Research Byte

Published in the RSAA Lunations
Vol1 Issue56 1–31 October 2024

GASKAP-HI, the Galactic Australian SKA Pathfinder (ASKAP) survey, is a high-resolution study of the HI (21 cm) and OH (18 cm) lines in the interstellar medium (ISM) of the Milky Way and Magellanic Systems. In this study, "Local HI Absorption towards the Magellanic Cloud foreground using ASKAP," we conducted an extensive analysis of the local neutral ISM using the largest Galactic HI absorption survey to date with the ASKAP Telescope at unprecedented high spatial resolution (30 arcsec).

We aimed to investigate the physical properties of cold (CNM) and warm (WNM) HI gas in the Solar neighborhood at high Galactic latitudes (−45, −25 degrees) toward the Magellanic Cloud foreground. The surveyed area encompasses the intersection of two prominent HI filaments: the vertical Reticulum filament and the horizontal Hydrus filament (see Figure 1).

Through the GASKAP-Hi survey, we measured the HI absorption along 2714 continuum background sources, achieving ~12 absorption measurements per square degree. This density of measurements, made possible by the GASKAP large field of view, provides a dense grid of HI absorption lines of sight, approaching the density of emission measurements (though still far from equal). We detected strong HI absorption at the 3-sigma threshold towards 462 background radio continuum sources, yielding an absorption detection rate of 17% and two detections per square degree. The unprecedented 30 arcsec angular resolution, corresponding to a linear scale of 0.04 pc (assuming distance to local gas clouds 𝑑 = 250 pc), enables GASKAP-HI to measure emission extremely close to the absorption, ensuring that the emission and absorption sample similar HI parcels along a given sky direction. By performing joint Gaussian decompositions for all pairs of GASKAP absorption-emission spectra, we directly determined HI optical depths, temperatures, and column densities for cold and warm gas components.

We then examined the correlation between CNM spin temperatures and optical depths, and the dust-gas relationship in Magellanic Clouds’ foreground. Our key conclusions are as follows:

• The thermal properties of cold HI gas in the Magellanic Cloud foreground are in excellent agreement with those previously observed along a wide range of Solar neighborhood environments, indicating that cold HI properties are widely prevalent throughout the local interstellar medium (see Figure 2, bottom).
• Across our region of interest, CNM accounts for ~30% of the total HI gas, with the CNM fraction increasing with column density toward the two filaments.
• We observe an anti-correlation between CNM temperature and its optical depth in the Magellanic Cloud foreground, which implies that CNM with lower optical depths leads to higher temperatures. Including findings from previous high-sensitivity absorption observations, a similar conclusion can be drawn for the CNM across the local Galactic ISM.
• Our findings reveal a general linear relation between HI column density and dust visual extinction, but an excess of dust extinction at higher column density, in agreement with previous results.
• The Galactic radiation field in front of Magellanic Clouds is relatively uniform, resulting in flat distributions when compared to GASKAP mean spin temperatures. Nevertheless, combining our data with those from previous high-sensitivity absorption studies suggests a hint of higher HI spin temperatures in regions with a stronger radiation field.