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

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’ ionized 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 Hα spectral line from the IFS observations and find that while some global Hα asymmetries can arise from disturbed ionized gas kinematics, the majority of asymmetric cases are driven by the distribution of Hα-emitting gas. When compared to the HI⁠, we find no evidence for a relationship between the global Hα and HI asymmetry. Further, a visual inspection reveals that cases where galaxies have qualitatively similar Hα and HI spectral profiles can be spurious, with the similarity originating from an irregular 2D Hα flux distribution. Our results highlight that comparisons between global Hα and HI asymmetry are not straightforward, and that many global HI asymmetries trace disturbances that do not significantly impact the central regions of galaxies.