a rare transitional object between dusty starbursts and passive systems in the first Gyr, offering a potential explanation for the apparent surplus of early massive quenched galaxies and constraining quenching timescales. The work demonstrates JWST's power in pinpointing rapid star-formation episodes and motivates targeted follow-up (NIRSpec, ALMA, MIRI) and broader searches to quantify this short-lived phase across cosmic time.

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 ($β=-0.6$) 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$α$ 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 ($β>-1$). UDS_43065 is clearly resolved yet compact in F444W ($r_e=400\pm10$ pc) indicating a very high stellar mass surface density of log$(Σ_\mathrm{eff}/(M_\odot\,\mathrm{kpc}^{-2}))=10.25\pm0.13$ 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.

Figures (5)

• Figure 1: Histograms of the F115W (left) and F444W (right) apparent magnitudes for our sample of 813 z$\sim$6 Lyman-break galaxies selected over $\approx$500 arcmin$^2$ of deep JWST/NIRCam imaging. The apparent magnitudes of UDS_43065, the primary object discussed in this work, are shown with vertical dotted red lines.
• Figure 2: (a) A comparison of two rest-UV continuum slope ($F_\lambda \propto \lambda^\beta$) measurements among our $z\sim6$ LBG sample. The x-axis shows the UV slope measurement using the three NIRCam broadbands F115W, F150W, and F200W, while the y-axis shows the measurement obtained when also folding in F090W. We show the 1$\sigma$ and 2$\sigma$ contours capturing the bulk of most of our sample, while outliers beyond this range are represented as individual data points. Only one object in our sample (UDS_43065) has an extremely red UV slope ($\beta > -1$) from both measurements. This galaxy is very bright in the UV (F115W = 26.0 mag) and hence has precise UV slope measurements firmly placing it in this extremely red regime. (b) An RGB color image of UDS_43065 illustrating its very red nature and resolved morphology ($r_e = 400\pm10 \, \text{pc}$).
• Figure 3: beagle and prospector SED fits for UDS_43065. Both codes are able to successfully reproduce the 14-band photometry, with best-fitting $\chi^2$ values of $\approx$20. The strong color between F200W and F277W is reproduced with a pronounced Balmer break ($F_{\nu,4200}/F_{\nu,3500} \approx 1.3-1.4$), implying that relatively old B and A-type stars are contributing substantially to the continuum light. Additionally, both beagle and prospector infer strong H-alpha emission (EW$\approx$400-500 Å) in the F410M band, a hallmark of active star formation fueled by young, hot O-type stars. The codes consistently infer a high rest-frame V-band dust attenuation of $A_V\approx1.5$ mag, as expected given the extremely red rest-UV color of this galaxy.
• Figure 4: The inferred SFHs of UDS_43065 from prospector (blue) and beagle (red). Both SED fits imply that UDS_43065 experienced a dramatic burst of star formation $\approx$5--10 Myr ago, with a SFR $\approx500–1000\,M_\odot$/yr, and has been relatively inactive since. Approximately 20--40% of the galaxy’s mass is inferred to have formed during this event.
• Figure 5: This plot depicts the prospector and beagle stellar mass estimates of the 24 star-forming Lyman-break $z\sim6$ galaxies in our sample with $M_\ast>3\times 10^9\,M_\odot$ from prospector. We identify two galaxies with $M_\ast >10^{10}\,M_\odot$ in both prospector and beagle, one being the exceptionally red object UDS_43065, shown here with a black star.