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 $\text{HCO}^{+}$, HCN, HNC, and C$_2$H 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 find no detections to a 3$\sigma$ significance, despite four sightlines having column densities surpassing the threshold for molecular formation in the diffuse regions of the Milky Way. Here we present our calculations for the upper limits of the column densities of each of these molecular absorption lines, ranging from $3 \times 10^{10}$ to $1 \times 10^{13}$ cm$^{-2}$. The non-detection of $\text{HCO}^{+}$ suggests that at least one of the following is true: (i) $X_{\text{HCO}^{+}{}, \mathrm{MS}}$ is significantly lower than the Milky Way value; (ii) that the widespread diffuse molecular gas observed by Rybarczyk (2022b, ApJ, 928, 79) in the Milky Way’s diffuse interstellar medium (ISM) does not have a direct analogue in the MS; (iii) the HI-to-$\text{H}_{2}$ transition occurs in the MS at a higher surface density in the MS than in the LMC or SMC; or (iv) molecular gas exists in the MS, but only in small, dense clumps.

We present the first unbiased survey of neutral hydrogen absorption in the Small Magellanic Cloud. The survey utilises pilot neutral hydrogen observations with the Australian Square Kilometre Array Pathfinder telescope as part of the Galactic Australian Square Kilometre Array Pathfinder neutral hydrogen project whose dataset has been processed with the Galactic Australian Square Kilometre Array Pathfinder-HI absorption pipeline, also described here. This dataset provides absorption spectra towards 229 continuum sources, a 275% increase in the number of continuum sources previously published in the Small Magellanic Cloud region, as well as an improvement in the quality of absorption spectra over previous surveys of the Small Magellanic Cloud. Our unbiased view, combined with the closely matched beam size between emission and absorption, reveals a lower cold gas faction (11%) than the 2019 ATCA survey of the Small Magellanic Cloud and is more representative of the Small Magellanic Cloud as a whole. We also find that the optical depth varies greatly between the Small Magellanic Cloud’s bar and wing regions. In the bar we find that the optical depth is generally low (correction factor to the optically thin column density assumption of $\mathcal{R}_{\mathrm{HI}} \sim 1.04$ ) but increases linearly with column density. In the wing however, there is a wide scatter in optical depth despite a tighter range of column densities.

We report a search for the NH3 (J,K) = (1,1) inversion line in the Large Magellanic Cloud (LMC) using the Parkes 64–m telescope. Candidate positions were chosen with the help of recent H i data from the Australia Telescope Compact Array and published 12CO data from the Swedish–ESO Submillimetre Telescope. No detections of NH3 in emission were found at the positions surveyed. Upper limits are approximately 25 to 74 mK.

We present preliminary results from the Southern Galactic Plane Survey (SGPS) Test Region and Parkes data. As part of the pilot project for the Southern Galactic Plane Survey, observations of a Test Region (325·5° ≤l ≤ 333·5°; −0·5° ≤ b ≤ 3·5°) were completed in December 1998. Single-dish observations of the full survey region (253° ≤ l ≤ 358 ° |b| ≤ 1°) with the Parkes Radio Telescope were completed in March 2000. We present a sample of SGPS H I data, with particular attention to the smallest-and largest-scale structures seen in absorption and emission, respectively. On the large scale, we detect many prominent H I shells. On the small scale, we note extremely compact, cold clouds seen in H I self-absorption. We explore how these two classes of objects probe opposite ends of the H I spatial power spectrum.

A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (H i) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of H i emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b| < 10°) at all declinations south of δ = +40°, spanning longitudes 167° through 360°to 79° at b = 0°, plus the entire area of the Magellanic Stream and Clouds, a total of 13 020 deg2. The brightness temperature sensitivity will be very good, typically σT≃ 1 K at resolution 30 arcsec and 1 km s−1. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.

The astrophysical drivers for far-infrared spectroscopy of the Galactic interstellar medium using a 15m class telescope on Dome A are compelling. For the diffuse, atomic phase, the most important lines in the far-IR spectrum are OI at 63μm and CII at 158μm. These are the dominant cooling lines of the cool, neutral medium, and they show rich spectral structure in Herschel observations at low latitudes. But theory predicts that they should both be highly sub-thermal in excitation, so that the level populations are not in equilibrium with the kinetic temperature of the gas. A large single dish telescope or an interferometer may be able to study the absorption and emission to determine the optical depth and column density of atoms and the physical conditions in the emission regions. Comparison of Herschel CII spectra with 21-cm absorption spectra indicates that a significant fraction of the 158μm flux may be coming from the atomic rather than the molecular phase.

The history of our understanding of the interstellar medium has a pre-modern era and an early-modern era. Several threads show continuity in observational approach and interpretation, reaching from the first decade of the last century to the present. The first 50 years were a period of pioneering work, done mostly by stellar astronomers, who chanced upon the ISM by a strange variety of pathways.

The Galactic Australian SKA Pathfinder (GASKAP) survey is one of several key science projects with ASKAP, a new radio telescope being built in Australia as a technology demonstrator for the Square Kilometer Array (SKA). GASKAP aims to survey about 12,779 square degrees of the Galaxy and the Magellanic System, at high spectral resolution (0.2 km s−1) and using several wavelengths: the λ21-cm HI line, the λ18-cm OH lines, and the comb of recombination lines around λ18-cm. The area covered by GASKAP includes all of the Galactic plane south of declination +40° with |b| < 10°, selected areas at higher latitudes covering important interstellar clouds in the disk and halo, the Large and Small Magellanic Clouds, and the Magellanic Bridge and Stream. Compared with previous surveys, GASKAP will achieve an order of magnitude or greater improvement in brightness sensitivity and resolution in various combinations of beam size and mapping speed matched to the astrophysical objectives.

The warm neutral medium, warm ionized medium, and cool neutral medium all show strong evidence for turbulence as a process dominating their structure and motions on a wide range of scales. The spatial power spectra of density fluctuations in all three phases are consistent with a Kolmogorov slope. Turbulence in the magnetic field in the diffuse medium can also be measured through the structure function of the Faraday rotation measure. With new surveys, new analysis techniques, and new telescopes, in the next few years it will be possible to measure the structure function of the magnetic field over a similarly wide range of scales. This will give a complete picture of the turbulence as a magneto-acoustic process.

H I 21cm-line self-absorption (HISA) reveals the shape and distribution of cold atomic clouds in the Galactic disk. Many of these clouds lack corresponding CO emission, despite being colder than purely atomic gas in equilibrium models. HISA requires background line emission at the same velocity, hence mechanisms that can produce such backgrounds. Weak, small-scale, and widespread absorption is likely to arise from turbulent eddies, while strong, large-scale absorption appears organized in cloud complexes along spiral arm shocks. In the latter, the gas may be evolving from an atomic to a molecular state prior to star formation, which would account for the incomplete HISA-CO agreement.

Surveys of λ21-cm absorption in the Magellanic System show that the cool phase of the HI is less abundant in the SMC than in the Milky Way, and may be so also in the LMC. The typical cool cloud temperature is colder than in the Milky Way, 30 to 40 K rather than 60 to 75 K. The lower abundance of cool phase HI can be traced to the lower heavy element abundances in the Magellanic environment. The cooler cloud temperatures are somewhat mysterious.

The velocity dispersion of the interstellar gas measured perpendicular to the disk in face-on spiral galaxies shows a remarkable consistency. Typically the width of the velocity distribution decreases monotonically with increasing galactic radius through the luminous, stellar disk; but in the outer disk where there is gas but no stars the velocity dispersion is συ ≃ 7 km s−1 with a scatter of 2 km s–1 at most from one position to another and from one galaxy to another. Trying to understand this number raises questions about the thermal state of the H I in outer, gaseous disks. The most important issue is to understand the dominant heating and cooling processes for the atomic gas.

The abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

The interstellar medium shows turbulence whose velocity and density spectra are surprisingly like a Kolmogoroff law, in spite of the fact that the turbulence is generated mainly by microscopic explosions, rather than a cascade of energy from larger to smaller scales.

In 1981 we conducted an extensive, low-latitutde, HI 21-cm absorption survey from the VLA. The goal of the survey was to answer the following questions:

(a) What is the vertical distribution of the cold HI clouds?

(b) Do HI clouds get systematically warmer at high |z|?

(c) Does the fraction of warm intercloud medium HI increase with high |z|?

(d) How much HI has been missed in previous HI emission surveys because of optical depth effects?

(e) What is the galactic distribution of cool HI clouds?

By observing the difference in optical depths between absorption spectra toward the two components of double sources we have measured the variations in opacity over lengths of less than 0.1 up to 10 pc inside diffuse interstellar clouds. Significant variations are detected on scales larger than about 0.2 pc, but not less. This may represent the minimum size for diffuse cloud structure. By comparing the variations of Gaussian fitted line parameters we find that variations in the internal velocity field of diffuse clouds explain the data rather better than tiny independent “cloudlets.”