A Quiescent Galaxy in a Gas-Rich Cosmic Web Node at z~3

TL;DR

The paper presents the discovery of a massive, quiescent galaxy (MQN01 J004131.9-493704) at $z\approx3.25$ embedded in a gas-rich cosmic web node, with an extensive cool CGM revealed by bright Ly$\alpha$, H$\alpha$, and [O III] emission. Multi-wavelength data show a stellar mass of $M_\star\approx1.1\times10^{11}\,M_\odot$ and a star formation rate well below the main sequence, coupled with very low molecular gas content ($M_{\mathrm{H}_2}<7\times10^{9}\,M_\odot$, $f_{\mathrm{H}_2}<0.06$). The CGM exhibits unusually high turbulence (Ly$\alpha$ $\sigma\sim4\!00$–$5\!00$ km s$^{-1}$ in the inner halo) and extended emission, and X-ray and radio data reveal a nearby AGN (ID2) with a jet whose orientation toward the galaxy likely sustains CGM turbulence and suppresses gas accretion. The results suggest that, in specific high-redshift, overdense environments, external AGN feedback can quench massive galaxies even amidst abundant surrounding gas, and they motivate broader CGM studies to understand quenching pathways at $z>3$.

Abstract

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.

Figures (14)

• Figure 1: The passive galaxy MQN01 J004131.9-493704 "Red Potato" at z=3.250 and its surrounding cool Ly$\alpha$-emitting gas reservoir. This massive and compact object was discovered at the center of an extended cool ($10^4$--$10^5$ K) gas reservoir, as traced by the bright Ly$\alpha$ emission spanning 10 arcsec or 80 kpc (contours) within an overdense region in terms of galaxies and AGN Pensabene2024Galbiati2025Travascio2025. An X-ray bright AGN ID2 from Travascio2025 was found at a projected distance of only 7 arcsec or 50 kpc and a line-of-sight velocity separation of 100 km/s ($z=3.251$), likely providing enough ionizing photons to illuminate the CGM of the Red Potato and the CGM of its host galaxy. The color image is made using the NIRCam/F322W2, NIRCam/F150W2, and ACS/F814W for the red, green, and blue channels. The Ly$\alpha$ surface brightness values are 10, 7.0, 5.5, 4.0, 3.0 ($\times 10^{-18}~$erg/s/cm$^2$/arcsec$^2$) from the outmost to the innermost contour around the Red Potato, and 4.0, 3.0 ($\times 10^{-18}~$erg/s/cm$^2$/arcsec$^2$) for the contours around the AGN-ID2.
• Figure 2: The spectrum and multi-band photometry suggests the quiescent nature of the Red Potato galaxy. Top: The image cutout of the Red Potato galaxy is shown on the left. Its NIRSpec spectrum, which is smoothed by a 5-pixel box kernel, shows strong Balmer and metal absorption lines (solid vertical lines), suggesting old stellar populations. This is also supported by the D4000 break captured by the multi-band photometry (filled circles). Best-fit models of the stellar continuum and photometric band fluxes from prospector are shown as the orange dotted line and open squares, respectively. Nebular emission lines are also present in the observed spectrum, which are however driven by an AGN-like radiation field which we think has an external origin (see Fig. \ref{['fig:nirspec_zoomins']} and Section \ref{['subsec:discussion_quenching_galaxy']}) rather than be due to internal star formation. A 2-$\sigma$ errorbar is also plotted for each photometric point, except for the case of CH4 where the 2-$\sigma$ upper limit is plotted instead (yellow arrow). Bottom: Zoomed-in views of regions where the absorption lines are present, including the H$\delta$, G, H$\gamma$, Fe I, H$\beta$ (blended with a weak emission component), and Mg b lines.
• Figure 3: Profiles of the Na D and nebular lines on the continuum-removed spectrum of the galaxy. Left panel: The Na D line profiles before (light gray) and after (black) removing the stellar continuum from prospector (orange dashed). The continuum-subtracted spectrum shows no residual absorption component from gas, suggesting the absence of cold ISM gas and outflows. The continuum model from ppxf is also shown (cyan solid) which is consistent with the prospector model. Middle panels: Hydrogen, [O III], and [N II] lines are detected from the continuum-subtracted spectrum. The black curves represent the spectrum smoothed with a boxcar kernel of 4 pixels and the light gray represent the native-resolution spectrum. Right panel: The observed emission lines are driven by an AGN-like spectrum rather than star formation, according to the BPT line ratio criteria by Kewley2001 and are consistent with the presence of the nearby AGN ID2 or, in general, with an elevated ionizing field produced by the large AGN overdensity in this region
• Figure 4: The full galaxy SED measured from the NIRSpec spectroscopy (gray curve) and multi-band photometry (filled circles), and the corresponding models from prospector (orange curve and open squares) are shown in the left panel. The prospector fitting results suggest a SFR below 10 M$_\sun$/yr, demonstrated by the best-fit SFH in the right panel, corresponding to sSFR $< 10^{-10} \mathrm{yr}^{-1}$. The low SFR is also supported by the faint rest-frame UV fluxes (F625W and F814W filters) and the non-detection of dust continuum emission from ALMA (not shown in figure). During the prospector fitting, the observed emission lines were neglected in the step of modeling the stellar population and SFH, considering that these lines are likely not produced by internal star formation (Fig. \ref{['fig:nirspec_zoomins']}). The transmission curve of each photometric filter is also shown at the top.
• Figure 5: Morphology and physical properties of the Red Potato galaxy, which is marked by large red circles in the middle and right panels. Left: The galaxy has a red and compact morphology in the JWST+HST color image. Middle: It is a massive system with $M_\star \simeq 10^{11}~M_\sun$ and at least one dex below the main sequence at its redshift of z=3.250 (blue curves; Speagle2014Popesso2023), according to the SED fitting and the SFR upper limits from UV+IR and H$\alpha$ (red downward arrows). Its SFR is also substantially lower than those of other massive galaxies discovered in the same proto-cluster MQN01 (light blue squares; see Galbiati2025Pensabene2025 and main text). Right: The galaxy is poor in molecular gas, with a gas fraction $f_\mathrm{H_2}\lesssim 0.06$, substantially smaller than those of the field star-forming galaxyes (SFGs, solid curve; Tacconi2018). Similarly low fractions, smaller than around 10%, are also found in other massive $z\sim 3$ quiescent galaxies (Scholtz2024Umehata2025b).