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The Formation and Evolution of Giant Molecular Clouds

  • Author(s): Imara, Nia
  • Advisor(s): Blitz, Leo
  • et al.

To adequately address topics such as stellar and galactic evolution, it is necessary to address the question of giant molecular cloud (GMC) formation and evolution, topics that continue to be actively debated in astrophysics. In this thesis, I present new studies on the kinematic properties of individual molecular clouds in the Galaxy and M33 and on their global properties in low-metallicity environments. My primary aim in analyzing the kinematic features of GMCs is to determine the extent to which they are explained by current formation theories. Clues pointing to the origins of GMCs are revealed by comparing the large-scale linear velocity gradients, which they are frequently observed to possess, with the gradients in the high-density atomic hydrogen (HI) from which they are expected to form. Using high-resolution CO13 observations of five Milky Way GMCs, I create intensity-weighted velocity maps from which I measure the maximum gradient magnitudes and directions of the clouds. I use data from the Leiden/Argentine/Bonn Galactic HI survey to identify and measure the properties of regions of atomic gas associated with the GMCs. If the molecular cloud gradients – ranging from 0.04 to 0.20 km s-1 pc-1 – are due to rotation, their angular momentum is always less than that in the surrounding HI. Though this result is consistent with the the hypothesis that GMCs form from large-scale instabilities, one must necessarily introduce some mechanism capable of reducing the angular momentum in order to explain the discrepancies in the molecular and atomic gas. The second key result is that – with the exception of the Orion A molecular cloud – there are large differences in the gradient directions of the molecular and atomic gas.

A continuation of this study is given for a much larger sample of GMCs in M33. The results are consistent with those in the Milky Way; in particular, the gradient directions of the GMCs are uncorrelated with the HI gradient directions. Additional findings include the observation that the local surface density of atomic gas slowly increases with GMC mass as &SigmaHI ∼ MGMC 0.27 ± 0.06. Also, the properties of high-density atomic hydrogen in which GMCs have not been observed generally has smaller gradients ( ∼ 0.03 km s-1 pc-1) than does the HI associated with GMCs (∼ 0.05 km s-1 pc-1). This suggests that high shear in atomic gas is either a prerequisite for or consequence of GMC formation.

Studying the properties of GMCs in different environments is another avenue for enhancing our understanding of their evolution. An extinction map of the low-metallicity Large Magellanic Cloud (LMC) is presented, using near-infrared photometry from the Two Micron All Sky Survey. A mean visual extinction of 0.38 mag is found, and an extended distribution of molecular gas is observed across the face of the galaxy that was previously undetected by CO observations. The CO-to-H2 conversion factor in the LMC, 9.3 ± 0.4 × 1020 cm-2 (K km s-2)-1, is nearly 5 times greater than the average value in the Milky Way. My work demonstrates that CO is not a good tracer of H2, and caution must be applied in using the Galactic X-factor in low-metallicity environments.

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