Extended [CII] gas emission in and around a massive quiescent galaxy at z=7.3

Abstract: We report the discovery of [CII] 158 micron emission in and around the most distant known massive quiescent galaxy RUBIES-UDS-QG-z7 at z = 7.27. Observed with ALMA in band 6, the [CII] line independently confirms the spectroscopic redshift from JWST/NIRSpec spectra at low and medium resolution. The emission extends over an effective radius R_eff,[CII] = 8 +/- 3 kpc, well beyond the compact stellar body traced by JWST/NIRCam (R_eff = 209 (+33/-24) pc), with a significant fraction of approximately 70% of the flux arising from a circumgalactic halo. No dust continuum is detected at rest-frame ~160 micron, setting an upper limit on the infrared luminosity of L_IR < 1.4 x 10^11 Lsun, overall consistent with expectations from rest-frame UV to near-infrared SED modeling under energy balance. Converting the galaxy-scale [CII] emission into cold gas mass, we find log(M_mol/Msun) = 9.53 (+0.32/-0.31) and log(M_HI/Msun) = 9.46-10.34, depending on the assumed calibration and metallicity. Despite being approximately 10x more gas-poor than typical star-forming galaxies at fixed redshift, stellar mass, and [CII] to gas mass conversion, RUBIES-UDS-QG-z7 retains a substantial cold gas reservoir with fractions f_gas >~ 20% and long depletion timescales across most assumptions. The extended [CII] halo carries approximately twice as much gas as the galaxy alone and shows a blueshifted velocity offset consistent with the tentative gas outflow detected in MgII absorption in previous work, suggesting a past episode of AGN-driven gas expulsion possibly linked to the suppression of star formation. The presence of a large gas reservoir in and around a massive quiescent galaxy just 700 Myr after the Big Bang implies that whatever mechanism is suppressing star formation must be remarkably effective at maintaining a low star formation efficiency on ~100 Myr timescales, even in the presence of abundant fuel.

• The paper reveals that extended [CII] emission spans an effective radius of 8±3 kpc, with only 30% overlapping the compact stellar core.
• The analysis demonstrates a low star formation rate with a molecular gas fraction ≥20%, implying long depletion times and efficient quenching despite abundant gas.
• The observed blueshifted, circumgalactic [CII] supports an AGN-driven feedback scenario where quenching occurs via gas heating and stabilization rather than total expulsion.

Extended [CII] Gas Emission in and around a Massive Quiescent Galaxy at $z = 7.3$

Introduction and Motivation

The detection and characterization of massive quiescent galaxies at redshifts $z>7$ directly probes the earliest phases of galaxy formation and rapid star formation suppression (quenching) in the first $\sim 700$ Myr of cosmic history. The presence of such systems, identified in large numbers by JWST, challenges prevailing theoretical models of star formation and feedback. A definitive resolution requires empirical constraints on the cold gas content—the immediate fuel for star formation—and its distribution relative to the stellar body, as only multi-phase gas mapping elucidates the interplay between residual star formation, gas exhaustion, expulsion, or stabilization. At these high redshifts, classical cold gas tracers (e.g., CO, HI 21-cm) are generally inaccessible; from $z\approx 4$, [CII] 158$\mu$m becomes the most luminous and informative ISM cooling line accessible by ALMA.

This study presents ALMA Band 6 observations of the [CII] line in the most distant known massive quiescent galaxy, RUBIES-UDS-QG-z7 at $z=7.27$, along with a comprehensive analysis of the gas and dust content, supported by deep JWST NIRCam and NIRSpec data.

Observational Strategy and Data Analysis

[CII] emission in RUBIES-UDS-QG-z7 was targeted by ALMA using a measurement strategy optimized for the recovery of extended, low surface brightness emission. Visibility-based morphokinematic modeling quantified the emission size, and flux extraction was validated through several tapering and weighting schemes to control for interferometric limitations.

The redshift was confirmed via coincidence between the [CII] centroid and independent JWST/NIRSpec rest-frame optical emission features ($z_{[CII]}=7.2749^{+0.0011}_{-0.0012}$), ensuring robust association with the stellar component. No continuum emission was detected down to $\sim18\,\mu$Jy at 1.3mm, setting upper limits on infrared luminosity and dust mass.

Key Results

Spatial Distribution and Luminosity of [CII] Emission

The [CII] line is both luminous and spatially extended. The effective radius of the [CII] emission is $R_{eff, [CII]}=8\pm3$ kpc, in stark contrast to the compact JWST-derived stellar radius $R_{eff, \star}\approx210$ pc. Notably, only about 30% of the [CII] flux is coincident with the central stellar body; the majority resides in a spatially extended, circumgalactic component. Kinematic analysis shows a velocity gradient and a blueshift ($z>7$0 km/s) of the halo relative to the systemic velocity.

The total [CII] luminosity is $z>7$1, with the compact component at $z>7$2.

Cold Gas Mass and Star Formation Efficiency

Translating [CII] luminosity to gas mass is subject to uncertainties in the dense gas fraction and metallicity, but adopting calibrations from Zanella et al. (2018) and Heintz et al. (2021), the central galaxy has $z>7$3, and $z>7$4, with metallicity dependence. These values correspond to a molecular gas fraction $z>7$5, and extended [CII] emission contains about twice as much gas as the galaxy alone.

Compared to main-sequence star-forming counterparts at the same epoch, RUBIES-UDS-QG-z7 is an order of magnitude more gas-poor, yet retains substantial reservoirs. Given the extremely low current SFR inferred from SED modeling and emission lines ($z>7$6 yr$z>7$7), the depletion time $z>7$8 Gyr, and all plausible combinations imply very low star formation efficiency on 10–100 Myr timescales.

Circumgalactic Gas, Outflows, and Quenching Mechanisms

The extended [CII] gas is blueshifted, and its kinematics are consistent with previously reported Mg II absorption (tracing neutral outflow), which is also blueshifted by $z>7$9 km/s. The mass and energetics of both features are consistent with AGN-phase outflow-driven expulsion in the recent past, plausibly linked to the suppression of star formation.

No evidence for ongoing AGN activity or recent major merger signatures is found in the stellar or dust emission. The cold gas is unlikely to be escaping the gravitational potential, implying that feedback must suppress star formation through heating or dynamical stabilization rather than wholesale gas removal.

Dust Content and SED Constraints

No dust continuum is detected; the limit is $\sim 700$0 and $\sim 700$1. Assuming a canonical gas-to-dust ratio, the dust-based gas mass is consistent with [CII]-based estimates. This non-detection aligns with SED-predicted $\sim 700$2, though higher-frequency and deeper continuum observations are needed to resolve model degeneracies, particularly for metallicity.

Implications for Galaxy Evolution and Future Prospects

The presence of an extended, massive cold gas reservoir around a compact, quiescent, massive galaxy at $\sim 700$3 demonstrates that rapid quenching can proceed without complete gas evacuation. Star formation is suppressed despite the presence of abundant molecular gas, requiring extremely efficient feedback or ISM stabilization, active already at less than a Gyr after the Big Bang. The blueshift and structure of the circumgalactic [CII] halo support an AGN-driven feedback scenario.

These results argue against simple gas exhaustion or rapid expulsion as the dominant quenching mechanism for massive galaxies at high redshift. Instead, they support models where quenching is driven by rapid heating, dynamical stabilization, or AGN-driven expulsion followed by fallback/reaccretion, as predicted in some cosmological frameworks.

High spatial and spectral resolution ALMA [CII] and multi-line follow-up, along with deep JWST grating observations, are essential to resolve the galaxy/halo decomposition, confirm neutral/ionized outflows, measure the gas-phase metallicity, and completely diagnose the gas-star-dust energetics.

Conclusion

This work presents the first detection of highly extended [CII] emission, accompanied by a substantial cold gas reservoir, in a massive, quiescent galaxy at $\sim 700$4. The compactness of the stellar body contrasted with the spatial scale of the cold gas, together with suppressed star formation, provide direct evidence that quenching mechanisms capable of decoupling gas availability from star formation efficiency must operate rapidly and efficiently at cosmic dawn. This system becomes a benchmark for testing feedback and quenching prescriptions in early massive galaxies and their subsequent evolution.