Found 4 talks width keyword molecular clouds

Gas kinematics on the scales of Giant Molecular Clouds (GMCs) are essential for probing the framework that links the large-scale organization of interstellar gas to cloud formation and subsequent star formation. I will present an overview of results from the PdBI Arcsecond Whirlpool Survey (PAWS, PI: E. Schinnerer), which has mapped CO(1-0) emission over 9 kpc in the nearby grand-design spiral galaxy M51 at 40 pc resolution, and is sensitive to giant molecular clouds (GMCs) with masses above 10^5 Msun. This unprecedented view challenges the conventional picture of how molecular gas is structured and organized in galaxies: clouds are not ‘universal’, but respond to their environment, resulting in a diversity of cloud properties that not only depend on (dynamical) environment but also vary from galaxy to galaxy. I will discuss how this sensitivity to environment emerges, in consideration of the stability of M51’s GMCs (including the effects of pressure, shear, turbulence) and our view of non-circular motions in the gas disk. As a result of the strong streaming motions that arise due to departures from axisymmetry in the gravitational potential (i.e. the nuclear bar and spiral arms), embedded clouds feel a reduced surface pressure, which can prevent collapse. This dynamical pressure naturally leads to changes in the efficiency of star formation and hence gas depletion time along the spiral arms. I will show that local reductions to cloud surface pressure in M51 dominate over shear and star formation feedback-driven turbulence in determining the observed radial variation the depletion time. I will also describe how incorporating 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.

I will review some recent results about the molecular content of galaxies and its dynamics, obtained from CO lines, dense tracers (HCN,HCO+), or the dust continuum emission. New data to constrain the conversion factor XCO will be discussed. The molecular surface density is essential to determine the star formation efficiency in galaxies, and the resolved Kennicutt-Schmidt law will be presented as a function of surface density and galaxy type. Large progress has been made on galaxy at moderate and high redshifts, allowing to interprete the star formation history and star formation efficiency as a function of gas content, or galaxy evolution. In massive galaxies, the gas fraction was higher in the past, and galaxy disks were more unstable and more turbulent. ALMA observations will allow the study of more normal galaxies at high z with higher spatial resolution and sensitivity.

In this talk I consider two questions. First, I investigate the formation of molecular clouds from diffuse interstellar gas. It has been argued that the midplane pressure controls the fraction of molecular hydrogen present, and thus the star formation rate. Alternatively, I and others have suggested that the gravitational instability of the disk controls both. I present numerical results demonstrating that the observed correlations between midplane pressure, molecular hydrogen fraction, and star formation rate can be explained within the gravitational instability picture. Second, I discuss how ionization affects the formation of massive stars. Although most distinctive observables of massive stars can be traced back to their ionizing radiation, it does not appear to have a strong effect on their actual formation. Rather, I present simulations suggesting that stars only ionize large volumes after their accretion has already been throttled by gravitational fragmentation in the accretion flow. At the same time these models can explain many aspects of the observations of ultracompact H II regions.