Galaxies that sit along the minor axis of a massive companion show a measurable excess of heavy elements in their gas, according to a study accepted in principle by Nature Communications and posted on arXiv on July 27. The authors, led by Cheqiu Lyu and Enci Wang of the University of Science and Technology of China, analyzed 1,433 galaxy pairs from the first-year data release of the Dark Energy Spectroscopic Instrument (DESI) and found that neighboring galaxies aligned with a massive galaxy’s minor axis carry a gas-phase metallicity excess of 14.6 percent plus or minus 3.7 percent at projected separations of 15 to 30 kiloparsecs, rising to 24.2 percent plus or minus 2.6 percent at 30 to 60 kiloparsecs. They interpret the pattern as chemical enrichment spread by galactic outflows, which simulations have long predicted escape preferentially perpendicular to the galactic disk.
The study is a test of a specific prediction. Stellar feedback and active galactic nuclei launch metal-enriched gas out of galaxies, and cosmological simulations describe these outflows as anisotropic: material leaves along the path of least resistance, forming bipolar cones perpendicular to the disk. If that picture is right, a neighbor located inside the outflow cone should accrete gas that is richer in metals than a neighbor of the same mass sitting along the disk plane. The authors say direct evidence of such enrichment in the interstellar medium of neighboring galaxies had been lacking, despite decades of observations showing that the circumgalactic medium around galaxies is denser and more metal-loaded along the minor axis.
The team built its sample from DESI’s Bright Galaxy Survey, starting from 376,083 star-forming galaxies with redshifts between 0.05 and 0.49, then selecting pairs with projected separations between 15 and 100 kiloparsecs and line-of-sight velocity differences below 500 kilometers per second. The more massive galaxy in each pair was designated the primary, and the analysis kept only primaries viewed close to edge-on, with an axis ratio below 0.5, so that the disk orientation could be measured reliably. Gas-phase metallicity, expressed as 12 plus log(O/H), was derived from strong emission lines using the N2S2H-alpha calibrator, and each neighbor’s offset from the parent sample’s mass-metallicity relation was computed as a function of the angle between the primary’s major axis and the vector to the neighbor.
The central result is a positive correlation between the neighbor’s metallicity offset and its azimuthal angle, meaning neighbors nearer the minor axis are systematically metal-richer. For the closest pairs, the Pearson correlation coefficient reaches 0.668 at 15 to 30 kiloparsecs and 0.905 at 30 to 60 kiloparsecs, both with p-values below 0.001. Monte Carlo permutation tests with 10,000 shuffles put the confidence at just over 92 percent for the inner bin, which the authors describe as a marginal detection, and above 98 percent for the 30 to 60 kiloparsec bin, which they describe as significant. Pairs at 60 to 100 kiloparsecs show a flat trend, acting as a control sample that suggests the enrichment does not reach beyond about 60 kiloparsecs. The signal is strongest around the most massive primaries, with stellar masses between about 10.2 and 11.0 in log solar masses, which the authors connect to deeper potential wells and more heavily enriched outflows.
The authors compared their measurements with mock galaxy pairs built from the IllustrisTNG cosmological simulation, using the TNG100-1 run at a redshift of 0.3 and 9,897 projected pairs. The simulation reproduces the observed trends qualitatively: a positive angular gradient at separations below 60 kiloparsecs, a flat signal at larger separations, and a stronger effect around more massive primaries. The simulated slopes are shallower than the observed ones, and the authors caution against direct quantitative comparison because the amplitude of the measured effect depends on the choice of metallicity calibrator, while simulated metallicities carry their own uncertainties in nucleosynthetic yields and sub-grid feedback. They tested four additional calibrators and found the qualitative signal survives in most cases.
The paper also examines alternative explanations. Gas stripping by the primary’s wind could raise a neighbor’s integrated metallicity by removing low-metallicity gas from its outskirts, but the authors argue stripping is largely isotropic with respect to the primary’s disk and would not produce the observed angular dependence. Interaction-triggered star formation is disfavored: the star formation rate offset of neighbors shows a tentative negative angular gradient, peaking along the major axis, which is the opposite of the metallicity pattern and spatially decoupled from it. Large-scale structure accretion is consistent with the data rather than contradictory, since pristine cosmic web gas tends to flow in along the major axis and would set the lower-metallicity baseline seen near the disk plane; the authors find no preferred spin alignment between pair members, which argues against a geometric bias from coherent filament formation.
The authors list several reasons the measured signal is probably a muted version of the underlying enrichment. Projected separations are lower limits on true three-dimensional distances, outflows are episodic rather than continuous, enrichment takes time to travel and mix, and the DESI fibers sample the central regions of each galaxy, where the metallicity is dominated by the galaxy’s own star formation, with a median coverage of about one effective radius. They also state plainly what the study does not establish: a definitive causal link between the outflow and the neighbor’s interstellar medium, which would require gas tracer particles in simulations to confirm the kinematic assimilation of the metal excess.
If the interpretation holds, the result provides an observational constraint on how much metal galactic winds carry and how far they spread, quantities that feedback models currently set somewhat freely. The authors frame the work as a complement to absorption-line studies of the circumgalactic medium, and as a data-driven basis for refining sub-grid feedback prescriptions in the next generation of galaxy formation models.
Sources
- Lyu, Wang et al., “Observational Evidence for Anisotropic Metal Excess around Galaxies”, arXiv:2607.24929, https://arxiv.org/abs/2607.24929
- arXiv listing for 2607.24929 (abstract, identifiers, submission history), https://arxiv.org/abs/2607.24929