Clare L. Dobbs

Why are most molecular clouds not gravitationally bound

The most recent observational evidence seems to indicate that giant molecular clouds are predominantly gravitationally unbound objects. In this paper we show that this is a natural consequence of a scenario in which cloud-cloud collisions and stellar feedback regulate the internal velocity dispersion of the gas, and so prevent global gravitational forces from becoming dominant. Thus, whilst the molecular gas is for the most part gravitationally unbound, local regions within the denser parts of the gas (within the clouds) do become bound and are able to form stars. We find that the observations, in terms of distributions of virial parameters and cloud structures, can be well modelled provided that the star formation efficiency in these bound regions is of the order of 5-10 per cent. We also find that in this picture the constituent gas of individual molecular clouds changes over relatively short time-scales, typically a few Myr.

Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure

We present hydrodynamical models of the grand design spiral M51 (NGC 5194), and its interaction with its companion NGC 5195. Despite the simplicity of our models, our simulations capture the present day spiral structure of M51 remarkably well, and even reproduce details such as a kink along one spiral arm, and spiral arm bifurcations. We investigate the oset between the stellar and gaseous spiral arms, and nd at most times (including the present day) there is no oset between the stars and gas to within our error bars. We also compare our simulations with recent observational analysis of M51. We compute the pattern speed versus radius, and like the observations, nd no single global pattern speed. We also show that the spiral arms cannot be tted well by logarithmic spirals. We interpret these ndings as evidence that M51 does not exhibit a quasi-steady density wave, as would be predicted by density wave theory. The internal structure of M51 derives from the complicated and dynamical interaction with its companion, resulting in spiral arms showing considerable structure in the form of short-lived kinks and bifurcations. Rather than trying to model such galaxies in terms of global spiral modes with

The formation of molecular clouds in spiral galaxies

ABSTRACT We present Smoothed Particle Hydrodynamics (SPH) simulations of molecular cloudformation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disk to a galactic potential. The spiral shock induces high densitiesin the gas, and considerable structure in the spiral arms, which we identify as molec-ular clouds. We regard the formation of these structures as due to the dynamics ofclumpy shocks, which perturb the flow of gas through the spiral arms. In addition, thespiral shocks induce a large velocity dispersion in the spiral arms, comparable with themagnitude of the velocity dispersion observed in molecular clouds. We estimate theformation of molecular hydrogen, by post-processing our results and assuming the gasis isothermal. Provided the gas is cold (T 6 100 K), the gas is compressed sufficientlyin the spiral shock for molecular hydrogen formation to occur in the dense spiral armclumps. These molecular clouds are largely confined to the spiral arms, since mostmolecular gas is photodissociated to atomic hydrogen upon leaving the arms.Key words: galaxies: spiral – hydrodynamics – ISM: clouds – ISM: molecules – stars:formation

THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY

The Plateau de Bure Interferometer Arcsecond Whirlpool Survey has mapped the molecular gas in the central similar to 9 kpc of M51 in its (CO)-C-12(1-0) line emission at a cloud-scale resolution of similar to 40 pc using both IRAM telescopes. We utilize this data set to quantitatively characterize the relation of molecular gas (or CO emission) to other tracers of the interstellar medium, star formation, and stellar populations of varying ages. Using two-dimensional maps, a polar cross-correlation technique and pixel-by-pixel diagrams, we find: (1) that (as expected) the distribution of the molecular gas can be linked to different components of the gravitational potential; (2) evidence for a physical link between CO line emission and radio continuum that seems not to be caused by massive stars, but rather depends on the gas density; (3) a close spatial relation between polycyclic aromatic hydrocarbon (PAH) and molecular gas emission, but no predictive power of PAH emission for the molecular gas mass; (4) that the I-H color map is an excellent predictor of the distribution (and to a lesser degree, the brightness) of CO emission; and (5) that the impact of massive (UV-intense) young star-forming regions on the bulk of the molecular gas in central similar to 9 kpc cannot be significant due to a complex spatial relation between molecular gas and star-forming regions that ranges from cospatial to spatially offset to absent. The last point, in particular, highlights the importance of galactic environment-and thus the underlying gravitational potential-for the distribution of molecular gas and star formation.

GMC formation by agglomeration and self gravity

We investigate the formation of giant molecular clouds (GMCs) in spiral galaxies through both agglomeration of clouds in the spiral arms, and self gravity. The simulations presented include two-fluid models, which contain both cold and warm gas, although there is no heating or cooling between them. We find agglomeration is predominant when both the warm and cold components of the interstellar medium are effectively stable to gravitational instabilities. In this case, the spacing (and consequently mass) of clouds and spurs along the spiral arms is determined by the orbits of the gas particles and correlates with their epicyclic radii (or equivalently spiral shock strength). Notably GMCs formed primarily by agglomeration tend to be unbound associations of many smaller clouds, which disperse upon leaving the spiral arms. These GMCs are likely to be more massive in galaxies with stronger spiral shocks or higher surface densities. GMCs formed by agglomeration are also found to exhibit both prograde and retrograde rotation, a consequence of the clumpiness of the gas. At higher surface densities, self gravity becomes more important in arranging both the warm and cold gas into clouds and spurs, and determining the properties of the most massive GMCs. These massive GMCs can be distinguished by their higher angular momentum, exhibit prograde rotation and are more bound. For a 20 M ⊙ pc -2 disc, the spacing between the GMCs fits both the agglomeration and self gravity scenarios, as the maximum unstable wavelength of gravitational perturbations in the warm gas is similar to the spacing found when GMCs form solely by agglomeration.

The exciting lives of giant molecular clouds

We present a detailed study of the evolution of giant molecular clouds (GMCs) in a galactic disc simulation. We follow individual GMCs (defined in our simulations by a total column density criterion), including their level of star formation, from their formation to dispersal. We find the evolution of GMCs is highly complex, and GMCs cannot be considered as isolated objects. GMCs often form from a combination of smaller clouds and ambient interstellar medium (ISM), and similarly disperse by splitting into a number of smaller clouds and ambient ISM. However some clouds emerge as the result of the disruption of a more massive GMC, rather than from the assembly of smaller clouds. Likewise in some cases, clouds accrete on to more massive clouds rather than disperse. Because of the difficulty of determining a precursor or successor of a given GMC, determining GMC histories and lifetimes is highly non-trivial. Using a definition relating to the continuous evolution of a cloud, we obtain lifetimes typically

THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS): ENVIRONMENTAL DEPENDENCE OF GIANT MOLECULAR CLOUD PROPERTIES IN M51*

Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated the largest extragalactic giant molecular cloud (GMC) catalog to date, containing 1507 individual objects. GMCs in the inner M51 disk account for only 54% of the total 12CO(1-0) luminosity of the survey, but on average they exhibit physical properties similar to Galactic GMCs. We do not find a strong correlation between the GMC size and velocity dispersion, and a simple virial analysis suggests that ~30% of GMCs in M51 are unbound. We have analyzed the GMC properties within seven dynamically motivated galactic environments, finding that GMCs in the spiral arms and in the central region are brighter and have higher velocity dispersions than inter-arm clouds. Globally, the GMC mass distribution does not follow a simple power-law shape. Instead, we find that the shape of the mass distribution varies with galactic environment: the distribution is steeper in inter-arm region than in the spiral arms, and exhibits a sharp truncation at high masses for the nuclear bar region. We propose that the observed environmental variations in the GMC properties and mass distributions are a consequence of the combined action of large-scale dynamical processes and feedback from high-mass star formation. We describe some challenges of using existing GMC identification techniques for decomposing the 12CO(1-0) emission in molecule-rich environments, such as M51s inner disk.

Spurs and feathering in spiral galaxies

We present smoothed particle hydrodynamic (SPH) simulations of the response of gas discs to a spiral potential. These simulations show that the commonly observed spurs and feathering in spiral galaxies can be understood as being due to structures present in the spiral arms that are sheared by the divergent orbits in a spiral potential. Thus, dense molecular cloud-like structures generate the perpendicular spurs as they leave the spiral arms. Subsequent feathering occurs as spurs are further sheared into weaker parallel structures as they approach the next spiral passage. Self-gravity of the gas is not included in these simulations, stressing that these features are purely due to the hydrodynamics in spiral shocks. Instead, a necessary condition for this mechanism to work is that the gas need be relatively cold (1000 K or less) in order that the shock is sufficient to generate structure in the spiral arms, and such structure is not subsequently smoothed by the gas pressure.

The ISM in spiral galaxies: can cooling in spiral shocks produce molecular clouds?

We investigate the thermodynamics of the interstellar medium (ISM) and the formation of molecular hydrogen through numerical simulations of spiral galaxies. The model follows the chemical, thermal and dynamical response of the disc to an external spiral potential. Self-gravity and magnetic fields are not included. The calculations demonstrate that gas can cool rapidly when subject to a spiral shock. Molecular clouds in the spiral arms arise through a combination of compression of the ISM by the spiral shock and orbit crowding. These results highlight that local self-gravity is not required to form molecular clouds. Self-shielding provides a sharp transition density, below which gas is essentially atomic, and above which the molecular gas fraction is >0.001. The time-scale for gas to move between these regimes is very rapid (≤1 Myr). From this stage, the majority of gas generally takes between 10 and 20 Myr to obtain high-H2 fractions (>50 per cent). These are, however, strict upper limits to the H2 formation time-scale, since our calculations are unable to resolve turbulent motions on scales smaller than the spiral arm, and do not include self-gravity. True cloud formation time-scales are therefore expected to be even shorter. The mass budget of the disc is dominated by cold gas residing in the spiral arms. Between 50 and 75 per cent of this gas is in the atomic phase. When this gas leaves the spiral arm and drops below the self-shielding limit, it is heated by the galactic radiation field. Consequently, most of the volume in the interarm regions is filled with warm atomic gas. However, some cold spurs and clumps can survive in interarm regions for periods comparable to the interarm passage time-scale. Altogether between 7 and 40 per cent of the gas in our disc is molecular, depending on the surface density of the calculation, with approximately 20 per cent molecular for a surface density comparable to the solar neighbourhood.

Gas Kinematics on Giant Molecular Cloud Scales in M51 with PAWS: Cloud Stabilization through Dynamical Pressure

We use the high spatial and spectral resolution of the PAWS CO(1-0) survey of the inner 9 kpc of the iconic spiral galaxy M51 to examine the effects of gas streaming motions on the star-forming properties of individual giant molecular clouds (GMCs). We compare our view of gas flows in M51--which arise due to departures from axisymmetry in the gravitational potential (i.e., the nuclear bar and spiral arms)--with the global pattern of star formation as traced by Hα and 24 μm emission. We find that the dynamical environment of GMCs strongly affects their ability to form stars, in the sense that GMCs situated in regions with large streaming motions can be stabilized, while similarly massive GMCs in regions without streaming go on to efficiently form stars. We argue that this is the result of reduced surface pressure felt by clouds embedded in an ambient medium undergoing large streaming motions, which prevent collapse. Indeed, the variation in gas depletion time expected based on the observed streaming motions throughout the disk of M51 quantitatively agrees with the variation in the observed gas depletion time scale. The example of M51 shows that streaming motions, triggered by gravitational instabilities in the form of bars and spiral arms, can alter the star formation law; this can explain the variation in gas depletion time among galaxies with different masses and morphologies. In particular, we can explain the long gas depletion times in spiral galaxies compared with dwarf galaxies and starbursts. We suggest that adding a dynamical pressure term to the canonical free-fall time produces a single star formation law that can be applied to all star-forming regions and galaxies across cosmic time.