SAMI-HI: the connection between global asymmetry in the ionised and neutral atomic hydrogen gas in galaxies

TL;DR

This study investigates the relationship between ionised and neutral hydrogen gas asymmetries in galaxies, revealing that global HI asymmetries often do not correspond to disturbances in the central ionised gas regions.

Contribution

It provides the first detailed comparison between global HI and Hα asymmetries using spatially resolved IFS data, clarifying their physical origins.

Findings

Most Hα asymmetries are due to gas distribution, not kinematics.

No clear correlation between Hα and HI asymmetries.

Global HI asymmetries often trace disturbances outside galaxy centers.

Abstract

Observations of the neutral atomic hydrogen (HI) gas in galaxies are predominantly spatially unresolved, in the form of a global HI spectral line. There has been substantial work on quantifying asymmetry in global HI spectra (`global HI asymmetry'), but due to being spatially unresolved, it remains unknown what physical regions of galaxies the asymmetry traces, and whether the other gas phases are affected. Using optical integral field spectrograph (IFS) observations from the Sydney AAO Multi-object IFS (SAMI) survey for which global HI spectra are also available (SAMI-HI), we study the connection between asymmetry in galaxies' ionised and neutral gas reservoirs to test if and how they can help us better understand the origin of global HI asymmetry. We reconstruct the global H$\alpha$ spectral line from the IFS observations and find that, while some global H$\alpha$ asymmetries can arise from disturbed ionised gas kinematics, the majority of asymmetric cases are driven by the distribution of H$\alpha$-emitting gas. When compared to the HI, we find no evidence for a relationship between the global H$\alpha$ and HI asymmetry. Further, a visual inspection reveals that cases where galaxies have qualitatively similar H$\alpha$ and HI spectral profiles can be spurious, with the similarity originating from an irregular 2D H$\alpha$ flux distribution. Our results highlight that comparisons between global H$\alpha$ and HI asymmetry are not straightforward, and that many global HI asymmetries trace disturbances that do not significantly impact the central regions of galaxies.