Neutral atomic hydrogen (HI), the most abundant gas in the interstellar medium (ISM), plays a crucial role in the evolution and dynamics of the ISM in galaxies. It provides the initial material for the formation of stars, serves as the building block of molecular clouds, influences the dynamics of the ISM, acts as a cooling agent and as a source of radiation shielding, and participates in the feedback mechanism that regulates star formation as well as galaxy evolution.

Nguyen et al. present the largest Galactic neutral hydrogen HI absorption survey to date, utilizing ASKAP at an unprecedented spatial resolution of 30 arcseconds. The GASKAP-HI pilot survey targeted 2,714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase over the 373 sources observed by previous similar surveys across the entire Milky Way. The aim of the survey was to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments of gas and dust in our Galaxy. The team detected strong HI absorption (in the spectrum of background radio sources) along 462 lines of sight, corresponding to an absorption detection rate of 17%.

The figure shows the locations of all 2,714 background radio continuum sources (brighter than 15 mJy at 1.4 GHz) in the GASKAP-HI Pilot survey, overlaid on the GASKAP peak brightness temperature map. The light blue outer boundary shows the GASKAP-HI 250-square-degree observing footprint (11 sources per square degree), the GASS peak brightness is shown in the background outside this footprint. Orange crosses mark 462 lines of sight with absorption detections, gray crosses indicate non-detections (2239), and red dots show cases of saturated absorption (12). The approximate directions toward the LMC and SMC are represented by magenta and green circles respectively. Two local HI filaments are depicted as gray dashed curves (The Reticulum filament in the vertical direction and the Hydrus filament in the horizontal direction).