We present first results from SIMPLIFI (Study of Interstellar Magnetic Polarization: a Legacy Investigation of Filaments), a SOFIA/HAWC+ 214 mu m polarimetric survey of Galactic molecular cloud filaments. We trace magnetic field morphology from the DR21 Main Ridge into surrounding subfilaments at similar to 0.1 pc resolution, extending polarimetric detections for the first time beyond high-column-density regions probed by prior submillimeter observations. We compare the plane-of-sky orientations of the magnetic field, B-pos , the projected gravitational acceleration, g(pos) , and the intensity gradient rotated by 90 degrees, del I-perpendicular to . The relative orientation of Bpos and del I-perpendicular to transitions from preferentially parallel in subfilaments to perpendicular in the DR21 Main Ridge at N(H-2) similar to 2 & times; 10(22) cm(-2), consistent with thresholds seen with Planck and expected in clouds formed from strongly magnetized, sub-Alfv & eacute;nic, magnetically subcritical gas ( M-A less than or similar to 1 , M/Phi(B)<[M/Phi(B)](cr) ). We find that the relative alignment between orientations shows region-to-region and pixel-to-pixel variations at fixed column density. Column density alone is thus not sufficient to encode changes in magnetic field structure. Theoretical models must account for additional drivers. Our central finding is that g(pos) and B-pos remain aligned throughout the cloud regardless of column density or environment, unlike the environment-dependent behavior of B-pos versus del I-perpendicular to and gpos versus del I-perpendicular to . This persistent alignment is consistent with magnetically guided accretion: subfilaments channel material along field lines at several 10(-3) M circle dot yr(-1), sufficient to assemble the Ridge within similar to 10(6) yr and sustain high-mass star formation. This framework also explains why observed radial velocities (approximate to 2 km s(-1)) fall well below freefall expectations (approximate to 8 km s(-1)): with the field nearly in the plane of the sky, only a small fraction of the accretion velocity projects along the line of sight.

Low-metallicity environments are subject to inefficient cooling. They also have low dust-to-gas ratios and therefore less efficient photoelectric (PE) heating than in solar-neighbourhood conditions, where PE heating is one of the most important heating processes in the warm neutral interstellar medium (ISM). We perform magnetohydrodynamic simulations of stratified ISM patches with a gas metallicity of 0.02 Z(circle dot) as part of the SILCC project. The simulations include non-equilibrium chemistry, heating, and cooling of the low-temperature ISM as well as anisotropic cosmic-ray (CR) transport, and stellar tracks. We include stellar feedback in the form of far-ultraviolet and ionizing (FUV and extreme ultraviolet, EUV) radiation, massive star winds, supernovae, and CR injection. From the local CR energy density, we compute a CR heating rate that is variable in space and time. In this way, we can compare the relative impact of PE and CR heating on the metal-poor ISM and find that CR heating can dominate over PE heating. Models with a uniform CR ionization rate of sigma = 3 x 10(-17)s(-1) suppress or severely delay star formation, since they provide a larger amount of energy to the ISM due to CR heating. Models with a variable CR ionization rate form stars predominantly in pristine regions with low PE heating and CR ionization rates where the metal-poor gas is able to cool efficiently. Because of the low metallicity, the amount of formed stars in all runs is not enough to trigger outflows of gas from the mid-plane.

The 'Diamond Ring' within Cygnus X, south-west of the DR21 ridge, stands out as a prominent, nearly circular structure in infrared (IR) and far-infrared (FIR) emission, spanning approximately 6 pc in diameter. It is enclosed by clumpy molecular clouds seen in CO lines and contains an H II region, visible in cm emission. It resembles a classical H II region associated with an expanding bubble seen routinely in the 158 mu m line of ionised carbon ([C II]). However, our recent observations utilising the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) under the FEEDBACK program for the spectrally resolved [C II] line have revealed a slightly tilted ring with a mass of similar to 10(3) M-circle dot, advancing at a velocity of similar to 1.3 km s(-1). The bulk emission of the gas has a line-of-sight (LOS) velocity around -2 km s(-1). The [C II] data revealed that the 'Diamond' of the Diamond Ring is an unrelated, dense gas clump at a LOS velocity of similar to 7 km s(-1). The driving source, which is also responsible for powering the associated H II region, is a B0.5e star, classified by our IR spectroscopy. This observation marks the first instance where we observe only a slowly expanding ring of [C II] emission and not an expanding 3D shell. We suggest that the H II region (along with its associated [C II] bubble), initially formed by a massive star, expanded outwards from a flat slab of molecular gas nearly in the plane of the sky. Presently, the [C II] ring is confined by the swept-up gas of the slab, while the parts of the shell moving in the directions perpendicular to the shell along the LOS have dissipated, resulting in a notable decrease in the expansion of the remaining ring. This scenario is supported by dedicated simulations that trace the evolution of the [C II] bubble. Our observations support the scenario of H II region evolution in 'flat' molecular clouds, reported earlier in the literature. In this geometry, we propose that the Diamond Ring represents the terminal phase of an expanding [C II] bubble driven by stellar winds and thermal pressure.

Context. Identified as parsec-size, gas clumps at the junction of multiple filaments, hub-filament systems (HFS) play a crucial role during the formation of young clusters and high-mass stars. These HFS still appear to be detached from most galactic filaments when compared in the mass-length (M-L) phase space. Aims. We aim to characterize the early evolution of HFS as part of the filamentary description of the interstellar medium (ISM). Methods. Combining previous scaling relations with new analytic calculations, we created a toy model to explore the different physical regimes described by the M-L diagram. Despite its simplicity, our model accurately reproduces several observational properties reported for filaments and HFS, such as their expected typical aspect ratio (A), mean surface density (Sigma), and gas accretion rate (m). Moreover, this model naturally explains the different mass and length regimes populated by filaments and HFS, respectively. Results. Our model predicts a dichotomy between filamentary (A >= 3) and spheroidal (A < 3) structures connected to the relative importance of their fragmentation, accretion, and collapse timescales. Individual filaments with low accretion rates are dominated by an efficient internal fragmentation. In contrast, the formation of compact HFS at the intersection of filaments triggers a geometric phase-transition, leading to the gravitational collapse of these structures at parsec-scales in similar to 1-2 Myr. In addition, this process also induces higher accretion rates.

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields, dust properties, and poses a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process needs to be studied thoroughly. The Magnetically enhanced Radiative Torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Our numerical modelling of dust polarisation using the MRAT theory demonstrated that the alignment efficiency of starlight polarisation (p_ ext/A_ V) and the degree of thermal dust polarisation (p_ em) first decrease slowly with increasing visual extinction (A_ V) and then falls steeply as ∝ A^-1_ V at large A_ V due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope (A^ loss_ V) increases with the maximum grain size. By applying physical profiles suitable for a starless core 109 in the Pipe Nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation at R-band (0.65 μm) and H-band (1.65 μm), as well as emission polarisation at submillimetre (870 μm). Successful modelling of observational data requires perfect alignment of large grains as evidence of the MRAT mechanism, and larger maximum size with higher elongation at higher A_ V. The latter reveals the first evidence for the new model of anisotropic grain growth induced by magnetic grain alignment.

Molecular clouds (MCs) are the birthplaces of new stars in galaxies. A key component of MCs are photodissociation regions (PDRs), where far-ultraviolet radiation plays a crucial role in determining the gas’s physical and chemical state. Traditional PDR models assume a chemical steady state (CSS), where the rates of H _2 formation and photodissociation are balanced. However, real MCs are dynamic and can be out of CSS. In this study, we demonstrate that combining H _2 emission lines observed in the far-ultraviolet or infrared with column density observations can be used to derive the rates of H _2 formation and photodissociation. We derive analytical formulae that relate these rates to observable quantities, which we validate using synthetic H _2 line emission maps derived from the SILCC-Zoom hydrodynamical simulation. Our method estimates integrated H _2 formation and dissociation rates with an accuracy ≈30% (on top of the uncertainties in the observed H _2 emission maps and column densities). Our simulations, valid for column densities N ≤ 2 × 10 ^22 cm ^−2 , cover a wide dynamic range of H _2 formation and photodissociation rates, showing significant deviations from CSS, with 74% of the MC’s mass deviating from CSS by a factor greater than 2. Our analytical formulae can effectively distinguish between regions in and out of CSS. When applied to actual H _2 line observations, our method can assess the chemical states of MCs, providing insights into their evolutionary stages and lifetimes. A NASA Small Explorer mission concept, Eos, will be proposed in 2025 and is specifically designed to conduct the types of observations outlined in this study.

We present magnetohydrodynamic simulations of star formation in the multiphase interstellar medium (ISM) to quantify the impact of non-ionizing far-ultraviolet (FUV) radiation within the SILCC PROJECT simulation framework. Our study incorporates the radiative transfer of ionizing radiation and self-consistent modelling of variable FUV radiation from star clusters, advancing beyond previous studies using static or simplified FUV fields. This enables a more accurate capture of the dynamic interaction between radiation and the evolving ISM alongside other stellar feedback channels. The interstellar radiation field (ISRF) near young star clusters can reach G(0) approximate to 10(4) (in Habing units), far exceeding the solar neighbourhood value of G(0) = 1.7. Despite these high intensities, FUV radiation minimally impacts the integrated star formation rate compared to ionizing radiation, stellar winds, and supernovae. A slight reduction in star formation burstiness is linked to increased photoelectric (PE) heating efficiency by the variable FUV field. Dust near star-forming regions can be heated up to 60 K via the PE effect, with a broad temperature distribution. PE heating rates in variable FUV models exhibit higher peaks but lower averages than static ISRF models. Simulations under solar neighbourhood conditions without stellar winds or ionizing radiation but with supernovae yield unexpectedly high star formation rates of similar to 0.1 M-circle dot yr(-1) kpc(-2). Our analysis reveals increased cold neutral medium volume-filling factors (VFF) outside stellar clusters, reduced thermally unstable gas, and sharper warm-cold gas separation. The variable FUV field also promotes a cold diffuse gas phase with a molecular component, exhibiting a VFF of similar to 5-10 per cent.

In this work we extend previous theoretical works to gain a better understanding of the origin of recently observed polarisation degree spectra of molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum around 350 $\mu$m. For this purpose, we present results of semi-analytical dust polarisation models. We benchmark our model against dust polarisation radiative transfer calculations performed with POLARIS. We show that V-shaped polarisation spectra can only be obtained if two dust phases, one dense and cold and one warm and dilute phase, are present along the line of sight. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbon grains are assumed to be not aligned with the magnetic field. We find that the V-shape is the stronger pronounced the larger the density and temperature contrast between both phases is. Moreover, the destruction of carbon grains by UV radiation in the warm and dilute phase leads to a significantly more pronounced V-shape in the polarisation spectrum. Reducing the alignment efficiency in the cold and dense phase also results in a more pronounced V-shape, its effect, however, is smaller than that of the UV-induced carbon grain destruction. Furthermore, we present a first, self-consistent polarisation spectrum obtained from a 3D, magneto-hydrodynamical molecular cloud simulation. The spectrum matches well with our semi-analytical prediction demonstrating the potential of such complex 3D simulations to study polarisation spectra. Comparing our model results with actual observations indicates that carbon grain destruction in illuminated regions might be required to match these observations. Reducing the alignment efficiency of silicate grains in the cold and dense phase would further improve the match between both data, however, it appears to not be a necessity.

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields and dust properties, while posing a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process must be studied thoroughly. The magnetically enhanced radiative torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Here, we aim to test the MRAT mechanism in starless cores using the multi-wavelength polarisation from optical to submillimetre. Our numerical modelling of dust polarisation using the MRAT theory demonstrates that the alignment efficiency of starlight polarisation (pext/AV) and the degree of thermal dust polarisation (pem) first decrease slowly with increasing visual extinction (AV) and then fall steeply as proportional to Av-1 at large AV due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope (AVloss) increases with maximum grain size. By applying physical profiles suitable for a starless core, 109 in the Pipe nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation in the R band (0.65 mu m) and the H band (1.65 mu m), as well as emission polarisation in the submillimetre (870 mu m). Successful modelling of observational data requires perfect alignment of large grains, which serves as evidence for the MRAT mechanism, and an increased maximum grain size with higher elongation at higher AV. The latter reveals the first evidence for a new model of anisotropic grain growth induced by magnetic grain alignment. This paper introduces the framework for probing the fundamental physics of grain alignment and dust evolution using multi-wavelength dust polarisation (GRADE-POL), and it is the first of our GRADE-POL series.

The gas-phase metallicity affects heating and cooling processes in the star-forming galactic interstellar medium (ISM) as well as ionizing luminosities, wind strengths, and lifetimes of massive stars. To investigate its impact, we conduct magnetohydrodynamic simulations of the ISM using the FLASH code as part of the SILCC project. The simulations assume a gas surface density of 10 M-circle dot pc(-2) and span metallicities from 1/50 to 1 Z(circle dot). We include non-equilibrium thermochemistry, a space-and time-variable far-UV background and cosmic ray ionization rate, metal-dependent stellar tracks, the formation of H II regions, stellar winds, type II supernovae, and cosmic ray injection and transport. With the metallicity decreasing over the investigated range, the star formation rate decreases by more than a factor of 10, the mass fraction of cold gas decreases from 60 percent to 2.3 percent, while the volume filling fraction of the warm gas increases from 20 per cent to 80 per cent. Furthermore, the fraction of H-2 in the densest regions drops by a factor of 4, and the dense ISM fragments into approximately five times fewer structures at the lowest metallicity. Outflow mass loading factors remain largely unchanged, with values close to unity, except for a significant decline at the lowest metallicity. Including the major processes that regulate ISM properties, this study highlights the strong impact of gas phase metallicity on the star-forming ISM.

In recent decades, significant attention has been dedicated to analytical and observational studies of the atomic hydrogen (HI) to molecular hydrogen (H2) transition in the interstellar medium. We focussed on the Draco diffuse cloud to gain deeper insights into the physical properties of the transition from HI to H2. We employed the total hydrogen column density probability distribution function (N-PDF) derived from Herschel dust observations and the N(HI)-PDF obtained from HI data collected by the Effelsberg HI survey. The N-PDF of the Draco cloud exhibits a double-log-normal distribution, whereas the N(HI)-PDF follows a single log-normal distribution. The HI-to-H2 transition is identified as the point where the two log-normal components of the dust N-PDF contribute equally; it occurs at Av = 0.33 (N=6.2e20 cm^-2). The low-column-density segment of the dust N-PDF corresponds to the cold neutral medium, which is characterized by a temperature of around 100 K. The higher-column-density part is predominantly associated with H2. The shape of the Draco N-PDF is qualitatively reproduced by numerical simulations. In the absence of substantial stellar feedback, such as radiation or stellar winds, turbulence exerts a significant influence on the thermal stability of the gas and can regulate the condensation of gas into denser regions and its subsequent evaporation. Recent observations of the ionized carbon line at 158 micron in Draco support this scenario. Using the KOSMA-tau photodissociation model, we estimate a gas density of n=50 cm^-3 and a temperature of 100 K at the location of the HI-to-H2 transition. Both the molecular and atomic gas components are characterized by supersonic turbulence and strong mixing, suggesting that simplified steady-state chemical models are not applicable under these conditions.

Aims. We analyse synthetic emission maps of the [CII] 158 mu m line and far-infrared (FIR) continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the observed [CII] deficit, that is, the drop in the [CII]/FIR intensity ratio caused by stellar activity. Methods. All simulations include stellar radiative feedback and the on-the-fly chemical evolution of hydrogen species, CO, and C+. We also account for further ionisation of C+ into C2+ inside HII regions, which is crucial to obtain reliable results. Results. Studying individual HII regions, we show that I-FIR is initially high in the vicinity of newly born stars, and then moderately decreases over time as the gas is compressed into dense and cool shells. In contrast, there is a large drop in I-CII over time, to which the second ionisation of C+ into C2+ contributes significantly. This leads to a large drop in I-[CII] /I-FIR inside HII regions, with I-[CII] /I-FIR decreasing from 10(-3)-10(-2) at scales above 10 pc to around 10(-6)-10(-4) at scales below 2 pc. However, projection effects can significantly affect the radial profile of I-[CII]/I-FIR, and their ratio, and can create apparent HII regions without any stars. Considering the evolution on MC scales, we show that the luminosity ratio, L-[CII]/L-FIR, decreases from values of greater than or similar to 10(-2) in MCs without star formation to values of around similar to 10(-3) in MCs with star formation. We attribute this decrease and thus the origin of the [CII] deficit to two main contributors: (i) the saturation of the [CII] line and (ii) the conversion of C+ into C2+ by stellar radiation. The drop in the L-[CII]/L-FIR ratio can be divided into two phases: (i) During the early evolution of HII regions, the saturation of [CII] and the further ionisation of C+ limit the increase in L-[CII], while L-FIR increases rapidly, leading to the initial decline of L-[CII]/L-FIR. (ii) In more evolved HII regions, L-CII stagnates and even partially drops over time due to the aforementioned reasons. L-FIR also stagnates as the gas gets pushed into the cooler shells surrounding the HII region. In combination, this keeps the global L-[CII]/L-FIR ratio at low values of similar to 10(-3).

Molecular hydrogen (H$_2$) formation and dissociation are key processes that drive the gas lifecycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds (SILCC) zoom-in simulation suite, we explore the utility of future observations of H$_2$ dissociation and formation for tracking the lifecycle of molecular clouds. The simulations used in this work include non-equilibrium H$_2$ formation, stellar radiation, sink particles, and turbulence. We find that, at early times in the cloud evolution, H$_2$ formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H$_2$ formation is also associated with a higher early star formation rate. For the clouds studied here, H$_2$ is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady-state about 2 Myrs after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories for the molecular cloud lifecycle and the star formation efficiency may be distinguished with observational measurements of H$_2$ fluorescence with a space-based high-resolution FUV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H$_2$ dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud's star-forming life, including cloud building, rapidly star-forming, H$_2$ chemical equilibrium, and cloud destruction.

ABSTRACT 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 μG. 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.

The interstellar medium is threaded by a hierarchy of filaments from large scales (~ 100 pc) to small scales (~ 0.1pc). The masses and lengths of these nested structures may reveal important constraints for cloud formation and evolution, but it is difficult to investigate from an evolutionary perspective using single observations. In this work, we extract simulated molecular clouds from the Cloud Factory galactic-scale ISM suite in combination with 3D Monte Carlo radiative transfer code POLARIS to investigate how filamentary structure evolves over time. We produce synthetic dust continuum observations in three regions with a series of snapshots and use the Filfinder algorithm to identify filaments in the dust derived column density maps. When the synthetic filaments mass and length are plotted on an M-L plot, we see a scaling relation of $L\propto M^{0.45}$ similar to that seen in observations, and find that the filaments are thermally supercritical. Projection effects systematically affect the masses and lengths measured for the filaments, and are particularly severe in crowded regions. In the filament Mass-Length (M-L) diagram we identify three main evolutionary mechanisms: accretion, segmentation, and dispersal. In particular we find that the filaments typically evolve from smaller to larger masses in the observational M-L plane, indicating the dominant role of accretion in filament evolution. Moreover, we find a potential correlation between line mass and filament growth rate. Once filaments are actively star forming they then segment into smaller sections, or are dispersed by internal or external forces.

Context. About 15%-60% of all supernova remnants are estimated to interact with dense molecular clouds. In these high-density environments, radiative losses are significant. The cooling radiation can be observed in forbidden lines at optical wavelengths. Aims. We aim to determine whether supernovae at different positions within a molecular cloud (with or without magnetic fields) can be distinguished based on their optical emission (e.g. H alpha (lambda 6563), H beta (lambda 4861), [O III] (lambda 5007), [S II] (lambda 6717, 6731), and [N II] (lambda 6583)) using machine learning (e.g. principle component analysis and k-means clustering). Methods. We have conducted a statistical analysis of the optical line emission of simulated supernovae interacting with molecular clouds that formed from the multi-phase interstellar medium modelled in the SILCC-Zoom simulations with and without magnetic fields. This work is based on the post-processing of simulations that have been carried out with the 3D (magneto)hydrodynamic code FLASH. Our dataset consists of 22 simulations. The supernovae were placed at a distance of either 25 pc or 50 pc from the molecular cloud's centre of mass. First, we calculated optical synthetic emission maps (taking into account dust attenuation within the simulation sub-cube) with a post-processing code based on MAPPINGS V cooling tables. Second, we analysed the dataset of synthetic observations using principle component analysis to identify clusters with the k-means algorithm. In addition, we made use of BPT diagrams as a diagnostic of shock-dominated regions. Results. We find that the presence or absence of magnetic fields has no statistically significant effect on the optical line emission. However, the ambient density distribution at the site of the supernova changes the entire evolution and morphology of the supernova remnant. Due to the different ambient densities in the 25 pc and 50 pc simulations, we are able to distinguish them in a statistically significant manner. Although, optical line attenuation within the supernova remnant can mimic this result depending on the attenuation model that is used. That is why, multi-dimensional analysis of optical emission line ratios in this work does not give extra information about the environmental conditions (ambient density and ambient magnetic field) of supernova remnant.

How molecular clouds fragment into dense structures that eventually form stars is an open question. We investigate the relative importance of gravity (both self-gravity and tidal forces) and the volume and surface terms of kinetic, thermal, and magnetic energy for the formation and evolution of molecular clouds and their substructures based on the SILCC-Zoom simulations. These simulations follow the self-consistent formation of cold molecular clouds down to scales of 0.1 pc from the diffuse supernova-driven interstellar medium in a stratified galactic disc. We study the time evolution of seven molecular clouds (of which five are magnetized) over similar to 2 Myr. Using a dendrogram, we identify hierarchical three-dimensional substructures inside the clouds with the aim of understanding their dynamics. The virial analysis shows that the dense gas is indeed dominated by the interplay of gravity and turbulence, while magnetic fields and thermal pressure are mostly important for fluffy, atomic structures. However, not all bound structures are gravitationally bound; some are held together by ram pressure aided by other surface terms. Overall, similar to 36 per cent of the clouds have >50 per cent of their mass in 'potentially gravity bound' structures. A subset of them (70 per cent) is 'potentially bound' by gravity on scales >15 pc. A detailed tidal analysis shows that the tidal tensor is highly anisotropic. Yet the tidal forces are generally not strong enough to disrupt either large-scale or dense substructures but cause their deformation. When comparing the tidal and crossing time-scales, we find that tidal forces do not appear to be the main driver of turbulence within the molecular clouds.

Aims. We present a new sub-grid model, HYACINTH - HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations - for computing the non-equilibrium abundances of H-2 and its carbon-based tracers, namely CO, C, and C+, in cosmological simulations of galaxy formation. Methods. The model accounts for the unresolved density structure in simulations using a variable probability distribution function of sub-grid densities and a temperature-density relation. Included is a simplified chemical network that has been tailored for hydrogen and carbon chemistry within molecular clouds and easily integrated into large-scale simulations with minimal computational overhead. As an example, we applied HYACINTH to a simulated galaxy at redshift z similar to 2.5 in post-processing and compared the resulting abundances with observations. Results. The chemical predictions from HYACINTH are in reasonable agreement with high-resolution molecular-cloud simulations at different metallicities. By post-processing a galaxy simulation with HYACINTH, we reproduced the H I - H-2 transition as a function of the hydrogen column density N-H for both Milky-Way-like and Large-Magellanic-Cloud-like conditions. We also matched the NCO versus N-H2 relation inferred from absorption measurements towards Milky-Way molecular clouds, although most of our post-processed regions occupy the same region as (optically) dark molecular clouds in the N-CO - N-H2 plane. Column density maps reveal that CO is concentrated in the peaks of the H-2 distribution, while atomic carbon more broadly traces the bulk of H-2 in our post-processed galaxy. Based on both the column density maps and the surface density profiles of the different gas species in the post-processed galaxy, we find that C+ maintains a substantially high surface density out to similar to 10 kpc as opposed to other components that exhibit a higher central concentration. This is similar to the extended [C II] emission found in some recent observations at high redshifts.

The interstellar medium contains filamentary structure over a wide range of scales. Understanding the role of this structure, both as a conduit of gas across the scales and a diagnostic tool of local physics, is a major focus of star formation studies. We review recent progress in studying filamentary structure in the ISM, interpreting its properties in terms of physical processes, and exploring formation and evolution scenarios. We include structures from galactic-scale filaments to tenth-of-a-parsec scale filaments, comprising both molecular and atomic structures, from both observational and theoretical perspectives. In addition to the literature overview, we assemble a large amount of catalogue data from different surveys and provide the most comprehensive census of filamentary structures to date. Our census consists of 22 803 filamentary structures, facilitating a holistic perspective and new insights. We use our census to conduct a meta-analysis, leading to a description of filament properties over four orders of magnitudes in length and eight in mass. Our analysis emphasises the hierarchical and dynamical nature of filamentary structures. Filaments do not live in isolation, nor they generally resemble static structures close to equilibrium. We propose that accretion during filament formation and evolution sets some of the key scaling properties of filaments. This highlights the role of accretion during filament formation and evolution and also in setting the initial conditions for star formation. Overall, the study of filamentary structures during the past decade has been observationally driven. While great progress has been made on measuring the basic properties of filaments, our understanding of their formation and evolution is clearly lacking. In this context, we identify a number of directions and questions we consider most pressing for the field.

To what extent magnetic fields affect how molecular clouds (MCs) fragment and create dense structures is an open question. We present a numerical study of cloud fragmentation using the SILCC-Zoom simulations. These simulations follow the self-consistent formation of MCs in a few hundred parsec sized region of a stratified galactic disc; and include magnetic fields, self-gravity, supernova-driven turbulence, as well as a non-equilibrium chemical network. To discern the role of magnetic fields in the evolution of MCs, we study seven simulated clouds, five with magnetic fields, and two without, with a maximum resolution of 0.1 parsec. Using a dendrogram we identify hierarchical structures which form within the clouds. Overall, the magnetised clouds have more mass in a diffuse envelope with a number density between 1-100 cm$^{-3}$. We find that six out of seven clouds are sheet-like on the largest scales, as also found in recent observations, and with filamentary structures embedded within, consistent with the bubble-driven MC formation mechanism. Hydrodynamic simulations tend to produce more sheet-like structures also on smaller scales, while the presence of magnetic fields promotes filament formation. Analysing cloud energetics, we find that magnetic fields are dynamically important for less dense, mostly but not exclusively atomic structures (typically up to $\sim 100 - 1000$~cm$^{-3}$), while the denser, potentially star-forming structures are energetically dominated by self-gravity and turbulence. In addition, we compute the magnetic surface term and demonstrate that it is generally confining, and some atomic structures are even magnetically held together. In general, magnetic fields delay the cloud evolution and fragmentation by $\sim$ 1 Myr.