When a galaxy group’s gas heats up, its galaxies may quiet down

Galaxies are not closed systems: to keep forming stars, they need a continuing supply of cold gas. In a growing galaxy group, that supply may arrive along narrow streams from the cosmic web. Theory predicts that, under some conditions, gas entering a sufficiently massive halo is heated by shocks instead of remaining cold. If less cold gas reaches galaxies, star formation may eventually fade—a process often called starvation.

A new study asks whether this expected shift from cold to hot gas accretion is reflected in the fraction of galaxies that have stopped forming stars. The authors examined 16 spectroscopically confirmed groups at redshifts 1.6 to 3.6, when the universe was much younger, with estimated halo masses from about 10^12.8 to 10^13.9 times the Sun’s mass. These are groups in the process of assembling, not necessarily settled, mature clusters.

Estimating which galaxies belong

The team began with galaxies in the COSMOS2020 catalogue, which includes measurements of light across many wavelengths. Only a small number of galaxies in each group had spectroscopic confirmation, so the researchers also used photometric redshifts—estimates based on a galaxy’s colours—to identify likely members. They statistically accounted for foreground and background galaxies that could be mistaken for group members, and weighted candidates by their estimated contamination.

To estimate whether a galaxy was still forming stars, the researchers fitted models to its observed light, or spectral energy distribution. They used several indicators together: the galaxy’s estimated star-formation rate compared with similar galaxies, the shape of its inferred star-formation history, and its ultraviolet-to-infrared colours. Because these measurements are uncertain, they estimated a probability of quiescence for each galaxy rather than treating every classification as certain. Those probabilities and membership weights were incorporated into estimates of each group’s quiescent fraction.

The researchers compared those fractions with each group’s position relative to a theoretically predicted boundary between hot and cold-in-hot accretion. The boundary depends on halo mass and redshift. A group’s position relative to it indicates which accretion regime theory predicts—not a direct measurement of the gas entering that group.

A pattern, with important caveats

The groups expected to be in the hot-accretion regime had quiescent fractions reaching roughly 50%; groups in the cold-accretion regime were consistent with having no quiescent members. Across the sample, the fraction tended to decline toward the cold-accretion side of the predicted boundary. The authors describe the correlation as moderate, with a statistical significance of about 2.1 sigma—not a decisive detection.

The spatial distribution adds a clue. In the more mature, hot-accreting groups, massive quiescent galaxies were concentrated toward the inner regions. In the highest stellar-mass bin, the study reports a 4.4-sigma excess of quiescent galaxies in the inner regions compared with the outskirts. This pattern is consistent with a possible inside-out process: as groups evolve, changes to their gas supply may affect galaxies nearer the centre. It does not directly show that this is what happened to any individual galaxy, or map the gas flows themselves.

The study also tested whether quiescence tracked the mass of a galaxy or the dominance of the group’s most massive member. It found no clear relationship between group quiescent fraction and how overmassive that leading galaxy was relative to a model expectation. Most groups also lacked a strongly dominant brightest group galaxy, suggesting that many were still assembling. These results make a group-scale environmental explanation plausible, but do not rule out internal processes such as black-hole feedback in individual galaxies.

What remains uncertain

Sixteen groups provide a small sample, and the predicted accretion state is inferred from halo mass and redshift rather than directly observed gas flows. Its relationship with quiescence is not independent of relationships with mass and redshift. The authors explicitly say their data cannot establish that accretion state is a stronger statistical predictor than halo mass alone. Correlation also cannot establish that hot gas caused star formation to stop.

Membership and galaxy properties rely substantially on photometry and model fitting, with statistical corrections for contamination. The paper reports that its trends persist under alternative contamination choices and quiescence criteria, but these checks cannot replace more complete spectroscopic membership data. The analysis is limited to galaxies within each group’s estimated virial radius, potentially missing quiescent galaxies in the outskirts or in the process of joining the group. Whether gas filaments survive inside these halos remains an open question.

The result is best read as evidence consistent with hot accretion contributing to starvation in these young groups—not as proof of a single quenching mechanism. The supplied manuscript is labelled “A&A proofs,” but its acceptance field contains placeholder dates, so its publication or review status cannot be established from this version. Larger samples and deeper observations could test the interpretation; the authors note that JWST data are reserved for future analysis.