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The Central Molecular Zone (CMZ; the central ∼500 pc of the Galaxy) is a kinematically unusual environment relative to the Galactic disc, with high-velocity dispersions and a steep size–linewidth relation of the molecular clouds. In addition, the CMZ region has a significantly lower star formation rate (SFR) than expected by its large amount of dense gas. An important factor in explaining the low SFR is the turbulent state of the star-forming gas, which seems to be dominated by rotational modes. However, the turbulence driving mechanism remains unclear. In this work, we investigate how the Galactic gravitational potential affects the turbulence in CMZ clouds. We focus on the CMZ cloud G0.253+0.016 ('the Brick'), which is very quiescent and unlikely to be kinematically dominated by stellar feedback. We demonstrate that several kinematic properties of the Brick arise naturally in a cloud-scale hydrodynamics simulation, that takes into account the Galactic gravitational potential. These properties include the line-of-sight velocity distribution, the steepened size–linewidth relation, and the predominantly solenoidal nature of the turbulence. Within the simulation, these properties result from the Galactic shear in combination with the cloud's gravitational collapse. This is a strong indication that the Galactic gravitational potential plays a crucial role in shaping the CMZ gas kinematics, and is a major contributor to suppressing the SFR, by inducing predominantly solenoidal turbulent modes. [ABSTRACT FROM AUTHOR]

We use new HCN(1–0) data from the ACA Large-sample Mapping Of Nearby galaxies in Dense gas (ALMOND) survey to trace the kpc-scale molecular gas density structure and CO(2–1) data from the Physics at High Angular resolution in Nearby GalaxieS–Atacama Large Millimeter/submillimeter Array (PHANGS–ALMA) to trace the bulk molecular gas across 25 nearby star-forming galaxies. At 2.1 kpc scale, we measure the density-sensitive HCN/CO line ratio and the star formation rate (SFR)/HCN ratio to trace the star formation efficiency in the denser molecular medium. At 150 pc scale, we measure structural and dynamical properties of the molecular gas via CO(2–1) line emission, which is linked to the lower resolution data using an intensity-weighted averaging method. We find positive correlations (negative) of HCN/CO (SFR/HCN) with the surface density, the velocity dispersion, and the internal turbulent pressure of the molecular gas. These observed correlations agree with expected trends from turbulent models of star formation, which consider a single free-fall time gravitational collapse. Our results show that the kpc-scale HCN/CO line ratio is a powerful tool to trace the 150 pc scale average density distribution of the molecular clouds. Lastly, we find systematic variations of the SFR/HCN ratio with cloud-scale molecular gas properties, which are incompatible with a universal star formation efficiency. Overall, these findings show that mean molecular gas density, molecular cloud properties, and star formation are closely linked in a coherent way, and observations of density-sensitive molecular gas tracers are a useful tool to analyse these variations, linking molecular gas physics to stellar output across galaxy discs. [ABSTRACT FROM AUTHOR]

We present new neutral atomic carbon [C i] (3P1! 3P0) mapping observations within the inner ~7 kpc and ~4 kpc of the disks of NGC3627 and NGC4321 at a spatial resolution of 190 pc and 270 pc, respectively, using the Atacama Large Millimeter/Submillimeter Array (ALMA) Atacama Compact Array (ACA). We combine these with the CO(21) data from PHANGS-ALMA, and literature [C i] and CO data for two other starburst and/or active galactic nucleus (AGN) galaxies (NGC 1808, NGC7469) with the aim of studying: (a) the spatial distributions of C i and CO emission; (b) the observed line ratio RCi=CO = I[Ci](10)=ICO(21) as a function of various galactic properties; and (c) the abundance ratio of [C i/CO]. We find excellent spatial correspondence between C i and CO emission and nearly uniform RCi=CO ~ 0:1 across the majority of the star-forming disks of NGC3627 and NGC4321. However, RCi=CO strongly varies from ~0:05 at the center of NGC4321 to >0:20:5 in NGC1808's starbursting center and NGC7469's center with an X-ray-luminous AGN. Meanwhile, RCi=CO does not obviously vary with hUi, which is in line with predictions from photodissociation-dominated region (PDR) models. We also find a mildly decreasing RCi=CO value with an increasing metallicity over 0:70:85 Z, which is consistent with the literature. Assuming various typical interstellar medium (ISM) conditions representing giant molecular clouds, active star-forming regions, and strong starbursting environments, we calculated the (non)local-thermodynamic-equilibrium radiative transfer and estimated the [C i/CO] abundance ratio to be ~0:1 across the disks of NGC3627 and NGC4321, similar to previous large-scale findings in Galactic studies. However, this abundance ratio likely experiences a substantial increase, up to ~1 and &15 in NGC1808's starburst and NGC7469's strong AGN environments, respectively. This result is in line with the expectations for cosmic-ray dominated region (CRDR) and X-ray dominated region (XDR) chemistry. Finally, we do not find robust evidence for a generally CO-dark-and-C i-bright gas in the disk areas we probed. [ABSTRACT FROM AUTHOR]

Collisions between giant molecular clouds (GMCs) are one of the pathways for massive star formation due to the high densities created. However, the enhancement of the star formation rate (SFR) is not well constrained. In this study, we perform a parameter study of cloud–cloud collisions and investigate how the resulting SFR depends on the details of set-up. Our parameter study explores variations in collision speed, magnetic field inclination (with respect to the collisional axis), and resolution, as defined by the number of cells per Jeans length. In all our collision simulations, we find a factor of 2–3 increase in the SFR compared to our no collision simulation, with star formation beginning sooner with (a) high collisional velocities, (b) parallel orientation between the magnetic field and collision axis, (c) and lower resolution. The mean virial parameter of high density (and thus possible star-forming) gas increases with collisional velocity, but has little variation with magnetic field inclination. The alignment of the velocity and magnetic field remains uniform in low-density environments but becomes more perpendicular with increasing density, indicating the compression of the magnetic field by collapsing gas. Comparing the trends in the SFR with other GMC collision studies, we find good agreement with studies that account for the gravitational boundedness of the gas in their star formation algorithm, but not with those that simply form stars above a prescribed density threshold. This suggests that the latter approach should be used with caution when modelling star formation on resolved cloud scales. [ABSTRACT FROM AUTHOR]

In the hierarchical view of star formation, giant molecular clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1″) PHANGS–ALMA catalogue of GMCs with the star cluster catalogues from PHANGS– HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4 − 6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young (≤10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the |$\gt \, 10$|  Myr stellar populations, indicating that the hierarchical structure dissolves over time. [ABSTRACT FROM AUTHOR]

The processes of star formation and feedback, regulating the cycle of matter between gas and stars on the scales of giant molecular clouds (GMCs; ∼100 pc), play a major role in governing galaxy evolution. Measuring the time-scales of GMC evolution is important to identify and characterize the specific physical mechanisms that drive this transition. By applying a robust statistical method to high-resolution CO and narrow-band H α imaging from the PHANGS survey, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions on GMC scales, across the discs of an unprecedented sample of 54 star-forming main-sequence galaxies (excluding their unresolved centres). We find that clouds live for about 1−3 GMC turbulence crossing times (5−30 Myr) and are efficiently dispersed by stellar feedback within 1−5 Myr once the star-forming region becomes partially exposed, resulting in integrated star formation efficiencies of 1−8 per cent. These ranges reflect physical galaxy-to-galaxy variation. In order to evaluate whether galactic environment influences GMC evolution, we correlate our measurements with average properties of the GMCs and their local galactic environment. We find several strong correlations that can be physically understood, revealing a quantitative link between galactic-scale environmental properties and the small-scale GMC evolution. Notably, the measured CO-visible cloud lifetimes become shorter with decreasing galaxy mass, mostly due to the increasing presence of CO-dark molecular gas in such environment. Our results represent a first step towards a comprehensive picture of cloud assembly and dispersal, which requires further extension and refinement with tracers of the atomic gas, dust, and deeply embedded stars. [ABSTRACT FROM AUTHOR]

Young massive stars play an important role in the evolution of the interstellar medium (ISM) and the self-regulation of star formation in giant molecular clouds (GMCs) by injecting energy, momentum, and radiation (stellar feedback) into surrounding environments, disrupting the parental clouds, and regulating further star formation. Information of the stellar feedback inheres in the emission we observe, however inferring the physical properties from photometric and spectroscopic measurements is difficult, because stellar feedback is a highly complex and non-linear process, so that the observational data are highly degenerate. On this account, we introduce a novel method that couples a conditional invertible neural network (cINN) with the WARPFIELD-emission predictor (WARPFIELD-EMP) to estimate the physical properties of star-forming regions from spectral observations. We present a cINN that predicts the posterior distribution of seven physical parameters (cloud mass, star formation efficiency, cloud density, cloud age which means age of the first generation stars, age of the youngest cluster, the number of clusters, and the evolutionary phase of the cloud) from the luminosity of 12 optical emission lines, and test our network with synthetic models that are not used during training. Our network is a powerful and time-efficient tool that can accurately predict each parameter, although degeneracy sometimes remains in the posterior estimates of the number of clusters. We validate the posteriors estimated by the network and confirm that they are consistent with the input observations. We also evaluate the influence of observational uncertainties on the network performance. [ABSTRACT FROM AUTHOR]

To investigate how molecular clouds react to different environmental conditions at a galactic scale, we present a catalogue of giant molecular clouds (GMCs) resolved down to masses of ∼10 M⊙ from a simulation of the entire disc of an interacting M51-like galaxy and a comparable isolated galaxy. Our model includes time-dependent gas chemistry, sink particles for star formation, and supernova feedback, meaning we are not reliant on star formation recipes based on threshold densities and can follow the physics of the cold molecular phase. We extract GMCs from the simulations and analyse their properties. In the disc of our simulated galaxies, spiral arms seem to act merely as snowplows, gathering gas, and clouds without dramatically affecting their properties. In the centre of the galaxy, on the other hand, environmental conditions lead to larger, more massive clouds. While the galaxy interaction has little effect on cloud masses and sizes, it does promote the formation of counter-rotating clouds. We find that the identified clouds seem to be largely gravitationally unbound at first glance, but a closer analysis of the hierarchical structure of the molecular interstellar medium shows that there is a large range of virial parameters with a smooth transition from unbound to mostly bound for the densest structures. The common observation that clouds appear to be virialized entities may therefore be due to CO bright emission highlighting a specific level in this hierarchical binding sequence. The small fraction of gravitationally bound structures found suggests that low galactic star formation efficiencies may be set by the process of cloud formation and initial collapse. [ABSTRACT FROM AUTHOR]

Feedback from massive stars plays a key role in molecular cloud evolution. After the onset of star formation, the young stellar population is exposed by photoionization, winds, supernovae, and radiation pressure from massive stars. Recent observations of nearby galaxies have provided the evolutionary timeline between molecular clouds and exposed young stars, but the duration of the embedded phase of massive star formation is still ill-constrained. We measure how long massive stellar populations remain embedded within their natal cloud, by applying a statistical method to six nearby galaxies at |$20{-}100~\mbox{${\rm ~pc}$}$| resolution, using CO, Spitzer 24 |$\rm \, \mu m$|⁠ , and H α emission as tracers of molecular clouds, embedded star formation, and exposed star formation, respectively. We find that the embedded phase (with CO and 24 |$\rm \, \mu m$|  emission) lasts for 2−7 Myr and constitutes |$17{-}47{{\ \rm per\ cent}}$| of the cloud lifetime. During approximately the first half of this phase, the region is invisible in H α, making it heavily obscured. For the second half of this phase, the region also emits in H α and is partially exposed. Once the cloud has been dispersed by feedback, 24 |$\rm \, \mu m$| emission no longer traces ongoing star formation, but remains detectable for another 2−9 Myr through the emission from ambient CO-dark gas, tracing star formation that recently ended. The short duration of massive star formation suggests that pre-supernova feedback (photoionization and winds) is important in disrupting molecular clouds. The measured time-scales do not show significant correlations with environmental properties (e.g. metallicity). Future JWST observations will enable these measurements routinely across the nearby galaxy population. [ABSTRACT FROM AUTHOR]

A unique filament is identified in the Herschel maps of the Orion A giant molecular cloud. The filament, which we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position–velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C18O (1−0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics code Athena++ to simulate the collision between two diffuse molecular clumps, each carrying an antiparallel magnetic field. The clump collision produces a narrow, straight, dense filament with a factor of >200 increase in density. The production of the dense gas is seven times faster than freefall collapse. The dense filament shows ring/fork-like structures in radiative transfer maps. Cores in the filament are confined by surface magnetic pressure. CMR can be an important dense-gas-producing mechanism in the Galaxy and beyond. [ABSTRACT FROM AUTHOR]

Molecular clouds are a fundamental ingredient of galaxies: they are the channels that transform the diffuse gas into stars. The detailed process of how they do it is not completely understood. We review the current knowledge of molecular clouds and their substructure from scales ∼ 1 kpc down to the filament and core scale. We first review the mechanisms of cloud formation from the warm diffuse interstellar medium down to the cold and dense molecular clouds, the process of molecule formation and the role of the thermal and gravitational instabilities. We also discuss the main physical mechanisms through which clouds gather their mass, and note that all of them may have a role at various stages of the process. In order to understand the dynamics of clouds we then give a critical review of the widely used virial theorem, and its relation to the measurable properties of molecular clouds. Since these properties are the tools we have for understanding the dynamical state of clouds, we critically analyse them. We finally discuss the ubiquitous filamentary structure of molecular clouds and its connection to prestellar cores and star formation. [ABSTRACT FROM AUTHOR]

Star formation is a complex multi-scale phenomenon that is of significant importance for astrophysics in general. Stars and star formation are key pillars in observational astronomy from local star forming regions in the Milky Way up to high-redshift galaxies. From a theoretical perspective, star formation and feedback processes (radiation, winds, and supernovae) play a pivotal role in advancing our understanding of the physical processes at work, both individually and of their interactions. In this review we will give an overview of the main processes that are important for the understanding of star formation. We start with an observationally motivated view on star formation from a global perspective and outline the general paradigm of the life-cycle of molecular clouds, in which star formation is the key process to close the cycle. After that we focus on the thermal and chemical aspects in star forming regions, discuss turbulence and magnetic fields as well as gravitational forces. Finally, we review the most important stellar feedback mechanisms. [ABSTRACT FROM AUTHOR]

We present here the first of a series of papers aimed at better understanding the evolution and properties of giant molecular clouds (GMCs) in a galactic context. We perform high-resolution, three-dimensional arepo simulations of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial simulations include a non-equilibrium, time-dependent, chemical network that follows the evolution of atomic and molecular hydrogen as well as carbon and oxygen self-consistently. Our calculations also treat gas self-gravity and subsequent star formation (described by sink particles), and coupled supernova feedback. In the densest parts of the simulated interstellar medium (ISM), we reach sub-parsec resolution, granting us the ability to resolve individual GMCs and their formation and destruction self-consistently throughout the galaxy. In this initial work, we focus on the general properties of the ISM with a particular focus on the cold star-forming gas. We discuss the role of the interaction with the companion galaxy in generating cold molecular gas and controlling stellar birth. We find that while the interaction drives large-scale gas flows and induces spiral arms in the galaxy, it is of secondary importance in determining gas fractions in the different ISM phases and the overall star formation rate. The behaviour of the gas on small GMC scales instead is mostly controlled by the self-regulating property of the ISM driven by coupled feedback. [ABSTRACT FROM AUTHOR]

We introduce a new suite of simulations, 'The Cloud Factory', which self-consistently forms molecular cloud complexes at high enough resolution to resolve internal substructure (up to 0.25 M⊙ in mass) all while including galactic-scale forces. We use a version of the arepo code modified to include a detailed treatment of the physics of the cold molecular ISM, and an analytical galactic gravitational potential for computational efficiency. The simulations have nested levels of resolution, with the lowest layer tied to tracer particles injected into individual cloud complexes. These tracer refinement regions are embedded in the larger simulation so continue to experience forces from outside the cloud. This allows the simulations to act as a laboratory for testing the effect of galactic environment on star formation. Here we introduce our method and investigate the effect of galactic environment on filamentary clouds. We find that cloud complexes formed after a clustered burst of feedback have shorter lengths and are less likely to fragment compared to quiescent clouds (e.g. the Musca filament) or those dominated by the galactic potential (e.g. Nessie). Spiral arms and differential rotation preferentially align filaments, but strong feedback randomizes them. Long filaments formed within the cloud complexes are necessarily coherent with low internal velocity gradients, which has implications for the formation of filamentary star-clusters. Cloud complexes formed in regions dominated by supernova feedback have fewer star-forming cores, and these are more widely distributed. These differences show galactic-scale forces can have a significant impact on star formation within molecular clouds. [ABSTRACT FROM AUTHOR]

We present a new approach to extract the power-law part of a density/column-density probability density function (ρ-pdf/ N -pdf) in star-forming clouds. This approach is based on the mathematical method bPlfit of Virkar & Clauset (2014, Annals of Applied Statistics, 8, 89) and it assesses the power-law part of an arbitrary distribution, without any assumptions about the other parts of this distribution. The slope and deviation point are derived as averaged values as the number of bins is varied. Neither parameter is sensitive to spikes and other local features of the tail. This adapted bPlfit method is applied to two different sets of data from numerical simulations of star-forming clouds at scales 0.5 and 500 pc, and it displays ρ-pdf and N -pdf evolution in agreement with a number of numerical and theoretical studies. Applied to Herschel data on the regions Aquila and Rosette, the method extracts pronounced power-law tails, consistent with those seen in simulations of evolved clouds. [ABSTRACT FROM AUTHOR]

Observations of molecular gas near the Galactic Centre (| l | < 10°, | b | < 1°) reveal the presence of a distinct population of enigmatic compact clouds that are characterized by extreme velocity dispersions (⁠|$\Delta v \gt 100\, {\rm km\, s^{-1}}$|⁠). These extended velocity features are very prominent in the data cubes and dominate the kinematics of molecular gas just outside the Central Molecular Zone (CMZ). The prototypical example of such a cloud is Bania Clump 2. We show that similar features are naturally produced in simulations of gas flow in a realistic barred potential. We analyse the structure of the features obtained in the simulations and use this to interpret the observations. We find that the features arise from collisions between material that has been infalling rapidly along the dust lanes of the Milky Way bar and material that belongs to one of the following two categories: (i) material that has 'overshot' after falling down the dust lanes on the opposite side; (ii) material which is part of the CMZ. Both types of collisions involve gas with large differences in the line-of-sight velocities, which is what produces the observed extreme velocity dispersions. Examples of both categories can be identified in the observations. If our interpretation is correct, we are directly witnessing (a) collisions of clouds with relative speeds of |$\sim 200\, {\rm km\, s^{-1}}$| and (b) the process of accretion of fresh gas onto the CMZ. [ABSTRACT FROM AUTHOR]

Observations of interstellar gas clouds are typically limited to two-dimensional (2D) projections of the intrinsically three-dimensional (3D) structure of the clouds. In this study, we present a novel method for relating the 2D projected fractal dimension (⁠|$\mathcal {D}_{\text{p}}$|⁠) to the 3D fractal dimension (⁠|$\mathcal {D}_{\text{3D}}$|⁠) of turbulent clouds. We do this by computing the fractal dimension of clouds over two orders of magnitude in turbulent Mach number |$(\mathcal {M} = 1\rm{-}100)$|⁠ , corresponding to seven orders of magnitude in spatial scales within the clouds. This provides us with the data to create a new empirical relation between |$\mathcal {D}_{\text{p}}$| and |$\mathcal {D}_{\text{3D}}$|⁠. The proposed relation is |$\mathcal {D}_{\text{3D}}(\mathcal {D}_{\text{p}}) = \Omega _1 \operatorname{erfc}(\xi _1 \operatorname{erfc}^{-1}[ (\mathcal {D}_{\text{p}} - \mathcal {D}_{\text{p,min}})/\Omega _2 ] + \xi _2) + \mathcal {D}_{\text{3D,min}}$|⁠ , where the minimum 3D fractal dimension, |$\mathcal {D}_{\text{3D,min}} = 2.06 \pm 0.35$|⁠ , the minimum projected fractal dimension, |$\mathcal {D}_{\text{p,min}} = 1.55 \pm 0.13$|⁠ , Ω1 = 0.47 ± 0.18, Ω2 = 0.22 ± 0.07, ξ1 = 0.80 ± 0.18, and ξ2 = 0.26 ± 0.19. The minimum 3D fractal dimension, |$\mathcal {D}_{\text{3D,min}} = 2.06 \pm 0.35$|⁠ , indicates that in the high |$\mathcal {M}$| limit the 3D clouds are dominated by planar shocks. The relation between |$\mathcal {D}_{\text{p}}$| and |$\mathcal {D}_{\text{3D}}$| of molecular clouds may be a useful tool for those who are seeking to understand the 3D structures of molecular clouds, purely based upon 2D projected data and shows promise for relating the physics of the turbulent clouds to the fractal dimension. [ABSTRACT FROM AUTHOR]

Star formation is an inefficient process and in general only a small fraction of the gas in a giant molecular cloud (GMC) is turned into stars. This is partly due to the negative effect of stellar feedback from young massive star clusters. Recently, we introduced a novel 1D numerical treatment of the effects of stellar feedback from young massive clusters on their natal clouds, which we named warpfield. Here, we present version 2 of the warpfield code, containing improved treatments of the thermal evolution of the gas and the fragmentation of the feedback-driven shell. As part of this update, we have produced new cooling and heating tables that account for the combined effects of photoionization and collisional ionization on the cooling rate, which we now make publicly available. We employ our updated version of warpfield to investigate the impact of stellar feedback on GMCs with a broad range of masses and surface densities and a variety of density profiles. We show that the minimum star formation efficiency (SFE) εmin, i.e. the SFE above which the cloud is destroyed by feedback, is mainly set by the average cloud surface density. An SFE of 1–6  per cent is generally sufficient to destroy a GMC. We also find an SFE per free-fall time εff ∼ 0.3  per cent, in good agreement with recent observations. Our results imply that feedback alone is sufficient to explain the low observed SFE of GMCs. Finally, we show that very massive clouds with steep density profiles – possible proxies of the giant clumps observed in galaxies at |$z$| ≈ 2 – are more resilient to feedback than typical GMCs, with εmin between 1 and 12  per cent. [ABSTRACT FROM AUTHOR]

Molecular clouds are turbulent structures whose star formation efficiency (SFE) is strongly affected by internal stellar feedback processes. In this paper, we determine how sensitive the SFE of molecular clouds is to randomized inputs in the star formation feedback loop, and to what extent relationships between emergent cloud properties and the SFE can be recovered. We introduce the yule suite of 26 radiative magnetohydrodynamic simulations of a 10 000 solar mass cloud similar to those in the solar neighbourhood. We use the same initial global properties in every simulation but vary the initial mass function sampling and initial cloud velocity structure. The final SFE lies between 6 and 23 per cent when either of these parameters are changed. We use Bayesian mixed-effects models to uncover trends in the SFE. The number of photons emitted early in the cluster's life and the length of the cloud provide the strongest predictors of the SFE. The H ii regions evolve following an analytic model of expansion into a roughly isothermal density field. The more efficient feedback is at evaporating the cloud, the less the star cluster is dispersed. We argue that this is because if the gas is evaporated slowly, the stars are dragged outwards towards surviving gas clumps due to the gravitational attraction between the stars and gas. While star formation and feedback efficiencies are dependent on non-linear processes, statistical models describing cloud-scale processes can be constructed. [ABSTRACT FROM AUTHOR]

The escape of ultraviolet photons from the densest regions of the interstellar medium (ISM) - giant molecular clouds (GMCs) - is a poorly constrained parameter which is vital to understanding the ionization of the ISM and the intergalactic medium. We characterize the escape fraction, fesc,GMC, from a suite of individual GMC simulations with masses in the range 104-6 M☉ using the adaptive-mesh refinement code FLASH. We find significantly different fesc,GMC depending on the GMC mass that can reach >90 per cent in the evolution of 5 × 104 and 105 M☉ clouds or remain low at ∼5 per cent for most of the lifetime of more massive GMCs. All clouds show fluctuations over short, sub-Myr time-scales produced by flickering HII regions. We combine our results to calculate the total escape fraction (fesc,tot) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution (dN/dM α Mα, where α is either -1.5 or -2.5) over fixed time intervals.We find typical fesc,tot values of 8 per cent for both the dwarf and spiral models. The fluctuations of fesc,tot, however, are much larger for the dwarf models with values as high as 90 per cent. The photons escaping from the 5 × 104 and 105 M☉ GMCs are the dominant contributors to fesc,tot in all cases. We also show that the accompanying star formation rates (SFRs) of our model (∼2 × 10-2 and 0.73 M☉yr-1) are consistent with observations of SFRs in dwarf starburst and spiral galaxies, respectively. [ABSTRACT FROM AUTHOR]

Spatial association of clumps from different tracers turns out to be a valuable tool to determine the physical properties of molecular clouds. It provides a reliable estimate for the X-factors, serves to trace the density of clumps seen in column densities only, and allows one to measure the velocity dispersion of clumps identified in dust emission. We study the spatial association between clump populations, extracted by use of the GAUSSCLUMPS technique from 12CO(1-0), 13CO(1-0) line maps and Herschel dust-emission maps of the star-forming region Rosette, and analyse their physical properties. All CO clumps that overlap with another CO or dust counterpart are found to be gravitationally bound and located in the massive star-forming filaments of the molecular cloud. They obey a single mass-size relationMcl ∝ Rγ cl with γ ∝ 3 (implying constant mean density) and display virtually no velocity-size relation.We interpret their population as low-density structures formed through compression by converging flows and still not evolved under the influence of self-gravity. The high-mass parts of their clump mass functions are fitted by a power law dNcl/d logMcl ∝ MГ cl and display a nearly Salpeter slope Г ~ -1.3. On the other hand, clumps extracted from the dust-emission map exhibit a shallowermass-size relation with γ =2.5 and mass functions with very steep slopes Г~-2.3 even if associated with CO clumps. They trace density peaks of the associated CO clumps at scales of a few tenths of pc where no single density scaling law should be expected. [ABSTRACT FROM AUTHOR]

Line emission is strongly dependent on the local environmental conditions in which the emitting tracers reside. In this work, we focus on modelling the CO emission from simulated giant molecular clouds (GMCs), and study the variations in the resulting line ratios arising from the emission from the J = 1-0, J = 2-1, and J = 3-2 transitions. We perform a set of smoothed particle hydrodynamics simulations with time-dependent chemistry, in which environmental conditions - including total cloud mass, density, size, velocity dispersion, metallicity, interstellar radiation field (ISRF), and the cosmic ray ionization rate (CRIR) - were systematically varied. The simulations were then post-processed using radiative transfer to produce synthetic emission maps in the three transitions quoted above. We find that the cloud-averaged values of the line ratios can vary by up to±0.3 dex, triggered by changes in the environmental conditions. Changes in the ISRF and/or in the CRIR have the largest impact on line ratios since they directly affect the abundance, temperature, and distribution of CO-rich gas within the clouds. We show that the standard methods used to convert CO emission to H2 column density can underestimate the total H2 molecular gas in GMCs by factors of 2 or 3, depending on the environmental conditions in the clouds. [ABSTRACT FROM AUTHOR]

Aims. We aim to test the hypothesis that radiation pressure from young star clusters acting on dust is the dominant feedback agent disrupting the largest star-forming molecular clouds and thus regulating the star-formation process. Methods. We performed multi-frequency, 3D, radiative transfer calculations including both scattering and absorption and re-emission to longer wavelengths for model clouds with masses of 104-107 M☉, containing embedded clusters with star formation efficiencies of 0.009-91%, and varying maximum grain sizes up to 200 μm. We calculated the ratio between radiative and gravitational forces to determine whether radiation pressure can disrupt clouds. Results. We find that radiation pressure acting on dust almost never disrupts star-forming clouds. Ultraviolet and optical photons from young stars to which the cloud is optically thick do not scatter much. Instead, they quickly get absorbed and re-emitted by the dust at thermal wavelengths. As the cloud is typically optically thin to far-infrared radiation, it promptly escapes, depositing little momentum in the cloud. The resulting spectrum is more narrowly peaked than the corresponding Planck function, and exhibits an extended tail at longer wavelengths. As the opacity drops significantly across the sub-mm and mm wavelength regime, the resulting radiative force is even smaller than for the corresponding single-temperature blackbody. We find that the force from radiation pressure falls below the strength of gravitational attraction by an order of magnitude or more for either Milky Way or moderate starbust conditions. Only for unrealistically large maximum grain sizes, and star formation efficiencies far exceeding 50% do we find that the strength of radiation pressure can exceed gravity. Conclusions. We conclude that radiation pressure acting on dust does not disrupt star-forming molecular clouds in any Local Group galaxies. Radiation pressure thus appears unlikely to regulate the star-formation process on either local or global scales. [ABSTRACT FROM AUTHOR]

We use hydrodynamical simulations to construct a new coherent picture for the gas flow in the Central Molecular Zone (CMZ), the region of our Galaxy within R ≲ 500 pc. We relate connected structures observed in (l, b, v) data cubes of molecular tracers to nuclear spiral arms. These arise naturally in hydrodynamical simulations of barred galaxies, and are similar to those that can be seen in external galaxies such as NGC 4303 or NGC 1097. We discuss a face-on view of the CMZ, including the positions of several prominent molecular clouds, such as Sgr B2, the 20 and 50 km s-1 clouds, the polar arc, Bania Clump 2 and Sgr C. Our model is also consistent with the larger scale gas flow, up to R ≃ 3 kpc, thus providing a consistent picture of the entire Galactic bar region. [ABSTRACT FROM AUTHOR]

We investigate how star formation is spatially organized in the grand-design spiral NGC 1566 from deep Hubble Space Telescope photometry with the Legacy ExtraGalactic UV Survey. Our contour-based clustering analysis reveals 890 distinct stellar conglomerations at various levels of significance. These star-forming complexes are organized in a hierarchical fashion with the larger congregations consisting of smaller structures, which themselves fragment into even smaller and more compact stellar groupings. Their size distribution, covering a wide range in length-scales, shows a power law as expected from scale-free processes. We explain this shape with a simple 'fragmentation and enrichment' model. The hierarchical morphology of the complexes is confirmed by their mass-size relation that can be represented by a power law with a fractional exponent, analogous to that determined for fractal molecular clouds. The surface stellar density distribution of the complexes shows a lognormal shape similar to that for supersonic non-gravitating turbulent gas. Between 50 and 65 per cent of the recently formed stars, as well as about 90 per cent of the young star clusters, are found inside the stellar complexes, located along the spiral arms.We find an age difference between young stars inside the complexes and those in their direct vicinity in the arms of at least 10 Myr. This time-scale may relate to the minimum time for stellar evaporation, although we cannot exclude the in situ formation of stars. As expected, star formation preferentially occurs in spiral arms. Our findings reveal turbulent-driven hierarchical star formation along the arms of a grand-design galaxy. [ABSTRACT FROM AUTHOR]

The carbon monoxide (CO) rotational transition lines are the most common tracers of molecular gas within giant molecular clouds (MCs).We study the ratio (R2-1/1-0) between CO's first two emission lines and examine what information it provides about the physical properties of the cloud. To study R2-1/1-0, we perform smooth particle hydrodynamic simulations with timedependent chemistry (using GADGET-2), along with post-process radiative transfer calculations on an adaptive grid (using RADMC-3D) to create synthetic emission maps of a MC. R2-1/1-0 has a bimodal distribution that is a consequence of the excitation properties of each line, given that J = 1 reaches local thermal equilibrium while J = 2 is still sub-thermally excited in the considered clouds. The bimodality of R2-1/1-0 serves as a tracer of the physical properties of different regions of the cloud, and it helps constrain local temperatures, densities and opacities. Additionally, this bimodal structure shows an important portion of the CO emission comes from diffuse regions of the cloud, suggesting that the commonly used conversion factor of R2-1/1-0 ~ 0.7 between both lines may need to be studied further. [ABSTRACT FROM AUTHOR]

We present a model for describing the general structure of molecular clouds (MCs) at early evolutionary stages in terms of their mass-size relationship. Sizes are defined through threshold levels at which equipartitions between gravitational, turbulent and thermal energy ∣W∣ ~ f(Ekin + Eth) take place, adopting interdependent scaling relations of velocity dispersion and density and assuming a lognormal density distribution at each scale. Variations of the equipartition coefficient 1 ≤ f ≤ 4 allow for modelling of star-forming regions at scales within the size range of typical MCs (≳4 pc). Best fits are obtained for regions with low or no star formation (Pipe, Polaris) as well for such with star-forming activity but with nearly lognormal distribution of column density (Rosette). An additional numerical test of the model suggests its applicability to cloud evolutionary times prior to the formation of first stars. [ABSTRACT FROM AUTHOR]

The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a more selfconsistent understanding of the interstellar medium (ISM) on small scales and its link to galaxy evolution. We present three-dimensional (magneto)hydrodynamic simulations of the ISM in a vertically stratified box including self-gravity, an external potential due to the stellar component of the galactic disc, and stellar feedback in the form of an interstellar radiation field and supernovae (SNe). The cooling of the gas is based on a chemical network that follows the abundances of H+, H, H2, C+, and CO and takes shielding into account consistently. We vary the SN feedback by comparing different SN rates, clustering and different positioning, in particular SNe in density peaks and at random positions, which has a major impact on the dynamics. Only for random SN positions the energy is injected in sufficiently low-density environments to reduce energy losses and enhance the effective kinetic coupling of the SNe with the gas. This leads to more realistic velocity dispersions (sHI 0.8s300-8000K ~ 10-20 km s-1, sHa 0.6s8000-3 105 K ~ 20-30 km s-1), and strong outflows with mass loading factors (ratio of outflow to star formation rate) of up to 10 even for solar neighbourhood conditions. Clustered SNe abet the onset of outflows compared to individual SNe but do not influence the net outflow rate. The outflows do not contain any molecular gas and are mainly composed of atomic hydrogen. The bulk of the outflowing mass is dense (~ 10-25-10-24 g cm-3) and slow (v ~ 20-40 km s-1) but there is a high-velocity tail of up to v ~ 500 km s-1 with ~ 10-28-10-27 g cm-3. [ABSTRACT FROM AUTHOR]

We show that hydrodynamic turbulent cloud simulations naturally produce large filaments made up of a network of smaller and coherent subfilaments. Such simulations resemble observations of filaments and fibres in nearby molecular clouds. The subfilaments are dynamical features formed at the stagnation points of the turbulent velocity field where shocks dissipate the turbulent energy. They are a ubiquitous feature of the simulated clouds, which appear from the beginning of the simulation and are not formed by gradual fragmentation of larger filaments. Most of the subfilaments are gravitationally subcritical and do not fragment into cores, however, there is also a significant fraction of supercritical subfilaments which break up into star-forming cores. The subfilaments are coherent along their length, and the residual velocities along their spine show that they are subsonically contracting without any ordered rotation on scales of ~0.1 pc. Accretion flows along the subfilaments can feed material into star-forming cores embedded within the network. The overall mass in subfilaments and the number of subfilaments increases as the cloud evolves. We propose that the formation of filaments and subfilaments is a natural consequence of the turbulent cascade in the complex multiphase interstellar medium. Subfilaments are formed by the high wavenumber, smallscale modes in the turbulent velocity field. These are then stretched by local shear motions and gathered together by a combination of low wavenumber modes and gravitational contraction on larger scales, and by doing so build up the extended filaments observed in column density maps. [ABSTRACT FROM AUTHOR]

We use the arepo moving mesh code to simulate the evolution of molecular clouds exposed to a harsh environment similar to that found in the galactic centre (GC), in an effort to understand why the star formation efficiency (SFE) of clouds in this environment is so small. Our simulations include a simplified treatment of time-dependent chemistry and account for the highly non-isothermal nature of the gas and the dust. We model clouds with a total mass of 1.3 × 105 M⊙ and explore the effects of varying the mean cloud density and the virial parameter, α = Ekin/|Epot|. We vary the latter from α = 0.5 to 8.0, and so many of the clouds that we simulate are gravitationally unbound. We expose our model clouds to an interstellar radiation field (ISRF) and cosmic ray flux (CRF) that are both a factor of 1000 higher than the values found in the solar neighbourhood. As a reference, we also run simulations with local solar neighbourhood values of the ISRF and the CRF in order to better constrain the effects of the extreme conditions in the GC on the SFE. Despite the harsh environment and the large turbulent velocity dispersions adopted, we find that all of the simulated clouds form stars within less than a gravitational free-fall time. Increasing the virial parameter from α = 0.5 to 8.0 decreases the SFE by a factor of ~4-10, while increasing the ISRF/CRF by a factor of 1000 decreases the SFE again by a factor of ~2-6. However, even in our most unbound clouds, the SFE remains higher than that inferred for real GC clouds. We therefore conclude that high levels of turbulence and strong external heating are not enough by themselves to lead to a persistently low SFE at the centre of the Galaxy. [ABSTRACT FROM AUTHOR]

We employ the Δ-variance analysis and study the turbulent gas dynamics of simulated molecular clouds (MCs). Our models account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of the gas. We investigate simulations using three different initial mean number densities of n0 = 30, 100 and 300 cm-3 that span the range of values typical for MCs in the solar neighbourhood. Furthermore, we model the CO line emission in a post-processing step using a radiative transfer code. We evaluate Δ-variance spectra for centroid velocity (CV) maps as well as for integrated intensity and column density maps for various chemical components: the total, H2 and 12CO number density and the integrated intensity of both the 12CO and 13CO (J = 1 → 0) lines. The spectral slopes of the Δ-variance computed on the CV maps for the total and H2 number density are significantly steeper compared to the different CO tracers. We find slopes for the line width-size relation ranging from 0.4 to 0.7 for the total and H2 density models, while the slopes for the various CO tracers range from 0.2 to 0.4 and underestimate the values for the total and H2 density by a factor of 1.5-3.0. We demonstrate that optical depth effects can significantly alter the Δ-variance spectra. Furthermore, we report a critical density threshold of ~100 cm-3 at which the Δ-variance slopes of the various CO tracers change sign. We thus conclude that carbon monoxide traces the total cloud structure well only if the average cloud density lies above this limit. [ABSTRACT FROM AUTHOR]

We present an analysis of probability distribution functions (pdfs) of column density in different zones of the star-forming region Perseus and its diffuse environment based on the map of dust opacity at 353 GHz available from the Planck archive. The pdf shape can be fitted by a combination of a lognormal function and an extended power-law tail at high densities, in zones centred at the molecular cloud Perseus. A linear combination of several lognormals fits very well the pdf in rings surrounding the cloud or in zones of its diffuse neighbourhood. The slope of the mean-density scaling law (ρ) L ∝ Lβ is steep (α = -1.93) in the former case and rather shallow (α = 0.77 ± 0.11) in the rings delineated around the cloud. We interpret these findings as signatures of two distinct physical regimes: (i) a gravoturbulent one which is characterized by nearly linear scaling of mass and practical lack of velocity scaling; and (ii) a predominantly turbulent one which is best described by steep velocity scaling and by invariant for compressible turbulence (ρ)L.../L, describing a scale-independent flux of the kinetic energy per unit volume through turbulent cascade. The gravoturbulent spatial domain can be identified with the molecular cloud Perseus while a relatively sharp transition to predominantly turbulent regime occurs in its vicinity. [ABSTRACT FROM AUTHOR]

The 13CO molecule is often used as a column density tracer in regions where the 12CO emission saturates. The 13CO column density is then related to that of 12CO by a uniform isotopic ratio. A similar approximation is frequently used when deriving 13CO emission maps from numerical simulations of molecular clouds. To test this assumption, we calculate the 12CO/13CO ratio self-consistently, taking the isotope-selective photodissociation and the chemical fractionation of CO into account. We model the coupled chemical, thermal and dynamical evolution and the emergent 13CO emission of isolated, starless molecular clouds in various environments. Selective photodissociation has a minimal effect on the ratio, while the chemical fractionation causes a factor of 2–3 decrease at intermediate cloud depths. The variation correlates with both the 12CO and the 13CO column densities. Neglecting the depth dependence results in ≤60 per cent error in 12CO column densities derived from 13CO. The same assumption causes ≤50 per cent disparity in the 13CO emission derived from simulated clouds. We show that the discrepancies can be corrected by a fitting formula. The formula is consistent with millimetre-wavelength isotopic ratio measurements of dense molecular clouds, but underestimates the ratios from the ultraviolet absorption of diffuse regions. [ABSTRACT FROM AUTHOR]

We use a suite of high-resolution molecular cloud simulations carried out with the moving mesh code arepo to explore the nature of star-forming filaments. The simulated filaments are identified and categorized from column density maps in the same manner as for recent Herschel observations. When fitted with a Plummer-like profile, the filaments are in excellent agreement with observations, and have shallow power-law profiles of p ∼ 2.2 without the need for magnetic support. When data within 1 pc of the filament centre are fitted with a Gaussian function, the average full width at half-maximum (FWHM) is ∼0.3 pc, in agreement with predictions for accreting filaments. However, if the fit is constructed using only the inner regions, as in Herschel observations, the resulting FWHM is only ∼0.2 pc. This value is larger than that measured in IC 5146 and Taurus, but is similar to that found in the Planck Galactic cold cores and in Cygnus X. The simulated filaments have a range of widths rather than a constant value. When the column density maps are compared to the 3D gas densities, the filaments seen in column density do not belong to a single structure. Instead, they are made up of a network of short ribbon-like sub-filaments reminiscent of those seen in Taurus. The sub-filaments are pre-existing within the simulated clouds, have radii similar to their Jeans radius, and are not primarily formed through fragmentation of the larger filament seen in column density. Instead, small filamentary clumps are swept together into a single column density structure by the large-scale collapse of the cloud. [ABSTRACT FROM AUTHOR]

Recent observational and theoretical investigations have emphasized the importance of filamentary networks within molecular clouds as sites of star formation. Since such environments are more complex than those of isolated cores, it is essential to understand how the observed line profiles from collapsing cores with non-spherical geometry are affected by filaments. In this study, we investigate line profile asymmetries by performing radiative transfer calculations on hydrodynamic models of three collapsing cores that are embedded in filaments. We compare the results to those that are expected for isolated cores. We model the five lowest rotational transition line (J = 1–0, 2–1, 3–2, 4–3 and 5–4) of both optically thick (HCN, HCO+) as well as optically thin (N2H+, H13CO+) molecules using constant abundance laws. We find that less than 50 per cent of simulated (1–0) transition lines show blue infall asymmetries due to obscuration by the surrounding filament. However, the fraction of collapsing cores that have a blue asymmetric emission line profile rises to 90 per cent when observed in the (4–3) transition. Since the densest gas towards the collapsing core can excite higher rotational states, upper level transitions are more likely to produce blue asymmetric emission profiles. We conclude that even in irregular, embedded cores one can trace infalling gas motions with blue asymmetric line profiles of optically thick lines by observing higher transitions. The best tracer of collapse motions of our sample is the (4–3) transition of HCN, but the (3–2) and (5–4) transitions of both HCN and HCO+ are also good tracers. [ABSTRACT FROM PUBLISHER]

Recent statistical analysis of two extragalactic observational surveys strongly indicate a sub-linear Kennicutt–Schmidt (KS) relationship between the star formation rate (ΣSFR) and molecular gas surface density (Σmol). Here, we consider the consequences of these results in the context of common assumptions, as well as observational support for a linear relationship between ΣSFR and the surface density of dense gas. If the CO traced gas depletion time ($\tau _{\rm dep}^{\rm CO}$) is constant, and if CO only traces star-forming giant molecular clouds (GMCs), then the physical properties of each GMC must vary, such as the volume densities or star formation rates. Another possibility is that the conversion between CO luminosity and Σmol, the XCO factor, differs from cloud-to-cloud. A more straightforward explanation is that CO permeates the hierarchical interstellar medium, including the filaments and lower density regions within which GMCs are embedded. A number of independent observational results support this description, with the diffuse gas comprising at least 30 per cent of the total molecular content. The CO bright diffuse gas can explain the sub-linear KS relationship, and consequently leads to an increasing $\tau _{\rm dep}^{\rm CO}$ with Σmol. If ΣSFR linearly correlates with the dense gas surface density, a sub-linear KS relationship indicates that the fraction of diffuse gas fdiff grows with Σmol. In galaxies where Σmol falls towards the outer disc, this description suggests that fdiff also decreases radially. [ABSTRACT FROM AUTHOR]

Observations of atomic or molecular lines can provide important information about the physical state of star-forming regions. In order to investigate the line profiles from dynamical collapsing massive star-forming regions (MSFRs), we model the emission from hydrodynamic simulations of a collapsing cloud in the absence of outflows. By performing radiative transfer calculations, we compute the optically thick HCO+ and optically thin N2H+ line profiles from two collapsing regions at different epochs. Due to large-scale collapse, theMSFRs have large velocity gradients, reaching up to 20 km s-1 pc-1 across the central core. The optically thin lines typically contain multiple velocity components resulting from the superposition of numerous density peaks along the line of sight. The optically thick lines are only marginally shifted to the blue side of the optically thin line profiles, and frequently do not have a central depression in their profiles due to self-absorption. As the regions evolve, the lines become brighter and the optically thick lines become broader. The lower-order HCO+ (1-0) transitions are better indicators of collapse than the higher-order (4-3) transitions. We also investigate how the beam sizes affect profile shapes. Smaller beams lead to brighter and narrower lines that are more skewed to the blue in HCO+ relative to the true core velocity, but show multiple components in N2H+. High-resolution observations (e.g., with Atacama Large Millimeter Array) can test these predictions and provide insights into the nature of MSFRs. [ABSTRACT FROM AUTHOR]

We study the influence of the choice of cooling function on the formation of molecular clouds in high-resolution three-dimensional simulations of converging flows. We directly compare the results obtained using the simple, parametrized cooling function introduced by Koyama and Inutsuka and used by a number of converging flow studies with the results of the detailed calculation of the non-equilibrium chemistry and thermal balance of the gas. We find that a number of the cloud properties, such as the mass and volume filling fractions of cold gas, are relatively insensitive to the choice of cooling function. On the other hand, the cloud morphology and the large-scale velocity distribution of the gas do strongly depend on the cooling function. We show that the differences that we see can largely be explained by differences in the way that Lyman α cooling is treated in the two complementary approaches, and that a proper non-equilibrium treatment of the ionization and recombination of the gas is necessary in order to model the high-temperature cooling correctly.We also investigate the properties of the dense clumps formed within the cloud. In agreement with previous models, we find that the majority of these clumps are not self-gravitating, suggesting that some form of large-scale collapse of the cloud may be required in order to produce gravitationally unstable clumps and hence stars. Overall, the physical properties of the dense clumps are similar in both simulations, suggesting that they do not depend strongly on the choice of cooling function. However, we do find a systematic difference of around 10 K in the mean temperatures of the clumps produced by the two models. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the first numerical simulations that self-consistently follow the formation of dense molecular clouds in colliding flows. Our calculations include a time-dependent model for the H2 and CO chemistry that runs alongside a detailed treatment of the dominant heating and cooling processes in the interstellar medium. We adopt initial conditions characteristic of the warm neutral medium and study two different flow velocities - a slow flow with vflow = 6.8 km s−1 and a fast flow with vflow = 13.6 km s−1. The clouds formed by the collision of these flows form stars, with star formation beginning after 16 Myr in the case of the slower flow, but after only 4.4 Myr in the case of the faster flow. In both flows, the formation of CO-dominated regions occurs only around 2 Myr before the onset of star formation. Prior to this, the clouds produce very little emission in the J = 1-0 transition line of CO and would probably not be identified as molecular clouds in observational surveys. In contrast, our models show that H2-dominated regions can form much earlier, with the timing depending on the details of the flow. In the case of the slow flow, small pockets of gas become fully molecular around 10 Myr before star formation begins, while in the fast flow, the first H2-dominated regions occur around 3 Myr before the first pre-stellar cores form. Our results are consistent with models of molecular cloud formation in which the clouds are dominated by 'dark' molecular gas for a considerable proportion of their assembly history. [ABSTRACT FROM AUTHOR]

ABSTRACT We derive the mass function of condensations (clumps) which were formed through a turbulent cascade over a range of spatial scales L≤ 20 pc during early, predominantly turbulent evolution of a molecular cloud. The approach rests upon the assumption of a statistical clump mass-density relationship n∝ m x with a scale dependence of the exponent x obtained from equipartition relations between various forms of energy of clumps. The derived clump mass function (ClMF) could be represented by series of two or three power laws, depending on the chosen equipartition relation, the velocity scaling index and the type of turbulent forcing. The high-mass ClMF exhibits an average slope Γ≃−1, typical for fractal clouds, whereas its intermediate-mass part is shallower or flattened, in agreement with some observational studies. [ABSTRACT FROM AUTHOR]

Observations are revealing the ubiquity of filamentary structures in molecular clouds. As cores are often embedded in filaments, it is important to understand how line profiles from such systems differ from those of isolated cores. We perform radiative transfer calculations on a hydrodynamic simulation of a molecular cloud in order to model line emission from collapsing cores embedded in filaments. We model two optically thick lines, CS(2-1) and HCN(1-0), and one optically thin line, N2H+ (1-0), from three embedded cores. In the hydrodynamic simulation, gas self-gravity, turbulence, and bulk flows create filamentary regions within which cores form. Though the filaments have large dispersions, the N2H+ (1-0) lines indicate subsonic velocities within the cores. We find that the observed optically thick line profiles of CS(2-1) and HCN(1-0) vary drastically with viewing angle. In over 50% of viewing angles, there is no sign of a blue asymmetry, an idealized signature of infall motions in an isolated spherical collapsing core. Profiles that primarily trace the cores, with little contribution from the surrounding filament, are characterized by a systematically higher HCN(1-0) peak intensity. The N2H+ (1-0) lines do not follow this trend. We demonstrate that red asymmetric profiles are also feasible in the optically thick lines, due to emission from the filament or one-sided accretion flows onto the core. We conclude that embedded cores may frequently undergo collapse without showing a blue asymmetric profile, and that observational surveys including filamentary regions may underestimate the number of collapsing cores if based solely on profile shapes of optically thick lines. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the impact of different initial conditions for the initial density profile and the initial turbulence on the formation process of protostellar clusters. We study the collapse of dense molecular cloud cores with three-dimensional adaptive mesh refinement simulations. We focus our discussion on the distribution of the gas among the protostellar objects in the turbulent dynamical cores. Despite the large variations in the initial configurations and the resulting gas and cluster morphology we find that all stellar clusters follow a very similar gas accretion behaviour. Once secondary protostars begin to form, the central region of a cluster is efficiently shielded from further accretion. Hence, objects located close to the centre are starved of material, as indicated by a strong decrease of the central accretion rate. This fragmentation-induced starvation occurs not only in rotationally supported discs and filaments, but also in more spherically symmetric clusters with complex chaotic motions. [ABSTRACT FROM AUTHOR]

ABSTRACT We study the mass-density relationship n ∝ mx in molecular cloud condensations (clumps), considering various equipartition relations between their gravitational, kinetic, internal and magnetic energies. Clumps are described statistically, with a density distribution that reflects a lognormal probability density function in turbulent cold interstellar medium. The clump mass-density exponent x derived at different scales L varies in most of the cases within the range −2.5 ≲ x≲−0.2, with a pronounced scale dependence and in consistency with observations. When derived from the global size-mass relationship for set of clumps, generated at all scales, the clump mass-density exponent has typical values −3.0 ≲ x(γglob) ≲−0.3 that depend on the forms of energy, included in the equipartition relations, and on the velocity scaling law, whereas the description of clump geometry is important when magnetic energy is taken into account. [ABSTRACT FROM AUTHOR]

Turbulence plays a major role in the formation and evolution of molecular clouds. Observationally, turbulent velocities are convolved with the density of an observed region. To correct for this convolution, we investigate the relation between the turbulence spectrum of model clouds, and the statistics of their synthetic observations obtained from principal component analysis (PCA). We apply PCA to spectral maps generated from simulated density and velocity fields, obtained from hydrodynamic simulations of supersonic turbulence, and from fractional Brownian motion (fBm) fields with varying velocity, density spectra, and density dispersion. We examine the dependence of the slope of the PCA pseudo-structure function, αPCA on intermittency, on the turbulence velocity (βv) and density (βn) spectral indexes, and on density dispersion. We find that PCA is insensitive to β n and to the log-density dispersion σ s, provided σs ≤ 2. For σs > 2, αPCA increases with σs due to the intermittent sampling of the velocity field by the density field. The PCA calibration also depends on intermittency. We derive a PCA calibration based on fBm structures with σs ≤ 2 and apply it to 367 13CO spectral maps of molecular clouds in the Galactic Ring Survey. The average slope of the PCA structure function, <αPCA> = 0.62 ± 0.2, is consistent with the hydrodynamic simulations and leads to a turbulence velocity exponent of (βv = 2.06 ± 0.6 for a non-intermittent, low density dispersion flow. Accounting for intermittency and density dispersion, the coincidence between the PCA slope of the GRS clouds and the hydrodynamic simulations suggests βv ≃ 1.9, consistent with both Burgers and compressible intermittent turbulence. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate how the X factor, the ratio of the molecular hydrogen column density () to velocity-integrated CO intensity ( W), is determined by the physical properties of gas in model molecular clouds (MCs). The synthetic MCs are results of magnetohydrodynamic simulations, including a treatment of chemistry. We perform radiative transfer calculations to determine the emergent CO intensity, using the large velocity gradient approximation for estimating the CO population levels. In order to understand why observations generally find cloud-averaged values of X = XGal∼ 2 × 1020 cm−2 K−1 km−1 s, we focus on a model representing a typical Milky Way MC. Using globally integrated and W reproduces the limited range in X found in observations and a mean value X = XGal= 2.2 × 1020 cm−2 K−1 km−1 s. However, we show that when considering limited velocity intervals, X can take on a much larger range of values due to CO line saturation. Thus, the X factor strongly depends on both the range in gas velocities and the volume densities. The temperature variations within individual MCs do not strongly affect X, as dense gas contributes most to setting the X factor. For fixed velocity and density structure, gas with higher temperatures T has higher W, yielding X ∝ T−1/2 for T ∼ 20-100 K. We demonstrate that the linewidth-size scaling relationship does not influence the X factor - only the range in velocities is important. Clouds with larger linewidths σ, regardless of the linewidth-size relationship, have a higher W, corresponding to a lower value of X, scaling roughly as X ∝σ−1/2. The 'mist' model, often invoked to explain a constant XGal consisting of optically thick cloudlets with well-separated velocities, does not accurately reflect the conditions in a turbulent MC. We propose that the observed cloud-averaged values of X ∼ XGal are simply a result of the limited range in , temperatures and velocities found in Galactic MCs - a nearly constant value of X therefore does not require any linewidth-size relationship, or that MCs are virialized objects. Since gas properties likely differ (albeit even slightly) from cloud to cloud, masses derived through a standard value of the X factor should only be considered as a rough first estimate. For temperatures T ∼ 10-20 K, velocity dispersions σ∼ 1-6 km s−1and cm−2, we find cloud-averaged values X ∼ 2-4 × 1020 cm−2 K−1 km−1 s for solar-metallicity models. [ABSTRACT FROM AUTHOR]

We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that (1) clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re-expand, oscillating on the scale of tens of Myr; (2) our suite of simulations with the initial magnetic field strength of 2, 3 and show that only supercritical or marginal critical streams lead to reasonable star forming rates. This result is not altered by the inclusion of AD; (3) the GMC's M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM; (4) the M2FR is a highly fluctuating function of position in the clouds. This implies that a significant fraction of a cloud's mass may remain magnetically supported, while SF occurs in the supercritical regions that are not supported; (5) in our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate to which it converts mass to stars. In such an approximately stationary regime, we find that the SFE provides a proxy of the supercritical mass fraction in the cloud; and (6) the low-M2FR regions exhibit buoyancy within the gravitationally contracting GMCs, so that the latter naturally segregate into a high-density, high-M2FR 'core' and a low-density, low-M2FR 'envelope', without the intervention of AD. [ABSTRACT FROM AUTHOR]

Theoretical and observational investigations have indicated that the abundance of carbon monoxide (CO) is very sensitive to intrinsic properties of the gaseous medium, such as density, metallicity and the background radiation field. CO observations are often employed to study the properties of molecular clouds (MCs), such as mass, morphology and kinematics. It is thus important to understand how well CO traces the total mass, which in MCs is predominantly due to molecular hydrogen (H). Recent hydrodynamic simulations by Glover & Mac Low have explicitly followed the formation and destruction of molecules in model MCs under varying conditions. These models have confirmed that CO formation strongly depends on the cloud properties. Conversely, the formation of H is primarily determined by the amount of time available for its formation. We apply radiative transfer calculations to these MC models in order to investigate the properties of CO line emission. We focus on integrated CO (-0) intensities emerging from individual clouds, including its relationship to the total, H and CO column densities, as well as the ' factor,' the ratio of H column density to CO intensity. Models with high CO abundances have a threshold CO intensity of 65 K km s at sufficiently large extinctions (or column densities). Clouds with low CO abundances show no such intensity thresholds. The distributions of total and H column densities are well described as lognormal functions, though the distributions of CO intensities and column densities are usually not lognormal. In general, the probability distribution functions of the integrated intensity do not follow the distribution functions of CO column densities. In the model with Milky Way-like conditions, the factor is in agreement with the near-constant value determined from observations. In clouds with lower metallicity, lower density or a higher background UV radiation field, the CO abundances are in general lower, and hence the factor can vary appreciably - sometimes by up to 4 orders of magnitude. In models with high densities, the CO line is fully saturated, so that the factor is directly proportional to the molecular column density. [ABSTRACT FROM AUTHOR]