GECO – GALAXY GECO EVOLUTION CIRCLE

For our next G.E. circle, we will have the presence of Simon Weng, who is going to present :

Peering down-the-barrel with DESI DR2: 10,000+ inflows at z<0.6 reveal how galaxies accrete cold gas

The depletion time (M­_H2/SFR) of cold gas reservoirs in galaxies suggests that ongoing accretion is required to sustain star formation. However, direct detections of gas inflows remain extremely rare. In this circle, I will present the detection of more than 50,000 Na I D down-the-barrel absorbers in galaxies at z < 0.6 from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2. We identify over 10,000 absorbers consistent with inflowing gas at velocities v > 50 km s⁻¹ relative to the systemic redshift of their host galaxies.

We find strong statistical evidence for a large population of slow, infalling gas with low velocity dispersion. These inflows are observed primarily in edge-on systems, consistent with streaming motions within the disc, although this geometry is difficult to distinguish from radial accretion from the circumgalactic medium onto the disc. We also find evidence for fountain flows: the median inflow velocity increases with inclination, suggesting a velocity component perpendicular to the disc.

The fastest inflows (≳ 200 km s⁻¹) are consistent with accreting satellites or merger events. This interpretation is supported by the fact that the infall velocity correlates more strongly with stellar velocity dispersion than with stellar mass, indicating a closer link to the depth of the gravitational potential. At such high velocities, Na I D is unlikely to survive in a hot CGM unless bound to another galaxy and shielded.

Absorbers are found predominantly in early-type galaxies. Outflows detected in passive systems likely trace prior star-formation episodes, while inflows in similarly passive galaxies may provide the fuel for future activity. We also identify more than 200 galaxies exhibiting both inflows and outflows simultaneously, demonstrating that gas accretion and ejection can occur concurrently. Together, the coexistence of inflows and outflows within individual galaxies, and their detection in both star-forming and passive systems, indicate that gas cycling operates across all evolutionary stages, providing a natural mechanism for stochastic star-formation histories.

In the future, we will balance the baryon budget within galaxies by computing whether the mass inflow rate is sufficient to sustain the star-formation rate and mass outflow rate.

The idea is to continue with a round table format, with a lively and informal discussion.