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

Context. The formation of high-mass star-forming regions from their parental gas cloud and the subsequent fragmentation processes lie at the heart of star formation research. Aims. We aim to study the dynamical and fragmentation properties at very early evolutionary stages of high-mass star formation. Methods. Employing the NOrthern Extended Millimeter Array and the IRAM 30 m telescope, we observed two young high-mass star-forming regions, ISOSS22478 and ISOSS23053, in the 1.3 mm continuum and spectral line emission at a high angular resolution (~0.8″). Results. We resolved 29 cores that are mostly located along filament-like structures. Depending on the temperature assumption, these cores follow a mass-size relation of approximately M ∝ r2.0 ± 0.3, corresponding to constant mean column densities. However, with different temperature assumptions, a steeper mass-size relation up to M ∝ r3.0 ± 0.2, which would be more likely to correspond to constant mean volume densities, cannot be ruled out. The correlation of the core masses with their nearest neighbor separations is consistent with thermal Jeans fragmentation. We found hardly any core separations at the spatial resolution limit, indicating that the data resolve the large-scale fragmentation well. Although the kinematics of the two regions appear very different at first sight – multiple velocity components along filaments in ISOSS22478 versus a steep velocity gradient of more than 50 km s−1 pc−1 in ISOSS23053 – the findings can all be explained within the framework of a dynamical cloud collapse scenario. Conclusions. While our data are consistent with a dynamical cloud collapse scenario and subsequent thermal Jeans fragmentation, the importance of additional environmental properties, such as the magnetization of the gas or external shocks triggering converging gas flows, is nonetheless not as well constrained and would require future investigation.

Context. Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas. Aims. We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. Methods. We studied the cold neutral medium (CNM) by means of H I self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length ~170 pc) and compared our results with molecular gas traced by 13CO emission. We fitted baselines of HISA features to H I emission spectra using first and second order polynomial functions. Results. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and 13CO is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with 13CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 1020 cm−2 while those of molecular hydrogen traced by 13CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H I emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H2 N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H I column density is observed at ~25 M⊙ pc−2. Conclusions. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced H2 column density peaks and signs of active star formation.

We present polarization and Faraday rotation for the supernova remnants (SNRs) G46.8 − 0.3, G43.3 − 0.2, G41.1 − 0.3, and G39.2 − 0.3 in the L-band (1–2 GHz) radio continuum in the H i/OH/Recombination line survey. We detect polarization from G46.8 − 0.3, G43.3 − 0.2, and G39.2 − 0.3 but find upper limits at the 1% level of Stokes I for G41.1 − 0.3. For G46.8 − 0.3 and G39.2 − 0.3, the fractional polarization varies on small scales from 1% to ∼6%. G43.3 − 0.2 is less polarized with fractional polarization ≲3%. We find upper limits at the 1% level for the brighter regions in each SNR with no evidence for associated enhanced Faraday depolarization. We observe significant variation in Faraday depth and fractional polarization on angular scales down to the resolution limit of 16″. Approximately 6% of our polarization detections from G46.8 − 0.3 and G39.2 − 0.3 exhibit two-component Faraday rotation and 14% of polarization detections in G43.3 − 0.2 are multicomponent. For G39.2 − 0.3, we find a bimodal Faraday depth distribution with a narrow peak and a broad peak for all polarization detections as well as for the subset with two-component Faraday rotation. We identify the narrow peak with the front side of the SNR and the broad peak with the back side. Similarly, we interpret the observed Faraday depth distribution of G46.8 − 0.3 as a superposition of the distributions from the front side and the back side. We interpret our results as evidence for a partially filled shell with small-scale magnetic field structure and internal Faraday rotation.

We present a study of the cold atomic hydrogen (HI) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of HI at 21 cm including HI self-absorption (HISA) and observational effects. We find that HI column densities, $N_\rm {HI}$, of ≳ 1022 cm−2 are frequently reached in molecular clouds with HI temperatures as low as ∼10 K. Hence, HISA observations assuming a fixed HI temperature tend to underestimate the amount of cold HI in molecular clouds by a factor of 3 – 10 and produce an artificial upper limit of $N_\rm {HI}$ around 1021 cm−2. We thus argue that the cold HI mass in molecular clouds could be a factor of a few higher than previously estimated. Also $N_\rm {HI}$-PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold HI in HISA observations is due to both the large HI temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold HI around 1 – 10 making optical depth corrections essential. We show that the high HI column densities (≳ 1022 cm−2) can in parts be attributed to the occurrence of up to 10 individual HI-H2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single HI-H2 transition, $N_\rm {HI}$ frequently exceeds 1021 cm−2, challenging 1-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that HI-H2 transition regions usually do not possess a 1-dimensional geometry. Finally, we show that the HI gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of ∼2.