Electron Density of Ionized Gas Outflows: Insights from the MaNGA Survey

Abstract

We investigate the properties of ionized gas outflows in nearby star-forming galaxies from the final Data Release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) Survey. Using spatially resolved spectroscopy, we search for signatures of ionized outflows within physically motivated outflow apertures. We find significant evidence for additional broad H$α$, [N II]$λλ$6550,6584, and [S II]$λλ$6718,6733 emission-line components in 115 galaxies, $\sim$3% of star-forming MaNGA galaxies. Our analysis suggests that outflow electron densities ($n_{\text{e, out}}$), mass-loss rates, and mass-loading factors are related to both the properties of the regions that launch and global galactic properties. In particular, we find correlations between $n_{\text{e, out}}$ and both the enclosed ($-$3.4$σ$) and global ($-$2.4$σ$) star-formation-rate surface densities, such that lower surface density regions tend to drive denser ionized gas outflows. We find several $>$$2σ$ correlations between the mass-loading factor and galactic properties using individual $n_{\text{e, out}}$ estimates, while more significant correlations ($>$$3σ$) only emerge when $n_{\text{e, out}}$ is fixed to the median value of the sample. Combining with literature $n_{\text{e, out}}$ measurements out to $z \sim 1.9$, $n_{\text{e, out}}$ exhibits no significant redshift evolution, in contrast to the strong evolution observed for gas electron densities of the interstellar medium (ISM). Ionized outflows remain denser than the ISM by a factor of $\sim$8.5 at $z \sim 0$ and $\sim$1.6 at $z \sim 1.9$. These results provide new constraints for feedback models and highlight the need for high-resolution IFU observations and synthetic line diagnostics from simulations to investigate the connection between outflow properties and local ISM conditions.