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

@misc{deugenio2026dusty,
  title = {{A Dusty Quenching-candidate AGN host at z=5.7: massive quiescent galaxies may quench already during the dust-obscured phase}},
  author = {Francesco D'Eugenio and Matilde Brazzini and Roberto Maiolino and Elena Bertola and Stefano Carniani and Xihan Ji and Ignas Juodžbalis and Yixiao Liu and Minjung Park and Eleonora Parlanti and Robert G. Pascalau and Gabriele Pezzulli and Jan Scholtz and Sandro Tacchella and Stefano Zibetti},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.07825},
  eprint = {2609.07825},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.07825},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - A Dusty Quenching-candidate AGN host at z=5.7: massive quiescent galaxies may quench already during the dust-obscured phase
AU  - Francesco D'Eugenio
AU  - Matilde Brazzini
AU  - Roberto Maiolino
AU  - Elena Bertola
AU  - Stefano Carniani
AU  - Xihan Ji
AU  - Ignas Juodžbalis
AU  - Yixiao Liu
AU  - Minjung Park
AU  - Eleonora Parlanti
AU  - Robert G. Pascalau
AU  - Gabriele Pezzulli
AU  - Jan Scholtz
AU  - Sandro Tacchella
AU  - Stefano Zibetti
PY  - 2026
DA  - 2026/09/07
PB  - arXiv
DO  - 10.48550/arxiv.2609.07825
UR  - https://arxiv.org/abs/2609.07825
AB  - 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± 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_⊙)=10.7±0.3, Lbol∼4×10^46 erg s^−1 , and black-hole mass log(M_BH/M_⊙)∼8.1 (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 (∼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.
ER  -