Population Of Molecular Clouds Research Articles

Two distinct molecular cloud populations detected in massive galaxies

ABSTRACT We present new ALMA observations of CO, CN, CS, HCN, and HCO$^{+}$ absorption seen against the bright and compact radio continuum sources of eight galaxies. Combined with archival observations, they reveal two distinct populations of molecular clouds, which we identify by combining CO emission and absorption profiles to unambiguously reveal each cloud’s direction of motion and likely location. In galaxy discs, we see clouds with low velocity dispersions, low line-of-sight velocities, and a lack of any systemic inflow or outflow. In galactic cores, we find high velocity dispersion clouds inflowing at up to 550 km s−1. This provides observational evidence in favour of cold accretion on to galactic centres, which likely contributes to the fuelling of active galactic nuclei. We also see a wide range in the CO(2-1)/CO(1-0) ratios of the absorption lines. This is likely the combined effect of hierarchical substructure within the molecular clouds and continuum sources which vary in size with frequency.

Gamma-rays and neutrinos from giant molecular cloud populations in the galactic plane

The recent IceCube detection of significant neutrino flux from the inner Galactic plane has provided us valuable insights on the spectrum of cosmic rays in our Galaxy. This flux can be produced either by a population of Galactic point sources or by diffused emission from cosmic ray interactions with the interstellar medium or by a mixture of both. In this work, we compute diffused gamma-ray and neutrino fluxes produced by a population of giant molecular clouds (GMCs) in our Galaxy, assuming different parametrizations of the Galactic diffused cosmic ray distribution. In particular, we take into account two main cases: (I) constant cosmic ray luminosity in our Galaxy, and (II) space-dependent cosmic ray luminosity, based on the supernovae distribution in our Galaxy. For Case-I, we found that the neutrino flux from GMCs is a factor of ∼ 10 below compared to π 0 and KRA γ best-fitted models of IceCube observations at 105 GeV. Instead, for Case-II the model can explain up to ∼ 90 % of the neutrino flux at that energy. Moreover, for this last scenario IceCube detector could be able to detect neutrino events from the Galactic centre regions. We then calculated gamma-ray and neutrino fluxes from individual GMCs and noticed that several current and future Cherenkov telescopes and neutrino observatories have the right sensitivities to study these objects. In particular, very neutrino-bright region such as Aquila Rift is favourable for detection by the IceCube-Gen2 observatory.

ALMA-LEGUS. II. The Influence of Subgalactic Environments on Molecular Cloud Properties

We compare the molecular cloud properties in subgalactic regions of two galaxies, barred spiral NGC 1313, which is forming many massive clusters, and flocculent spiral NGC 7793, which is forming significantly fewer massive clusters despite having a similar star formation rate to NGC 1313. We find that there are larger variations in cloud properties between different regions within each galaxy than there are between the galaxies on a global scale, especially for NGC 1313. There are higher masses, line widths, pressures, and virial parameters in the arms of NGC 1313 and the center of NGC 7793 than in the interarm and outer regions of the galaxies. The massive cluster formation of NGC 1313 may be driven by its greater variation in environment, allowing more clouds with the necessary conditions to emerge, although no one parameter seems primarily responsible for the difference in star formation. Meanwhile NGC 7793 has clouds that are as massive and have as much kinetic energy as the clouds in the arms of NGC 1313, but have densities and pressures more similar to those in the interarm regions and so are less inclined to collapse and form stars. The cloud properties in NGC 1313 and NGC 7793 suggest that spiral arms, bars, interarm regions, and flocculent spirals each represent distinct environments with regard to molecular cloud populations. We see surprisingly little difference in surface density between the regions, suggesting that the differences in surface densities frequently seen between arm and interarm regions in lower-resolution studies are indicative of the sparsity of molecular clouds, rather than differences in their true surface density.

ALMA-LEGUS. I. The Influence of Galaxy Morphology on Molecular Cloud Properties

We present a comparative study of the molecular gas in two galaxies from the Legacy ExtraGalactic UV Survey (LEGUS) sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >104 M ⊙). Using Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) observations of the two galaxies with the same sensitivity and resolution (13 pc), we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size–line width relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than with the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences is that NGC 1313 has higher surface densities and lower freefall times.

Exploring extreme conditions for star formation: A deep search for molecular gas in the Leo ring

Aims. We carried out sensitive searches for the 12CO J = 1–0 and J = 2–1 lines in the giant extragalactic HI ring in Leo to investigate the star formation process within environments where gas metallicities are close to solar, but physical conditions are different than those typical of bright galaxy disks. Our aim is to check the range of validity of known scaling relations. Methods. We used the IRAM-30 m telescope to observe 11 regions close to HI gas peaks or where sparse young massive stars have been found. For all pointed observations we reached spectral noise between 1 and 5 mK for at least one of the observed frequencies at 2 km s−1 spectral resolution. Results. We marginally detect two 12CO J = 1–0 lines in the star-forming region Clump 1 of the Leo ring, whose radial velocities are consistent with those of Hα lines, but whose line widths are much smaller than observed for virialized molecular clouds of similar mass in galaxies. The low signal-to-noise ratio, the small line widths, and the extremely low number densities inferred by virialized cloud models suggest that a more standard population of molecular clouds, still undetected, might be in place. Using upper limits to the CO lines, the most sensitive pointed observations show that the molecular gas mass surface density is lower than expected from the extrapolation of the molecular Kennicutt–Schmidt relation established in the disk of galaxies. The sparse stellar population in the ring, possibly forming ultra diffuse dwarf galaxies, might then be the result of a short molecular gas depletion time in this extreme environment.

Building the molecular cloud population: the role of cloud mergers

ABSTRACT We study the physical drivers of slow molecular cloud mergers within a simulation of a Milky Way-like galaxy in the moving-mesh code arepo, and determine the influence of these mergers on the mass distribution and star formation efficiency of the galactic cloud population. We find that 83 per cent of these mergers occur at a relative velocity below 5 km s−1, and are associated with large-scale atomic gas flows, driven primarily by expanding bubbles of hot, ionized gas caused by supernova explosions and galactic rotation. The major effect of these mergers is to aggregate molecular mass into higher-mass clouds: mergers account for over 50 per cent of the molecular mass contained in clouds of mass M &amp;gt; 2 × 106 M⊙. These high-mass clouds have higher densities, internal velocity dispersions and instantaneous star formation efficiencies than their unmerged, lower mass precursors. As such, the mean instantaneous star formation efficiency in our simulated galaxy, with its merger rate of just 1 per cent of clouds per Myr, is 25 per cent higher than in a similar population of clouds containing no mergers.

First Detection of the Molecular Cloud Population in the Extended Ultraviolet Disk of M83

We report a CO(J = 3−2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with the Atacama Large Millimeter/submillimeter Array. The observed 1 kpc2 region is at about 1.24 times the optical radius (R 25) of the disk, where CO(J = 2–1) was previously not detected. The detection and nondetection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A—with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3−2), range from 8.2 × 102 to 2.3 × 104 M ⊙ and from 2.7 × 102 to 7.5 × 103 M ⊙, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is common. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The common cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3−2) is an efficient tracer of molecular clouds even in low-metallicity environments.

Does absorption against AGN reveal supermassive black hole accretion?

ABSTRACT Galaxies often contain large reservoirs of molecular gas that shape their evolution. This can be through cooling of the gas – which leads to star formation, or accretion on to the central supermassive black hole – which fuels active galactic nucleus (AGN) activity and produces powerful feedback. Molecular gas has been detected in early-type galaxies on scales of just a few tens to hundreds of solar masses by searching for absorption against their compact radio cores. Using this technique, ALMA has found absorption in several brightest cluster galaxies, some of which show molecular gas moving towards their galaxy’s core at hundreds of km s−1. In this paper, we constrain the location of this absorbing gas by comparing each galaxy’s molecular emission and absorption. In four galaxies, the absorption properties are consistent with chance alignments between the continuum and a fraction of the molecular clouds visible in emission. In four others, the properties of the absorption are inconsistent with this scenario. In these systems, the absorption is likely produced by a separate population of molecular clouds in close proximity to the galaxy core and with high inward velocities and velocity dispersions. We thus deduce the existence of two types of absorber, caused by chance alignments between the radio core and: (i) a fraction of the molecular clouds visible in emission, and (ii) molecular clouds close to the AGN, in the process of accretion. We also present the first ALMA observations of molecular emission in S555, Abell 2390, RXC J1350.3+0940, and RXC J1603.6+1553 – with the latter three having $M_{\rm {mol}} \gt 10^{10}\, \rm {M}_{\odot }$.

Molecular Cloud Populations in the Context of Their Host Galaxy Environments: A Multiwavelength Perspective

We present a rich, multiwavelength, multiscale database built around the PHANGS–ALMA CO (2 − 1) survey and ancillary data. We use this database to present the distributions of molecular cloud populations and subgalactic environments in 80 PHANGS galaxies, to characterize the relationship between population-averaged cloud properties and host galaxy properties, and to assess key timescales relevant to molecular cloud evolution and star formation. We show that PHANGS probes a wide range of kpc-scale gas, stellar, and star formation rate (SFR) surface densities, as well as orbital velocities and shear. The population-averaged cloud properties in each aperture correlate strongly with both local environmental properties and host galaxy global properties. Leveraging a variable selection analysis, we find that the kpc-scale surface densities of molecular gas and SFR tend to possess the most predictive power for the population-averaged cloud properties. Once their variations are controlled for, galaxy global properties contain little additional information, which implies that the apparent galaxy-to-galaxy variations in cloud populations are likely mediated by kpc-scale environmental conditions. We further estimate a suite of important timescales from our multiwavelength measurements. The cloud-scale freefall time and turbulence crossing time are ∼5–20 Myr, comparable to previous cloud lifetime estimates. The timescales for orbital motion, shearing, and cloud–cloud collisions are longer, ∼100 Myr. The molecular gas depletion time is 1–3 Gyr and shows weak to no correlations with the other timescales in our data. We publish our measurements online, and expect them to have broad utility to future studies of molecular clouds and star formation.

Why most molecular clouds are gravitationally dominated

ABSTRACT Observational and theoretical evidence suggests that a substantial population of molecular clouds (MCs) appear to be unbound, dominated by turbulent motions. However, these estimations are made typically via the classical virial parameter $\alpha _{\rm vir}^{\rm class}$, which is an observational proxy to the virial ratio between the kinetic and the gravitational energy. This parameter intrinsically assumes that MCs are isolated, spherical, and with constant density. However, MCs are embedded in their parent galaxy and thus are subject to compressive and disruptive tidal forces from their galaxy, exhibit irregular shapes, and show substantial substructure. We, therefore, compare the typical estimations of $\alpha _{\rm vir}^{\rm class}$ to a more precise definition of the virial parameter, $\alpha _{\rm vir}^{\rm full}$, which accounts not only for the self-gravity (as $\alpha _{\rm vir}^{\rm class}$), but also for the tidal stresses, and thus, it can take negative (self-gravity) and positive (tides) values. While we recover the classical result that most of the clouds appear to be unbound, having $\alpha _{\rm vir}^{\rm class}\gt 2$, we show that, with the more detailed definition considering the full gravitational energy, (i) 50 per cent of the total population is gravitationally bound, however, (ii) another 20 per cent is gravitationally dominated, but with tides tearing them apart; (iii) the source of those tides does not come from the galactic structure (bulge, halo, spiral arms), but from the molecular cloud complexes in which clouds reside, and probably (iv) from massive young stellar complexes, if they were present. (v) Finally, our results also suggest that, interstellar turbulence can have, at least partially, a gravitational origin.

The Dense Gas Mass Fraction and the Relationship to Star Formation in M51

Observations of 12CO J = 1 – 0 and HCN J = 1 – 0 emission from NGC 5194 (M51) made with the 50 m Large Millimeter Telescope and the SEQUOIA focal plane array are presented. Using the HCN-to-CO ratio, we examine the dense gas mass fraction over a range of environmental conditions within the galaxy. Within the disk, the dense gas mass fraction varies along the spiral arms but the average value over all spiral arms is comparable to the mean value of interarm regions. We suggest that the near-constant dense gas mass fraction throughout the disk arises from a population of density-stratified, self-gravitating molecular clouds and the required density threshold to detect each spectral line. The measured dense gas fraction significantly increases in the central bulge in response to the effective pressure, P e , from the weight of the stellar and gas components. This pressure modifies the dynamical state of the molecular cloud population and, possibly, the HCN-emitting regions in the central bulge from self-gravitating to diffuse configurations in which P e is greater than the gravitational energy density of individual clouds. Diffuse molecular clouds comprise a significant fraction of the molecular gas mass in the central bulge, which may account for the measured sublinear relationships between the surface densities of the star formation rate and molecular and dense gas.

On the initial mass–radius relation of stellar clusters

ABSTRACT Young stellar clusters across nearly five orders of magnitude in mass appear to follow a power-law mass–radius relationship (MRR), $R_{\star }\propto M_{\star }^{\alpha }$, with α ≈ 0.2–0.33. We develop a simple analytic model for the cluster mass–radius relation. We consider a galaxy disc in hydrostatic equilibrium, which hosts a population of molecular clouds that fragment into clumps undergoing cluster formation and feedback-driven expansion. The model predicts a mass–radius relation of $R_{\star }\propto M_{\star }^{1/2}$ and a dependence on the kpc-scale gas surface density $R_{\star }\propto \Sigma _{\rm g}^{-1/2}$, which results from the formation of more compact clouds (and cluster-forming clumps within) at higher gas surface densities. This environmental dependence implies that the high-pressure environments in which the most massive clusters can form also induce the formation of clusters with the smallest radii, thereby shallowing the observed MRR at high-masses towards the observed $R_{\star }\propto M_{\star }^{1/3}$. At low cluster masses, relaxation-driven expansion induces a similar shallowing of the MRR. We combine our predicted MRR with a simple population synthesis model and apply it to a variety of star-forming environments, finding good agreement. Our model predicts that the high-pressure formation environments of globular clusters at high redshift naturally led to the formation of clusters that are considerably more compact than those in the local Universe, thereby increasing their resilience to tidal shock-driven disruption and contributing to their survival until the present day.

Molecular Clouds with CO-dark Envelopes in the Extended Ultraviolet (XUV) Disk of M83

AbstractWe report a CO(3-2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1 kpc2 region is at about 1.24R25 from the disk center, where CO(2-1) was previously not detected. The detection and non-detection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A – with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud masses, estimated from CO(3-2), range from 8.2×102 to 2.3×104M⊙. The most massive clouds appear similar to Orion A in SF activity as well as in gas mass. The common cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3-2) is an efficient tracer of molecular clouds even in low-metallicity environments. This study is published in the Astronomical Journal, entitled “First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83" by J. Koda, L. Watson, F. Combes, M. Rubio, S. Boissier, M. Yagi, D. Thilker, A. M Lee, Y. Komiyama, K. Morokuma-Matsui, and C. Verdugo.

The SEDIGISM survey: molecular clouds in the inner Galaxy

ABSTRACT We use the 13CO (2–1) emission from the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium) high-resolution spectral-line survey of the inner Galaxy, to extract the molecular cloud population with a large dynamic range in spatial scales, using the Spectral Clustering for Interstellar Molecular Emission Segmentation (scimes) algorithm. This work compiles a cloud catalogue with a total of 10 663 molecular clouds, 10 300 of which we were able to assign distances and compute physical properties. We study some of the global properties of clouds using a science sample, consisting of 6664 well-resolved sources and for which the distance estimates are reliable. In particular, we compare the scaling relations retrieved from SEDIGISM to those of other surveys, and we explore the properties of clouds with and without high-mass star formation. Our results suggest that there is no single global property of a cloud that determines its ability to form massive stars, although we find combined trends of increasing mass, size, surface density, and velocity dispersion for the sub-sample of clouds with ongoing high-mass star formation. We then isolate the most extreme clouds in the SEDIGISM sample (i.e. clouds in the tails of the distributions) to look at their overall Galactic distribution, in search for hints of environmental effects. We find that, for most properties, the Galactic distribution of the most extreme clouds is only marginally different to that of the global cloud population. The Galactic distribution of the largest clouds, the turbulent clouds and the high-mass star-forming clouds are those that deviate most significantly from the global cloud population. We also find that the least dynamically active clouds (with low velocity dispersion or low virial parameter) are situated further afield, mostly in the least populated areas. However, we suspect that part of these trends may be affected by some observational biases (such as completeness and survey limitations), and thus require further follow up work in order to be confirmed.

Molecular Gas Properties on Cloud Scales across the Local Star-forming Galaxy Population

Abstract Using the PHANGS–ALMA CO(2–1) survey, we characterize molecular gas properties on ∼100 pc scales across 102,778 independent sightlines in 70 nearby galaxies. This yields the best synthetic view of molecular gas properties on cloud scales across the local star-forming galaxy population obtained to date. Consistent with previous studies, we observe a wide range of molecular gas surface densities (3.4 dex), velocity dispersions (1.7 dex), and turbulent pressures (6.5 dex) across the galaxies in our sample. Under simplifying assumptions about subresolution gas structure, the inferred virial parameters suggest that the kinetic energy of the molecular gas typically exceeds its self-gravitational binding energy at ∼100 pc scales by a modest factor (1.3 on average). We find that the cloud-scale surface density, velocity dispersion, and turbulent pressure (1) increase toward the inner parts of galaxies, (2) are exceptionally high in the centers of barred galaxies (where the gas also appears less gravitationally bound), and (3) are moderately higher in spiral arms than in inter-arm regions. The galaxy-wide averages of these gas properties also correlate with the integrated stellar mass, star formation rate, and offset from the star-forming main sequence of the host galaxies. These correlations persist even when we exclude regions with extraordinary gas properties in galaxy centers, which contribute significantly to the inter-galaxy variations. Our results provide key empirical constraints on the physical link between molecular cloud populations and their galactic environment.

Major impact from a minor merger

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T &gt; 300 K) and high gas densities (n(H2) &gt; 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

Deep Chandra Observations of ESO 428-G014. II. Spectral Properties and Morphology of the Large-scale Extended X-Ray Emission

Abstract We present a deep Chandra spectral and spatial study of the kpc-scale diffuse X-ray emission of the Compton-thick (CT) active galactic nucleus (AGN) ESO 428-G014. The entire spectrum is best fit with composite photoionization + thermal models. The diffuse emission is more extended at lower energies (&lt;3 keV). The smaller extent of the hard continuum and Fe Kα profiles implies that the optically thicker clouds responsible for this scattering may be relatively more prevalent closer to the nucleus. These clouds must not prevent soft ionizing X-rays from the AGN escaping to larger radii, in order to have photoionized ISM at larger radii. This suggests that at smaller radii, there may be a larger population of molecular clouds to scatter the hard X-rays, as in the Milky Way. The diffuse emission is also significantly extended in the cross-cone direction, where the AGN emission would be mostly obscured by the torus in the standard AGN model. Our results suggest that the transmission of the obscuring region in the cross-cone direction is ∼10% of that in the cone direction. In the 0.3–1.5 keV band, the ratio of cross-cone to cone photons increases to ∼84%, suggesting an additional soft diffuse emission component disjoint from the AGN. This could be due to hot ISM trapped in the potential of the galaxy. The luminosity of this component, ∼5 × 1038 erg s−1, is roughly consistent with the thermal component suggested by the spectral fits in the 170–900 pc annulus.

Simulating the UV escape fractions from molecular cloud populations in star-forming dwarf and spiral galaxies

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 H ii 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 ∝ 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.

Feeding versus Falling: The Growth and Collapse of Molecular Clouds in a Turbulent Interstellar Medium

Abstract In order to understand the origin of observed molecular cloud (MC) properties, it is critical to understand how clouds interact with their environments during their formation, growth, and collapse. It has been suggested that accretion-driven turbulence can maintain clouds in a highly turbulent state, preventing runaway collapse and explaining the observed non-thermal velocity dispersions. We present 3D, adaptive-mesh-refinement, magnetohydrodynamical simulations of a kiloparsec-scale, stratified, supernova-driven, self-gravitating, interstellar medium (ISM), including diffuse heating and radiative cooling. These simulations model the formation and evolution of a MC population in the turbulent ISM. We use zoom-in techniques to focus on the dynamics of the mass accretion and its history for individual MCs. We find that mass accretion onto MCs proceeds as a combination of turbulent flow and near free-fall accretion of a gravitationally bound envelope. Nearby supernova explosions have a dual role, compressing the envelope and increasing mass accretion rates, but also disrupting parts of the envelope and eroding mass from the cloud’s surface. It appears that the inflow rate of kinetic energy onto clouds from supernova explosions is insufficient to explain the net rate of change of the cloud kinetic energy. In the absence of self-consistent star formation, the conversion of gravitational potential into kinetic energy during contraction seems to be the main driver of non-thermal motions within clouds. We conclude that although clouds interact strongly with their environments, bound clouds are always in a state of gravitational contraction, close to runaway, and their properties are a natural result of this collapse.

The varying mass distribution of molecular clouds across M83

The work of Adamo et al. (2015) showed that the mass distributions of young massive stellar clusters were truncated above a maximum-mass scale in the nearby galaxy M83 and that this truncation mass varies with galactocentric radius. Here, we present a cloud-based analysis of ALMA CO($1\to 0$) observations of M83 to search for such a truncation mass in the molecular cloud population. We identify a population of 873 molecular clouds in M83 that is largely similar to those found in the Milky Way and Local Group galaxies, though clouds in the centre of the galaxy show high surface densities and enhanced turbulence, as is common for clouds in high-density nuclear environments. Like the young massive clusters, we find a maximum-mass scale for the molecular clouds that decreases radially in the galaxy. We find the most massive young massive cluster tracks the most massive molecular cloud with the cluster mass being $10^{-2}$ times that of the most massive molecular cloud. Outside the nuclear region of M83 ($R_{g}>0.5$ kpc), there is no evidence for changing internal conditions in the population of molecular clouds, with the average internal pressures, densities, and free-fall times remaining constant for the cloud population over the galaxy. This result is consistent with the bound cluster formation efficiency depending only on the large-scale properties of the ISM, rather than the internal conditions of individual clouds.