We present observations of the four $^2 \Pi _{3/2}\,J=3/2$ ground-rotational state transitions of the hydroxyl molecule (OH) along 107 lines of sight both in and out of the Galactic plane: 92 sets of observations from the Arecibo telescope and 15 sets of observations from the Australia Telescope Compact Array (ATCA). Our Arecibo observations included off-source pointings, allowing us to measure excitation temperature ($T_{\rm ex}$) and optical depth, while our ATCA observations give optical depth only. We perform Gaussian decomposition using the Automated Molecular Excitation Bayesian line-fitting Algorithm ‘Amoeba’ (Petzler, Dawson, & Wardle 2021, ApJ, 923, 261) fitting all four transitions simultaneously with shared centroid velocity and width. We identify 109 features across 38 sightlines (including 58 detections along 27 sightlines with excitation temperature measurements). While the main lines at 1665 and 1667 MHz tend to have similar excitation temperatures (median $|\Delta T_{\rm ex}({\rm main})|=0.6\,$K, 84% show $|\Delta T_{\rm ex}({\rm main})|<2\,$K), large differences in the 1612 and 1720 MHz satellite line excitation temperatures show that the gas is generally not in LTE. For a selection of sightlines, we compare our OH features to associated (on-sky and in velocity) Hi cold gas components (CNM) identified by Nguyen et al. (2019, ApJ, 880, 141) and find no strong correlations. We speculate that this may indicate an effective decoupling of the molecular gas from the CNM once it accumulates.

We examine the physical conditions required for the formation of H2 in the solar neighborhood by comparing H i emission and absorption spectra toward 58 lines of sight at b < −5∘ to CO(1–0) and dust data. Our analysis of CO-associated cold and warm neutral medium (CNM and WNM) shows that the formation of CO-traced molecular gas is favored in regions with high column densities where the CNM becomes colder and more abundant. In addition, our comparison to the one-dimensional steady-state H i-to-H2 transition model of Bialy et al. (2016) suggests that only a small fraction of the clumpy CNM participates in the formation of CO-traced molecular gas. Another possible interpretation would be that missing physical and chemical processes in the model could play an important role in H2 formation.

Emission lines from CO and Hi are the standard tracers of molecular and atomic interstellar medium, respectively. In the past two decades, a consensus has formed that a substantial fraction of Galactic molecular gas evades detection by these two tracers, thus giving rise to the empirical concept of dark molecular gas (DMG). Largely based on the experience and evidence garnered from the Arecibo Millennium survey, we have formed an international consortium, the Pacific Rim Interstellar Matter Observers (PRIMO), to pursue alternative tracers of DMG, particularly absorption against background radio sources (quasars). PRIMO have carried out observing programs at Arecibo, JVLA, Delingha 13.7m, ATCA and ALMA, among others. Our observations reveal abundant hydrides, namely OH and CH, in DMG clouds. The historical difficulty of mapping OH can be explained by the measured OH excitation temperature $f(T_{{\rm{ex}}}^{{\rm{OH}}}) \propto \frac{1}{{\sqrt {2\pi } \sigma }}{\rm{exp}}[ - {(ln(T_{ex}^{OH}) - ln(3.4\;K))^2}/(2{\sigma ^2})],$

which is a modified log-normal function peaking close to the numerical value of the L-band Galactic continuum background (synchrotron + CMB). Both OH and CH are shown to be better tracers of molecular hydrogen than CO, particularly in the intermediate extinction regions (Av ~ 0.05-2 magnitude), where DMG dominates. http://ism.bao.ac.cn/primo

The I-GALFA survey is mapping HI 21 cm emission in the inner parts of our Milky Way Galaxy using the Arecibo L-band Feed Array (ALFA). Examples of various H i features such as supershells and chimneys are shown.

Many Zeeman-spitting measurements in the diffuse Galactic Interstellar Medium have been made of the 21-cm line in both absorption and emission. Typical field strength is about 6 μG, with enhancement in shocked regions; the magnetic, turbulent, and cosmic-ray pressures are comparable and considerably larger than the thermal gas pressure. For PhotoDissociation Regions, Carbon recombination lines show intriguing results for single-dish measurements. OH Megamasers in Ultra-Luminous Infrared Galaxies show easily detectable fields whose strength ranges up to at least 20 mG. Upper limits for several damped Ly-α systems range down to a few μG. The z=0.692 system against 3C286 was reported in the literature to have a large field strength, but this result is wrong.

The low masses of irregular galaxies change the behavior of their interstellar medium (ISM) compared to that of normal spirals, so the role of magnetic fields in the ISM in irregulars may be very different than in spirals. We present high-resolution and high-sensitivity observations of the magnetic fields of two irregular galaxies: NGC 4214 and NGC 1569.

An ongoing search for Zeeman splitting in the 1667 MHz OH megamaser emission from luminous star-forming galaxies has yielded numerous detections. These results, in addition to being the first extragalactic measurement of the Zeeman effect in an emission line, suggest that OH megamasers are excellent extragalactic magnetometers. We review the progress of our survey and discuss future observations.

We begin with a brief review of Zeeman-splitting fundamentals and the importance of circular polarization, i.e. Stokes V. We then turn to modern results in several areas, emphasizing the diffuse interstellar medium in the Galaxy. The median field in the Cold Neutral Medium is determined from HI absorption lines and is about 6 μG; the magnetic and turbulent pressures are comparable. Using HI emission lines the field has been mapped in several areas: the field reverses across the Orion Molecular Cloud; the 3-d field structure has been determined in the ρ Oph region; and in regions having shock-like morphology the field is generally stronger, strong enough to limit further compression. We briefly present new field measurements for: photo-dissociation regions at the edges of HII regions, determined from carbon recombination lines; Ultra Luminous Infrared Galaxies, from OH megamasers; and the 3C 286 damped Lyman-α absorption system, determined from the 21-cm line in absorption. We show the sidelobe response of the Green Bank Telescope, which is surprisingly severe and makes the telescope less than optimum for Zeeman-splitting measurements of HI emission lines. Finally, we compare two techniques for determining field strengths, i.e. Zeeman splitting and the Chandrasekhar-Fermi method, and show why the latter usually gives higher field strengths – and sometimes unrealistically high fields.

As a part of the ongoing H i survey by the consortium for Galactic studies with the Arecibo L-band Feed Array (GALFA-HI), we have recently imaged the tip of the MS and found several long filamentary structures. This demonstrates that the northern portion of the MS, which has been interacting with the Galactic halo for a long time, is more extended than previously thought and in the form of highly organized H i structures. The observed filaments, and especially the kinematic dichotomy of H i clouds observed for the first time, agree with predictions by the Connors, Kawata & Gibson (2006) tidal model. However, specific time-stamps in the history of the Magellanic System are required to explain these phenomena. The 20-degree long filaments are accompanied by a large population of small H i clouds. We investigate the observed properties of these clouds and explore various instabilities that affect a warm tail of gas trailing through the Galactic halo. Interestingly, if the observed H i structure is mainly due to thermal instability, then the tip of the MS is at a distance of ~70 kpc.

The magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars (Beck 2005). The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, i.e., Faraday rotation, yield an average value B ≈ 3 μG (Han et al. 2006). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars (Kronberg et al. 2008) suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain.

Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z =0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy (Heiles et al. 2004). This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past, rather than stronger (Parker 1970).

The full text of this paper was published in Nature (Wolfe et al. 2008).

We detected significant Zeeman splitting in the 1667 MHz OH megamaser emission from four ultraluminous galaxies. These detections, in addition to being the first extragalactic detection of the Zeeman effect in an emission line, suggest that OH megamasers are excellent extragalactic magnetometers.

There are almost no direct observational indicators of the magnetic field inside the local bubble. Just outside the bubble, the best tracers are stellar polarization and HI Zeeman splitting. These show that the local field does not follow the large-scale Galactic field. Here we discuss whether the deformation of the large-scale field by the local HI shells is consistent with the observations. We concentrate on the Loop 1 region, and find that the field lines are well-explained by this idea; in addition, the bright radio filaments of Radio Loop 1 delineate particular field lines that are “lit up” by an excess of relativistic electrons.

This paper concentrates on reviewing magnetic fields in dense regions; see Heiles (1988, 90) for a review of fields in diffuse regions. The past few years have increased our observational knowledge of magnetic fields in dense regions by an enormous factor—not because there are many measurements, but because we started from zero. The observable is polarization, which is small and subject to systematic errors. The fact that the advances have occurred only recently is a result of several factors: technological development; the interest and commitment of experimentally-minded astronomers; and the maturing of molecular and infrared astronomy to the point that really new results require either new insights or more difficult techniques.

An impressively large amount of data was presented at this meeting and a review by an ordinary human cannot hope to do full justice to either the authors or the research. Accordingly, I shall concentrate on those aspects that particularly piqued my interest and apologize to those authors whom I overlook. I shall give references in the usual style except for references to papers presented at this meeting, for which I shall simply mention the author(s) with no date.

More than 300 measurements of magnetic field strengths in H I regions now exist. Interpretation of about 100 shows that magnetic pressure either is comparable with or dominates other pressures near at least some dark clouds and in at least some H I shells. There appears to be direct evidence for Alfvén waves, but this needs to be confirmed by additional examples. In many regions the field is highly uniform, but in other regions it varies considerably over 30 arcminutes or less. The field directions derived from Zeeman splitting and from Faraday rotation do not correlate well.

Prerequisite to the determination of the Galactic and Extragalactic contributions to the infrared background is an accurate removal of the zodiacal emission. The all-sky observations by IRAS in 1983 were made with a variety of observing conditions and with higher sensitivity and angular resolution than previous infrared and optical observations of the zodiacal background. These facts alone mandate a better understanding of the optical properties and spatial distribution of interplanetary dust.

Recent developments, both observational and theoretical, require a reevaluation of the effects of clustered supernovae on the two-dimensional porosity parameter Q2D and the rates of mass injection into the halo Ṁ of both cold and hot gas. Clustered supernovae produce two types of bubble. Most clusters produce breakthrough bubbles, which do no more than break through the dense gas disk. But large clusters produce enough energy to make blowout bubbles, which blow gas up into the halo. We calculate area filling factors and mass injection rates into the halo for different types of galaxy. We relate our calculations to two observables, the area covered by H I ‘holes’ and the area covered by giant H II regions. We also reiterate the difficulty of producing the very largest supershells by clusered supernovae.

At the conclusion of IAU Colloquium 101 a panel discussion was held. The panellists were asked to give their opinions on significant new developments presented at the Colloquium and to outline questions to be answered in the future. The editors have summarized the discussion.

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?