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The extraplanar diffuse ionised gas (eDIG) is a key component for understanding the feedback processes that connect galactic discs and their halos. In this paper, we present the second study of the Bidimensional Exploration of the warm-Temperature Ionised Gas (BETIS) project, the aim of which is to explore the possible ionisation mechanisms and characteristics of the eDIG. We use a sample of eight edge-on galaxies observed with the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph (IFS) and apply the methodology developed in the first paper of the BETIS project for obtaining binned emission line maps. We find that the vertical and radial profiles of the [N II]/Hα, [S II]/Hα, [O III]/Hβ, and [O I]/Hα ratios depict a complex ionisation structure within galactic halos – which is influenced by the spatial distribution of H II regions across the galactic plane as observed from our line of sight–, with Lyman continuum photon leakage from OB associations constituting the main ionisation source. Moreover, the electron temperature and S+/S ionisation ratio also exhibit a dependency on the distribution of H II regions within the galactic discs. Our analysis excludes low-mass, hot, and evolved stars (HOLMES) as viable candidates for secondary ionisation sources to elucidate the unusual behaviour of the line ratios at greater distances from the galactic midplane. In contrast, we ascertain that shocks induced in the interstellar medium by star formation(SF)-related feedback mechanisms represent a promising secondary ionisation source of the eDIG. We present a suite of models integrating ionisation mechanisms arising from fast shocks and photoionisation associated with star formation. When applied to the classical Baldwin–Phillips–Terlevich (BPT) diagrams, these models reveal that the ionisation budget of the eDIG ranges from 20% to 50% across our sample, with local variations of up to 20% within individual galaxy halos. This contribution correlates with the presence of filaments and other structural components observed within galaxy halos. The presence of shocks is additionally supported by the observation of a high density of high [O I]/Hα ratios, which is characteristic of shock-compressed ionised gas, and is likely induced by feedback from regions of intense SF within the galactic disc. These results demonstrate consistency across all galaxies analysed in this sample. [ABSTRACT FROM AUTHOR]

Context. Magnetohydrodynamic (MHD) turbulence plays a critical role in many key astrophysical processes, such as star formation, acceleration of cosmic rays, and heat conduction. However, its properties are still poorly understood. Aims. In this work, we explore how to extract the intermittency of compressible MHD turbulence from synthetic and real observations. Methods. We used three statistical methods, namely the probability distribution function, kurtosis, and scaling exponent of the multi-order structure function, to reveal the intermittency of MHD turbulence. Results. Our numerical results demonstrate that: (1) the synchrotron polarization intensity statistics can be used to probe the intermittency of magnetic turbulence, by which we can distinguish different turbulence regimes; (2) the intermittency of MHD turbulence is dominated by the slow mode in the sub-Alfvénic turbulence regime; and (3) the Galactic interstellar medium (ISM) in the low latitude region corresponds to the sub-Alfvénic and supersonic turbulence regime. Conclusions.We have successfully measured the intermittency of the Galactic ISM from synthetic and realistic observations. [ABSTRACT FROM AUTHOR]

Context. Predicting the star formation rate (SFR) in galaxies is crucial to understand their evolution and morphology. To do so requires a fine understanding of how dense structures of gas are created and collapse. In that, turbulence and gravity play a major role. Aims. Within the gravo-turbulent framework, we assume that turbulence shapes the interstellar medium (ISM), creating density fluctuations that, if gravitationally unstable, will collapse and form stars. The goal of this work is to quantify how different regimes of turbulence, characterized by the strength and compressibility of the driving, shape the density field. We are interested in the outcome in terms of SFR and how it compares with existing analytical models for the SFR. Methods. We ran a series of hydrodynamical simulations of turbulent gas. The simulations were first conducted without gravity, so that the density and velocity were shaped by the turbulence driving. Gravity was then switched on, and the SFR was measured and compared with analytical models. The physics included in these simulations was very close to the one assumed in the classical gravo-turbulent SFR analytical models, which makes the comparison straightforward. Results. We found that the existing analytical models convincingly agree with simulations at low Mach number, but we measure a much lower SFR in the simulation with a high Mach number. We develop, in a companion paper, an updated physically motivated SFR model that reproduces well the inefficient high Mach regime of the simulations. Conclusions. Our work demonstrates that accurate estimations of the turbulent-driven replenishment time of dense structures and the dense gas spatial distribution are necessary to correctly predict the SFR in the high Mach regime. The inefficient high-Mach regime is a possible explanation for the low SFR found in dense and turbulent environments such as the centre of our Milky Way and other galaxies. [ABSTRACT FROM AUTHOR]

Three-dimensional dust density maps are crucial for understanding the structure of the interstellar medium of the Milky Way and the processes that shape it. However, constructing these maps requires large data sets and the methods used to analyse them are computationally expensive and difficult to scale up. As a result, it has only recently become possible to map kiloparsec-scale regions of our Galaxy at parsec-scale grid sampling. We present all-sky three-dimensional dust density and extinction maps of the Milky Way out to 2.8 kpc in distance from the Sun using the fast and scalable Gaussian Process algorithm Dustribution. The sampling of the three-dimensional map is l, b, d  = 1° × 1° × 1.7 pc. The input extinction and distance catalogue contains 120 million stars with photometry and astrometry from Gaia DR2, 2MASS and AllWISE. This combines the strengths of optical and infrared data to probe deeper into the dusty regions of the Milky Way. We compare our maps with other published 3D dust maps. All maps quantitatively agree at the 0.001 mag pc−1 scale with many qualitatively similar features, although each map also has its own features. We recover Galactic features previously identified in the literature. Moreover, we also see a large under-density that may correspond to an inter-arm or -spur gap towards the Galactic Centre. [ABSTRACT FROM AUTHOR]

To advance our understanding of the evolution of the interstellar medium (ISM) of our Galaxy, numerical models of Milky Way (MW) type galaxies are widely used. However, most models only vaguely resemble the MW (e.g. in total mass), and often use imposed analytic potentials (which cannot evolve dynamically). This poses a problem in asserting their applicability for the interpretation of observations of our own Galaxy. The goal of this work is to identify a numerical model that is not only an MW-type galaxy, but one that can mimic some of the main observed structures of our Galaxy, using dynamically evolving potentials, so that it can be used as a base model to study the ISM cycle in a galaxy like our own. This paper introduces a suite of 15 MW-type galaxy models developed using the arepo numerical code, that are compared to Galactic observations of |$^{12}$| CO and H  i emission via longitude–velocity plots, from where we extract and compare the skeletons of major galactic features and the terminal gas velocities. We found that our best-fitting model to the overall structure, also reproduces some of the more specific observed features of the MW, including a bar with a pattern speed of |$30.0 \pm 0.2$| km s |$^{-1}$|  kpc |$^{-1}$|⁠ , and a bar half-length of |$3.2 \pm 0.8$|  kpc. Our model shows large streaming motions around spiral arms, and strong radial motions well beyond the inner bar. This model highlights the complex motions of a dynamic MW-type galaxy and has the potential to offer valuable insight into how our Galaxy regulates the ISM and star formation. [ABSTRACT FROM AUTHOR]

Recent advancements in 3D dust mapping have transformed our understanding of the Milky Way's local interstellar medium, enabling us to explore its structure in three spatial dimensions for the first time. In this Letter, we use the most recent 3D dust map by Edenhofer et al. to study the well-known Chameleon, Musca, and Coalsack cloud complexes, located about 200 pc from the Sun. We find that these three complexes are not isolated but rather connect to form a surprisingly well-defined half-ring, constituting a single C-shaped cloud with a radius of about 50 pc, a thickness of about 45 pc, and a total mass of about 5 × 104M⊙, or 9 × 104M⊙ if including everything in the vicinity of the C-shaped cloud. Despite the absence of an evident feedback source at its center, the dynamics of young stellar clusters associated with the C structure suggest that a single supernova explosion about 4 Myr–10 Myr ago likely shaped this structure. Our findings support a single origin story for these cloud complexes, suggesting that they were formed by feedback-driven gas compression, and offer new insights into the processes that govern the birth of star-forming clouds in feedback-dominated regions, such as the Scorpius-Centaurus association. [ABSTRACT FROM AUTHOR]

Faraday tomography observations with the Low Frequency Array (LOFAR) have unveiled a remarkable network of structures in polarized synchrotron emission at high Galactic latitudes. The observed correlation between LOFAR structures, dust polarization, and HI emission suggests a connection to the neutral interstellar medium (ISM). We investigated this relationship by estimating the rotation measure (RM) of the warm neutral (partially ionized) medium (WNM) in the local ISM. Our work combines UV spectroscopy from FUSE and dust polarization observations from Planck with LOFAR data. We derived electron column densities from UV absorption spectra toward nine background stars, within the field of published data from the LOFAR two-meter sky survey. The associated RMs were estimated using a local magnetic field model fitted to the dust polarization data of Planck. A comparison with Faraday spectra at the position of the stars suggests that LOFAR structures delineate a slab of magnetized WNM and synchrotron emission, located ahead of the bulk of the warm ionized medium. This conclusion establishes an astrophysical framework for exploring the link between Faraday structures and the dynamics of the magnetized multiphase ISM. It will be possible to test it on a larger sample of stars when maps from the full northern sky survey of LOFAR become available. [ABSTRACT FROM AUTHOR]

Context. We conducted a follow-up study on the analysis of the LOw Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) mosaic in the high-latitude outer Galaxy presented in the first paper of this series. Here, we focus on the search for alignment between the magnetic field traced by dust, HI filaments, starlight optical linear polarisation, and linear depolarised structures (depolarisation canals) observed in low-frequency synchrotron polarisation. This alignment was previously found in several smaller fields observed with LOFAR, offering valuable insights into the nature of the interstellar medium and the 3D spatial distribution of the diffuse ionised medium. Aims. We aim to determine whether the alignment of the interstellar medium (ISM) phases observed through multiple tracers is a common occurrence or an exception. Additionally, in areas where depolarisation canals align with the magnetic field, we use starlight polarisation to constrain the distance to the structures associated with the observed canals. Methods. We employed the Rolling Hough Transform (RHT) and projected Rayleigh statistics (PRS) to identify and quantify the alignment between the different tracers. We used these tools to detect linear features in the data and quantify the significance of the orientation trends between pairs of tracers. Results. On the scale of the whole mosaic, we did not find any evidence of a universal alignment among the three tracers. However, in one particular area, the western region (Dec between 29° and 70° and RA between 7h44m and 9h20m), we do find a significant alignment between the magnetic field, depolarisation canals, and HI filaments. Based on this alignment, we used the starlight polarisation of stars with known parallax distances to estimate that the minimum distance to the structures observed by LOFAR in this region lies within the range of 200 to 240 pc. We associate these structures with the edge of the Local Bubble. [ABSTRACT FROM AUTHOR]

The dynamic spectra of pulsars frequently exhibit diverse interference patterns, often associated with parabolic arcs in the Fourier-transformed (secondary) spectra. Our approach differs from previous ones in two ways: first, we extend beyond the traditional Fresnel–Kirchhoff method by using the Green's function of the Helmholtz equation, i.e. we consider spherical waves originating from three-dimensional space, not from a two-dimensional screen. Secondly, the discrete structures observed in the secondary spectrum result from discrete scatterer configurations, namely plasma concentrations in the interstellar medium, and not from the selection of points by the stationary phase approximation. Through advanced numerical techniques, we model both the dynamic and secondary spectra, providing a comprehensive framework that describes all components of the latter spectra in terms of physical quantities. Additionally, we provide a thorough analytical explanation of the secondary spectrum. [ABSTRACT FROM AUTHOR]

We suggest that filaments in star-forming regions undergo frequent mergers. As stellar nurseries, filaments play a vital role in understanding star formation and mergers could pave the way for understanding the formation of more complex filamentary systems, such as networks and hubs. We compare the physical properties derived from hydrodynamic ramses simulations of merging filaments to those obtained from ALMA observations towards the LDN 1641-North region in Orion. We find similarities in the distributions of line-mass, column density, and velocity dispersion. Such common features support the hypothesis of filament mergers shaping the structure of the interstellar medium. [ABSTRACT FROM AUTHOR]

Widespread, high altitude, diffuse ionized gas with scale heights of around a kiloparsec is observed in the Milky Way and other star-forming galaxies. Numerical radiation-magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium show that gas can be driven to high altitudes above the galactic mid-plane, but the degree of ionization is often less than inferred from observations. For computational expediency, ionizing radiation from massive stars is often included as a post-processing step assuming ionization equilibrium. We extend our simulations of an Milky Way-like interstellar medium to include the combined effect of supernovae and photoionization feedback from mid-plane OB stars and a population of hot evolved low mass stars. The diffuse ionized gas has densities below 0.1 |${\rm \,cm^{-3}}$|⁠ , so recombination time-scales can exceed millions of years. Our simulations now follow the time-dependent ionization and recombination of low density gas. The long recombination time-scales result in diffuse ionized gas that persists at large altitudes long after the deaths of massive stars that produce the vast majority of the ionized gas. The diffuse ionized gas does not exhibit the large variability inherent in simulations that adopt ionization equilibrium. The vertical distribution of neutral and ionized gas is close to what is observed in the Milky Way. The volume filling factor of ionized gas increases with altitude resulting in the scale height of free electrons being larger than that inferred from H  |$\alpha$| emission, thus reconciling the observations of ionized gas made in H  |$\alpha$| and from pulsar dispersion measurements. [ABSTRACT FROM AUTHOR]

Context. High-resolution 3D maps of interstellar dust are critical for probing the underlying physics shaping the structure of the interstellar medium, and for foreground correction of astrophysical observations affected by dust. Aims. We aim to construct a new 3D map of the spatial distribution of interstellar dust extinction out to a distance of 1.25 kpc from the Sun. Methods. We leveraged distance and extinction estimates to 54 million nearby stars derived from the Gaia BP/RP spectra. Using the stellar distance and extinction information, we inferred the spatial distribution of dust extinction. We modeled the logarithmic dust extinction with a Gaussian process in a spherical coordinate system via iterative charted refinement and a correlation kernel inferred in previous work. In total, our posterior has over 661 million degrees of freedom. We probed the posterior distribution using the variational inference method MGVI. Results. Our 3D dust map has an angular resolution of up to 14′ (Nside = 256), and we achieve parsec-scale distance resolution, sampling the dust in 516 logarithmically spaced distance bins spanning 69 pc to 1250 pc. We generated 12 samples from the variational posterior of the 3D dust distribution and release the samples alongside the mean 3D dust map and its corresponding uncertainty. Conclusions. Our map resolves the internal structure of hundreds of molecular clouds in the solar neighborhood and will be broadly useful for studies of star formation, Galactic structure, and young stellar populations. It is available for download in a variety of coordinate systems online and can also be queried via the publicly available dustmaps Python package. [ABSTRACT FROM AUTHOR]

Young Stellar Objects (YSO) are newly formed stars from molecular clouds. They stay close to where they were born and serve as good tracers to study gas and star formation. During cloud evolution, young massive stars can disrupt the surrounding gas through stellar feedback, changing the gas distribution. We study the distribution of the gas around a sample of YSO associations located at |$d \lesssim 3.5 \,\,\rm kpc$| from the Sun by comparing the location and morphology between 12CO (J  = 1–0) emission, Planck 870  |$\mu$| m maps and YSO associations. Based on the spatial distribution of the gas compared to that of the YSOs, we classify the YSO associations into six types: direct, close, bubble, complex, diffuse, and clean associations. The complex associations are large structures consisting of both gas-rich and gas-poor segments. We study the velocity dispersion-size relation towards different association types. From the ratio between different types, we estimate a feedback time of ≈ 1.7 Myr in the solar neighbourhood. The sample sets a solid foundation to explore the relationship between interstellar medium evolution, star formation, and Galaxy structure. [ABSTRACT FROM AUTHOR]

We present the preliminary results of 4 yr of observations of the scattering of pulsar radiation in the interstellar medium using the Low Frequency Array (LOFAR) PL611 station located in Lazy near Krakow. In this work we show the initial results for six pulsars from our observing campaign. We used the HBA antennas of the station, with a central frequency of 154 MHz and a 72 MHz bandwidth and we were able to detect the frequency-dependent change in the pulse profiles. Splitting the bandwidth into a number of separate sub-bands we obtained independent scatter time estimates, which allowed for the estimation of the scattering frequency scaling slope for each individual observation. Our project has been accumulating data for more than 4 yr and as a result we are in a position to study the time variability of the scattering parameters over this period. We detected significant changes in the scatter time and its frequency scaling for at least two of pulsars. The average values of the frequency scaling index for the six pulsars selected for this work are below the range of values predicted by the simple thin screen model of interstellar scattering. This is in accordance with previous results shown for both higher dispersion measure pulsars observed in the past, as well as the more recent LOFAR observations and other projects similar in the observing frequency. We also discuss the advantages of using individual LOFAR stations (or similar instruments) for this kind of research. [ABSTRACT FROM AUTHOR]

The Sun is located close to the Galactic mid-plane, meaning that we observe the Galaxy through significant quantities of dust. Moreover, the vast majority of the Galaxy's stars also lie in the disc, meaning that dust has an enormous impact on the massive astrometric, photometric and spectroscopic surveys of the Galaxy that are currently underway. To exploit the data from these surveys we require good three-dimensional maps of the Galaxy's dust. We present a new method for making such maps in which we form the best linear unbiased predictor of the extinction at an arbitrary point based on the extinctions for a set of observed stars. This method allows us to avoid the artificial inhomogeneities (so-called 'fingers of God') and resolution limits that are characteristic of many published dust maps. Moreover, it requires minimal assumptions about the statistical properties of the interstellar medium. In fact, we require only a model of the first and second moments of the dust density field. The method is suitable for use with directly measured extinctions, such as those provided by the Rayleigh–Jeans colour excess method, and inferred extinctions, such as those provided by hierarchical Bayesian models like StarHorse. We test our method by mapping dust in the region of the giant molecular cloud Orion A. Our results indicate a foreground dust cloud at a distance of 350 pc, which has been identified in work by another author. [ABSTRACT FROM AUTHOR]

Cosmic rays (CRs) with energies ≪ TeV comprise a significant component of the interstellar medium (ISM). Major uncertainties in CR behaviour on observable scales (much larger than CR gyroradii) stem from how magnetic fluctuations scatter CRs in pitch angle. Traditional first-principles models, which assume these magnetic fluctuations are weak and uniformly scatter CRs in a homogeneous ISM, struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. We therefore explore a new category of 'patchy' CR scattering models, wherein CRs are pre-dominantly scattered by intermittent strong scattering structures with small volume-filling factors. These models produce the observed rigidity dependence with a simple size distribution constraint, such that larger scattering structures are rarer but can scatter a wider range of CR energies. To reproduce the empirically inferred CR scattering rates, the mean free path between scattering structures must be |$\ell _{\rm mfp}\sim 10\, {\rm pc}$| at GeV energies. We derive constraints on the sizes, internal properties, mass/volume-filling factors, and the number density any such structures would need to be both physically and observationally consistent. We consider a range of candidate structures, both large scale (e.g. H ii  regions) and small scale (e.g. intermittent turbulent structures, perhaps even associated with radio plasma scattering) and show that while many macroscopic candidates can be immediately ruled out as the primary CR scattering sites, many smaller structures remain viable and merit further theoretical study. We discuss future observational constraints that could test these models. [ABSTRACT FROM AUTHOR]

To understand physical properties of the interstellar medium (ISM) on various scales, we investigate it at parsec resolution on the kiloparsec scale. Here, we report on the sub-kpc scale gas density histogram (GDH) of the Milky Way. The GDH is a density probability distribution function (PDF) of the gas volume density. Using this method, we are free from an identification of individual molecular clouds and their spatial structures. We use survey data of 12CO and 13CO (J = 1–0) emission in the Galactic plane (l = 10○–50○) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). We make a GDH for every channel map of 2○ × 2○ area including the blank sky component, and without setting cloud boundaries. This is a different approach from previous works for molecular clouds. The GDH fits well to a single or double lognormal distribution, which we name the low-density lognormal (L-LN) and high-density lognormal (H-LN) components, respectively. The multi-lognormal components suggest that the L-LN and H-LN components originate from two different stages of structure formation in the ISM. Moreover, we find that both the volume ratios of H-LN components to total (f H) and the width of the L-LN along the gas density axis (σL) show coherent structure in the Galactic-plane longitude-velocity diagram. It is possible that these GDH parameters are related to strong galactic shocks and other weak shocks in the Milky Way. [ABSTRACT FROM AUTHOR]

Observing 3D magnetic fields, including orientation and strength, within the interstellar medium is vital but notoriously difficult. However, recent advances in our understanding of anisotropic magnetohydrodynamic (MHD) turbulence demonstrate that MHD turbulence and 3D magnetic fields leave their imprints on the intensity features of spectroscopic observations. Leveraging these theoretical frameworks, we propose a novel Convolutional Neural Network (CNN) model to extract this embedded information, enabling the probe of 3D magnetic fields. This model examines the plane-of-the-sky magnetic field orientation (ϕ), the magnetic field's inclination angle (γ) relative to the line-of-sight, and the total magnetization level (M |$_{\rm A}^{-1}$|⁠) of the cloud. We train the model using synthetic emission lines of 13CO (J  = 1–0) and C18O (J  = 1–0), generated from 3D MHD simulations that span conditions from sub-Alfvénic to super-Alfvénic molecular clouds. Our tests confirm that the CNN model effectively reconstructs the 3D magnetic field topology and magnetization. The median uncertainties are under 5° for both ϕ and γ, and less than 0.2 for M A in sub-Alfvénic conditions (M A ≈ 0.5). In super-Alfvénic scenarios (M A ≈ 2.0), they are under 15° for ϕ and γ, and 1.5 for M A. We applied this trained CNN model to the L1478 molecular cloud. Results show a strong agreement between the CNN-predicted magnetic field orientation and that derived from Planck 353 GHz polarization. The CNN approach enabled us to construct the 3D magnetic field map for L1478, revealing a global inclination angle of ≈76° and a global M A of ≈1.07. [ABSTRACT FROM AUTHOR]

We propose to employ emission line luminosities obtained via optical spectroscopy to estimate the content of neutral hydrogen (H  i) in galaxies. We use the optical spectroscopy data from the Mapping of Nearby Galaxies at APO (MaNGA) survey released in the frames of public DR17 of the Sloan Digital Sky Survey (SDSS). We compare the H  i mass estimated by us for a large sample of SDSS/MaNGA galaxies with direct H  i measurements from the Arecibo Legacy Fast ALFA survey and find a tight correlation between the masses with the correlation coefficient (CC) of 0.91 and the rms scatter of 0.15 dex for the logarithmic mass. The obtained relationship is verified via another sample of MaNGA galaxies with H  i masses measured with the Green Bank Telescope. Despite the coarser angular resolution of the radio data, the relation between the estimated and measured directly H  i mass is tight as well – in this case CC = 0.74 and the rms is 0.29 dex. The established relations allow us to estimate the total mass of neutral hydrogen as well as the spatial distribution of H  i surface density in galaxies from optical spectroscopic observations only in a simple and efficient way. [ABSTRACT FROM AUTHOR]

We explore the dependence of dust attenuation, as traced by the |$\rm H_{\alpha }/\rm H_{\beta }$| Balmer decrement, on galactic properties by using a large sample of Sloan Digital Sky Survey spectra. We use both partial correlation coefficients and random forest analysis to distinguish those galactic parameters that directly and primarily drive dust attenuation in galaxies, from parameters that are only indirectly correlated through secondary dependencies. We find that, once galactic inclination is controlled for, dust attenuation depends primarily on stellar mass, followed by metallicity and velocity dispersion. Once the dependence on these quantities is taken into account, there is no dependence on the star formation rate. While the dependence on stellar mass and metallicity was expected based on simple analytical equations for the interstellar medium, the dependence on velocity dispersion was not predicted, and we discuss possible scenarios to explain it. We identify a projection of this multidimensional parameters space which minimizes the dispersion in terms of the Balmer decrement and which encapsulates the primary and secondary dependences of the Balmer decrement into a single parameter defined as the reduced mass μ = log M⋆ + 3.67[O/H] + 2.96log (σ v /100 km s−1). We show that the dependence of the Balmer decrement on this single parameter also holds at high redshift, suggesting that the processes regulating dust production and distribution do not change significantly through cosmic epochs at least out to z ∼ 2. [ABSTRACT FROM AUTHOR]

Context. Carbon monoxide (CO) is a poor tracer of H2 in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions. Aims. We present a novel, indirect method for constraining H2 column densities (NH2) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction, AV (H2), derived from dust emission and the H I column density (NH I) are due to the presence of molecular gas. Methods. We employed archival (NH2) data, obtained from the UV spectra of stars, and calculated AV(H2) toward these sight lines using 3D extinction maps. The following relation fits the data: log NH2 = 1.38742 (log AV(H2))3 − 0.05359 (log AV(H2))2 + 0.25722 log AV(H2) + 20.67191. This relation is useful for constraining NH2 in the diffuse ISM as it requires only NH I and dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed a NH2 map of our Galaxy and compared it to the CO integrated intensity (WCO) distribution. Results. We find that the average ratio (XCO) between NH2 and WCO is approximately equal to 2 × 1020 cm−2 (K km s−1)−1, consistent with previous estimates. However, we find that the XCO factor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions with NH2 ≳ 1020 cm−2, we estimate that the average H2 fractional abundance, fH2 = 2 NH2/(2NH2 + NH I), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (AV ≥ 1 mag), hence limiting fH2 in these directions. Conclusions. More than 50% of the lines of sight with NH2 ≥ 1020 cm−2 are untraceable by CO with a J = 1−0 sensitivity limit WCO = 1 K km s−1. [ABSTRACT FROM AUTHOR]

Observations of the hot X-ray emitting interstellar medium in the Milky Way are important for studying the stellar feedback and for understanding the formation and evolution of galaxies. We present measurements of the soft X-ray background emission for 130 Suzaku observations at 75° < l < 285° and | b | > 15°. With the standard soft X-ray background model consisting of the local hot bubble and of the Milky Way halo, residual structures remain at 0.7–1 keV in the spectra of some regions. Adding a collisional-ionization-equilibrium component with a temperature of ∼0.8 keV, much higher than the virial temperature of the Milky Way, significantly reduces the derived C-statistic for 56 out of 130 observations. The emission measure of the 0.8 keV component varies by more than an order of magnitude: assuming the solar abundance, the median value is |$3 \times 10^{-4}\, \rm {cm^{-6}\ pc}$| and the 16th–84th percentile range is |$(1\!-\!8) \times 10^{-4}\, \rm {cm^{-6}\ pc}$|⁠. Regions toward the Orion–Eridanus superbubble, having a large cavity extending from the Ori OB1 association, have the highest emission measures of the 0.8 keV component. While the scatter is large, the emission measures tend to be higher toward lower galactic latitudes. We discuss possible biases caused by the solar wind charge exchange, stars, and background groups. The 0.8 keV component is probably heated by supernovae in the Milky Way disk, possibly related to Galactic fountains. [ABSTRACT FROM AUTHOR]

The onset of star formation is set by the collapse of filaments in the interstellar medium. From a theoretical point of view, an isolated cylindrical filament forms cores via the edge effect. Due to the self-gravity of a filament, the strong increase in acceleration at both ends leads to a pile-up of matter which collapses into cores. However, this effect is rarely observed. Most theoretical models consider a sharp density cut-off at the edge of the filament, whereas a smoother transition is more realistic and would also decrease the acceleration at the ends of the filament. We show that the edge effect can be significantly slowed down by a density gradient, although not completely avoided. However, this allows perturbations inside the filament to grow faster than the edge. We determine the critical density gradient for which the time-scales are equal and find it to be of the order of several times the filament radius. Hence, the density gradient at the ends of a filament is an essential parameter for fragmentation and the low rate of observed cases of the edge effect could be naturally explained by shallow gradients. [ABSTRACT FROM AUTHOR]

Context. Three-dimensional (3D) kinetic maps of the Milky Way interstellar medium are an essential tool in studies of its structure and of star formation. Aims. We aim to assign radial velocities to Galactic interstellar clouds now spatially localized based on starlight extinction and star distances from Gaia and stellar surveys. Methods. We developed an automated search for coherent projections on the sky of clouds isolated in 3D extinction density maps on the one hand, and regions responsible for CO radio emissions at specific Doppler shifts on the other hand. The discrete dust structures were obtained by application of the Fellwalker algorithm to a recent 3D extinction density map. For each extinction cloud, a technique using a narrow sliding spectral window moved along the contour-bounded CO spectrum and geometrical criteria was used to select the most likely velocity interval. Results. We applied the new contour-based technique to the 3D extinction density distribution within the volume encompassing the Taurus, Auriga, Perseus, and California molecular complexes. From the 45 clouds issued from the decomposition, 42 were assigned a velocity. The remaining structures correspond to very weak CO emission or extinction. We used the non-automated assignments of radial velocities to clouds of the same region presented in Paper I and based on KI absorption spectra as a validation test. The new fully automated determinations were found to be in good agreement with these previous measurements. Conclusions. Our results show that an automated search based on cloud-contour morphology can be efficient and that this novel technique may be extended to wider regions of the Milky Way and at larger distance. We discuss its limitations and potential improvements after combination with other techniques. [ABSTRACT FROM AUTHOR]

Many recent numerical studies have argued that cosmic rays (CRs) from supernovae (SNe) or active galactic nuclei (AGNs) could play a crucial role in galaxy formation, in particular by establishing a CR-pressure-dominated circumgalactic medium (CGM). But explicit CR-magnetohydrodynamics (CR-MHD) remains computationally expensive, and it is not clear whether those results can be applied to simulations that do not explicitly treat magnetic fields or resolved interstellar medium phase structure. We therefore present an intentionally extremely simplified 'sub-grid' model for CRs, which attempts to capture the key qualitative behaviors of greatest interest for those interested in simulations or semi-analytical models including some approximate CR effects on galactic (≳ kpc) scales, while imposing negligible computational overhead. The model is numerically akin to some recently developed sub-grid models for radiative feedback, and allows for a simple constant parametrization of the CR diffusivity and/or streaming speed; it allows for an arbitrary distribution of sources (proportional to black hole accretion rates or star–particle SNe rates or gas/galaxy star formation rates), and interpolates between the limits where CRs escape the galaxies with negligible losses and those where CRs lose most of their energy catastrophically before escape (relevant in e.g. starburst galaxies). The numerical equations are solved trivially alongside gravity in most codes. We compare this to explicit CR-MHD simulations and discuss where the (many) sub-grid approximations break down, and what drives the major sources of uncertainty. [ABSTRACT FROM AUTHOR]

Density fluctuations in the ionized interstellar medium have a profound effect on radio pulsar observations, through angular scattering, intensity scintillations, and small changes in time delays from dispersion. Here we show that it is possible to recover the variations in dispersive delays that originate from a dominant scattering region using measurements of the dynamic spectrum of intensity scintillations, provided that the pulsar velocity and scattering region location are known. We provide a theoretical framework for the technique, which involves estimating the phase gradient from the dynamic spectra and integrating that gradient to obtain phase variations. It can be used to search for "extreme scattering events" (ESEs) in pulsars for which precision dispersion delay measurements are not otherwise possible, or to separate true dispersion variations from apparent variability caused by frequency-dependent pulse shape changes. We demonstrate that it works in practice by recovering an ESE in PSR J1603−7202, which is known from precision dispersion delay measurements from pulsar timing. For this pulsar, we find that the phase gradients also track the long-term variations in electron column density observed by pulsar timing, indicating that the column density variations and the scattering are dominated by the same thin scattering screen. We identify a sudden increase in the scintillation strength and magnitude of phase gradients over ∼days in 2010, indicating a compact structure. A decrease in the electron density in 2012 was associated with persistent phase gradients and preceded a period of decreased scintillation strength and an absence of scintillation arcs. [ABSTRACT FROM AUTHOR]

The problem of missing baryons in the local universe remains an open question. One proposed alternative is that at low redshifts missing baryons are in the form of the warm-hot intergalactic medium (WHIM). In order to test this idea, we present a detailed analysis of X-ray high-resolution spectra of six extragalactic sources, Mrk 421, 1ES 1028+511, 1ES 1553+113, H2356−309, PKS 0558−504, and PG 1116+215, obtained with the XMM–Newton Reflection Grating Spectrometer to search for signals of WHIM and/or circumgalactic medium X-ray-absorbing gas. We fit the X-ray absorption with the IONeq model, allowing us to take into account the presence of X-ray spectral features due to the multiphase component of the local interstellar medium (ISM). An additional IONeq component is included to model the WHIM absorption, instead of the traditional Gaussian absorption line modelling. We found no statistical improvement in the fits when including such component in any of the sources, concluding that we can safely reject a successful detection of WHIM absorbers towards these lines of sights. Our simulation shows that the presence of the multiphase ISM absorption features prevents the detection of low-redshift WHIM absorption features in the >17 Å spectral region for moderate exposures using high-resolution spectra. [ABSTRACT FROM AUTHOR]

Diffuse 1-ray line emission traces freshly produced radioisotopes in the interstellar gas, providing a unique perspective on the entire Galactic cycle of matter from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium (ISM). We aim to construct a model of nucleosynthesis ejecta on a galactic scale that is specifically tailored to complement the physically most important and empirically accessible features of 1ray measurements in the MeV range, in particular for decay 1 rays such as 26Al, 60Fe, or 44Ti. Based on properties of massive star groups, we developed a Population SYnthesis COde (PSYCO), which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate (SFR), density profiles, or stellar evolution models. As a result, we obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the population synthesis calculations of individual massive star groups. Based on a variety of stellar evolution models, supernova (SN) explodabilities, and density distributions, we find that the measured 26Al distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a SFR of 4-8 M yr-1 and a spiral-arm-dominated density profile with a scale height of at least 700 pc. Our model requires that most massive stars indeed undergo a SN explosion. This corresponds to a SN rate in the Milky Way of 1.8-2.8 per century, with quasi-persistent 26Al and 60Fe masses of 1.2-2.4 Mand 1-6 M, respectively. Comparing the simulated morphologies to SPI data suggests that a frequent merging of superbubbles may take place in the Galaxy, and that an unknown yet strong foreground emission at 1.8MeV could be present. [ABSTRACT FROM AUTHOR]

We use a dynamical model of galactic fountain to study the neutral extraplanar gas (EPG) in the nearby spiral galaxy NGC 2403. We have modelled the EPG as a combination of material ejected from the disc by stellar feedback (i.e. galactic fountain) and gas accreting from the inner circumgalactic medium (CGM). This accretion is expected to occur because of cooling/condensation of the hot CGM (corona) triggered by the fountain. Our dynamical model reproduces the distribution and kinematics of the EPG H  i emission in NGC 2403 remarkably well and suggests a total EPG mass of |$4.7^{+1.2}_{-0.9}\times 10^8\, \mathrm{M_\odot }$|⁠ , with a typical scale height of around 1 kpc and a vertical gradient of the rotation velocity of |$-10.0\pm 2.7\, \mathrm{km\, s^{-1}\, kpc^{-1}}$|⁠. The best-fitting model requires a characteristic outflow velocity of |$50\pm 10\, \mathrm{km\, s^{-1}}$|⁠. The outflowing gas starts out mostly ionized and only becomes neutral later in the trajectory. The accretion rate from the condensation of the inner hot CGM inferred by the model is 0.8  |$\mathrm{M}_\odot \, \mathrm{yr}^{-1}$|⁠ , approximately equal to the star-formation rate in this galaxy (0.6  |$\mathrm{M}_\odot \, \mathrm{yr}^{-1}$|⁠). We show that the accretion profile, which peaks at a radius of about 4.5 kpc, predicts a disc growth rate compatible with the observed value. Our results indicate that fountain-driven corona condensation is a likely mechanism to sustain star formation, as well as the disc inside-out growth in local disc galaxies. [ABSTRACT FROM AUTHOR]

Long-term studies of binary millisecond pulsars (MSPs) provide precise tests of strong-field gravity, and can be used to measure neutron-star masses. PSR J1909-3744, a binary MSP has been the subject of several pulsar timing analyses. The edge-on orbit enables measurement of its mass using the Shapiro delay; however, there is degeneracy in the sense of the inclination angle, i , and multiple solutions for the longitude of ascending node, Ω. Radio pulsars scintillate due to inhomogeneities in the ionized interstellar medium (IISM). This can result in scintillation arcs in the power spectrum of the dynamic spectrum that can use these to study the interstellar medium and constrain binary pulsar orbits. Here, we study the scintillation of PSR J1909-3744 using observations from the 64-m Parkes Radio Telescope (Murriyang) over ≈13 yr, using techniques to study scintillation in a lower signal-to-noise regime. By monitoring annual and orbital variations of the arc-curvature measurements, we are able to characterize the velocity of the IISM. We find that the statistics of the IISM remained stationary over this time and a slightly anisotropic model (axial ratio ≳1.2) is preferred. We measure the relative distance to a single dominant thin scattering screen at s  = 0.49 ± 0.04, or D s = 590 ± 50 pc, with an angle of anisotropy ζ = 85 ± 6° (East of North) and velocity in the direction of anisotropy V IISM,ζ = 14 ± 10 km s−1. By combining a physical model of the IISM and current pulsar timing results, we also constrain Ω = 225 ± 3° and i  = 86.46 ± 0.05°. [ABSTRACT FROM AUTHOR]

We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5−1.8 (G214.5) using Herschel data. We find that G214.5 has a mass of ∼16 000 M⊙, yet hosts only 15 potentially protostellar 70- |$\mu$| m sources, making it highly quiescent compared to equally massive clouds such as Serpens and Mon R2. We show that G214.5 has a unique morphology, consisting of a narrow 'Main filament' running north–south and a perpendicular 'Head' structure running east–west. We identify 33 distinct massive clumps from the column density maps, 8 of which are protostellar. However, the star formation activity is not evenly spread across G214.5 but rather predominantly located in the Main filament. Studying the Main filament in a manner similar to previous works, we find that G214.5 is most like a 'Bone' candidate GMF, highly elongated and massive, but it is colder and narrower than any such GMF. It also differs significantly due to its low fraction of high column density gas. Studying the radial profile, we discover that G214.5 is highly asymmetric and resembles filaments which are known to be compressed externally. Considering its environment, we find that G214.5 is co-incident, spatially and kinematically, with a H  i superbubble. We discuss how a potential interaction between G214.5 and the superbubble may explain G214.5's morphology, asymmetry and, paucity of dense gas and star formation activity, highlighting the intersection of a bubble-driven interstellar medium paradigm with that of a filament paradigm for star formation. [ABSTRACT FROM AUTHOR]

Accurately reproducing the thin cold gas discs observed in nearby spiral galaxies has been a long standing issue in cosmological simulations. Here, we present measurements of the radially resolved H  i  scale height in 22 non-interacting Milky Way-mass galaxies from the FIREbox cosmological volume. We measure the H  i  scale heights using five different approaches commonly used in the literature: fitting the vertical volume density distribution with a Gaussian, the distance between maximum and half-maximum of the vertical volume density distribution, a semi-empirical description using the velocity dispersion and the galactic gravitational potential, the analytic assumption of hydrostatic equilibrium, and the distance from the midplane which encloses ≳60 per cent of the H  i  mass. We find median H  i  scale heights, measured using the vertical volume distribution, that range from ∼100 pc in the galactic centres to ∼800 pc in the outskirts and are in excellent agreement with recent observational results. We speculate that the presence of a realistic multiphase interstellar medium, including cold gas, and realistic stellar feedback are the drivers behind the realistic H  i  scale heights. [ABSTRACT FROM AUTHOR]

We present the first simulations evolving resolved spectra of cosmic rays (CRs) from MeV–TeV energies (including electrons, positrons, (anti)protons, and heavier nuclei), in live kinetic-magnetohydrodynamics galaxy simulations with star formation and feedback. We utilize new numerical methods including terms often neglected in historical models, comparing Milky Way analogues with phenomenological scattering coefficients ν to Solar-neighbourhood [Local interstellar medium (LISM)] observations (spectra, B/C, e+/e−, |$\mathrm{\bar{p}}/\mathrm{p}$|⁠ , 10Be/9Be, ionization, and γ-rays). We show it is possible to reproduce observations with simple single-power-law injection and scattering coefficients (scaling with rigidity R), similar to previous (non-dynamical) calculations. We also find: (1) The circumgalactic medium in realistic galaxies necessarily imposes an |$\sim 10\,$| kpc CR scattering halo, influencing the required ν(R). (2) Increasing the normalization of ν(R) re-normalizes CR secondary spectra but also changes primary spectral slopes, owing to source distribution and loss effects. (3) Diffusive/turbulent reacceleration is unimportant and generally sub-dominant to gyroresonant/streaming losses, which are sub-dominant to adiabatic/convective terms dominated by |$\sim 0.1-1\,$|  kpc turbulent/fountain motions. (4) CR spectra vary considerably across galaxies; certain features can arise from local structure rather than transport physics. (5) Systematic variation in CR ionization rates between LISM and molecular clouds (or Galactic position) arises naturally without invoking alternative sources. (6) Abundances of CNO nuclei require most CR acceleration occurs around when reverse shocks form in SNe, not in OB wind bubbles or later Sedov–Taylor stages of SNe remnants. [ABSTRACT FROM AUTHOR]

The interstellar medium is highly structured, presenting a range of morphologies across spatial scales. The large data sets resulting from observational surveys and state-of-the-art simulations studying these hierarchical structures means that identification and classification must be done in an automated fashion to be efficient. Here we present RJ-plots, an improved version of the automated morphological classification technique J -plots developed by Jaffa et al. This method allows clear distinctions between quasi-circular/elongated structures and centrally over/underdense structures. We use the recent morphological SEDIGISM catalogue of Neralwar et al. to show the improvement in classification resulting from RJ-plots, especially for ring-like and concentrated cloud types. We also find a strong correlation between the central concentration of a structure and its star formation efficiency and dense gas fraction, as well as a lack of correlation with elongation. Furthermore, we use the accreting filament simulations of Clarke, Williams & Walch to highlight a multiscale application of RJ-plots, finding that while spherical structures become more common at smaller scales they are never the dominant structure down to r ∼ 0.03 pc. [ABSTRACT FROM AUTHOR]

The interstellar medium (ISM) is an inseparable part of the Milky Way ecosystem whose evolutionary history remains a challenging question. We trace the evolution of the molecular ISM using a sample of young stellar objects (YSO) association–molecular cloud complex (YSO–MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements of the YSO associations and the CO observations of the associated molecular clouds. Based on the 3D velocities, we simulate the motions of the YSO–MC complexes in the Galactic potential and forecast the ISM evolution by tracing the motions of the individual complexes, and reveal the roles of shear and stellar feedback in determining ISM evolution: Galactic shear stretches Galactic-scale molecular cloud complexes, such as the G120 Complex, into Galactic-scale filaments, and it also contributes to the destruction of the filaments; while stellar feedback creates interconnected superbubbles whose expansion injects peculiar velocities into the ISM. The Galactic-scale molecular gas clumps are often precursors of the filaments and the Galactic-scale filaments are transient structures under a constant stretch by shear. This evolutionary sequence sets a foundation to interpret other gas structures. [ABSTRACT FROM AUTHOR]

We study the relationship between the scale height of the molecular gas disc and the turbulent velocity dispersion of the molecular interstellar medium within a simulation of a Milky Way-like galaxy in the moving-mesh code arepo. We find that the vertical distribution of molecular gas can be described by a Gaussian function with a uniform scale height of ∼50 pc. We investigate whether this scale height is consistent with a state of hydrostatic balance between gravity and turbulent pressure. We find that the hydrostatic prediction using the total turbulent velocity dispersion (as one would measure from kiloparsec-scale observations) gives an overestimate of the true molecular disc scale height. The hydrostatic prediction using the velocity dispersion between the centroids of discrete giant molecular clouds (cloud–cloud velocity dispersion) leads to more accurate estimates. The velocity dispersion internal to molecular clouds is elevated by the locally enhanced gravitational field. Our results suggest that observations of molecular gas need to reach the scale of individual molecular clouds in order to accurately determine the molecular disc scale height. [ABSTRACT FROM AUTHOR]

The mechanisms that maintain turbulence in the interstellar medium (ISM) are still not identified. This work investigates how we can distinguish between two fundamental driving mechanisms: the accumulated effect of stellar feedback versus the energy injection from galactic scales. We perform a series of numerical simulations describing a stratified star-forming ISM subject to self-consistent stellar feedback. Large-scale external turbulent driving, of various intensities, is added to mimic galactic driving mechanisms. We analyse the resulting column density maps with a technique called Multi-scale non-Gaussian segmentation, which separates the coherent structures and the Gaussian background. This effectively discriminates between the various simulations and is a promising method to understand the ISM structure. In particular, the power spectrum of the coherent structures flattens above 60 pc when turbulence is driven only by stellar feedback. When large-scale driving is applied, the turn-over shifts to larger scales. A systematic comparison with the Large Magellanic Cloud (LMC) is then performed. Only 1 out of 25 regions has a coherent power spectrum that is consistent with the feedback-only simulation. A detailed study of the turn-over scale leads us to conclude that regular stellar feedback is not enough to explain the observed ISM structure on scales larger than 60 pc. Extreme feedback in the form of supergiant shells likely plays an important role but cannot explain all the regions of the LMC. If we assume ISM structure is generated by turbulence, another large-scale driving mechanism is needed to explain the entirety of the observations. [ABSTRACT FROM AUTHOR]

To understand the physical properties of the interstellar medium (ISM) in various scales, we should investigate it with pc-scale resolution over kpc scale coverage. Here, we report the sub-kpc scale Gas Density Histogram (GDH) of the Milky Way. GDH is a histogram of averaged density and corresponds to the probability density distribution (PDF) of gas volume density. We use galactic plain survey data (l =10∘− 50∘) at 12CO and 13CO (J = 1 − 0) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). With this method and data, we are free from spatial structure and molecular cloud identification. GDH can be well fitted with single or double log-normal distribution; which we call as the low-density log-normal (L-LN) and high-density log-normal (H-LN) components. We found both the H-LN fraction (f H) and L-LN width (σL) along the gas density axis show a coherent structure on the longitude-velocity diagram. It suggests that there is a relationship between the ISM property and kpc scale structure in the Milky Way. [ABSTRACT FROM AUTHOR]

Phenomenological models of cosmic ray (CR) transport in the Milky Way can reproduce a wide range of observations assuming that CRs scatter off of magnetic-field fluctuations with spectrum ∝  k −δ and δ ∼ [1.4, 1.67]. We study the extent to which such models can be reconciled with current microphysical theories of CR transport, specifically self-confinement due to the streaming instability and/or extrinsic turbulence due to a cascade of magnetohydrodynamic (MHD) fast modes. We first review why it is that on their own neither theory is compatible with observations. We then highlight that CR transport is a strong function of local plasma conditions in the multiphase interstellar medium, and may be diffusive due to turbulence in some regions and streaming due to self-confinement in others. A multiphase combination of scattering mechanisms can in principle reproduce the main trends in the proton spectrum and the boron-to-carbon ratio. However, models with a combination of scattering by self-excited waves and fast-mode turbulence require significant fine-tuning due to fast-mode damping, unlike phenomenological models that assume undamped Kolmogorov turbulence. The assumption that fast modes follow a weak cascade is also not well justified theoretically, as the weak cascade is suppressed by wave steepening and weak-shock dissipation even in subsonic turbulence. These issues suggest that there may be a significant theoretical gap in our understanding of MHD turbulence. We discuss a few topics at the frontier of MHD turbulence theory that bear on this (possible) gap and that may be relevant for CR scattering. [ABSTRACT FROM AUTHOR]

We discovered rapid intra-day variability in radio source PMN J1726+0639 at GHz frequencies, during a survey to search for such variability with the Australia Telescope Compact Array. Follow-up observations were conducted over 2 yr and revealed a clear, repeating annual cycle in the rate, or characteristic time-scale, of variability, showing that the observed variations can be attributed to scintillations from interstellar plasma inhomogeneities. The strong annual cycle includes an apparent 'standstill' in April and another in September. We fit kinematic models to the data, allowing for finite anisotropy in the scintillation pattern. The cycle implies a very high degree of anisotropy, with an axial ratio of at least 13:1, and the fit is consistent with a purely one-dimensional scintillation pattern. The position angle of the anisotropy, and the transverse velocity component are tightly constrained. The parameters are inconsistent with expectations from a previously proposed model of scattering associated with plasma filaments radially oriented around hot stars. We note that evidence for a foreground interstellar cloud causing anomalous Ca  ii absorption towards the nearby star Rasalhague (α Oph) has been previously reported, and we speculate that the interstellar scintillation of PMN J1726+0639 might be associated with this nearby cloud. [ABSTRACT FROM AUTHOR]

The interstellar medium (ISM) is a turbulent, highly structured multiphase medium. State-of-the-art cosmological simulations of the formation of galactic discs usually lack the resolution to accurately resolve those multiphase structures. However, small-scale density structures play an important role in the life cycle of the ISM, and determine the fraction of cold, dense gas, the amount of star formation, and the amount of radiation and momentum leakage from cloud-embedded sources. Here, we derive a statistical model to calculate the unresolved small-scale ISM density structure from coarse-grained, volume-averaged quantities such as the gas clumping factor , |$\mathcal {C}$|⁠ , and mean density 〈ρ〉 V . Assuming that the large-scale ISM density is statistically isotropic, we derive a relation between the three-dimensional clumping factor, |$\mathcal {C}_\rho$|⁠ , and the clumping factor of the 4 |$\pi$| column density distribution on the cloud surface, |$\mathcal {C}_\Sigma$|⁠ , and find |$\mathcal {C}_\Sigma =\mathcal {C}_\rho ^{2/3}$|⁠. Applying our model to calculate the covering fraction, i.e. the 4 |$\pi$| sky distribution of optically thick sightlines around sources inside interstellar gas clouds, we demonstrate that small-scale density structures lead to significant differences at fixed physical ISM density. Our model predicts that gas clumping increases the covering fraction by up to 30 per cent at low ISM densities compared to a uniform medium. On the other hand, at larger ISM densities, gas clumping suppresses the covering fraction and leads to increased scatter such that covering fractions can span a range from 20 per cent to 100 per cent at fixed ISM density. All data and example code are publicly available at GitHub. [ABSTRACT FROM AUTHOR]

The evolution of the molecular interstellar medium is controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. AL a part of the ISM-6D project, using Gaia astrometric measurements towards a sample of young stellar objects (YSOs), we study the morphology and kinematic structure of the associated molecular gas. We identify 150 YSO associations with distance |$d \lesssim 3 \,\,\rm kpc$|⁠. The YSO associations are elongated, with a median aspect ratio of 1.97, and are oriented parallel to the disc mid-plane, with a median angle of 30°. The turbulence in the molecular clouds as probed by the YSOs is isotropic, and the velocity dispersions are related to the sizes by σ v , 2D = 0.74 (r /pc)0.67 (km s−1) . The slope is on the steeper side, yet consistent with previous measurements. The energy dissipation rate of turbulence |$\dot{\epsilon } = \sigma _{v,{\rm 3D}}^3 /L$| decreases with the Galactocentric distance, with a gradient of 0.2 |$\rm dex \,\, kpc^{-1}$|⁠ , which can be explained if turbulence is driven by cloud collisions. In this scenario, the clouds located in the inner Galaxy have higher chances to accrete smaller clouds and are more turbulent. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. If the alignment between density structures and the Galactic-disc mid-plane is due to shear, we expect |$t_{\rm cloud} \gtrsim t_{\rm shear}\approx 30\,\, \rm Myr$|⁠. This cloud lifetime is longer than the turbulence crossing time, and a continuous energy injection is required to maintain the turbulence. [ABSTRACT FROM AUTHOR]

The infrared dark clouds (IRDCs) are molecular clouds with relatively greater values in their magnetic-field strengths. For example, the IRDC G34.43+00.24 (G34) has magnetic-field strength of the order of a few hundred micro-Gauss. In this study, we investigate if the dynamic motions of charged particles in an IRDC such as G34 can produce this magnetic-field strength inside it. The observations show that the line-of-sight velocity of G34 has global gradient. We assume that the measured global velocity gradient can correspond to the cloud rotation. We attribute a large-scale current density to this rotating cloud by considering a constant value for the incompleteness of charge neutrality and the velocity differences between the positive and negative particles with very low ionization fractions. We use the numerical package fishpack to obtain the magnetic-field strength and its morphology on the plane-of-sky within G34. The results show that the magnetic-field strengths are of the order of several hundred micro-Gauss, and its morphology in the plane-of-sky is somewhat consistent with the observational results. We also obtain the relationship between magnetic-field strength and density in G34. The results show that with increasing density, the magnetic-field strength increases approximately as a power-law function. The amount of power is approximately equal to 0.45, which is suitable for molecular clouds with strong magnetic fields. Therefore, we can conclude that the dynamical motion of IRDCs, and especially their rotations, can amplify the magnetic-field strengths within them. [ABSTRACT FROM AUTHOR]

We use high-resolution maps of the molecular interstellar medium (ISM) in the centres of 86 nearby galaxies from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) and Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) surveys to investigate the physical mechanisms setting the morphology of the ISM at molecular cloud scales. We show that early-type galaxies tend to have smooth, regular molecular gas morphologies, while the ISM in spiral galaxy bulges is much more asymmetric and clumpy when observed at the same spatial scales. We quantify these differences using non-parametric morphology measures (Asymmetry, Smoothness, and Gini), and compare these measurements with those extracted from idealized galaxy simulations. We show that the morphology of the molecular ISM changes systematically as a function of various large-scale galaxy parameters, including galaxy morphological type, stellar mass, stellar velocity dispersion, effective stellar mass surface density, molecular gas surface density, star formation efficiency, and the presence of a bar. We perform a statistical analysis to determine which of these correlated parameters best predicts the morphology of the ISM. We find the effective stellar mass surface (or volume) density to be the strongest predictor of the morphology of the molecular gas, while star formation and bars maybe be important secondary drivers. We find that gas self-gravity is not the dominant process shaping the morphology of the molecular gas in galaxy centres. Instead effects caused by the depth of the potential well, such as shear, suppression of stellar spiral density waves, and/or inflow, affect the ability of the gas to fragment. [ABSTRACT FROM AUTHOR]

Diffuse interstellar bands (DIBs) are absorption features arising from the interstellar medium, for which the carriers remain unclear. In this work, a total of 3115 stars in the regions of NGC 2264, the Rosette Nebula, and S147 are selected from LAMOST-MRS to study the spatial and velocity distributions of the DIB λ6614 carrier. From the spatial distribution, we find that DIB λ6614 carrier clouds appear to be widely and inhomogeneously distributed in the three target regions. They are either closer or further in distance, or much wider than the three nebulae, and there are gaps within them. The radial velocities of the DIB λ6614 carrier clouds are on average about 13, 20, and 10 km s−1 slower than the radial velocities of the nebulae in the NGC 2264, Rosette Nebula, and S147 regions, respectively. Furthermore, there is probably a high-extinction region from 0.6 to 0.8 kpc in the NGC 2264 direction, where the DIB λ6614 carrier vanishes. In the S147 region, the DIB λ6614 cloud is possibly discontinuous, and the tomographic evidence suggests that there are two DIB λ6614 clouds. The first cloud is at 0.3–0.9 kpc, which is in or near the Local Arm, while the second one emerges at about 1.8 kpc from the Sun and may be associated with S147 and in the Perseus Arm. [ABSTRACT FROM AUTHOR]

In recent years, it has been speculated that in extreme low-metallicity galactic environments, stars form in regions that lack H2. In this paper, we investigate how changing the metallicity and ultraviolet (UV) field strength of a galaxy affects the star formation within, and the molecular gas Kennicutt–Schmidt (KS) relation. Using extremely high-resolution arepo simulations of isolated dwarf galaxies, we independently vary the metallicity and UV field to between 1 per cent and 10 per cent solar neighbourhood values. We include a non-equilibrium, time-dependent chemical network to model the molecular composition of the interstellar medium and include the effects of gas shielding from an ambient UV field. Crucially, our simulations directly model the gravitational collapse of gas into star-forming clumps and cores and their subsequent accretion using sink particles. In this first publication, we find that reducing the metallicity and UV field by a factor of 10 has no effect on star formation and minimal effect on the cold, dense star-forming gas. The cold gas depletion times are almost an order of magnitude longer than the molecular gas depletion time due to the presence of star formation in H  i dominated cold gas. We study the H2 KS relationship that arises naturally within the simulations and find a near-linear power-law index of N  = 1.09 ± 0.014 in our fiducial |$10{{\ \rm per\ cent}}$| solar metallicity model. As the metallicity and UV field are reduced, this becomes moderately steeper, with a slope of N  = 1.24 ± 0.022 for our |$1{{\ \rm per\ cent}}$| solar metallicity and |$1{{\ \rm per\ cent}}$| solar UV field model. [ABSTRACT FROM AUTHOR]

The interstellar medium is characterized by an intricate filamentary network that exhibits complex structures. These show a variety of different shapes (e.g. junctions, rings, etc.) deviating strongly from the usually assumed cylindrical shape. A possible formation mechanism are filament mergers that we analyse in this study. Indeed, the proximity of filaments in networks suggests mergers to be rather likely. As the merger has to be faster than the end dominated collapse of the filament along its major axis, we expect three possible results: (a) The filaments collapse before a merger can happen, (b) the merged filamentary complex shows already signs of cores at the edges, or (c) the filaments merge into a structure which is not end-dominated. We develop an analytic formula for the merging and core-formation time-scale at the edge and validate our model via hydrodynamical simulations with the adaptive-mesh-refinement-code ramses. This allows us to predict the outcome of a filament merger, given different initial conditions which are the initial distance and the respective line-masses of each filament as well as their relative velocities. [ABSTRACT FROM AUTHOR]

Evidence of triggered star formation at large spatial scales involving stellar clusters is scarce. We investigate a Galactic region (l  = 130 |${_{.}^{\circ}}$| 0, b  = 0 |${_{.}^{\circ}}$| 35) populated by several open stellar clusters that according to the last Gaia data release, are located at a distance of about 2.9 kpc. By analysing the interstellar medium (ISM) at infrared, centimeter, and millimeter wavelengths towards this group of clusters we discovered a shell of material of about 2° in size at the same distance. We suggest that the shell, mainly observed at 12 μ m and in the H  i  emission at 21 cm, was generated by the action of massive stars belonging to clusters Berkeley 7 and UBC 414, which lie at its centre. Five clusters (MWSC0152, Czernik 6, Czernik 7, Berkeley 6, NGC 663, and NGC 654) lie at the border of this shell. From the comparison between the dynamical time of the discovered H  i  shell and the analysis of the ages of stellar populations in these clusters, we conclude that the expansion of the shell could have triggered in the past the formation of stars in some of them. We point out that in order to find physical evidence supporting a genetic connection between stellar clusters, it is necessary not only to study the individual clusters and their stellar populations, but also to investigate their surrounding ISM at a large spatial scale. [ABSTRACT FROM AUTHOR]

We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy-merger simulations (stellar mass ratio = 2.5:1), which employs the 'Feedback In Realistic Environments-2' model (fire-2). This framework resolves star formation, feedback processes, and the multiphase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (H  i) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR enhancement in the central kiloparsec is fuel-driven (55 per cent for the secondary, 71 per cent for the primary) – while central SFR suppression is efficiency-driven (91 per cent for the secondary, 97 per cent for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially resolved galaxy surveys – capable of examining vast and diverse samples of interacting systems – coupled with multiwavelength campaigns aimed to capture their internal ISM structure. [ABSTRACT FROM AUTHOR]

Based on high-resolution 3D data cubes from a magnetohydrodynamic (MHD) turbulence simulation, we study how to reveal the direction of the magnetic field within the optically thick interstellar medium by using the velocity gradient technique (VGT), correlation function anisotropy (CFA), and principal component analysis of anisotropies (PCAA). Considering the CO molecular tracers as a tracing method for radiative transfer processes, we find that the VGT and CFA can successfully trace the orientation of mean magnetic fields, which is in good agreement with the low-resolution numerical results obtained in the case of an optically thin medium. Similar to the simulation of an optically thin ISM, our simulations show that PCCA is still unusable in optically thick media. The synergetic application of the VGT and CFA to high-resolution spectroscopic observations is expected to yield valuable information on the interstellar magnetic field. [ABSTRACT FROM AUTHOR]