A quiescent galaxy in a gas-rich cosmic web node at z~3

Recent JWST observations have unveiled a large number of quiescent galaxies at $z\gtrsim3$, bringing potential challenges to current galaxy formation models. Since star formation is expected to be fed by external gas accretion, the knowledge about the circumgalactic media (CGM) of these galaxies is essential to understanding how they quench. In this work, we present the discovery of a massive and passive galaxy ($M_\star\simeq10^{11},M_\odot$) within the MQN01 structure at z~3.25, containing one of the largest overdensities of galaxies and active galactic nuclei (AGN) found so far at $z\gtrsim3$. The passive galaxy has a star-formation rate of $4^{+6}{-2}~M\odot$/yr, placing it more than 1 dex below the star-forming main sequence, and has no detectable molecular gas ($M_\mathrm{H2}<7\times10^{9},M_\odot$). Surprisingly, it is located at the center of a large cool gas reservoir, as traced by bright Ly$α$ and H$α$ emission. By taking advantage of deep multi-wavelength information unique to this field, including deep Chandra X-ray data, we argue that the inefficient gas accretion from the CGM onto this galaxy over the last few hundreds of Myr, as suggested by the observations, could be caused by an AGN jet of a nearby star-forming galaxy located at a projected distance of 48 kpc. In particular, we argue that the jet feedback may have maintained a high level of CGM turbulence around the passive galaxy and thus caused a reduced gas accretion over the required time-scales. In addition, the elevated ionizing field provided by the AGN overdensity, including the nearby AGN, can illuminate the passive galaxy’s cool CGM and make it visible through fluorescent emission. Our study demonstrates that the star formation rates of high-redshift galaxies could be substantially reduced and maintained at a low level even within gas-rich and overdense environments in particular situations.

💡 Research Summary

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In this paper the authors report the discovery of a massive, quiescent galaxy—nicknamed the “Red Potato”—embedded in a gas‑rich cosmic‑web node (the MQN01 structure) at redshift z ≈ 3.25. The work exploits an unprecedented suite of multi‑wavelength data: deep JWST NIRCam imaging (F150W2, F322W2), JWST NIRSpec M​SA spectroscopy (F170LP/G235H, R ≈ 2000‑3700), HST ACS/WFC3 imaging, VLT/MUSE integral‑field spectroscopy, ALMA Band 6 continuum and CO(4‑3) line observations, a 634 ks Chandra ACIS exposure, and ASKAP 0.8 GHz/1.4 GHz radio maps. All data are reduced with state‑of‑the‑art pipelines, astrometrically aligned to Gaia, and photometry is performed with PSF‑matched apertures.

Physical properties of the Red Potato
Spectral‑energy‑distribution (SED) fitting with the Prospector code yields a stellar mass of M★ = 1.1 + 0.4 − 0.4 × 10¹¹ M⊙. The NIRSpec spectrum shows a strong Balmer break, deep Balmer and metal absorption lines (Hδ, Hγ, Mg b, Fe I), and a D4000 index consistent with an old (>1 Gyr) stellar population. The inferred star‑formation rate (SFR) from the SED is 4 + 6 − 2 M⊙ yr⁻¹, placing the galaxy more than one dex below the star‑forming main sequence at this epoch. Independent SFR limits from Hα (< 16 M⊙ yr⁻¹) and UV+IR (< 7 M⊙ yr⁻¹ assuming an SMG template) are consistent with a truly quiescent system. The galaxy’s half‑light radius is compact (≈ 1 kpc at rest‑frame 1.5 µm) and its integrated stellar velocity dispersion is σ = 268 ± 20 km s⁻¹, indicating a massive, dispersion‑supported system.

ALMA does not detect the 1.2 mm dust continuum (5σ < 0.2 mJy) nor CO(4‑3) line emission (5σ < 2.8 × 10⁹ K km s⁻¹ pc²). Assuming a Milky‑Way CO‑to‑H₂ conversion factor (αCO = 4 M⊙ (K km s⁻¹ pc²)⁻¹) this translates into an upper limit on the molecular gas mass M_H₂ < 7 × 10⁹ M⊙, i.e. a gas fraction f_H₂ < 0.06. The galaxy is also undetected in the deep Chandra data (2σ limits of 1.7 × 10⁻¹⁶ erg s⁻¹ cm⁻² in 0.5‑2 keV and 2.9 × 10⁻¹⁶ erg s⁻¹ cm⁻² in 2‑7 keV), implying no luminous X‑ray AGN. Radio maps from ASKAP show no significant emission at the galaxy position after careful astrometric correction using a bright QSO in the field.

A massive cool circum‑galactic reservoir
VLT/MUSE reveals an extended Ly α nebula surrounding the Red Potato, with a surface‑brightness profile ranging from 10⁻¹⁸ to 3 × 10⁻¹⁸ erg s⁻¹ cm⁻² arcsec⁻² and a spatial extent of ≈ 80 kpc (≈ 10 arcsec). Hα emission is co‑spatial, confirming the presence of a ≈ 10⁴‑10⁵ K cool gas component. The nebula is part of a much larger Ly α “blob” that permeates the MQN01 node, which also hosts a remarkable overdensity of galaxies (≈ 30 spectroscopically confirmed star‑forming members) and active galactic nuclei (≈ 8 X‑ray AGN).

Interpretation: why is the galaxy quenched despite abundant CGM?
The authors propose two complementary mechanisms.

1. AGN‑driven jet turbulence – A bright X‑ray AGN (ID 2) lies at a projected distance of ≈ 48 kpc and a line‑of‑sight velocity offset of ≈ 100 km s⁻¹ from the Red Potato. Radio detections at 0.8 GHz and 1.4 GHz suggest a powerful jet. The jet is hypothesized to inject turbulence into the surrounding CGM, raising its velocity dispersion and preventing the cool gas from efficiently accreting onto the central galaxy over the past few hundred Myr. This “maintenance‑mode” feedback can keep the galaxy star‑formation suppressed even though a large reservoir of cool gas is present.

2. Fluorescent illumination by the AGN overdensity – The dense population of AGN in the node produces a strong ionizing photon field. This field can photo‑ionize the neutral hydrogen in the CGM, causing the observed Ly α and Hα fluorescence. The nebular emission therefore does not necessarily imply that the gas is actively feeding the galaxy; rather it is being illuminated externally.

The combination of a turbulent, jet‑heated CGM and an intense external ionizing background provides a natural explanation for the coexistence of a massive, gas‑rich environment and a quiescent central galaxy.

Broader implications
The discovery challenges the conventional picture that massive galaxies at z > 3 in the most overdense regions should be vigorously star‑forming, fueled by copious cold streams. It demonstrates that feedback from neighboring AGN can regulate gas accretion on scales of tens of kiloparsecs, effectively decoupling the host galaxy’s star‑formation activity from the ambient gas reservoir. This scenario adds a new dimension to semi‑analytic and hydrodynamic models of high‑redshift galaxy evolution, which must now account for inter‑galaxy feedback and environmental ionization when predicting the abundance of quiescent systems.

Conclusions

• A massive (M★ ≈ 10¹¹ M⊙), compact, quiescent galaxy at z ≈ 3.25 is identified within the MQN01 node.
• Its SFR is ≈ 4 M⊙ yr⁻¹ (≳ 1 dex below the main sequence) and its molecular gas fraction is < 6 %.
• The galaxy sits at the centre of an ≈ 80 kpc Ly α/Hα nebula, indicating a large cool CGM reservoir.
• No internal AGN or significant molecular gas is detected; instead, a nearby star‑forming galaxy hosts an AGN jet that likely stirs the CGM.
• The dense AGN population provides a strong ionizing field that makes the CGM visible via fluorescence.
• The results imply that AGN‑driven turbulence and external ionization can maintain quiescence even in the most gas‑rich, overdense environments at early cosmic times.