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We present 850 |$\mu$| m polarized observations of the molecular cloud G34.26 + 0.15 taken using the POL-2 polarimeter mounted on the James Clerk Maxwell Telescope (JCMT). G34.26 + 0.15 is a hub–filament system with ongoing high-mass star formation, containing multiple H  ii regions. We extend the histogram of relative orientations technique with an alternative application that considers the alignment of the magnetic field to the filaments and a H  ii region boundary, denoted as the filament alignment factor (⁠|$\xi _{F}$|⁠) and the ellipse alignment factor (⁠|$\xi _{E}$|⁠), respectively. Using these metrics, we find that, although in general the magnetic field aligns parallel to the filamentary structure within the system in the north-west, the magnetic field structure of G34.26 + 0.15 has been radically reshaped by the expansion of an evolved H  ii region in the south-east, which itself may have triggered further high-mass star formation in the cloud. Thus, we suggest high-mass star formation is occurring through both mass accretion as per the hub–filament model from one side and compression of gas under stellar feedback from the other. We also use HARP (Heterodyne Array Receiver Program) observations of C |$^{18}$| O from the CHIMPS survey to estimate the magnetic field strength across the cloud, finding strengths of |$\sim$| 0.5–1.4 mG. [ABSTRACT FROM AUTHOR]

Context. Magnetic turbulence is classified as weak or strong based on the relative amplitude of the magnetic field fluctuations compared to the mean field. These two classes have different energy transport properties. Aims. The purpose of this study is to analyze turbulence in the interstellar medium (ISM) based on this classification. Specifically, we examined the ISM of simulated galaxies to detect evidence of strong magnetic turbulence and provide statistics on the associated magnetic coherent structures (MCoSs), such as current sheets, that arise in this context. Methods. We analyzed magnetohydrodynamic galaxy simulations with different initial magnetic field structures (either completely ordered or completely random) and recorded statistics on the magnetic field fluctuations (δB/B0) and the MCoSs, which are defined here as regions where the current density surpasses a certain threshold. We also studied the MCoS sizes and kinematics. Results. The magnetic field disturbances in both models follow a log-normal distribution, peaking at values close to unity; this distribution turns into a power law at large values (δB/B0 > 1), which is consistent with strong magnetic turbulence The current densities are widely distributed, with non-power-law deviations from a log-normal at the largest values. These deviating values of the current density define MCoSs. We find that, in both models, MCoSs are fractally distributed in space, with a typical volume-filling factor of about 10%, and tend to coincide with peaks of star formation density. Their fractal dimension is close to unity on sub-kiloparsec scales, and between 2 and 3 on larger scales. These values are consistent with MCoSs having a sheet-like or filament-like morphology. Conclusions. Our work challenges the prevailing paradigm of weak magnetic turbulence in the ISM by demonstrating that strong magnetic disturbances can occur even when the initial magnetic field is completely ordered. This strong magnetic turbulence arises self-consistently from differential rotation and supernova feedback. Our findings provide a foundation for a magnetic turbulence description of the galactic ISM that includes strong fluctuations of the magnetic field. [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]

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 use high-resolution magnetohydrodynamic simulations of isolated disc galaxies to investigate the co-evolution of magnetic fields with a self-regulated, star-forming interstellar medium (ISM). The simulations are conducted using the ramses adaptive mesh refinement code on the standard agora initial condition, with gas cooling, star formation, and feedback. We run galaxies with a variety of initial magnetic field strengths. The fields evolve and achieve approximate saturation within 500 Myr, but at different levels. The galaxies reach a quasi-steady state, with slowly declining star formation due to both gas consumption and increase in the field strength at intermediate ISM densities. We connect this behaviour to differences in the gas properties and overall structure of the galaxies. Stronger magnetic fields limit supernova bubble sizes. Different cases support the ISM using varying combinations of magnetic pressure, turbulence, and thermal energy. Initially, |$\gtrsim\!\! 1\ \mu \mathrm{ G}$| magnetic fields evolve modestly and dominate support at all radii. Conversely, initially weaker fields grow through feedback and turbulence but never dominate the support. This is reflected in the stability of the gas disc. This interplay determines the overall distribution of star formation in each case. We conclude that an initially weak field can grow to produce a realistic model of a local disc galaxy, but starting with typically assumed field strengths (⁠|$\gtrsim\!\! 1\ \mu \mathrm{ G}$|⁠) will not. [ABSTRACT FROM AUTHOR]

Context. The amplification of astrophysical magnetic fields takes place via dynamo instability in turbulent environments. Vorticity is usually present in any dynamo, but its role is not yet fully understood. Aims. This work is an extension of previous research on the effect of an irrotational subsonic forcing on a magnetized medium in the presence of rotation or a differential velocity profile. We aim to explore a wider parameter space in terms of Reynolds numbers, the magnetic Prandtl number, the forcing scale, and the cooling timescale in a Newtonian cooling. We studied the effect of imposing that either the acceleration or the velocity forcing function be curl-free and evaluated the terms responsible for the evolution vorticity. Methods. We used direct numerical simulations to solve the fully compressible, resistive magnetohydrodynamic equations with the Pencil Code. We studied both isothermal and non-isothermal regimes and addressed the relative importance of different vorticity source terms. Results. We report no small-scale dynamo for the models that do not include shear. We find a hydro instability, followed by a magnetic one, when a shearing velocity profile is applied. The vorticity production is found to be numerical in the purely irrotational acceleration case. Non-isothermality, rotation, shear, and density-dependent forcing, when included, contribute to increasing the vorticity. Conclusions. As in our previous study, we find that turbulence driven by subsonic expansion waves can amplify the vorticity and magnetic field only in the presence of a background shearing profile. The presence of a cooling function makes the instability occur on a shorter timescale. We estimate critical Reynolds and magnetic Reynolds numbers of 40 and 20, respectively. [ABSTRACT FROM AUTHOR]

Context. Ultrahigh-energy cosmic rays (UHECRs) are charged particles with energies surpassing 1018 eV. Their sources remain elusive because they are obscured by deflections caused by the Galactic magnetic field (GMF). This challenge is further complicated by our limited understanding of the 3D structure of the GMF because current GMF observations primarily consist of quantities that are integrated along the line of sight (LOS). Nevertheless, data from upcoming stellar polarization surveys along with Gaia stellar parallax data are expected to yield local GMF measurements. Aims. This study is the second entry in our exploration of a Bayesian inference approach to the local GMF that uses synthetic local GMF observations that emulate forthcoming local GMF measurements, and attempts to use them to reconstruct its 3D structure. The ultimate aim is to trace back observed UHECRs and thereby update our knowledge about their possible origin. Methods. In this proof-of-concept work, we assumed as ground truth a magnetic field produced by a dynamo simulation of the Galactic ISM. We employed methods of Bayesian statistical inference in order to sample the posterior distribution of the GMF within part of the Galaxy. By assuming a known rigidity and arrival direction of an UHECR, we traced its trajectory back through various GMF configurations drawn from the posterior distribution. Our objective was to rigorously evaluate the performance of our algorithm in scenarios that closely mirror the setting of expected future applications. In pursuit of this, we conditioned the posterior to synthetically integrated LOS measurements of the GMF, in addition to synthetic local plane of sky-component measurements. Results. Our results demonstrate that for all locations of the observed arrival direction on the plane of sky, our algorithm is able to substantially update our knowledge on the original arrival direction of UHECRs with a rigidity of E/Z = 5 × 1019 eV, even without any LOS information. When the integrated data are included in the inference, the regions of the celestial sphere in which the maximum error occurs are greatly reduced. The maximum error is diminished by a factor of about 3 even in these regions in the specific setting we studied. Additionally, we are able to identify the regions in which the largest error is expected to occur. [ABSTRACT FROM AUTHOR]

Context. Together with gravity, turbulence, and stellar feedback, magnetic fields (B-fields) are thought to play a critical role in the evolution of molecular clouds and star formation processes. The polarization of thermal dust emission is a popular tracer of B-fields in star-forming regions. Aims. We aim to map the morphology and measure the strength of B-fields of the nearby molecular cloud, rho Ophiuchus-A (ρ Oph-A), to understand the role of B-fields in regulating star formation and in shaping the cloud. Methods. We analyzed the far-infrared (FIR) polarization of thermal dust emission observed by SOFIA/HAWC+ at 89 and 154 μm toward the densest part of ρ Oph-A, which is irradiated by the nearby B3/4 star, Oph-S1. These FIR polarimetric maps cover an area of ~4.5′ × 4.5′ (corresponding to 0″.18 × 0″.18 pc2) with an angular resolution of 7.8″ and 13.6″ respectively. Results. The ρ Oph-A cloud exhibits well-ordered B-fields with magnetic orientations that are mainly perpendicular to the ridge of the cloud toward the densest region. We obtained a map of B-field strengths in the range of 0.2–2.5 mG, using the Davis-Chandrasekhar-Fermi (DCF) method. The B-fields are strongest at the densest part of the cloud, which is associated with the starless core SM1, and then decrease toward the outskirts of the cloud. By calculating the map of the mass-to-flux ratio, Alfvén Mach number, and plasma β parameter in ρ Oph-A, we find that the cloud is predominantly magnetically sub-critical, sub-Alfvénic, which implies that the cloud is supported by strong B-fields that dominate over gravity, turbulence, and thermal gas energy. The measured B-field strengths at the two densest subsregions using other methods that account for the compressible mode are relatively lower than that measured with the DCF method. However, these results do not significantly change our conclusions on the roles of B-fields relative to gravity and turbulence on star formation. Our virial analysis suggests that the cloud is gravitationally unbound, which is consistent with the previous detection of numerous starless cores in the cloud. By comparing the magnetic pressure with the radiation pressure from the Oph-S1 star, we find that B-fields are sufficiently strong to support the cloud against radiative feedback and to regulate the shape of the cloud. [ABSTRACT FROM AUTHOR]

Context. Magnetic fields permeate the diffuse interstellar medium (ISM) of the Milky Way, and are essential to explain the dynamical evolution and current shape of the Galaxy. Magnetic fields reveal themselves via their influence on the surrounding matter, and as such are notoriously hard to measure independently of other tracers. Aims. In this work, we attempt to disentangle an all-sky map of the line-of-sight (LoS)-parallel component of the Galactic magnetic field from the Faraday effect, utilizing several tracers of the Galactic electron density, ne. Additionally, we aim to produce a Galactic electron dispersion measure map and quantify several tracers of the structure of the ionized medium of the Milky Way. Methods. The method developed to reach these aims is based on information field theory, a Bayesian inference framework for fields, which performs well when handling noisy and incomplete data and constraining high-dimensional-parameter spaces. We rely on compiled catalogs of extragalactic Faraday rotation measures and Galactic pulsar dispersion measures, a well as data on bremsstrahlung and the hydrogen α spectral line to trace the ionized medium of the Milky Way. Results. We present the first full sky map of the LoS-averaged Galactic magnetic field. Within this map, we find LoS-parallel and LoS-averaged magnetic field strengths of up to 4 µG, with an all-sky root mean square of 1.1 µG, which is consistent with previous local measurements and global magnetic field models. Additionally, we produce a detailed electron dispersion measure map that agrees with existing parametric models at high latitudes but suffers from systematic effects in the disk. Further analysis of our results with regard to the 3D structure of ne reveals that it follows a Kolmogorov-type turbulence for most of the sky. From the reconstructed dispersion measure and emission measure maps, we construct several tracers of variability in ne along the LoS. Conclusions. This work demonstrates the power of consistent joint statistical analysis including multiple datasets and physical quantities and defines a road map toward a full three-dimensional joint reconstruction of the Galactic magnetic field and the ionized ISM. [ABSTRACT FROM AUTHOR]

Magnetic fields are an energetically important component of star formation galaxies, but it is often difficult to measure their properties from observations. One of the complexities stems from the fact that the magnetic fields, especially in spiral galaxies, have a two-scale nature: a large-scale field, coherent over kpc scales, and a small-scale random field, with a scale of |${\lesssim} 100 \, {\rm pc}$|⁠. Moreover, it is known that the strength of small- and large-scale fields is comparable and this makes it even harder to find their imprints in radio polarization observations such as the Faraday rotation measure (RM), which is the integral over the path-length of the product of the thermal electron density and the parallel component of the magnetic field to the line of sight. Here, we propose and demonstrate the use of second-order structure functions of RM computed with multiple higher-order stencils as a powerful analysis to separate the small- and large-scale magnetic field components. In particular, we provide new methods and calibrations to compute the scale and the strength of the large-scale magnetic field in the presence of small-scale magnetic fluctuations. We then apply the method to find the scale of large-scale magnetic fields in the nearby galaxies M51 and NGC 6946, using archival data, and further discuss the need for computing the RM structure functions with higher-order stencils. With multiple modern radio polarization observatories and eventually the Square Kilometre Array, RM observations will significantly improve in quantity and quality, and the higher-order stencil structure function techniques developed here can be used to extract information about multiscale magnetic fields in galaxies. [ABSTRACT FROM AUTHOR]

Magnetic fields play an important role in shaping and regulating star formation in molecular clouds. Here, we present one of the first studies examining the relative orientations between magnetic (B) fields and the dust emission, gas column density, and velocity centroid gradients on the 0.02 pc (core) scales, using the BISTRO and VLA+GBT observations of the NGC 1333 star-forming clump. We quantified these relative orientations using the Project Rayleigh Statistic (PRS) and found preferential global parallel alignment between the B field and dust emission gradients, consistent with large-scale studies with Planck. No preferential global alignments, however, are found between the B field and velocity gradients. Local PRS calculated for subregions defined by either dust emission or velocity coherence further revealed that the B field does not preferentially align with dust emission gradients in most emission-defined subregions, except in the warmest ones. The velocity-coherent structures, on the other hand, also showed no preferred B field alignments with velocity gradients, except for one potentially bubble-compressed region. Interestingly, the velocity gradient magnitude in NGC 1333 ubiquitously features prominent ripple-like structures that are indicative of magnetohydrodynamic (MHD) waves. Finally, we found B field alignments with the emission gradients to correlate with dust temperature and anticorrelate with column density, velocity dispersion, and velocity gradient magnitude. The latter two anticorrelations suggest that alignments between gas structures and B fields can be perturbed by physical processes that elevate velocity dispersion and velocity gradients, such as infall, accretions, and MHD waves. [ABSTRACT FROM AUTHOR]

Morphological evolution of expanding shells of fast-mode magnetohydrodynamic (MHD) waves through an inhomogeneous ISM is investigated in order to qualitatively understand the complicated morphology of shell-type supernova remnants (SNR). Interstellar clouds with high Alfvén velocity act as concave lenses to diverge the MHD waves, while those with slow Alfvén velocity act as convex lenses to converge the waves to the focal points. By combination of various types of clouds and fluctuations with different Alfvén velocities, sizes, or wavelengths, the MHD-wave shells attain various morphological structures, exhibiting filaments, arcs, loops, holes, and focal strings, mimicking old and deformed SNRs. [ABSTRACT FROM AUTHOR]

We simulate the formation of molecular clouds in colliding flows of warm neutral medium with the adaptive mesh refinement code flash in eight simulations with varying initial magnetic field strength, between 0.01–5  μ G. We include a chemical network to treat heating and cooling and to follow the formation of molecular gas. The initial magnetic field strength influences the fragmentation of the forming cloud because it prohibits motions perpendicular to the field direction and hence impacts the formation of large-scale filamentary structures. Molecular clump and core formation occurs anyhow. We identify 3D clumps and 3D cores, which are defined as connected, CO-rich regions. Additionally, 3D cores are heavily shielded. While we do not claim those 3D objects to be directly comparable to observations, this enables us to analyse their full virial state. With increasing field strength, we find more fragments with a smaller average mass; yet the dynamics of the forming clumps and cores only weakly depends on the initial magnetic field strength. The molecular clumps are mostly unbound, probably transient objects, which are weakly confined by ram pressure or thermal pressure, indicating that they are swept up by the turbulent flow. They experience significant fluctuations in the mass flux through their surface, such that the Eulerian reference frame shows a dominant time-dependent term due to their indistinct nature. We define the cores to encompass highly shielded molecular gas. Most cores are in gravitational-kinetic equipartition and are well described by the common virial parameter |$\alpha _\mathrm{vir}$|⁠ , while some undergo minor dispersion by kinetic surface effects. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive multiwavelength investigation of the intermediate-age open cluster NGC 1912. We identified 401 member stars for NGC 1912 using K-nearest neighbour and Gaussian Mixture Model machine learning techniques. The fundamental parameters determined for the cluster are metallicity (z) = 0.0141 |$\pm$| 0.0006, log(age) = 8.519 |$\pm$| 0.040, binary fraction (⁠|$b_{\text{frac}}$|⁠) = 0.445 |$\pm$| 0.035, visual extinction (⁠|$A_V$|⁠) = 0.864 |$\pm$| 0.033 mag, total-to-selective extinction ratio (⁠|$R_V$|⁠) = 3.007 |$\pm$| 0.096, distance = 1096 |$\pm$| 15 pc. The central density (⁠|$\rho _o$|⁠), core radius (⁠|$r_c$|⁠), tidal radius (⁠|$r_t$|⁠), and background density (⁠|$\rho _{\mathrm{bg}}$|⁠) for the cluster are obtained as 0.643 |$\pm$| 0.067 stars arcmin |$^{-2}$| , 7.743 |$\pm$| 0.784 arcmin, 84.633 |$\pm$| 7.188 arcmin, and 0.001 |$\pm$| 0.000 stars arcmin |$^{-2}$| , respectively. Dynamical mass segregation study revealed a concentration of some apparently massive stars towards the cluster centre. Polarimetric analysis of 90 stars in |$BVR_cI_c$| wavelength bands revealed that the observed polarization in NGC 1912 is not solely attributed to interstellar dust. The Galactic magnetic field is dominant in this region. Dust grain sizes within the intracluster region are found to be relatively smaller than that of the general ISM. The extinction map revealed the existence of a potential 'interstellar bubble' located close to the cluster region. Towards the periphery of the probable bubble region, which displays an ovoid-like geometry, 16 probable young stellar objects have been detected, suggesting the ongoing triggering of secondary star formation events in this region. [ABSTRACT FROM AUTHOR]

Context. Maps of polarized dust emission of molecular clouds reveal the morphology of the magnetic field associated with star-forming regions. In particular, polarization maps of hub-filament systems show the distortion of magnetic field lines induced by gas flows onto and inside filaments. Aims. We aim to understand the relation between the curvature of magnetic field lines associated with filaments in hub-filament systems and the properties of the underlying gas flows. Methods. We consider steady-state models of gas with finite electrical resistivity flowing across a transverse magnetic field. We derive the relation between the bending of the field lines and the flow parameters represented by the Alfvén Mach number and the magnetic Reynolds number. Results. We find that, on the scale of the filaments, the relevant parameter for a gas of finite electrical resistivity is the magnetic Reynolds number, and we derive the relation between the deflection angle of the field from the initial direction (assumed perpendicular to the filament) and the value of the electrical resistivity, due to either Ohmic dissipation or ambipolar diffusion. Conclusions. Application of this model to specific observations of polarized dust emission in filamentary clouds shows that magnetic Reynolds numbers of a few tens are required to reproduce the data. Despite significant uncertainties in the observations (the flow speed, the geometry and orientation of the filament), and the idealization of the model, the specific cases considered show that ambipolar diffusion can provide the resistivity needed to maintain a steady state flow across magnetic fields of significant strength over realistic time scales. [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. Regularly spaced star-forming regions along the spiral arms of nearby galaxies provide insight into the early stages and initial conditions of star formation. The regular separation of these star-forming regions suggests spiral arm instability as their origin. Aims. We explore the effects of magnetic fields on the spiral arm instability. Methods. We use 3D global magnetohydrodynamical simulations of isolated spiral galaxies, comparing three different initial plasma β values (ratios of the thermal to magnetic pressure) of β = ∞, 50, and 10. We perform a Fourier analysis to calculate the separation of the over-dense regions that formed as a result of the spiral instability. We then compare the separations with observations. Results. We find that the spiral arms in the hydro case (β = ∞) are unstable. The fragments are initially connected by gas streams that are reminiscent of the Kelvin-Helmholtz instability. For β = 50, the spiral arms also fragment, but the fragments separate earlier and tend to be slightly elongated in the direction perpendicular to the spiral arms. However, in the β = 10 run, the arms are stabilised against fragmentation by magnetic pressure. Despite the difference in the initial magnetic field strengths of the β = 50 and 10 runs, the magnetic field is amplified to βarm ∼ 1 inside the spiral arms for both runs. The spiral arms in the unstable cases (hydro and β = 50) fragment into regularly spaced over-dense regions. We determine their separation to be ∼0.5 kpc in the hydro and ∼0.65 kpc in the β = 50 case. These two values agree with the observed values found in nearby galaxies. We find a smaller median characteristic wavelength of the over-densities along the spiral arms of 0.73−0.36+0.31 kpc in the hydro case compared to 0.98−0.46+0.49 kpc in the β = 50 case. Moreover, we find a higher growth rate of the over-densities in the β = 50 run compared to the hydro run. We observe magnetic hills and valleys along the fragmented arms in the β = 50 run, which is characteristic of the Parker instability. [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]

We have mapped the magnetic field (B -field) for a region of about 30 pc around the centre of our Galaxy, which encompasses the circumnuclear disc (CND), the minispiral, and the 20 and 50 km s−1 molecular clouds, using thermal dust polarization observations obtained from SOFIA/HAWC+ and JCMT/SCUPOL. We decompose the spectra of 12CO (J = 3 → 2) transition from this region into individual cloud components and find the polarization observed at different wavelengths might be tracing completely different layers of dust along the line of sight. We use modified Davis–Chandrasekhar–Fermi methods to measure the strength of B -field projected in the plane of the sky (⁠|$B_{{}_{\mathrm{POS}}}$|⁠). The mean |$B_{{}_{\mathrm{POS}}}$| of the CND and the minispiral, probed at 53 μ m is of the order of ∼2 mG. |$B_{{}_{\mathrm{POS}}}\!\!\!\lt \!1$| mG close to the Galactic Centre, in the region of the ionized mini-cavity within the CND, and increases outwards. However, the longer wavelength polarization at 216 μ m appears to come from a dust layer that is cooler and behind the CND and has a stronger B -field of about 7 mG. The B -field strength is lowest along the Eastern Arm of the minispiral, which is also the only region with Alfvén Mach number, |$\mathcal {M}_{\mathrm{A}}\gt 1$| and mass-to-flux ratio, λ ≳ 1. Such an observed weak B -field could be a result of the low resolution of the observation, where the tangled B -fields due to the strong turbulence in the high density clumps of the CND are lost within the beam size of the observation. [ABSTRACT FROM AUTHOR]

We describe a novel method to compute the components of dynamo tensors from direct magnetohydrodynamic (MHD) simulations. Our method relies upon an extension and generalization of the standard Högbom CLEAN algorithm widely used in radio astronomy to systematically remove the impact of the strongest beams on to the corresponding image. This generalization, called the Iterative Removal of Sources (IROS) method, has been adopted here to model the turbulent electromotive force (EMF) in terms of the mean magnetic fields and currents. Analogous to the CLEAN algorithm, IROS treats the time series of the mean magnetic field and current as beams that convolve with the dynamo coefficients which are treated as (clean) images to produce the EMF time series (the dirty image). We apply this method to MHD simulations of galactic dynamos, to which we have previously employed other methods of computing dynamo coefficients such as the test-field method, the regression method, as well as local and non-local versions of the singular value decomposition (SVD) method. We show that our new method reliably recovers the dynamo coefficients from the MHD simulations. It also allows priors on the dynamo coefficients to be incorporated easily during the inversion, unlike in earlier methods. Moreover, using synthetic data, we demonstrate that it may serve as a viable post-processing tool in determining the dynamo coefficients, even when the power of additive noise to the EMF is twice as much the actual EMF. [ABSTRACT FROM AUTHOR]

Context. Hadronic interactions between cosmic rays (CRs) and interstellar gas have been probed in γ rays across the Galaxy. A fairly uniform CR distribution is observed up to a few hundred parsecs from the Sun, except in the Eridu cloud, which shows an unexplained 30–50% deficit in GeV to TeV CR flux. Aims. To explore the origin of this deficit, we studied the Reticulum cloud, which shares notable traits with Eridu: a comparable distance in the low-density region of the Local Valley and a filamentary structure of atomic hydrogen extending along a bundle of ordered magnetic-field lines that are steeply inclined to the Galactic plane. Methods. We measured the γ-ray emissivity per gas nucleon in the Reticulum cloud in the 0.16–63 GeV energy band using 14 years of Fermi-LAT data. We also derived interstellar properties that are important for CR propagation in both the Eridu and Reticulum clouds, at the same parsec scale. Results. The γ-ray emissivity in the Reticulum cloud is fully consistent with the average spectrum measured in the solar neighbourhood, but this emissivity, and therefore the CR flux, is 1.57 ± 0.09 times larger than in Eridu across the whole energy band. The difference cannot be attributed to uncertainties in gas mass. Nevertheless, we find that the two clouds are similar in many respects: both have magnetic-field strengths of a few micro-Gauss in the plane of the sky; both are in approximate equilibrium between magnetic and thermal pressures; they have similar turbulent velocities and sonic Mach numbers; and both show magnetic-field regularity with a dispersion in orientation lower than 10°–15° over large zones. The gas in Reticulum is colder and denser than in Eridu, but we find similar parallel diffusion coefficients around a few times 1028 cm2 s−1 in both clouds if CRs above 1 GV in rigidity diffuse on resonant, self-excited Alfvén waves that are damped by ion-neutral interactions. Conclusions. The loss of CRs in Eridu remains unexplained, but these two clouds provide important test cases to further study how magnetic turbulence, line tangling, and ion-neutral damping regulate CR diffusion in the dominant gas phase of the interstellar medium. [ABSTRACT FROM AUTHOR]

Magnetic fields play a crucial role in star formation, yet tracing them becomes particularly challenging, especially in the presence of outflow feedback in protostellar systems. We targeted the star-forming region L1551, notable for its apparent outflows, to investigate the magnetic fields. These fields were probed using polarimetry observations from the Planck satellite at 353 GHz/849 μm, the Stratospheric Observatory for Infrared Astronomy's (SOFIA) High-resolution Airborne Wide-band Camera (HAWC+) measurement at 214 μm, and the James Clerk Maxwell Telescope's (JCMT) Submillimetre Common-User POLarimeter (SCUPOL) 850 μm survey. Consistently, all three measurements show that the magnetic fields twist towards the protostar IRS 5. Additionally, we utilized the velocity gradient technique on the 12CO (J = 1–0) emission data to distinguish the magnetic fields directly associated with the protostellar outflows. These were then compared with the polarization results. Notably, in the outskirts of the region, these measurements generally align. However, as one approaches the centre of IRS 5, the measurements tend to yield mostly perpendicular relative orientations. This suggests that the outflows might be dynamically significant from a scale of ∼0.2 pc, causing the velocity gradient to change direction by 90°. Furthermore, we discovered that the polarization fraction p and the total intensity I correlate as p  ∝  I −α. Specifically, α is approximately 1.044 ± 0.06 for SCUPOL and around 0.858 ± 0.15 for HAWC+. This indicates that the outflows could significantly impact the alignment of dust grains and magnetic fields in the L1551 region. [ABSTRACT FROM AUTHOR]

The Snake is a remarkable Galactic Centre radio filament with a morphology characterized by two kinks along its ∼20 arcmin extent. The major and minor kinks are located where the filament is most distorted from a linear magnetized structure running perpendicular to the Galactic plane. We present Chandra , VLA, and MeerKAT data and report the detection of an X-ray and radio source at the location of the major kink. High-resolution radio images of the major kink reveal a compact source with a steep spectrum with spectral index α ∼ −2.7 surrounded by extended emission. The radio luminosity and steep spectrum of the compact source are consistent with a pulsar. We also show flattening of the spectrum and enhanced synchrotron emissivity away from the position of the major kink along the Snake, which suggests injection of relativistic particles along the Snake. We argue that the major kink is created by a fast-moving (∼500–1000 km s−1) object punching into the Snake, distorting its magnetic structure, and producing X-ray emission. X-ray emission pinpoints an active acceleration site where the interaction is taking place. A secondary kink is argued to be induced by the impact of the high-velocity object producing the major kink. [ABSTRACT FROM AUTHOR]

Cosmic rays (CRs) may drive outflows and alter the phase structure of the circumgalactic medium, with potentially important implications on galaxy formation. However, these effects ultimately depend on the dominant mode of transport of CRs within and around galaxies, which remains highly uncertain. To explore potential observable constraints on CR transport, we investigate a set of cosmological fire -2 CR-magnetohydrodynamic simulations of L* galaxies which evolve CRs with transport models motivated by self-confinement (SC) and extrinsic turbulence (ET) paradigms. To first order, the synchrotron properties diverge between SC and ET models due to a CR physics-driven hysteresis. SC models show a higher tendency to undergo 'ejective' feedback events due to a runaway buildup of CR pressure in dense gas due to the behaviour of SC transport scalings at extremal CR energy densities. The corresponding CR wind-driven hysteresis results in brighter, smoother, and more extended synchrotron emission in SC runs relative to ET and constant diffusion runs. The differences in synchrotron arise from different morphology, interstellar medium gas, and B properties, potentially ruling out SC as the dominant mode of CR transport in typical star-forming L* galaxies, and indicating the prospect for non-thermal radio continuum observations to constrain CR transport physics. [ABSTRACT FROM AUTHOR]

Dust polarization observations at optical wavelengths help understand the dust grain properties and trace the plane-of-the-sky component of the magnetic field. In this study, we make use of published optical polarization data acquired with AIMPOL along with distances (d) and extinction (A V) data. We study the variation of polarization efficiency (P / A V) as a function of A V in the diffuse interstellar medium (ISM) and intracluster mediums (ICM) using the already published polarization data of six clusters. Among these clusters, NGC 2281, NGC 1664, and NGC 1960 are old; while Stock 8, NGC 1931, and NGC 1893 are young. We categorize stars towards each cluster into foreground, background, and cluster members by employing two clustering algorithms GMM and DBSCAN. Thus, classified field stars and cluster members are used to reveal the polarization properties of ISM and ICM dust, respectively. We find that the dust grains located in the diffuse ISM show higher polarization efficiencies when compared to those located in the ICM of younger clusters. [ABSTRACT FROM AUTHOR]

In this work, we report the role of turbulence (measured by the |$^{12}\mathrm{ CO}$| linewidth, |$\Delta V$|⁠) on the offset between the orientation of local magnetic field in the low-density regions of dark clouds (⁠|$\theta ^{\mathrm{ env}}_\mathrm{ B}$|⁠) and the position angle of the Galactic Plane (GP; |$\theta _{\mathrm{ GP}}$|⁠) in 15 low-mass isolated molecular clouds. We find a dependency of |$\Delta V$| on offset, (⁠|$\theta _{\mathrm{ off}} =|\theta ^{\mathrm{ env}}_\mathrm{ B}-\theta _{\mathrm{ GP}}|$|⁠), represented by a second-order polynomial equation, |$\theta _{\mathrm{ off}}=(3.328 \pm 1.187) {\Delta V}^2 - (0.991 \pm 5.541) {\Delta V} + (4.767 \pm 5.309)$|⁠. In regions where the majority of low turbulence is observed, characterized by |$\Delta V \ \lt\ 3.1 \ \mathrm{ km\,s}^{-1}$|⁠ , the envelope magnetic fields closely align with the orientation of the GP (⁠|$\theta _{\mathrm{ off}}\ \lt\ 20^\circ$|⁠). Conversely, in areas exhibiting high turbulence with |$\Delta V \ \gt\ 3.1 \ \mathrm{ km\,s}^{-1}$| (for clouds CB130 and CB188), the envelope magnetic field tends to misalign with the GP (⁠|$\theta _{\mathrm{ off}}\ \gt\ 50^\circ$|⁠). This suggests that turbulence could be one of the important factors influencing the alignment of the local magnetic field with the GP. We have also explored the potential link between gas density of clouds (⁠|$\rho$|⁠) and turbulence (⁠|$\Delta V$|⁠) for the first time, utilizing data from 10 isolated clouds. This analysis suggests a possible correlation, with the observed trend quantified as, |$\Delta V \propto \rho ^{-0.19}$|⁠. [ABSTRACT FROM AUTHOR]

Magnetic fields are a defining yet enigmatic aspect of the interstellar medium, with their three-dimensional (3D) mapping posing a substantial challenge. In this study, we harness the innovative velocity gradient technique (VGT), underpinned by magnetohydrodynamic turbulence theories, to map the magnetic field structure by applying it to the atomic neutral hydrogen (H  i) emission line and the molecular tracer 12CO. We construct the tomography of the magnetic field in the low-mass star-forming region L1688, utilizing two approaches: (1) VGT-H  i combined with the Galactic rotational curve, and (2) stellar polarization paired with precise star parallax measurements. Our analysis reveals that the magnetic field orientations deduced from stellar polarization undergo a distinct directional change in the vicinity of L1688, providing evidence that the misalignment between VGT-H  i and stellar polarization stems from the influence of the molecular cloud's magnetic field on the polarization of starlight. When comparing VGT-12CO to stellar polarization and Planck polarization data, we observe that VGT-12CO effectively reconciles the misalignment noted with VGT-H  i , showing statistical alignment with Planck polarization measurements. This indicates that VGT-12CO could be integrated with VGT-H  i , offering vital insights into the magnetic fields of molecular clouds, thereby enhancing the accuracy of our 3D magnetic field reconstructions. [ABSTRACT FROM AUTHOR]

The relative importance of magnetic fields, turbulence, and gravity in the early phases of star formation is still not well understood. We report the first high-resolution dust polarization observations at 850  |$\mu$| m around the most massive clump, located at the hub of the Giant Molecular Cloud G148.24+00.41, using SCUBA-2/POL-2 at the James Clerk Maxwell Telescope. We find that the degree of polarization decreases steadily towards the denser portion of the cloud. Comparing the intensity gradients and local gravity with the magnetic field orientations, we find that local gravity plays a dominant role in driving the gas collapse as the magnetic field orientations and gravity vectors seem to point towards the dense clumps. We also find evidence of U-shaped magnetic field morphology towards a small-scale elongated structure associated with the central clump, hinting at converging accretion flows towards the clump. Our observation has resolved the massive clump into multiple substructures. We study the magnetic field properties of two regions, central clump (CC) and northeastern elongated structure (NES). Using the modified Davis–Chandrasekhar–Fermi method, we determine that the magnetic field strengths of CC and NES are ∼24.0 ± 6.0  |$\mu$| G and 20.0 ± 5.0  |$\mu$| G, respectively. The mass-to-flux ratios are found to be magnetically transcritical/supercritical, while the Alfv |$\acute{\text{e}}$| n Mach number indicates a trans-Alfv |$\acute{\text{e}}$| nic state in both regions. These results, along with Virial analysis, suggest that at the hub of G148.24+00.41, gravitational energy has an edge over magnetic and kinetic energies. [ABSTRACT FROM AUTHOR]

Synchrotron emission is one of few observable tracers of galactic magnetic fields (B) and cosmic rays (CRs). Much of our understanding of B in galaxies comes from utilizing synchrotron observations in conjunction with several simplifying assumptions of equipartition models, however, it remains unclear how well these assumptions hold, and what B these estimates physically represent. Using Feedback in Realistic Environments project simulations which self-consistently evolve CR proton, electron, and positron spectra from MeV to TeV energies, we present the first synthetic synchrotron emission predictions from simulated L * galaxies with 'live' spectrally resolved CR-magnetohydrodynamic. We find that synchrotron emission can be dominated by relatively cool and dense gas, resulting in equipartition estimates of B with fiducial assumptions underestimating the 'true' B in the gas that contributes the most emission by factors of 2–3 due to small volume-filling factors. Motivated by our results, we present an analytical framework that expands upon equipartition models for estimating B in a multiphase medium. Comparing our spectrally resolved synchrotron predictions to simpler spectral assumptions used in galaxy simulations with CRs, we find that spectral evolution can be crucial for accurate synchrotron calculations towards galactic centres, where loss terms are large. [ABSTRACT FROM AUTHOR]

Context. The fragmentation of high-mass star-forming regions depends on a variety of physical parameters, including density, the magnetic field, and turbulent gas properties. Aims. We evaluate the importance of the density and magnetic field structures in relation to the fragmentation properties during high-mass star formation. Methods. Observing the large parsec-scale Stokes I millimeter dust continuum emission with the IRAM 30 m telescope and the intermediate-scale (<0.1 pc) polarized submillimeter dust emission with the Submillimeter Array toward a sample of 20 high-mass star-forming regions allows us to quantify the dependence of the fragmentation behavior of these regions on the density and magnetic field structures. Results. Based on the IRAM 30 m data, we infer density distributions n ∝ r−p of the regions with typical power-law slopes p around ~1.5. There is no obvious correlation between the power-law slopes of the density structures on larger clump scales (~1 pc) and the number of fragments on smaller core scales (<0.1 pc). Comparing the large-scale single-dish density profiles to those derived earlier from interferometric observations at smaller spatial scales, we find that the smaller-scale power-law slopes are steeper, typically around ~2.0. The flattening toward larger scales is consistent with the star-forming regions being embedded in larger cloud structures that do not decrease in density away from a particular core. The magnetic fields of several regions appear to be aligned with filamentary structures that lead toward the densest central cores. Furthermore, we find different polarization structures; some regions exhibit central polarization holes, whereas other regions show polarized emission also toward the central peak positions. Nevertheless, the polarized intensities are inversely related to the Stokes I intensities, following roughly a power-law slope of ∝ SI−0.62. We estimate magnetic field strengths between ~0.2 and ~4.5 mG, and we find no clear correlation between magnetic field strength and the fragmentation level of the regions. A comparison of the turbulent to magnetic energies shows that they are of roughly equal importance in this sample. The mass-to-flux ratios range between ~2 and ~7, consistent with collapsing star-forming regions. Conclusions. Finding no clear correlations between the present-day large-scale density structure, the magnetic field strength, and the smaller-scale fragmentation properties of the regions, indicates that the fragmentation of high-mass star-forming regions may not be affected strongly by the initial density profiles and magnetic field properties. However, considering the limited evolutionary range and spatial scales of the presented CORE analysis, future research directions should include density structure analysis of younger regions that better resemble the initial conditions, as well as connecting the observed intermediate-scale magnetic field structure with the larger-scale magnetic fields of the parental molecular clouds. [ABSTRACT FROM AUTHOR]

Gamma-ray bursts (GRBs) are one of the most exciting sources that offer valuable opportunities for investigating the evolution of energy fraction given to magnetic fields and particles through microphysical parameters during relativistic shocks. The delayed onset of GeV–TeV radiation from bursts detected by the Fermi Large Area Telescope and Cherenkov Telescopes provide crucial information in favour of the external-shock model. Derivation of the closure relations (CRs) and the light curves in external shocks requires knowledge of GRB afterglow physics. In this manuscript, we derive the CRs and light curves in a stratified medium with variations of microphysical parameters of the synchrotron and synchrotron self-Compton (SSC) afterglow model radiated by an electron distribution with a hard and soft spectral index. Using Markov chain Monte Carlo simulations, we apply the current model to investigate the evolution of the spectral and temporal indices of those GRBs reported in the Second Gamma-ray Burst Catalog (2FLGC), which comprises 29 bursts with photon energies above 10 GeV and of those bursts (GRB 180720B, 190114C, 190829A, and 221009A) with energetic photons above 100 GeV, which can hardly be modelled with the CRs of the standard synchrotron scenario. The analysis shows that (i) the most likely afterglow model using synchrotron and SSC emission on the 2FLGC corresponds to the constant-density scenario, and (ii) variations of spectral (temporal) index keeping the temporal (spectral) index constant could be associated with the evolution of microphysical parameters, as exhibited in GRB 190829A and GRB 221009A. [ABSTRACT FROM AUTHOR]

We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival Green Bank Telescope (GBT), Murchison Widefield Array, and Very Large Array (VLA) images at 333, 88, and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud associated with the halo of the Sgr B complex on a scale of ∼60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic ray electrons. Correlation of the Fe  i 6.4 keV K α line and synchrotron emission provides compelling evidence that the low energy cosmic ray electrons are responsible for producing the K α line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has a mean spectral index α ∼ −1 ± 1, which gives the magnetic field strength ∼100 µG for cloud densities n H = 104–105 cm−3, and estimated cosmic ray ionization rates between 10−13 and 10−14 s−1. Furthermore, the energy spectrum of primary cosmic ray electrons is constrained to be E −3 ± 1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and γ-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic ray electron fluxes at the Galactic centre play in production of radio synchrotron emission, the Fe  i K α line emission at 6.4 keV, and ∼GeV γ-ray emission throughout the Central Molecular Zone. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the stability analysis of two regions, OMC-3 and OMC-4, in the massive and long molecular cloud complex of Orion A. We obtained 214 $\mu$m HAWC + /SOFIA polarization data, and we make use of archival data for the column density and C18O (1–0) emission line. We find clear depolarization in both observed regions and that the polarization fraction is anticorrelated with the column density and the polarization-angle dispersion function. We find that the filamentary cloud and dense clumps in OMC-3 are magnetically supercritical and strongly subvirial. This region should be in the gravitational collapse phase and is consistent with many young stellar objects (YSOs) forming in the region. Our histogram of relative orientation (HRO) analysis shows that the magnetic field is dynamically sub-dominant in the dense gas structures of OMC-3. We present the first polarization map of OMC-4. We find that the observed region is generally magnetically subcritical except for an elongated dense core, which could be a result of projection effect of a filamentary structure aligned close to the line of sight. The relative large velocity dispersion and the unusual positive shape parameters at high column densities in the HROs analysis suggest that our viewing angle may be close to axes of filamentary substructures in OMC-4. The dominating strong magnetic field in OMC-4 is unfavourable for star formation and is consistent with much fewer YSOs than in OMC-3.

Optical and infrared polarization mapping and recent Planck observations of the filametary cloud L1495 in Taurus show that the large-scale magnetic field is approximately perpendicular to the long axis of the cloud. We use the HAWC + polarimeter on SOFIA to probe the complex magnetic field in the B211 part of the cloud. Our results reveal a dispersion of polarization angles of 36 °, about five times that measured on a larger scale by Planck. Applying the Davis-Chandrasekhar-Fermi (DCF) method with velocity information obtained from Institut de Radioastronomie Millimétrique 30 m C 18 O(1-0) observations, we find two distinct sub-regions with magnetic field strengths differing by more than a factor 3. The quieter sub-region is magnetically critical and sub-Alfvé nic; the field is comparable to the average field measured in molecular clumps based on Zeeman observations. The more chaotic, super-Alfvé nic sub-region shows at least three velocity components, indicating interaction among multiple substructures. Its field is much less than the average Zeeman field in molecular clumps, suggesting that the DCF value of the field there may be an underestimate. Numerical simulation of filamentary cloud formation shows that filamentary substructures can strongly perturb the magnetic field. DCF and true field values in the simulation are compared. Pre-stellar cores are observed in B211 and are seen in our simulation. The appendices give a deri v ation of the standard DCF method that allows for a dispersion in polarization angles that is not small, present an alternate deri v ation of the structure function version of the DCF method, and treat fragmentation of filaments.

We characterize the kinematic and magnetic properties of H  i filaments located in a high Galactic latitude region (165° < α < 195° and 12° < δ < 24°). We extract three-dimensional filamentary structures using fil3d from the Galactic Arecibo L -Band Feed Array H  i (GALFA-H  i) survey 21-cm emission data. Our algorithm identifies coherent emission structures in neighbouring velocity channels. Based on the mean velocity, we identify a population of local and intermediate velocity cloud (IVC) filaments. We find the orientations of the local (but not the IVC) H  i filaments are aligned with the magnetic field orientations inferred from Planck 353 GHz polarized dust emission. We analyse position–velocity diagrams of the velocity-coherent filaments, and find that only 15 per cent of filaments demonstrate significant major-axis velocity gradients with a median magnitude of 0.5 km s−1 pc−1, assuming a fiducial filament distance of 100 pc. We conclude that the typical diffuse H  i filament does not exhibit a simple velocity gradient. The reported filament properties constrain future theoretical models of filament formation. [ABSTRACT FROM AUTHOR]

The hydrogen atoms penetrate the heliosphere from the local interstellar medium, and while being ionized, they form the population of pickup protons. The distribution of pickup protons is modified by the adiabatic heating (cooling) induced by the solar wind plasma compression (expansion). In this study, we emphasize the importance of the adiabatic energy change in the inner heliosheath that is usually either neglected or considered improperly. The effect of this process on the energy and spatial distributions of pickup protons and energetic neutral atoms (ENAs), which originate in the charge exchange of pickup protons, has been investigated and quantified using a kinetic model. The model employs the global distributions of plasma and hydrogen atoms in the heliosphere from the simulations of a kinetic-magnetohydrodynamic model of solar wind interaction with the local interstellar medium. The findings indicate that the adiabatic energy change is responsible for the broadening of the pickup proton velocity distribution and the significant enhancement of ENA fluxes (up to ∼5 and ∼20 times in the upwind and downwind directions at energies ∼1–2 keV for an observer at 1 au). It sheds light on the role of adiabatic energy change in explaining the discrepancies between the ENA flux observations and the results of numerical simulations. [ABSTRACT FROM AUTHOR]

Studies of dense core morphologies and their orientations with respect to gas flows and the local magnetic field have been limited to only a small sample of cores with spectroscopic data. Leveraging the Green Bank Ammonia Survey alongside existing sub-millimeter continuum observations and Planck dust polarization, we produce a cross-matched catalogue of 399 dense cores with estimates of core morphology, size, mass, specific angular momentum, and magnetic field orientation. Of the 399 cores, 329 exhibit 2D vLSR maps that are well fit with a linear gradient, consistent with rotation projected on the sky. We find a best-fit specific angular momentum and core size relationship of J / M  ∝  R 1.82 ± 0.10, suggesting that core velocity gradients originate from a combination of solid body rotation and turbulent motions. Most cores have no preferred orientation between the axis of core elongation, velocity gradient direction, and the ambient magnetic field orientation, favouring a triaxial and weakly magnetized origin. We find, however, strong evidence for a preferred anti-alignment between the core elongation axis and magnetic field for protostellar cores, revealing a change in orientation from starless and prestellar populations that may result from gravitational contraction in a magnetically-regulated (but not dominant) environment. We also find marginal evidence for anti-alignment between the core velocity gradient and magnetic field orientation in the L1228 and L1251 regions of Cepheus, suggesting a preferred orientation with respect to magnetic fields may be more prevalent in regions with locally ordered fields. [ABSTRACT FROM AUTHOR]

We report on the near-infrared polarimetric observations of G11.11–0.12 (hereafter G11) obtained with SIRPOL on the 1.4 m IRSF telescope. The starlight polarisation of the background stars reveals the on-sky component of magnetic fields in G11, and these are consistent with the field orientation observed from polarised dust emission at |$850\, \mu$| m. The magnetic fields in G11 are perpendicular to the filament, and are independent of the filament's orientation relative to the Galactic plane. The field strength in the envelope of G11 is in the range |$50-100\, \mu$| G, derived from two methods. The analyses of the magnetic fields and gas velocity dispersion indicate that the envelope of G11 is supersonic but sub-Alfvénic. The critical mass-to-flux ratio in the envelope of G11 is close to 1 and increases to ≳ 1 on the spine of G11. The relative weights on the importance of magnetic fields, turbulence and gravity indicate that gravity dominates the dynamical state of G11, but with significant contribution from magnetic fields. The field strength, | B |, increases slower than the gas density, n , from the envelope to the spine of G11, characterized by | B |∝ n 0.3. The observed strength and orientation of magnetic fields in G11 imply that supersonic and sub-Alfvénic gas flow is channelled by the strong magnetic fields and is assembled into filaments perpendicular to the magnetic fields. The formation of low-mass stars is enhanced in the filaments with high column density, in agreement with the excess of low-mass protostars detected in the densest regions of G11. [ABSTRACT FROM AUTHOR]

We present Faraday rotation measure (RM) values derived at L and P bands as well as some 60 Stokes-parameter profiles, both determined from our long-standing Arecibo dual-frequency pulsar polarimetry programmes. Many of the RM measurements were carried out towards the inner Galaxy and the anticentre on pulsars with no previous determination, while others are remeasurements intended to confirm or improve the accuracy of existing values. Stokes-parameter profiles are displayed for the 58 pulsars for which no meaningful Stokes profile at lower frequency is available and 4 without a high-frequency pair. This is a population that includes many distant pulsars in the inner Galaxy. A number of these polarized pulse profiles exhibit clear interstellar-scattering tails; none the less, we have attempted to interpret the associated emission-beam structures and to provide morphological classifications and geometrical models where possible. [ABSTRACT FROM AUTHOR]

We measure the mean-squared polarization fraction of a sample of 6282 Galactic cold clumps at 353 GHz, consisting of Planck Galactic cold clump (PGCC) catalogue category 1 objects [flux densities measured with signal-to-noise ratio (S/N) > 4]. At 353 GHz, we find the mean-squared polarization fraction, which we define as the mean-squared polarization divided by the mean-squared intensity, to be (4.79 ± 0.44) × 10−4 equation to an |$11\, \sigma$| detection of polarization. We test if the polarization fraction depends on the clumps' physical properties, including flux density, luminosity, Galactic latitude, and physical distance. We see a trend towards increasing polarization fraction with increasing Galactic latitude, but find no evidence that polarization depends on the other tested properties. The Simons Observatory, with angular resolution of order 1 arcmin and noise levels between 22 and |$54\, \mu$| K−arcmin at high frequencies, will substantially enhance our ability to determine the magnetic field structure in Galactic cold clumps. At |$\ge 5\, \sigma$| significance, we predict the Simons Observatory will detect at least ∼12 000 cold clumps in intensity and ∼430 cold clumps in polarization. This number of polarization detections would represent a two orders of magnitude increase over the current Planck results. We also release software that can be used to mask these Galactic cold clumps in other analyses. [ABSTRACT FROM AUTHOR]

The turbulent small-scale dynamo (SSD) is likely to be responsible for the magnetization of the interstellar medium (ISM) that we observe in the Universe today. The SSD efficiently converts kinetic energy E kin into magnetic energy E mag and is often used to explain how an initially weak magnetic field with E mag ≪ E kin is amplified, and then maintained at a level E mag ≲ E kin. Usually, this process is studied by initializing a weak seed magnetic field and letting the turbulence grow it to saturation. However, in this Part I of the Growth or Decay series, using three-dimensional, visco-resistive magnetohydrodynamical turbulence simulations up to magnetic Reynolds numbers of 2000, we show that the same final state in the integral quantities, energy spectra, and characteristic scales of the magnetic field can also be achieved if initially E mag ∼ E kin or even if initially E mag ≫ E kin. This suggests that the final saturated state of the turbulent dynamo is set by the turbulence and the material properties of the plasma, independent of the initial structure or amplitude of the magnetic field. We discuss the implications this has for the maintenance of magnetic fields in turbulent plasmas and future studies exploring the dynamo saturation. [ABSTRACT FROM AUTHOR]

This work presents the magnetic field geometry in a complex of three cometary (with head–tail morphology) globules, namely LDN 323, LDN 328, and LDN 331, using R -band polarization measurements of background stars. These observations were combined with a Planck sky survey to study the large-scale morphology of the magnetic fields in the region. The distances of the target stars were adopted from the Gaia catalogue. The variation of the degree of polarization and polarization position angle with distances of stars is analysed. The field geometry is mostly found to follow the cometary shape of the cloud, with some randomness at certain locations. For studying the correlation between cloud morphology and magnetic field orientations, a modified version of the histogram of relative orientation (HRO) analysis was employed. [ABSTRACT FROM AUTHOR]

Context. The main driving forces supplying energy to the interstellar medium (ISM) are supernova explosions and stellar winds. Such localized sources are assimilable to curl-free velocity fields as a first approximation. They need to be combined with other physical processes to replicate real galactic environments, such as the presence of turbulence and a dynamo-sustained magnetic field in the ISM. Aims. This work is focused on the effect of an irrotational forcing on a magnetized flow in the presence of rotation, baroclinicity, shear, or a combination of any of the three. It follows an earlier analysis with a similar focus, namely, subsonic spherical expansion waves in hydrodynamic simulations. By including magnetic field in the model, we can evaluate the occurrence of dynamo on both small and large scales. We aim to identify the minimum ingredients needed to trigger a dynamo instability as well as the relation between dynamo and the growth of vorticity. Methods. We used the Pencil code to run resistive magnetohydrodynamic direct numerical simulations, exploring the ranges of values of several physical and numerical parameters of interest. We explored Reynolds numbers up to a few hundreds. We analyzed the temporal evolution of vorticity, kinetic, and magnetic energy, as well as their features in Fourier space. Results. We report the absence of a small-scale dynamo in all cases where only rotation is included, regardless of the given equation of state and rotation rate. Conversely, the inclusion of a background sinusoidal shearing profile leads to an hydrodynamic instability that produces an exponential growth of the vorticity at all scales, starting from small ones. This is know as vorticity dynamo. The onset of this instability occurs after a rather long temporal evolution of several thousand turbulent turnover times. The vorticity dynamo in turn drives an exponential growth of the magnetic field, first at small scales, followed by large ones. The instability is then saturated and the magnetic field approximately reaches equipartition with the turbulent kinetic energy. During the saturation phase, we can observe a winding of the magnetic field in the direction of the shearing flow. By varying the intensity of the shear, we see that the growth rates of this instability change. The inclusion of the baroclinic term has the main effect of delaying the onset of the vorticity dynamo, but then leads to a more rapid growth. Conclusions. Our work demonstrates how even purely irrotational forcing may lead to dynamo action in the presence of shear, thus amplifying the field to an equipartition level. At the same time, we confirm that purely irrotational forcing alone does not lead to any growth in terms of the vorticity, nor the magnetic field. This picture does not change in the presence of rotation or baroclinicity, at least up to a resolution of 2563 mesh points. To further generalize such a conclusion, we will need to explore how this setup works both at higher magnetic Reynolds numbers and with different prescriptions of the irrotational forcing. [ABSTRACT FROM AUTHOR]

Star formation is a complex process that typically occurs in dense regions of molecular clouds mainly regulated by magnetic fields, magnetohydrodynamic (MHD) turbulence, and self-gravity. However, it remains a challenging endeavour to trace the magnetic field and determine regions of gravitational collapse, where the star is forming. Based on the anisotropic properties of MHD turbulence, a new technique termed velocity gradient technique (VGT) has been proposed to address these challenges. In this study, we apply the VGT to two regions of the giant California Molecular Cloud (CMC), namely, L1478 and L1482, and analyse the difference in their physical properties. We use the 12CO (J = 2–1), 13CO (J = 2–1), and C18O (J = 2–1) emission lines observed with the Heinrich Hertz Submillimeter Telescope. We compare VGT results calculated in the resolutions of 3.3 and 10 arcmin to Planck polarization at 353 GHz and 10 arcmin to determine areas of MHD turbulence dominance and self-gravity dominance. We show that the resolution difference can introduce misalignment between the two measurements. We find the VGT-measured magnetic fields globally agree with those from Planck in L1478, suggesting self-gravity's effect is insignificant. The best agreement appears in VGT-12CO. As for L1482, the VGT measurements are statistically perpendicular to the Planck polarization indicating the dominance of self-gravity. This perpendicular alignment is more significant in VGT-13CO and VGT-C18O. [ABSTRACT FROM AUTHOR]

The Davis–Chandrasekhar–Fermi (DCF) method is widely employed to estimate the mean magnetic field strength in astrophysical plasmas. In this study, we present a numerical investigation using the DCF method in conjunction with a promising new diagnostic tool for studying magnetic fields: the polarization of spectral lines resulting from the atomic alignment effect. We obtain synthetic spectropolarimetry observations from 3D magnetohydrodynamic (MHD) turbulence simulations and estimate the mean magnetic field projected onto the plane of the sky using the DCF method with ground-state-alignment (GSA) polarization maps and a modification to account for the driving scale of turbulence. We also compare the method to the classical DCF approach using dust polarization observations. Our observations indicate that the modified DCF method correctly estimates the plane-of-sky projected magnetic field strengths for sub-Alfvénic turbulence using a newly proposed correction factor of ξ′ ∈ 0.35–0.75. We find that the field strengths are accurately obtained for all magnetic field inclination and azimuth angles. We also observe a minimum threshold for the mean magnetic field inclination angle with respect to the line of sight, θB ∼ 16°, for the method. The magnetic field dispersion traced by the polarization from the spectral lines is comparable in accuracy to dust polarization, while mitigating some of the uncertainties associated with dust observations. The measurements of the DCF observables from the same atomic/ionic line targets ensure the same origin for the magnetic field and velocity fluctuations and offer a possibility of tracing the 3D direction of the magnetic field. [ABSTRACT FROM AUTHOR]

We present the results of a set of radiation magnetohydrodynamic simulations of turbulent molecular clouds in which we vary the initial strength of the magnetic field within a range (1 ≲ μ ≲ 5) consistent with observations of local giant molecular clouds (GMCs). We find that as we increase the strength of the magnetic field, star formation transitions from unimodal (the baseline case, μ = 5, with a single burst of star formation and Salpeter IMF) to bimodal. This effect is clearest in the most strongly magnetized GMC (μ = 1): a first burst of star formation with duration, intensity, and IMF comparable to the baseline case, is followed by a second star formation episode in which only low-mass stars are formed. Overall, due to the second burst of star formation, the strongly magnetized case results in a longer star formation period and a higher efficiency of star formation. The second burst is produced by gas that is not expelled by radiative feedback, instead remains trapped in the GMC by the large-scale B -field, producing a nearly one-dimensional flow of gas along the field lines. The trapped gas has a turbulent and magnetic topology that differs from that of the first phase and strongly suppresses gas accretion onto protostellar cores, reducing their masses. We speculate that this star formation bimodality may be an important ingredient to understand the origin of multiple stellar populations observed in massive globular clusters. [ABSTRACT FROM AUTHOR]

We present a study of the effects of magnetic fields on the formation of massive, self-gravitationally bound cores (MBCs) in high-speed cloud–cloud collisions (CCCs). Extending our previous work, we perform magnetohydrodynamic simulations of the high-speed (20–40 km s−1) collisions between two magnetized (initial 4  |$\mu\!$| G), turbulent clouds of different sizes in the range of 7–20 pc. We show that a magnetic field hinders the growth of cores, particularly in the case of short-duration collisions where cores are not seen to reach a highly bound state. In such cases, a shocked region created by the collision rapidly expands into the ambient medium owing to the enhanced magnetic pressure, resulting in the destruction of the highly unbound cores and suppression of gas accretion. This negative effect on the formation of MBCs has not been reported in previous hydrodynamic simulations of CCCs. Together with our previous work, we conclude that magnetic fields provide two competing effects on the MBC formation in CCCs; while they promote the mass accumulation onto cores during a collision, they operate to destroy cores or hinder the core growth after the collision. The duration of such collisions determines which effect prevails, resulting in a maximum collision speed for the MBC formation within given colliding clouds. Our results agree with the observed trend among CCC samples in the column density range probed in the simulations presented here; CCCs with higher relative velocities require higher column densities for the formation of massive stars. [ABSTRACT FROM AUTHOR]

We study the alignment and rotational disruption of dust grains at the centre of our Galaxy using polarized thermal dust emission observed by SOFIA/HAWC+ and JCMT/SCUPOL at 53, 216, and 850 µm. We analysed the relationship between the observed polarization degree with total emission intensity, dust temperature, gas column density, and polarization angle dispersion. Polarization degree from this region follows the predictions of the RAdiative Torque (RAT) alignment theory, except at high temperatures and long wavelengths where we found evidence for the rotational disruption of grains as predicted by the RAdiative Torque Disruption mechanism. The grain alignment and disruption sizes were found to be around 0.1 and 1 µm,  respectively. The maximum polarization degree observed was around p ∼ 13 per cent at 216 µm and comes from a region of high dust temperature, low column density, and ordered magnetic field. Magnetically enhanced RAT alignment (MRAT) was found to be important for grain alignment due to the presence of a strong magnetic field and can induce perfect alignment even when grains contain small iron clusters. We estimated the mass fraction of aligned grains using a parametric model for the fraction of the grains at high- J attractors and found it to correlate weakly with the observed polarization degree. We observe a change in the polarization ratio, from p 216µm/ p 850µm < 1 to p 216µm/ p 850µm > 1 at T d ≳ 35 K, which suggests a change in the grain model from a composite to a separate population of carbon and silicate grains as implied by previous numerical modelling. [ABSTRACT FROM AUTHOR]

The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization in the quest for the curl-like B-mode polarization from primordial gravitational waves and the low-multipole E-mode polarization associated with the reionization of the Universe. We used the new Planck PR3 maps to characterize Galactic dust emission at high latitudes as a foreground to the CMB polarization and use end-to-end simulations to compute uncertainties and assess the statistical significance of our measurements. We present Planck EE, BB, and TE power spectra of dust polarization at 353 GHz for a set of six nested high-Galactic-latitude sky regions covering from 24 to 71% of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over sky regions and a difference between the values for the EE and BB spectra, which for the largest sky region are αEE = -2.42 ± 0.02 and αBB = -2.54 ± 0.02, respectively. The spectra show that the TE correlation and E/B power asymmetry discovered by Planck extend to low multipoles that were not included in earlier Planck polarization papers due to residual data systematics. We also report evidence for a positive TB dust signal. Combining data from Planck and WMAP, we have determined the amplitudes and spectral energy distributions (SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six sky regions as a function of multipole. This quantifies the challenge of the component-separation procedure that is required for measuring the low-ℓ reionization CMB E-mode signal and detecting the reionization and recombination peaks of primordial CMB B modes. The SED of polarized dust emission is fit well by a single-temperature modified black-body emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean dust spectral index for dust polarization is βdP = 1.53±0.02. The difference between indices for polarization and total intensity is βdP-βdI = 0.05±0.03. By fitting multi-frequency cross-spectra between Planck data at 100, 143, 217, and 353 GHz, we examine the correlation of the dust polarization maps across frequency. We find no evidence for a loss of correlation and provide lower limits to the correlation ratio that are tighter than values we derive from the correlation of the 217- and 353 GHz maps alone. If the Planck limit on decorrelation for the largest sky region applies to the smaller sky regions observed by sub-orbital experiments, then frequency decorrelation of dust polarization might not be a problem for CMB experiments aiming at a primordial B-mode detection limit on the tensor-to-scalar ratio r≃ 0.01 at the recombination peak. However, the Planck sensitivity precludes identifying how difficult the component-separation problem will be for more ambitious experiments targeting lower limits on r.

The study of polarized dust emission has become entwined with the analysis of the cosmic microwave background (CMB) polarization in the quest for the curl-like B-mode polarization from primordial gravitational waves and the low-multipole E-mode polarization associated with the reionization of the Universe. We used the new Planck PR3 maps to characterize Galactic dust emission at high latitudes as a foreground to the CMB polarization and use end-to-end simulations to compute uncertainties and assess the statistical significance of our measurements. We present Planck EE, BB, and TE power spectra of dust polarization at 353 GHz for a set of six nested high-Galactic-latitude sky regions covering from 24 to 71% of the sky. We present power-law fits to the angular power spectra, yielding evidence for statistically significant variations of the exponents over sky regions and a difference between the values for the EE and BB spectra, which for the largest sky region are αEE = -2.42 ± 0.02 and αBB = -2.54 ± 0.02, respectively. The spectra show that the TE correlation and E/B power asymmetry discovered by Planck extend to low multipoles that were not included in earlier Planck polarization papers due to residual data systematics. We also report evidence for a positive TB dust signal. Combining data from Planck and WMAP, we have determined the amplitudes and spectral energy distributions (SEDs) of polarized foregrounds, including the correlation between dust and synchrotron polarized emission, for the six sky regions as a function of multipole. This quantifies the challenge of the component-separation procedure that is required for measuring the low-ℓ reionization CMB E-mode signal and detecting the reionization and recombination peaks of primordial CMB B modes. The SED of polarized dust emission is fit well by a single-temperature modified black-body emission law from 353 GHz to below 70 GHz. For a dust temperature of 19.6 K, the mean dust spectral index for dust polarization is βdP = 1.53±0.02. The difference between indices for polarization and total intensity is βdP-βdI = 0.05±0.03. By fitting multi-frequency cross-spectra between Planck data at 100, 143, 217, and 353 GHz, we examine the correlation of the dust polarization maps across frequency. We find no evidence for a loss of correlation and provide lower limits to the correlation ratio that are tighter than values we derive from the correlation of the 217- and 353 GHz maps alone. If the Planck limit on decorrelation for the largest sky region applies to the smaller sky regions observed by sub-orbital experiments, then frequency decorrelation of dust polarization might not be a problem for CMB experiments aiming at a primordial B-mode detection limit on the tensor-to-scalar ratio r≃ 0.01 at the recombination peak. However, the Planck sensitivity precludes identifying how difficult the component-separation problem will be for more ambitious experiments targeting lower limits on r.