Population Of Giant Molecular Clouds Research Articles

The impact of H II regions on giant molecular cloud properties in nearby galaxies sampled by PHANGS ALMA and MUSE

Context. The final stages of molecular cloud evolution involve cloud disruption due to feedback by massive stars, with recent literature suggesting the importance of early (i.e., pre-supernova) feedback mechanisms. Aims. We aim to determine whether feedback from massive stars in H II regions has a measurable impact on the physical properties of molecular clouds at a characteristic scale of ~ 100 pc, and whether the imprint of feedback on the molecular gas depends on the local galactic environment. Methods. We identified giant molecular clouds (GMCs) associated with H II regions for a sample of 19 nearby galaxies from catalogs of GMCs and H II regions released by the PHANGS-ALMA and PHANGS-MUSE surveys, using the overlap of the CO and Hα emission as the key criterion for physical association. We compared the distributions of GMC and H II region properties for paired and non-paired objects. We investigated correlations between GMC and H II region properties among galaxies and across different galactic environments to determine whether GMCs that are associated with H II regions have significantly distinct physical properties compared to the parent GMC population. Results. We identify trends between the Hα luminosity of an H II region and the CO peak brightness and molecular mass of GMCs that we tentatively attribute to a direct physical connection between the matched objects, and which arise independently of the underlying environmental variations of GMC and H II region properties within galaxies. The study of the full sample nevertheless hides a large galaxy-to-galaxy variability. Conclusions. At the ~100 pc scales accessed by the PHANGS-ALMA and PHANGS-MUSE data, pre-supernova feedback mechanisms in H II regions have a subtle but measurable impact on the properties of the surrounding molecular gas, as inferred from CO observations.

Long GMC Lifetimes: Using the Method and Data of Meidt et al. (2015) with a Correction

Abstract Meidt et al. derived short lifetimes of 20–30 Myr for giant molecular clouds (GMCs) in M51. Their novel approach utilizes a decline of the GMC population during their inter-arm passage from one spiral arm to the next. Using the inter-arm travel time t travel as a fiducial clock, they converted the decline rate to GMC lifetimes. They implicitly adopted zero as the pattern speed of the spiral arms, resulting in a very short t travel. However, it is well established that the spiral arm pattern in M51 is rotating, and that t travel should be measured with respect to the rotating spiral pattern. Here we use the same method and data of Meidt et al. and re-evaluate GMC lifetimes by accounting for the pattern speed given by Meidt et al. This correction gives a longer t travel, and consequently longer GMC lifetimes of 60–500 Myr.

The effects of subgrid models on the properties of giant molecular clouds in galaxy formation simulations

ABSTRACT Recent cosmological hydrodynamical simulations are able to reproduce numerous statistical properties of galaxies that are consistent with observational data. Yet, the adopted subgrid models strongly affect the simulation outcomes, limiting the predictive power of these simulations. In this work, we perform a suite of isolated galactic disc simulations under the SMUGGLE framework and investigate how different subgrid models affect the properties of giant molecular clouds (GMCs). We employ astrodendro, a hierarchical clump-finding algorithm, to identify GMCs in the simulations. We find that different choices of subgrid star formation efficiency, ϵff, and stellar feedback channels, yield dramatically different mass and spatial distributions for the GMC populations. Without feedback, the mass function of GMCs has a shallower power-law slope and extends to higher mass ranges compared to runs with feedback. Moreover, higher ϵff results in faster molecular gas consumption and steeper mass function slopes. Feedback also suppresses power in the two-point correlation function (TPCF) of the spatial distribution of GMCs. Specifically, radiative feedback strongly reduces the TPCF on scales below 0.2 kpc, while supernova feedback reduces power on scales above 0.2 kpc. Finally, runs with higher ϵff exhibit a higher TPCF than runs with lower ϵff, because the dense gas is depleted more efficiently, thereby facilitating the formation of well-structured supernova bubbles. We argue that comparing simulated and observed GMC populations can help better constrain subgrid models in the next generation of galaxy formation simulations.

Physical properties and scaling relations of molecular clouds: the effect of stellar feedback

Using hydrodynamical simulations of entire galactic discs similar to the Milky Way, reaching 4.6pc resolution, we study the origins of observed physical properties of giant molecular clouds (GMCs). We find that efficient stellar feedback is a necessary ingredient in order to develop a realistic interstellar medium (ISM), leading to molecular cloud masses, sizes, velocity dispersions and virial parameters in excellent agreement with Milky Way observations. GMC scaling relations observed in the Milky Way, such as the mass-size ($M$--$R$), velocity dispersion-size ($\sigma$--$R$), and the $\sigma$--$R\Sigma$ relations, are reproduced in a feedback driven ISM when observed in projection, with $M\propto R^{2.3}$ and $\sigma\propto R^{0.56}$. When analysed in 3D, GMC scaling relations steepen significantly, indicating potential limitations of our understanding of molecular cloud 3D structure from observations. Furthermore, we demonstrate how a GMC population's underlying distribution of virial parameters can strongly influence the scatter in derived scaling relations. Finally, we show that GMCs with nearly identical global properties exist in different evolutionary stages, where a majority of clouds being either gravitationally bound or expanding, but with a significant fraction being compressed by external ISM pressure, at all times.

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.

Resonant thickening of self-gravitating discs: imposed or self-induced orbital diffusion in the tightly wound limit

The secular thickening of a self-gravitating stellar galactic disc is investigated using the dressed collisionless Fokker–Planck equation and the inhomogeneous multicomponent Balescu–Lenard equation. The thick WKB limits for the diffusion fluxes are found using the epicyclic approximation, while assuming that only radially tightly wound transient spirals are sustained by the disc. This yields simple quadratures for the drift and diffusion coefficients, providing a clear understanding of the positions of maximum vertical orbital diffusion within the disc, induced by fluctuations either external or due to the finite number of particles. These thick limits also offer a consistent derivation of a thick disc Toomre parameter, which is shown to be exponentially boosted by the ratio of the vertical to radial scaleheights. Dressed potential fluctuations within the disc statistically induce a vertical bending of a subset of resonant orbits, triggering the corresponding increase in vertical velocity dispersion. When applied to a tepid stable tapered disc perturbed by shot noise, these two frameworks reproduce qualitatively the formation of ridges of resonant orbits towards larger vertical actions, as found in direct numerical simulations, but overestimates the time-scale involved in their appearance. Swing amplification is likely needed to resolve this discrepancy, as demonstrated in the case of razor-thin discs. Other sources of thickening are also investigated, such as fading sequences of slowing bars, or the joint evolution of a population of giant molecular clouds within the disc.

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

Abstract Recent radio observations show that giant molecular cloud (GMC) mass functions noticeably vary across galactic disks. High-resolution magnetohydrodynamics simulations show that multiple episodes of compression are required for creating a molecular cloud in the magnetized interstellar medium. In this article, we formulate the evolution equation for the GMC mass function to reproduce the observed profiles, for which multiple compressions are driven by a network of expanding shells due to H ii regions and supernova remnants. We introduce the cloud–cloud collision (CCC) terms in the evolution equation in contrast to previous work (Inutsuka et al.). The computed time evolution suggests that the GMC mass function slope is governed by the ratio of GMC formation timescale to its dispersal timescale, and that the CCC effect is limited only in the massive end of the mass function. In addition, we identify a gas resurrection channel that allows the gas dispersed by massive stars to regenerate GMC populations or to accrete onto pre-existing GMCs. Our results show that almost all of the dispersed gas contributes to the mass growth of pre-existing GMCs in arm regions whereas less than 60% contributes in inter-arm regions. Our results also predict that GMC mass functions have a single power-law exponent in the mass range <105.5 (where represents the solar mass), which is well characterized by GMC self-growth and dispersal timescales. Measurement of the GMC mass function slope provides a powerful method to constrain those GMC timescales and the gas resurrecting factor in various environments across galactic disks.

Gaps in globular cluster streams: giant molecular clouds can cause them too

Abstract As a result of their internal dynamical coherence, thin stellar streams formed by disrupting globular clusters (GCs) can act as detectors of dark matter (DM) substructure in the Galactic halo. Perturbations induced by close flybys amplify into detectable density gaps, providing a probe both of the abundance and of the masses of DM subhaloes. Here, we use N-body simulations to show that the Galactic population of giant molecular clouds (GMCs) can also produce gaps (and clumps) in GC streams, and so may confuse the detection of DM subhaloes. We explore the cases of streams analogous to the observed Palomar 5 and GD1 systems, quantifying the expected incidence of structure caused by GMC perturbations. Deep observations should detect such disturbances regardless of the substructure content of the Milky Way's halo. Detailed modelling will be needed to demonstrate that any detected gaps or clumps were produced by DM subhaloes rather than by molecular clouds.

Giant Molecular Cloud Populations in Nearby Galaxies

AbstractWe present new results from a comparative analysis of the resolved giant molecular cloud (GMC) populations in six nearby galaxies. We show that the GMCs in denser environments–M51, the centre of NGC6946–have greater CO surface brightness and higher velocity dispersions relative to their size than GMCs in less dense environments. We find systematic differences in the GMC mass distribution among galaxies, such that more of the molecular gas in the low-mass galaxies (M33, the Large Magellanic Cloud) and the outer disk of M31 is located in low mass clouds. Using the number density of GMCs in the interarm regions of M51, we argue that GMC destruction in this region is regulated by shear, and that cloud lifetimes there are finite and short, ~20 to 30 Myr. Our results indicate the importance of galactic environment on the evolution of GMCs, and on a galaxy's global pattern of star formation.

Evidence for the non-destruction of the Most Massive Molecular Clouds even after they have given Birth to Massive Star Clusters

AbstractWe have combined observations of the Antennae galaxies from the radio interferometer ALMA and from the optical interferometer GHαFaS (Galaxy Hα Fabry-Perot System). The two sets of observations have comparable angular and spectral resolutions, enabling us to identify 142 giant molecular clouds (GMCs) and 303 Hii regions. We have measured, and compared, their basic physical properties (radius, velocity dispersion, luminosity). We study the mass functions of Hii regions and GMCs, where a double population of Hii regions due to a double population of GMCs is found. Using the velocity dispersion measurements, we claim that the difference between the regimes is the nature of the dominant binding force. For the regions in the lower mass range, the dominant force is the external pressure, while in the higher mass range it is the internal gravity of the clouds. We have applied the same method to similar observations (GHαFaS & ALMA) of Arp 236 and NGC 1614 finding similar results.

GIANT MOLECULAR CLOUDS IN THE EARLY-TYPE GALAXY NGC 4526

We present a high spatial resolution ($\approx 20$ pc) of $^{12}$CO($2-1$) observations of the lenticular galaxy NGC4526. We identify 103 resolved Giant Molecular Clouds (GMCs) and measure their properties: size $R$, velocity dispersion $\sigma_v$, and luminosity $L$. This is the first GMC catalog of an early-type galaxy. We find that the GMC population in NGC4526 is gravitationally bound, with a virial parameter $\alpha \sim 1$. The mass distribution, $dN/dM \propto M^{-2.39 \pm 0.03}$, is steeper than that for GMCs in the inner Milky Way, but comparable to that found in some late-type galaxies. We find no size-linewidth correlation for the NGC4526 clouds, in contradiction to the expectation from Larson's relation. In general, the GMCs in NGC4526 are more luminous, denser, and have a higher velocity dispersion than equal size GMCs in the Milky Way and other galaxies in the Local Group. These may be due to higher interstellar radiation field than in the Milky Way disk and weaker external pressure than in the Galactic center. In addition, a kinematic measurement of cloud rotation shows that the rotation is driven by the galactic shear. For the vast majority of the clouds, the rotational energy is less than the turbulent and gravitational energy, while the four innermost clouds are unbound and will likely be torn apart by the strong shear at the galactic center. We combine our data with the archival data of other galaxies to show that the surface density $\Sigma$ of GMCs is not approximately constant as previously believed, but varies by $\sim 3$ orders of magnitude. We also show that the size and velocity dispersion of GMC population across galaxies are related to the surface density, as expected from the gravitational and pressure equilibrium, i.e. $\sigma_v R^{-1/2} \propto \Sigma^{1/2}$.

Observing simulations: molecular clouds and their journey in the galaxy

In order to have a global picture of the cycle of matter in galaxies, we need to understand the interplay of large-scale galactic phenomena with the formation of giant molecular clouds (GMCs) and, ultimately, their subsequent star formation (SF). In this work, we study the population of GMCs within a smoothed particle hydrodynamics (SPH) simulation of a spiral galaxy, and investigate the link between the GMC properties and position with respect to spiral arms, both directly from the simulation (with the 3D densities of H 2 and CO) and from an observer's perspective (with CO emission in PPV space).

A COMPARATIVE STUDY OF GIANT MOLECULAR CLOUDS IN M51, M33, AND THE LARGE MAGELLANIC CLOUD

We compare the properties of giant molecular clouds (GMCs) in M51 identified by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with GMCs identified in wide-field, high resolution surveys of CO emission in M33 and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger, brighter and have higher velocity dispersions relative to their size than equivalent structures in M33 and the LMC. These differences imply that there are genuine variations in the average mass surface density of the different GMC populations. To explain this, we propose that the pressure in the interstellar medium surrounding the GMCs plays a role in regulating their density and velocity dispersion. We find no evidence for a correlation between size and linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into GMCs, although moderately robust correlations are apparent when regions of contiguous CO emission (with no size limitation) are used. Our work demonstrates that observational bias remains an important obstacle to the identification and study of extragalactic GMC populations using CO emission, especially in molecule-rich galactic environments.

RESOLVED GIANT MOLECULAR CLOUDS IN NEARBY SPIRAL GALAXIES: INSIGHTS FROM THE CANON CO (1-0) SURVEY

We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 $\times$ 10$^{5}$ \Msun in the inner disks of five large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and $^{12}$CO (J=1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H$_{2}$ mass (or \xcons) for each galaxy is 1-2 \xcounits, all within a factor of two of the Milky Way disk value ($\sim$2 \xcounits). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-\xco trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.

The JCMT Nearby Galaxies Legacy Survey - IV. Velocity dispersions in the molecular interstellar medium in spiral galaxies

An analysis of large-area CO J= 3–2 maps from the James Clerk Maxwell Telescope for 12 nearby spiral galaxies reveals low velocity dispersions in the molecular component of the interstellar medium. The three lowest luminosity galaxies show a relatively flat velocity dispersion as a function of radius while the remaining nine galaxies show a central peak with a radial fall-off within 0.2–0.4r25. Correcting for the average contribution due to the internal velocity dispersions of a population of giant molecular clouds, the average cloud–cloud velocity dispersion across the galactic discs is 6.1 ± 1.0 km s−1 (standard deviation of 2.9 km s−1), in reasonable agreement with previous measurements for the Galaxy and M33. The cloud–cloud velocity dispersion derived from the CO data is on average two times smaller than the H i velocity dispersion measured in the same galaxies. The low cloud–cloud velocity dispersion implies that the molecular gas is the critical component determining the stability of the galactic disc against gravitational collapse, especially in those regions of the disc which are H2 dominated. The cloud–cloud velocity dispersion shows a significant positive correlation with both the far-infrared luminosity, which traces the star formation activity, and the K-band absolute magnitude, which traces the total stellar mass. For three galaxies in the Virgo cluster, smoothing the data to a resolution of 4.5 kpc (to match the typical resolution of high-redshift CO observations) increases the measured velocity dispersion by roughly a factor of 2, comparable to the dispersion measured recently in a normal galaxy at z= 1. This comparison suggests that the mass and star formation rate surface densities may be similar in galaxies from z= 0 to 1 and that the high star formation rates seen at z= 1 may be partly due to the presence of physically larger molecular gas discs.

MULTI-TRANSITION STUDY OF M51'S MOLECULAR GAS SPIRAL ARMS

Two selected regions in the molecular gas spiral arms in M51 were mapped with the Owens Valley Radio Observatory (OVRO) mm-interferometer in the 12CO(2–1), 13CO(1–0), C18O(1–0), HCN(1–0), and HCO+(1–0) emission lines. The CO data have been combined with the 12CO(1–0) data from Aalto et al. covering the central 3.5 kpc to study the physical properties of the molecular gas. All CO data cubes were short spacing corrected using IRAM 30 m (12CO(1–0): NRO 45 m) single-dish data. A large velocity gradient analysis finds that the giant molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180 pc (120 pc) resolution with an average kinetic temperature of Tkin = 20(16) K and H2 density of n(H2) = 120(240) cm−3 when assuming virialized clouds (a constant velocity gradient ). The associated conversion factor between H2 mass and CO luminosity is close to the Galactic value for most regions analyzed. Our findings suggest that the GMC population in the spiral arms of M51 is similar to those of the Milky Way and therefore the strong star formation occurring in the spiral arms has no strong impact on the molecular gas in the spiral arms. Extinction inferred from the derived H2 column density is very high (AV about 15–30 mag), about a factor of 5–10 higher than the average value derived toward H ii regions. Thus, a significant fraction of the ongoing star formation could be hidden inside the dust lanes of the spiral arms. A comparison of MIPS 24 μm and Hα data, however, suggests that this is not the case and most of the GMCs studied here are not (yet) forming stars. We also present low (45) resolution OVRO maps of the HCN(1–0) and HCO+(1–0) emission at the location of the brightest 12CO(1–0) peak.

Giant Molecular Clouds in M31. I. Molecular Cloud Properties

We present Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometer observations of giant molecular clouds (GMCs) along a spiral arm in M31. The observations consist of a survey using the compact configuration of the interferometer and follow-up, higher resolution observations on a subset of the detections in the survey. The data are processed using an analysis algorithm designed to extract GMCs and correct their derived properties for observational biases, thereby facilitating comparison with Milky Way data. The algorithm identifies 67 GMCs, of which 19 have a sufficient signal-to-noise ratio to accurately measure their properties. The GMCs in this portion of M31 are indistinguishable from those found in the Milky Way, having a similar size-line width relationship and distribution of virial parameters, confirming the results of previous, smaller studies. The velocity gradients and angular momenta of the GMCs are comparable to the values measured in M33 and the Milky Way, and in all cases are below expected values based on the local galactic shear. The studied region of M31 has an interstellar radiation field, metallicity, Toomre Q parameter, and midplane volume density similar to those of the inner Milky Way, so the similarity of GMC populations between the two systems is not surprising.

Testing an Identification Algorithm for Extragalactic OB Associations Using a Galactic Sample

We have used a Galactic sample of OB stars and associations to test the performance of an automatic grouping algorithm designed to identify extragalactic OB associations. The algorithm identifies the known Galactic OB associations correctly when the search radius (78 pc) is defined by the observed stellar surface density. Galactic OB associations identified with a 78 pc search radius have diameters that are $\sim$3 times larger than OB associations identified with a 22 pc search radius in M33. Applying the smaller search radius to the Galactic data matches both the sizes and the number of member stars between the two galaxies quite well. Thus, we argue that this and similar algorithms should be used with a constant physical search radius, rather than one which varies with the stellar surface density. Such an approach would allow the identification of differences in the giant molecular cloud populations and star formation efficiency under most circumstances.

The far-infrared luminosity of molecular clouds in the Galaxy

A detailed analysis is presented of the IR emission and CO properties of a large sample of giant molecular clouds (GMCs) with and without H II regions. It is found that high-mass star-forming clouds have significantly higher peak dust temperatures, but averaged over the entire cloud volume, their IR color temperatures are not very different from those of the general GMC population and the mean disk temperature. The results indicate that the bulk of the IR emission in GMCs is excited by embedded stars within the clouds rather than the exterior interstellar radiation field, that only a small fraction of the IR luminosity originates from stars which ionize the H II region. An overall higher level of star-formation activity for the H II region GMCs than the general population is indicated.

A latitude survey of carbon monoxide emission near the Galactic center

view Abstract Citations (21) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Latitude Survey of Carbon Monoxide Emission near the Galactic Center Bania, T. M. Abstract A new (C-12)O survey of the inner Galaxy is reported together with some additional observations of (C-13)O in selected directions. Most of the CO emissivity in this zone could be produced by only three structures; the nuclear disk/bar, the 3 kpc arm, and the +135 km/s feature. Selected giant molecular clouds (GMCs) near the (1,v)-loci predicted by kinematic ring models for the 3 kpc and +135 km/s arms have been observed in (C-13)O in order to estimate their physical properties. Except for 10 km/s CO line widths typical of clouds near the Galactic center, these objects have properties very similar to those derived for the GMC population in the outer Galactic disk. The clouds are typically 100 pc in diameter with H2 masses of a million solar masses, and they have excitation temperatures of 7 K. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164558 Bibcode: 1986ApJ...308..868B Keywords: Carbon Monoxide; Emission Spectra; Galactic Nuclei; Milky Way Galaxy; Molecular Clouds; Astronomical Spectroscopy; Gas Dynamics; Spectral Line Width; Astrophysics; GALAXIES: MILKY WAY; GALAXIES: NUCLEI; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (4) NED (1)