The Evolutionary Sequence of Giant Molecular Clouds in M51
Final Report Abstract
The project has been focused on the exploitation of the high resolution imaging of the molecular gas in the central disk of the nearby face-on, grand-design spiral galaxy M51 from the IRAM Large Programme PAWS. The aim has been to better understand the properties of the molecular gas and its relation to star formation on the scale of Giant Molecular Clouds (GMCs). Scientific Highlights from the IRAM Large Project PAWS (PdBI Arcsecond Whirlpool Survey) PAWS has imaged the molecular gas and thus the GMC population in the central 9 kpc of M 51 (at a distance of 7.6 Mpc) with a spatial resolution of 40pc and a 5σ mass sensitivity of 2 × 105 M⊙. The angular resolution represents an order of magnitude improvement in beam area compared to previous interferometric maps obtained at OVRO, BIMA and CARMA. The area surveyed by a 60- pointing mosaic incorporates three distinct environments, the central (star-bursting) disk including a nuclear stellar bar, the spiral arms and the inter-arm (relatively quiescent) regions. In the Milky Way, most H2 is found in clouds with masses in the range 105 − 106 M⊙. For M 51, we find that unexpectedly only about half of the CO emission is arising from compact structures or GMCs and that the remaining emission can not be accounted for by smaller densely packed clouds alone. Further there is strong kinematic evidence for a significant fraction of the CO emission coming from a thick molecular disk. The average mass of the over 1,500 detected GMCs is about 106 M⊙. As each galactic environment covers a substantial area, we could build up sufficient statistics on the GMC population with well over 100 GMCs available for each of our seven environments. We find that GMC properties (peak temperature, line width) vary with galactic environment and higher cloud masses are found in the center and spiral arms compared to the inter-arm region. This environmental dependance of GMC properties is also strongly reflected in the GMC mass spectra as well as Probability Distribution Functions which are independent of any decomposition algorithm used. Thus our analysis implies a strong influence of the galactic environment onto the hierarchical structure of the molecular gas. Interestingly, the relationship between molecular gas emission and star formation is non-trivial ranging from spatial coincidence, off-set distribution to a clear lack of evidence for star formation associated with molecular gas emission. We propose that dynamical pressure associated with the spiral density wave has a strong effect onto the stability of GMCs and thus also their star formation efficiency. This model could naturally explain some of the scatter observed in the relation between the surface density of the star formation rate and the (molecular) gas surface density.
Publications
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A Comparative Study of Giant Molecular Clouds in M 51, M 33 and the Large Magellanic Cloud. 2013, ApJ, 779, 46
Hughes, A., Meidt, S. E., Colombo, D., Schinnerer, E., et al.
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Gas Kinematics on GMC Scales in M 51 with PAWS: Cloud Stabilization through Dynamical Pressure. 2013, ApJ, 779, 45
Meidt, S., Schinnerer, E., Garcίa-Burillo, S., et al.
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Probability Distribution Functions of CO(1-0) Brightness and Integrate Intensity in M 51: the PAWS View. 2013, ApJ, 779, 44
Hughes, A., Meidt, S. E., Schinnerer, E., et al.
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The PdBI Arcsecond Whirlpool Survey (PAWS): A Cloud-Scale/Multi-Wavelength View of the Interstellar Medium in a Grand-Design Spiral Galaxy. 2013, ApJ, 779, 42
Schinnerer, E., Meidt, S. E., Pety, J., et al.
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The PAWS Project: Multi-Phase Cold Gas Kinematic of M 51. 2014, ApJ, 784, 4
Colombo, D., Colombo, D., Meidt, S. E., Schinnerer, E., et al.
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The PdBI Arcsecond Whirlpool Survey (PAWS): Environmental Dependency of Giant Molecular Cloud Properties in M 51. 2014, ApJ, 784, 3
Colombo, D., Hughes, A., Schinnerer, E., et al.
