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We present the largest Galactic neutral hydrogen H i absorption survey to date, utilizing the Australian SKA Pathfinder Telescope at an unprecedented spatial resolution of 30 arcsec. This survey, GASKAP-H i , unbiasedly targets 2714 continuum background sources over 250 square degrees in the direction of the Magellanic Clouds, a significant increase compared to a total of 373 sources observed by previous Galactic absorption surveys across the entire Milky Way. We aim to investigate the physical properties of cold (CNM) and warm (WNM) neutral atomic gas in the Milky Way foreground, characterized by two prominent filaments at high Galactic latitudes (between |$-45^{\circ }$| and |$-25^{\circ }$|⁠). We detected strong H i absorption along 462 lines of sight above the 3 |$\sigma$| threshold, achieving an absorption detection rate of 17 per cent. GASKAP-H i 's unprecedented angular resolution allows for simultaneous absorption and emission measurements to sample almost the same gas clouds along a line of sight. A joint Gaussian decomposition is then applied to absorption-emission spectra to provide direct estimates of H i optical depths, temperatures, and column densities for the CNM and WNM components. The thermal properties of CNM components are consistent with those previously observed along a wide range of Solar neighbourhood environments, indicating that cold H i properties are widely prevalent throughout the local interstellar medium. Across our region of interest, CNM accounts for |$\sim$| 30 per cent of the total H i gas, with the CNM fraction increasing with column density towards the two filaments. Our analysis reveals an anticorrelation between CNM temperature and its optical depth, which implies that CNM with lower optical depth leads to a higher temperature. [ABSTRACT FROM AUTHOR]

Neutral hydrogen (H i) bubbles and shells are common in the interstellar medium (ISM). Studying their properties provides insight into the characteristics of the local ISM as well as the galaxy in which the bubbles reside. We report the detection of magnetic fields associated with superbubbles in the nearby irregular galaxy, the Small Magellanic Cloud (SMC). Using the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) pilot survey, we obtain a high-density grid (⁠|$\approx 25 \, \rm sources\, deg^{-2}$|⁠) of Faraday rotation measure (RM) from polarized sources behind the SMC. This provides a sufficiently large number of RM measurements to study the magnetic properties of three of the largest H i shells previously identified in the SMC. The RM profiles as a function of distance from the shell centre show characteristic patterns at angular scales comparable to the shell size. We demonstrate that this can be explained by magnetohydrodynamic simulation models of bubbles expanding in magnetized environments. From the observations, we estimate the line-of-sight magnetic field strength at the edges of the shells is enhanced by |$\sim 1\, \rm \mu G$| with respect to their centres. This is an order of magnitude larger than the field strength in the ambient medium (⁠|$\sim 0.1\, \rm \mu G$|⁠) estimated based on the expansion velocity of the shells. This paper highlights the power of densely mapped RM grids in studying the magnetic properties of galactic substructures beyond the Milky Way. [ABSTRACT FROM AUTHOR]

We present initial results from the Polarization Sky Survey of the Universe's Magnetism (POSSUM), analysing 22 817 Faraday rotation measures (RMs) with median uncertainties of 1.2 rad m |$^{-2}$| across 1520 deg2 to study magnetized gas associated with 55 nearby galaxy groups (⁠|$z\lesssim 0.025$|⁠) with halo masses between |$10^{12.5}$| and |$10^{14.0}$| M |$_\odot$|⁠. We identify two distinct gas phases: the intragroup medium (IGrM) within 0–2 splashback radii and the warm-hot intergalactic medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e. Galactic foreground RM-subtracted) RMs by |$6.9\pm 1.8$| rad m |$^{-2}$| and |$4.2 \pm 1.2$| rad m |$^{-2}$|⁠ , respectively. Estimated magnetic field strengths are several μ G within the IGrM and 0.1–1 μ G in the WHIM. We estimate the plasma |$\beta$| in both phases, and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that 'missing baryons' in the WHIM likely extend beyond the gravitational radii of group-mass haloes to Mpc scales, consistent with large-scale, outflow-driven 'magnetized bubbles' seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetized thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel'dovich effect methods, and when combined with fast radio burst dispersion measures, data from the full POSSUM survey – comprising approximately a million RMs – will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetized universe. [ABSTRACT FROM AUTHOR]

The interstellar medium (ISM) is ubiquitously turbulent across many physically distinct environments within the Galaxy. Turbulence is key in controlling the structure and dynamics of the ISM, regulating star formation, and transporting metals within the Galaxy. We present the first observational measurements of turbulence in neutral hydrogen entrained in the hot nuclear wind of the Milky Way. Using recent MeerKAT observations of two extra-planar H i clouds above (gal. lat. |$\, \sim 7.0^{\circ }$|⁠) and below (gal. lat. |$\, \sim -3.9^{\circ }$|⁠) the Galactic disc, we analyse centroid velocity and column density maps to estimate the velocity dispersion (σ v ,3D), the turbulent sonic Mach number (⁠|$\mathcal {M}$|⁠), the volume density dispersion (⁠|$\sigma _{\rho /\rho _0}$|⁠), and the turbulence driving parameter (b). We also present a new prescription for estimating the spatial temperature variations of H i in the presence of related molecular gas. We measure these turbulence quantities on the global scale of each cloud, but also spatially map their variation across the plane-of-sky extent of each cloud by using a roving kernel method. We find that the two clouds share very similar characteristics of their internal turbulence, despite their varying latitudes. Both clouds are in the sub-to-trans-sonic Mach regime, and have primarily compressively driven (b ∼ 1) turbulence. Given that there is no known active star formation present in these clouds, this may be indicative of the way the cloud–wind interaction injects energy into the entrained atomic material on parsec scales. [ABSTRACT FROM AUTHOR]

Faraday rotation measures (RMs) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sightlines. We introduce a simple test using Galactic diffuse polarized emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium or foreground interstellar medium. We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magellanic Leading Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow depolarized filaments in the |$2.3\, \rm GHz$|   S -band Polarization All Sky Survey image that overlaps with the reported RM excess. We suggest that there is a steep gradient in Faraday rotation in a foreground screen arising from the Antlia SNR. The estimated strength of the line-of-sight component of the magnetic field is |$B_{\parallel }\sim 5\, \rm \mu G$|⁠ , assuming that the excess of RM is entirely an outcome of the magnetized supernova shell. Our analysis indicates that the overlap between the RM excess and the Magellanic Leading Arm is only a remarkable coincidence. We suggest for future RM grid studies that checking Galactic diffuse polarization maps is a convenient way to identify local Faraday screens. [ABSTRACT FROM AUTHOR]

We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40′′ and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm and the terminal velocities of the Molecular Ring, around l ≈ 28° and vLSR ≈ 100 km s−1, (ii) toward l ≈ 45° and vLSR ≈ 50 km s−1, (iii) around the Riegel-Crutcher cloud, and (iv) toward the positive and negative terminal velocities. A comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of large-scale Galactic dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behavior of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe. [ABSTRACT FROM AUTHOR]

Sharpless 2–27 (Sh2–27) is a nearby H  ii region excited by ζOph. We present observations of polarized radio emission from 300 to 480 MHz towards Sh2–27, made with the Parkes 64 m Radio Telescope as part of the Global Magneto-Ionic Medium Survey. These observations have an angular resolution of 1.35°, and the data are uniquely sensitive to magneto-ionic structure on large angular scales. We demonstrate that background polarized emission towards Sh2–27 is totally depolarized in our observations, allowing us to investigate the foreground. We analyse the results of Faraday tomography, mapping the magnetized interstellar medium along the 165 pc path to Sh2–27. The Faraday dispersion function in this direction has peaks at three Faraday depths. We consider both Faraday thick and thin models for this observation, finding that the thin model is preferred. We further model this as Faraday rotation of diffuse synchrotron emission in the Local Bubble and in two foreground neutral clouds. The Local Bubble extends for 80 pc in this direction, and we find a Faraday depth of |$-0.8 \pm 0.4\,$| rad m−2. This indicates a field directed away from the Sun with a strength of |$-2.5\pm 1.2\, \mu$| G. The near and far neutral clouds are each about 30 pc thick, and we find Faraday depths of |$-6.6\pm 0.6\,$| rad m−2 and |$+13.7\pm 0.8\,$| rad m−2, respectively. We estimate that the line-of-sight magnetic strengths in the near and far cloud are |$B_{\parallel , \text{near}} \approx -15\, \mu \text{G}$| and |$B_{\parallel , \text{far}} \approx +30\, \mu \text{G}$|⁠. Our results demonstrate that Faraday tomography can be used to investigate the magneto-ionic properties of foreground features in front of nearby H  ii regions. [ABSTRACT FROM AUTHOR]

H i self-absorption (HISA) clouds are clumps of cold neutral hydrogen (H i) visible in front of warm background gas, which makes them ideal places to study the properties of the cold atomic component of the interstellar medium. The Riegel–Crutcher (R–C) cloud is the most striking HISA feature in the Galaxy. It is one of the closest HISA clouds to us and is located in the direction of the Galactic Centre, which provides a bright background. High-resolution interferometric measurements have revealed the filamentary structure of this cloud; however, it is difficult to accurately determine the temperature and the density of the gas without optical depth measurements. In this paper, we present new H i absorption observations with the Australia Telescope Compact Array against 46 continuum sources behind the R–C cloud to directly measure the optical depth of the cloud. We decompose the complex H i absorption spectra into Gaussian components using an automated machine learning algorithm. We find 300 Gaussian components, from which 67 are associated with the R–C cloud (0 < $v$ LSR < 10 km s−1,  full width at half maximum < 10 km s−1). Combining the new H i absorption data with H i emission data from previous surveys, we calculate the spin temperature and find it to be between 20 and 80 K. Our measurements uncover a temperature gradient across the cloud with spin temperatures decreasing towards positive Galactic latitudes. We also find three new OH absorption lines associated with the cloud, which support the presence of molecular gas. [ABSTRACT FROM AUTHOR]

We present new parsec-scale resolution data from a multiphase study of the ISM in the walls of Galactic supershells. HI synthesis images and CO survey data reveal a wealth of substructure, including dense-tipped fingers and extended molecular clouds embedded in shell walls. We briefly consider formation scenarios for these features, and suggest that both the interaction of an expanding shell with pre-existing dense clouds, as well as in-situ formation of CNM and molecular gas, are likely to be relevant. Future work will also examine the role of instabilities in structure formation and breakup, and will investigate the presence of high-altitude gas associated with supershells and chimneys. [ABSTRACT FROM AUTHOR]

Bubbles and Superbubbles are among the largest and most energetic structures in the Interstellar Medium (ISM). It is thought that most bubbles are formed from the combined effects of stellar winds and supernovae, which plow through the ISM, sweeping up matter in to dense atomic hydrogen (H I) shell walls and heating and ionising the gas left behind. Although this explanation works clearly for some of the smaller objects, it is hard to understand how many superbubbles apparently require hundreds to thousands of massive stars to explain their sizes. In some galaxies, like the Large Magellanic Cloud, the H I structure of the disk is dominated by H I shells. Similarly, the ISM of the Milky Way is riddled with tens, perhaps hundreds, of H I shells. In the Milky Way, as well as in the nearest disk galaxies, there is clear evidence of superbubbles that have grown so large as to break out of the disk. It would seem, then, that not only do these objects have a significant impact on the disk, they provide an invaluable source of energy for the halo. In this review I present some of the latest results on bubbles and superbubbles in disk galaxies and their relationship to the halo. [ABSTRACT FROM AUTHOR]

RX J 1713.7-3946 is the most remarkable TeV γ-ray supernova remnant (SNR) that emits γ-rays in the highest energy range. We have made a new combined analysis of CO and H I in the SNR and derived the total protons in the interstellar medium (ISM). We have found that the inclusion of the H I gas provides a significantly better spatial match between the TeV γ-rays and ISM protons than the H2 gas alone. In particular, the southeastern rim of the γ-ray shell has a counterpart only in the H I. The finding shows that the ISM proton distribution is consistent with the hadronic scenario that cosmic-ray (CR) protons react with ISM protons to produce the γ-rays. This provides another step forward for the hadronic origin of the γ-rays by offering one of the necessary conditions missing in the previous hadronic interpretations. We argue that the highly inhomogeneous distribution of the ISM protons is crucial in the origin of the γ-rays. Most of the neutral gas was likely swept up by the stellar wind of an OB star prior to the supernova (SN) explosion to form a low-density cavity and a swept-up dense wall. The cavity explains the low-density site where the diffusive shock acceleration of charged particles takes place with suppressed thermal X-rays, whereas the CR protons can reach the target protons in the wall to produce the γ-rays. The present finding allows us to estimate the total CR proton energy to be ~ 1048 erg, 0.1% of the total energy of the SN explosion. [ABSTRACT FROM AUTHOR]

The interstellar medium of the Milky Way is multiphase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1,000 kilometres (ref. 4). Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine, because observations have lacked the sensitivity and resolution to image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q, U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse, ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm, ionized medium has a relatively low sonic Mach number, Ms???2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions. [ABSTRACT FROM AUTHOR]

We analyse the distribution of the interstellar matter in the environs of the Wolf-Rayet star LSS 3982 (= WR 85, WN6+OB?) linked to the optical ring nebula RCW 118. Our study is based on neutral hydrogen 21-cm line data belonging to the Southern Galactic Plane Survey (SGPS). The analysis of the H i data allowed the identification of a neutral hydrogen interstellar bubble related to WR 85 and the 25-arcmin-diameter ring nebula RCW 118. The H i bubble was detected at a systemic velocity of −21.5 km s−1, corresponding to a kinematical distance of 2.8 ± 1.1 kpc, compatible with the stellar distance. The neutral structure is about 25 arcmin in radius or 21 ± 8 pc, and is expanding at 9 ± 2 km s−1. The associated ionized and neutral masses amount to . The carbon monoxide (CO) emission distribution depicts a region lacking CO coincident in position and velocity with the H i structure. The 9.3-arcmin-diameter inner optical nebula appears to be related to the approaching part of the neutral atomic shell. The H i void and shell are the neutral gas counterparts of the optical bubble and have very probably originated in the action of the strong stellar wind of the central star during the O-type and WR phases on the surrounding interstellar medium. The H i bubble appears to be in the momentum conserving stage. [ABSTRACT FROM AUTHOR]