Gas around galaxies echoes a peak in star-building efficiency

A question beyond the stars

A galaxy’s stellar mass tells only part of its story. Its surrounding dark-matter halo provides a gravitational framework, while gas outside the visible galaxy can supply future star formation, circulate after being expelled, or become heated.

Astronomers describe the connection between stellar and halo mass using the stellar-to-halo mass relation. The ratio of stellar mass to halo mass reaches a maximum around galaxies containing roughly 30 billion solar masses in stars. Smaller and larger systems have proportionally less mass locked in stars. Accounting for the cosmic fraction of ordinary matter turns that ratio into an estimate of baryon-conversion efficiency—not a measure of the galaxy’s current star-formation rate.

In a preprint submitted to The Astrophysical Journal, Zeyu Chen and colleagues ask whether the surrounding cool gas shows a corresponding characteristic mass. Their evidence comes from observations, rather than a controlled experiment or a galaxy-formation simulation.

Using quasars as background lights

The team used Dark Energy Spectroscopic Instrument Year 1 spectra. When light from a distant quasar passes near a foreground galaxy, intervening gas can absorb particular wavelengths. The researchers studied two absorption lines from singly ionized magnesium, known as Mg II, which traces cool, metal-enriched gas at approximately 10,000 kelvin.

Individual signals can be weak. Instead of starting with a catalogue of detected absorbers, the researchers aligned quasar spectra to foreground-galaxy redshifts and combined them using median stacking. Millions of galaxy–quasar pairs provide the statistical basis for measurements spanning nearly a thousandfold range in stellar mass.

Their primary analysis covered projected separations of 20–180 kiloparsecs. These are distances across the sky, not full three-dimensional distances. The outer boundary supplies a common comparison aperture; it is not assumed to mark every galaxy’s halo edge.

The team measured absorption equivalent width, which summarizes the strength of a spectral absorption feature, and corrected stacked line widths for instrumental broadening and galaxy-redshift uncertainties.

Three linked patterns

First, magnesium absorption weakens with increasing distance from galaxies, but its radial profile changes systematically with stellar mass. Lower-mass galaxies have steeper profiles; more massive galaxies have shallower ones. The high-mass reduction in absorption relative to the intermediate-mass maximum is strongest at small projected separations. The pattern therefore involves changing profile shape, not simply stronger or weaker versions of one universal profile.

Second, absorption measured in each of three distance intervals rises toward intermediate stellar mass and then falls, producing a broad maximum near 30 billion solar masses. That turnover is measured from the stacks themselves, rather than imposed through the stellar-to-halo mass relation.

An empirical model using that relation describes emission-line and Bright Galaxy Survey measurements with a scatter of about 0.15 dex, equivalent to a multiplicative factor of roughly 1.4. It is not equally successful everywhere: the most massive luminous red galaxies show absorption averaging about 1.7 times the simplest model’s prediction. A more flexible scaling improves the overall fit slightly but leaves a systematic high-mass discrepancy.

Third, the stacked absorption widths change relative to an estimated gravitational velocity scale. At low stellar mass, widths commonly exceed the adopted line-of-sight virial expectation; at high mass, they fall below it. The ratio crosses unity near the same intermediate stellar mass.

A connection, not a unique explanation

The authors interpret the low-mass motions as consistent with feedback-driven circulation, recycled accretion or other bulk flows. Heating by hot halos or active galactic nuclei could help explain reduced inner magnesium absorption in massive systems. These are physical interpretations, not separately demonstrated mechanisms.

Important qualifications remain. Stacked widths combine motions within absorption systems with velocity offsets between gas and galaxies; they do not resolve the dynamics of individual halos. Halo masses and gravitational reference velocities are inferred using an adopted stellar-to-halo mass relation, making the normalized kinematic comparison model-dependent.

Magnesium absorption is also not a direct inventory of all surrounding gas. Changes in ionization and the fraction of sightlines containing absorbing gas can affect the signal.

The team checked halo-size normalization, another ionic tracer and redshift subdivisions. The principal trends persisted, although current star-formation activity supplies additional information about absorption strength. The result is a coherent observational benchmark: gas structure, absorption strength and ensemble motions change near a familiar galaxy mass scale, without yet establishing a single cause.