Galaxy Group COSMOS-Gr30 Holds a Fraction of Its Gas

The galaxies in COSMOS-Gr30 are running low on the raw material of stars. A team led by Jonathan Freundlich of the University of Strasbourg Astronomical Observatory measured the molecular gas in the group and found gas fractions of only 20 to 40 percent of those in typical main-sequence galaxies. The paper is published in Astronomy & Astrophysics.

COSMOS-Gr30 sits about 7 billion light-years away, at a redshift of about 0.7, which puts the light the team collected roughly halfway back through cosmic history. The group is exceptionally dense, with at least 44 confirmed galaxies, and it sits at the intersection of several cosmic web filaments. That address is the point of the study: the epoch between the peak of star formation and today is when crowded environments are suspected of shutting galaxies down.

Molecular gas is the cold gas that collapses into stars, and it doesn't announce itself directly. The team tracked it through carbon monoxide, observing the CO(2-1) emission line with two instruments in Europe: the NOEMA interferometer on the Plateau de Bure in the French Alps and the IRAM 30-meter telescope near Granada in Spain. Carbon monoxide glows where molecular hydrogen hides, so its strength works as a scale for the gas reservoir.

The detections are thin, and the paper doesn't hide that. The team detected carbon monoxide in four galaxies of the group at a signal-to-noise ratio above five and derived upper limits for the remaining members with secure spectroscopic redshifts. Stacking the measurements is what produces the headline figure: gas contents reduced by about 0.5 dex against field scaling relations, which works out to that 20 to 40 percent range.

Some of the missing gas is visible nearby, in a different form. Earlier observations with the MUSE instrument on the Very Large Telescope revealed a large structure of ionized gas around eight of the group's galaxies, including a host of an active galactic nucleus, spread across roughly 10,000 square kiloparsecs. The 30-meter observations cap how much of that expelled material is still cold enough to make stars: the molecular gas tied to the structure is below 2 x 10^10 solar masses, so less than a third of the gas in the space between the galaxies sits in a cold, star-forming phase.

The suspects are the usual environmental ones. Tidal interactions pull at galaxies as they pass each other. Ram pressure can strip gas as a galaxy moves through the material around it. Strangulation cuts off the fresh supply a galaxy would otherwise keep drawing in. The study doesn't convict one mechanism, but it shows the combined effect plainly: in a dense group at this epoch, the reservoirs drain far below what isolated galaxies keep.

The caveats travel with the result. The authors say the uncertainties are significant, and they name the weak points: the star-formation rates are uncertain, and so is the conversion from carbon monoxide emission to molecular gas mass. Four firm detections is a small base for a group of 44. What the stacked measurement establishes is the direction and rough size of the deficit, not a precise ledger for each galaxy.

The value of the result is the timing. Star formation across the universe peaked long before this group and has fallen since, and environment is one of the standing explanations for why individual galaxies stop. COSMOS-Gr30 catches the draining in progress, 7 billion years back, with the expelled gas still glowing between the galaxies that lost it.