Multi-phase HI clouds in the Small Magellanic Cloud halo

¹ Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 24 rue Lhomond, 75005 Paris Cedex 05, France
² AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
³ Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
⁴ School of Mathematical and Physical Sciences and Astrophysics and Space Technologies Research Centre, Macquarie University, 2109, NSW, Australia
⁵ Australia Telescope National Facility, CSIRO Space and Astronomy, PO Box 76, Epping NSW 1710, Australia
⁶ School of Physical Sciences and Nanotechnology, Yachay Tech University, Hacienda San José S/N, 100119, Urcuquí, Ecuador
⁷ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy
⁸ School of Natural Sciences, Private Bag 37, University of Tasmania, Hobart, TAS, 7001, Australia
⁹ Department of Physics & Astronomy, Western Kentucky University, 1906 College Heights Blvd., Bowling Green, KY 42101, USA
¹⁰ International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley 6102, WA, Australia
¹¹ CSIRO Space and Astronomy, 26 Dick Perry Avenue, Kensington, 6151, WA, Australia
¹² Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Department of Astronomy, University of Wisconsin, Madison, WI 53706-15821, USA
¹⁵ Lennard-Jones Laboratories, Keele University, ST5 5BG, UK

^★ Corresponding author; francesbw.astronomy@outlook.com

Received: 17 October 2024
Accepted: 28 November 2024

Abstract

Context. The Galactic ASKAP collaboration (GASKAP) is undertaking an HI emission survey of the 21cm line to map the Magellanic system and the Galactic plane with the Australian Square Kilometre Array Pathfinder (ASKAP). One of the first areas observed in the Pilot Phase I of the survey was the Small Magellanic Cloud (SMC). Previous surveys of the SMC have uncovered new structures in the periphery of the SMC, along relatively low column density lines of sight.

Aims. In this work we aimed to uncover the phase distribution of three distinct structures in the periphery of the SMC. This work will add to the constraints we have on the existence and survival of the cold neutral medium (CNM) in the SMC.

Methods. We used ROHSA, a Gaussian decomposition algorithm, to model the emission across each cloud and classify the HI emission into their respective phases based on the linewidths of the fitted Gaussians. We created maps of velocity and column density of each phase of the HI across these three clouds. We measured the HI mass and CNM number density for each cloud. We also compared the HI results across the different phases with other gas tracers.

Results. We find that in two clouds, the ends of each cloud are almost completely CNM dominated. Analysis of these two clouds indicates they are experiencing a compressive force from the direction of the SMC main body. In the third cloud we find a uniform CNM distribution along one wall of what is likely a supershell structure. Comparison with previous measurements of CO clumps in two of the clouds show the CO and HI are co-moving within a few km s⁻¹ in regions of high HI column density, particularly when considering just the CNM.