A Dusty Quenching-candidate AGN host at z=5.7: massive quiescent galaxies may quench already during the dust-obscured phase

Francesco D'Eugenio, Matilde Brazzini, Roberto Maiolino, Elena Bertola, Stefano Carniani, Xihan Ji, Ignas Juodžbalis, Yixiao Liu, Minjung Park, Eleonora Parlanti, Robert G. Pascalau, Gabriele Pezzulli, Jan Scholtz, Sandro Tacchella, Stefano Zibetti

First listed 2026-09-09 | Last updated 2026-09-07

Abstract

We present a spectro-photometric analysis of RUBIES-EGS-9809, a broad-line AGN host at z=5.7 with a Balmer break and a dominant point source. The source is dust reddened ($A_V>$3 mag), shows hot- and cold-dust emission, and is X-ray Compton thick, with X-ray-to-bolometric ratio consistent with luminous AGN. The large $A_V$ agrees with strong NaI absorption (EW=17$\pm 2 Å$), possibly from a neutral-gas outflow (3 $σ$), while broad [O III] traces an ionized outflow, suggesting AGN feedback may be affecting the host. We estimate a stellar mass log(M*/M$_\odot$)=10.7$\pm$0.3, Lbol$\sim4\times10^{46}$ erg s$^{-1}$ , and black-hole mass log(M$_{BH}/M_\odot)\sim8.1$ (subject to large systematics). The resolved Balmer break (0.1 arcsec, 0.6 kpc) is consistent with a stellar origin, implying the galaxy is already old in stars while still heavily obscured. This shows that massive galaxies can host evolved stellar populations during their compact, dust-obscured phase, consistent with formation in an earlier dusty starburst. SED modelling suggests a declining SFR, but low-resolution spectroscopy alone cannot constrain the recent SFR, so we treat the source as a strong quenching candidate rather than a secure post-starburst system. Because quenching follows the decline in SFR, star-formation-driven outflows are disfavoured, leaving AGN feedback as a plausible driver. We speculate that such rapid feedback ($\sim$100 Myr timescale) may precipitate quenching while the post-quenching phase stays dust-enshrouded, concealing the UV-bright quenched phase that would otherwise be easily detected, if dust free. By the time dust clears, the UV-luminous stars may have already dwindled, leaving a classic dust-free quiescent galaxy. This 'dusty path' to quiescence may explain the lack of intermediate-age quenched progenitors (ages 50-100 Myr) linking dusty starbursts to massive quiescent systems at z=3-7.

Short digest

D’Eugenio et al. present a spectro-photometric analysis of RUBIES-EGS-9809, a compact broad-line AGN host at z=5.7 that is deeply reddened (A_V > 3 mag), dust-rich, and Compton-thick in X-rays. Its spatially resolved 0.6-kpc Balmer break and inferred stellar mass of log(M*/M_sun)=10.7±0.3 indicate an evolved stellar population embedded in an obscured system, while strong Na I absorption and broad [O III] point to multiphase outflows plausibly powered by the AGN. Although the declining recent SFH cannot be securely established from the low-resolution spectra, the authors identify a strong quenching candidate and propose that quenching can occur before dust clears, hiding the expected UV-bright 50–100 Myr transition population between dusty starbursts and dust-free quiescent galaxies.

Key figures to inspect

• Figure 1. Use this as the observational overview: the NIRCam morphology and paired RUBIES/CAPERS spectra establish the compact target, weak dust-suppressed UV continuum, Balmer break, broad emission, and conspicuous absorption that motivate the quenching-candidate interpretation.
• Figure 2. This multi-band image-model decomposition shows the wavelength-dependent morphology, with more extended rest-frame UV emission and a nearly unresolved rest-frame optical component, providing key spatial context for separating stellar light from the dominant AGN point source.
• Figure 5. The joint prism and G395M spectral fits provide the paper’s direct line-profile evidence for an outflow component, linking the broad rest-optical emission to the proposed AGN-driven, multiphase feedback episode.
• Figure 8. This SFH comparison places RUBIES-EGS-9809 alongside massive quiescent galaxies and the dusty quenching system Hyde, showing why its assembled stellar population is consistent with a progenitor already approaching quiescence rather than an ordinary active starburst.
• Figure 10. The concluding cartoon distills the proposed dusty path to quiescence: star formation declines while the galaxy remains obscured, AGN feedback can remove gas and dust, and the system emerges only later as an already evolved dust-poor quiescent galaxy.