Kavli Affiliate: Susan E. Clark
| First 5 Authors: Hongxing Chen, Snežana Stanimirović, Nickolas M. Pingel, James Dempsey, Frances Buckland-Willis
| Summary:
Cold neutral hydrogen (HI) is a crucial precursor for molecular gas formation
and can be studied via HI absorption. This study investigates HI absorption in
low column density regions of the Small and Large Magellanic Clouds (SMC and
LMC) using the Galactic-ASKAP HI (GASKAP-HI) survey, conducted by the
Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC
directions in the outer regions and 18 LMC directions, with 4 in the outskirts
and 14 within the main disk. Using the radiative transfer method, we decompose
the emission and absorption spectra into individual cold neutral medium (CNM)
and warm neutral medium (WNM) components. In the SMC, we find HI peak optical
depths of 0.09-1.16, spin temperatures of ~20-50 K, and CNM fractions of 1-11%.
In the LMC, optical depths range from 0.03 to 3.55, spin temperatures from ~10
to 100 K, and CNM fractions from 1% to 100%. The SMC’s low CNM fractions likely
result from its low metallicity and large line-of-sight depth. Additionally,
the SMC’s outskirts show lower CNM fractions than the main body, potentially
due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell
motions dominate the kinematics of the majority of CNM clouds in this study and
likely supply cold HI to the Magellanic Stream. In the LMC, high CNM fraction
clouds are found near supergiant shells, where thermal instability induced by
stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide
(CO) has been detected, enhanced dust shielding in these areas helps maintain
the cold HI.
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