Kavli Affiliate: Susan E. Clark
| First 5 Authors: Hiep Nguyen, N. M. McClure-Griffiths, James Dempsey, John M. Dickey, Min-Young Lee
| Summary:
We present the largest Galactic neutral hydrogen HI absorption survey to
date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented
spatial resolution of 30”. This survey, GASKAP-HI, unbiasedly targets 2,714
continuum background sources over 250 square degrees in the direction of the
Magellanic Clouds, a significant increase compared to a total of 373 sources
observed by previous Galactic absorption surveys across the entire Milky Way.
We aim to investigate the physical properties of cold (CNM) and warm (WNM)
neutral atomic gas in the Milky Way foreground, characterized by two prominent
filaments at high Galactic latitudes (between $-45^{circ}$ and $-25^{circ}$).
We detected strong HI absorption along 462 lines of sight above the 3$sigma$
threshold, achieving an absorption detection rate of 17%. GASKAP-HI’s
unprecedented angular resolution allows for simultaneous absorption and
emission measurements to sample almost the same gas clouds along a line of
sight. A joint Gaussian decomposition is then applied to absorption-emission
spectra to provide direct estimates of HI optical depths, temperatures, and
column densities for the CNM and WNM components. The thermal properties of CNM
components are consistent 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. 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. Our analysis reveals
an anti-correlation between CNM temperature and its optical depth, which
implies that CNM with lower optical depth leads to a higher temperature.
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