Gas Flows and Scatter in Galaxy Scaling Relations ✛
Context
This study aligns with work linking galaxy evolution to their gaseous environment. Nearby studies have examined galactic outflows, cold gas coronae, and metal enrichment of neighbors, but direct observational comparisons of measured gas flows with galaxy positions on scaling relations have been rare. This work fills that gap using the largest sample of detected neutral gas inflows and outflows to date.
Methods
The authors used spectra of about 52,700 galaxies from the DESI DR2 survey, in which neutral gas flows were detected via sodium absorption lines (Na I D). Galaxies were divided into groups by the line-of-sight velocity of the flow: outflows (faster than −50 km/s), slow inflows (0–100 km/s), and fast inflows (faster than 100 km/s). For each group, they constructed control samples matched by redshift, stellar mass, and in some analyses also by star formation rate and other parameters, using multivariate resampling (1000 realizations). They then compared the specific star formation rate (sSFR), gas metallicity, and the Dn4000 parameter (sensitive to stellar population age and metallicity) between the flow groups and control samples, as well as relative to the fitted SFMS and MZR relations.
Results
Galaxies with outflows showed elevated sSFR by 0.25–0.40 dex and elevated central metallicity by 0.04–0.06 dex in the low-redshift sample. Slow inflows are also associated with enhanced star formation (by 0.20–0.30 dex) but without a significant shift in metallicity. Fast inflows show only a weak enhancement in sSFR (about 0.1 dex) and slightly reduced metallicity (approximately −0.03 dex). Relative to the fitted SFMS, outflows lie about 1σ above, slow inflows about 0.8σ above, and fast inflows only about 0.25σ above. Differences in Dn4000 are small but systematic: galaxies with outflows have a weaker break, while those with inflows have a stronger break than control samples.
Implications
For the first time on a large statistical basis, this work shows that detected neutral gas flows are associated with measurable offsets from the SFMS and MZR. Unlike neighboring studies that either examined flows without linking them to scaling relations or estimated scatter indirectly, here individual flow detections are directly compared with galaxy positions on the diagrams. This confirms that the baryon cycle contributes to the scatter in fundamental relations and clarifies which phases of the cycle correspond to different flow types.
Future development
The topic could develop toward accounting for active galactic nuclei (AGN), which dominate among classified objects with flows but were excluded from the main analysis. A joint analysis of stellar and AGN winds, as well as a transition to integral field spectroscopy for spatially resolving flows, would build a more complete picture of the baryon cycle. Additionally, estimates of mass inflow and outflow rates together with gas reservoirs would help test whether inflows can actually sustain the observed star formation.
Impact
The results will influence galaxy evolution models by clarifying the connection between gas flows and scatter in the mass–metallicity and star formation relations. They are also important for interpreting data from large spectroscopic surveys such as DESI and for future studies with integral field spectroscopy.
Next steps
Immediate next steps include analyzing AGN-hosting galaxies with flows, measuring mass flow rates and gas reservoirs, and applying full spectral fitting methods to separate star formation histories.
Key open problems
This work directly addresses the unresolved problem of the origin of scatter in the mass–metallicity relation and the star-forming main sequence. It shows that gas flows can explain part of this scatter but leaves open the question of the quantitative contribution and the role of AGN. It also remains unclear how long the inflow and outflow phases last and how synchronized they are with star formation.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
Using DESI DR2 data for more than 6000 star-forming galaxies, this work compares neutral gas flows detected via Na I D lines with deviations from the star-forming main sequence and the mass–metallicity relation. The authors identify outflows, slow and fast inflows, build control samples, and find systematic differences in sSFR and metallicity. The results support the regulator model, where gas flows reflect phases of the baryon cycle and contribute to the scatter in scaling relations.
Where its strength is
For the first time on a large sample, individual detections of neutral gas flows are directly linked to the position of galaxies on the SFMS and MZR. This allows a quantitative estimate of the contribution of different baryon cycle phases to the scatter in fundamental relations.
Where it could go next
- One could compare measured outflow velocities with the acceleration models proposed in the work on dust and acceleration of galactic outflows, to test which mechanisms (radiation pressure, cosmic rays) are consistent with the observed offsets on the SFMS.based on: Cosmic Sandblaster: How Dust Unveiled the Acceleration of Galactic Outflows arXiv:2606.25164
- It would be interesting to compare the metallicity of galaxies with outflows with predictions of intergalactic medium enrichment from the study on cross-enrichment of neighbors by metals; this could clarify how outflows spread metals and affect the chemical evolution of the environment.based on: Cosmic Cross-Pollination: How Galaxies Enrich Their Neighbors with Metals arXiv:2607.24929v1
- For galaxies with fast inflows, the reduced metallicity may be related to accretion of low-metallicity gas; comparison with the work on neutral gas leaving massive galaxies in the early Universe will help understand how inflows and outflows coexist at different masses and redshifts.based on: Galactic Exhalation: Neutral Gas Leaves Massive Galaxies in the Early Universe arXiv:2607.20606v1
Worth trying
- Test whether outflow velocity correlates with the sSFR excess relative to the control sample, using DESI data.
- Estimate what fraction of the MZR scatter can be explained by the presence of fast inflows, via analysis of variance.
- Compare Dn4000 with dust content to separate the influence of age from metallicity in groups with flows.
Why this may matter
The connection between gas flows and the scatter in scaling relations is important for understanding the regulation of star formation and the chemical evolution of galaxies; quantitative estimates of the contribution of different baryon cycle phases can refine galaxy formation models.
Nearby works in our archive
- Galactic Exhalation: Neutral Gas Leaves Massive Galaxies in the Early Universesimilarity 0.62
- Cosmic Sandblaster: How Dust Unveiled the Acceleration of Galactic Outflowssimilarity 0.58
- Stars Are Born Where There Is Gassimilarity 0.57
- Chaos Reigns in the Thick of Stellar Filamentssimilarity 0.56
- Cosmic Cross-Pollination: How Galaxies Enrich Their Neighbors with Metalssimilarity 0.56
The closest work in the archive sits at 0.62 by meaning — the usual level for our corpus.
About 200 galaxies in the sample show simultaneous presence of both an outflow and a slow inflow, indicating complex, multiphase gas dynamics.
Key numbers
- number of galaxies with flows: 52 696
- sSFR offset for outflows: 0.25–0.40 dex
- metallicity offset for outflows: 0.04–0.06 dex
- sSFR offset for slow inflows: 0.20–0.30 dex
- metallicity offset for fast inflows: approximately −0.03 dex