A neutral hydrogen absorption study of cold gas in the outskirts of the Magellanic Clouds using the GASKAP-HI survey

Abstract

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.

25 pages, 10 figures, accepted for publication in AJ

References in corpus (20)

• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Australian Square Kilometre Array Pathfinder: I. System Description
• Clustering of Local Group distances: publication bias or correlated measurements? I. The Large Magellanic Cloud
• Clustering of Local Group distances: publication bias or correlated measurements? III. The Small Magellanic Cloud
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• Supergiant Shells and Molecular Cloud Formation in the LMC
• The Physical Nature of Neutral Hydrogen Intensity Structure
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• The Magellanic Corona and the formation of the Magellanic Stream
• Introducing TIGRESS-NCR: I. Co-Regulation of the Multiphase Interstellar Medium and Star Formation Rates
• Observations of a Magellanic Corona
• GASKAP-HI Pilot Survey Science I: ASKAP Zoom Observations of HI Emission in the Small Magellanic Cloud
• The Quest for the Missing Dust: II -- Two Orders of Magnitude of Evolution in the Dust-to-Gas Ratio Resolved Within Local Group Galaxies
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• GASKAP-HI Pilot Survey Science III: An unbiased view of cold gas in the Small Magellanic Cloud
• Probing the cold neutral medium through HI emission morphology with the scattering transform
• Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows
• Cold HI ejected into the Magellanic Stream