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Atomic hydrogen (Hi) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive Hi absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of Hi, its structure, and its association with magnetic fields. We give an overview of the Hi phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: ▪ The mass fraction of the cold neutral medium is ≲40% on average, increasing with A V due to the increase of mean gas density. ▪ The cold disk extends to at least R ∼ 25 kpc. ▪ Approximately 40% of the Hi is warm, with structural characteristics that derive from feedback events. ▪ Cold Hi is highly filamentary, whereas warm Hi is more smoothly distributed. We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on Hi properties. 

The Magellanic Stream (MS), a tail of diffuse gas formed from tidal and ram pressure interactions between the Small and Large Magellanic Clouds (SMC and LMC) and the Halo of the Milky Way, is primarily composed of neutral atomic hydrogen (HI). The deficiency of dust and the diffuse nature of the present gas make molecular formation rare and difficult, but if present, could lead to regions potentially suitable for star formation, thereby allowing us to probe conditions of star formation similar to those at high redshifts. We search for HCO+ ,HCN,HNC,andC2H using the highest sensitivity observations of molecular absorption data from the Atacama Large Millimeter Array (ALMA) to trace these regions, comparing with HI archival data from the Galactic Arecibo L-Band Feed Array (GALFA) HI Survey and the Galactic All Sky Survey (GASS) to compare these environments in the MS to the HI column density threshold for molecular formation in the Milky Way. We also compare the line of sight locations with confirmed locations of stars, molecular hydrogen, and OI detections, though at higher sensitivities than the observations presented here. 

We present observations of linear polarisation in the southern radio lobe of Centaurus A, conducted during commissioning of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square degree region in a single 12-h pointing over a 240 MHz band centred on 913 MHz. Our observations achieve an angular resolution of 26 × 33 arcseconds (480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a full-band sensitivity of 85 μ Jy beam − 1 . The resulting maps of polarisation and Faraday rotation are amongst the most detailed ever made for radio lobes, with order 10 5 resolution elements covering the source. We describe several as-yet unreported observational features of the lobe, including its detailed peak Faraday depth structure, and intricate networks of depolarised filaments. These results demonstrate the exciting capabilities of ASKAP for widefield radio polarimetry.