Star formation is slow in the sense that the gas consumption time is much longer than the dynamical time It is also inefficient; star formation in local galaxies takes place in giant molecular clouds (GMCs), but the fraction of a GMC converted to stars is very small, GMC ~ 5% In luminous starbursts, the GMC lifetime is shorter than the main-sequence lifetime of even the most massive stars, so that supernovae can play no role in GMC disruption We investigate the disruption of GMCs across a wide range of galaxies from normal spirals to the densest starbursts; we take into account the effects of H II gas pressure, shocked stellar winds, protostellar jets, and radiation pressure produced by the absorption and scattering of starlight on dust grains In the Milky Way, a combination of three mechanisms—jets, H II gas pressure, and radiation pressure—disrupts the clouds In more rapidly star-forming galaxies such as "clump" galaxies at high-redshift, ultra-luminous infrared galaxies (ULIRGs), and submillimeter galaxies, radiation pressure dominates natal cloud disruption We predict the presence of ~10-20 clusters with masses ~107 M ☉ in local ULIRGs such as Arp 220 and a similar number of clusters with M * ~ 108 M ☉ in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters We find that GMC = πGΣGMC c/(2(L/M *)) for GMCs that are optically thin to far-infrared radiation, where ΣGMC is the GMC gas surface density The efficiency in optically thick systems continues to increase with ΣGMC, but more slowly, reaching ~35% in the most luminous starbursts The disruption of bubbles by radiation pressure stirs the interstellar medium (ISM) to velocities of ~10 km s–1 in normal galaxies and to ~100 km s–1 in ULIRGs like Arp 220, consistent with observations Thus, radiation pressure may play a dominant dynamical role in the ISM of star-forming galaxies

http://iopscience.iop.org/article/10.1088/0004-637X/709/1/191/pdf

The Disruption of Giant Molecular Clouds by Radiation Pressure & the Efficiency of Star Formation in Galaxies

This paper presents a summary of all 27 available sensitive Zeeman measurements of magnetic field strengths in molecular clouds together with other relevant physical parameters. From these data input parameters to magnetic star formation theory are calculated, and predictions of theory are compared with observations. Results for this cloud sample are the following: (1) Internal motions are supersonic but approximately equal to the Alfv?n speed, which suggests that supersonic motions are likely MHD waves. (2) The ratio of thermal to magnetic pressures ?p ? 0.04, implying that magnetic fields are important in the physics of molecular clouds. (3) The mass-to-magnetic flux ratio is about twice critical, which suggests but does not require that static magnetic fields alone are insufficient to support clouds against gravity. (4) Kinetic and magnetic energies are approximately equal, which suggests that static magnetic fields and MHD waves are roughly equally important in cloud energetics. (5) Magnetic field strengths scale with gas densities as |B| ?? with ? ? 0.47; this agrees with the prediction of ambipolar diffusion driven star formation, but this scaling may also be predicted simply by Alfv?nic motions. The measurements of magnetic field strengths in molecular clouds make it clear that magnetic fields are a crucial component of the physics governing cloud evolution and star formation.

Magnetic Fields in Molecular Clouds - Observations Confront Theory

We develop and implement numerical methods for including stellar feedback in galaxy-scale numerical simulations. Our models include simplified treatments of heating by Type I and Type II supernovae, gas recycling from young stars and asymptotic giant branch winds, heating from the shocked stellar winds, H ii photoionization heating and radiation pressure from stellar photons. The energetics and time dependence associated with the feedback are taken directly from stellar evolution models. We implement these stellar feedback models in smoothed particle hydrodynamic simulations with pc-scale resolution, modelling galaxies from Small Magellanic Cloud (SMC) like dwarfs and Milky Way (MW) analogues to massive z∼ 2 star-forming discs. In the absence of stellar feedback, gas cools rapidly and collapses without limit into dense sub-units, inconsistent with observations. By contrast, in all cases with feedback, the interstellar medium (ISM) quickly approaches a statistical steady state in which giant molecular clouds (GMCs) continuously form, disperse and re-form, leading to a multiphase ISM. In this paper, we quantify the properties of the ISM and GMCs in this self-regulated state. In a companion paper we study the galactic winds driven by stellar feedback.



Our primary results on the structure of the ISM in star-forming galaxies include the following.



1
Star-forming galaxies generically self-regulate so that the cool, dense gas maintains Toomre’s Q∼ 1. Most of the volume is occupied by relatively diffuse hot gas, while most of the mass is in dense GMC complexes created by self-gravity. The phase structure of the gas and the gas mass fraction at high densities are much more sensitive probes of the physics of stellar feedback than integrated quantities such as the Toomre Q or gas velocity dispersion.

2
Different stellar feedback mechanisms act on different spatial (and density) scales. Radiation pressure and H ii gas pressure are critical for preventing runaway collapse of dense gas in GMCs. Shocked supernova ejecta and stellar winds dominate the dynamics of the volume-filling hot gas. However, this gas primarily vents out of the star-forming disc and contributes only modestly to the mid-plane ISM pressure.

3
The galaxy-averaged star formation rate is determined by feedback, with different mechanisms dominating in different galaxy types. For a given feedback efficiency, restricting star formation to molecular gas or modifying the cooling function has little effect on the star formation rate in the galaxies we model (including an SMC-mass dwarf). By contrast, changing the feedback mechanisms or assumed feedback efficiencies directly translates to shifts off of the observed Kennicutt–Schmidt relation.

4
Self-gravity leads to GMCs with an approximately self-similar mass function ∝M−2, with a high-mass cut-off determined by the characteristic Jeans/Toomre mass of the system. In all of our galaxy models, GMCs live for a few dynamical times before they are disrupted by stellar feedback. The net star formation efficiency in GMCs ranges from ∼1 per cent in dwarfs and MW-like spirals to nearly ∼10 per cent in gas-rich rapidly star-forming galaxies. GMCs are approximately virialized, but there is a large dispersion in the virial parameter for a given GMC mass, and lower mass GMCs tend to be preferentially unbound.

The structure of the interstellar medium of star‐forming galaxies

We report a study of the relation between dust and gas over a 100 deg 2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12 CO and 13 CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200 �� corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N (CO) is linear between AV � 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions, the linear relation is flattened for AV 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N (CO) by ∼30%–70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data � 23 mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4 × 10 4 cm −3 and a CO depletion age of about 4.2 × 10 5 yr. At visual extinctions smaller than ∼3 mag, we find that the CO fractional abundance is reduced by up to two orders of magnitude. The data show a large scatter suggesting a range of physical conditions of the gas. We estimate the H2 mass of Taurus to be about 1.5 × 10 4 M� , independently derived from the AV and N (CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1 × 10 20 cm −2 (K km s −1 ) −1 , which applies even in the region where the [CO]/[H2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log-normal function but shows tails at large and low column densities. The length scale at which the high column density tail starts to be noticeable is about 0.4 pc.

/pdf/the-relation-between-gas-and-dust-in-the-taurus-molecular-24mzjya2a0.pdf

The relation between gas and dust in the taurus molecular cloud

Context. We are engaged in a multi-wavelength study of several Galactic H ii regions that exhibit signposts of triggered star formation on their borders, where the collect and collapse process could be at work.Aims. When addressing the question of triggered star formation, it is critical to ensure the real association between the ionized gas and the neutral material observed nearby. In this paper we stress this point and present CO observations of the RCW 82 star forming region.Methods. The velocity distribution of the molecular gas is combined with the study of young stellar objects (YSOs) detected in the direction of RCW 82. We discuss the YSO's evolutionary status using near- and mid-IR data. The spatial and velocity distributions of the molecular gas are used to discuss the possible scenarios for the star formation around RCW 82.Results. Several massive molecular condensations, together with star formation sites, are observed on the borders of RCW 82. The shapes of the three brightest condensations suggest that they were pre-existent, i.e. not formed through the collect and collapse process. A thin layer of molecular material is observed surrounding the ionized gas, adjacent to the ionization front. This results from the sweeping up of neutral material around the expanding region. Several Class I YSOs are detected in the direction of this layer.Conclusions. The numerous YSOs observed towards the bright molecular condensations bordering (and velocity-associated with) the ionized gas reveal the intense star formation activity in RCW 82. But this region is probably too young to have triggered star formation via the collect and collapse process.

https://www.aanda.org/articles/aa/pdf/2009/06/aa11050-08.pdf

Triggered star formation on the borders of the Galactic Hii region RCW 82

We present the first paper in a series detailing the results of 13CO observations of a 1 deg2 region of the giant molecular cloud (GMC) complex associated with the H ii region RCW 106. The 13CO observations are also the first stage of a multimolecular line study of the same region. These observations were amongst the first made using the new on-the-fly mapping capability of the Australia Telescope National Facility Mopra Telescope. In the configuration used, the instrument provided a full width at half-maximum (FWHM) beam size of 33 arcsec and a velocity resolution of 0.17 km s1. The gas emission takes the form of a string of knots, oriented along an axis that extends from the north-west (NW) to the south-east (SE) of the field of the observations, and which is surrounded by a more extended, diffuse emission. We analyse the 2D integrated 13CO emission using the clumpfind algorithm and identify 61 clumps. We compare the gas data in the GMC with the dust data provided by 21-?m Midcourse Space Experiment (MSX) and 1.2-mm Swedish European Southern Observatory Submillimetre Telescope (SEST) images that we both regridded to the cell spacing of the Mopra data and smoothed to the same resolution. The 13CO emission is more diffuse and extended than the dust emission revealed at the latter two wavebands, which both have a much higher contrast between the peaks and the extended emission. From comparison of their centre positions, we find that only 50 per cent of the 13CO clump fits to the data are associated with any dust clumps. Using the clump fits, the total local thermodynamic equilibrium gas mass above the 3? level measured from the molecular data is 2.7 × 105 M, whereas that measured from the smoothed 1.2-mm SEST dust data is 2.2 × 105 M

https://hal.archives-ouvertes.fr/hal-00023328/document

Molecular line mapping of the giant molecular cloud associated with RCW 106 - I.13CO

We present a fully sampled C 18 O (1-0) map towards the southern giant molecular cloud (GMC) associated with the H II region RCW 106, and use it in combination with previous 1 3 CO (1-0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant 13 CO optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the HII region G333.6-0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the 13 CO and C 18 O data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near -1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.

/pdf/molecular-line-mapping-of-the-giant-molecular-cloud-32frfm7jyu.pdf

Molecular line mapping of the giant molecular cloud associated with RCW 106 - II. Column density and dynamical state of the clumps

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Molecular line mapping of the giant molecular cloud associated with RCW 106 - I.13CO

Molecular line mapping of the giant molecular cloud associated with RCW 106 - II. Column density and dynamical state of the clumps