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Molecular clouds and star formation in the Inner Galaxy

Rigby, AJ (2016) Molecular clouds and star formation in the Inner Galaxy. Doctoral thesis, Liverpool John Moores University.

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A detailed understanding of the process of star formation is crucial for modern astrophysics. Stars form from the gravitational collapse of molecular gas clouds; it is the process by which cold molecular gas is transformed into the stars and planets that make up the many billions of galaxies in the observable Universe. However, there are a number of open questions that have yet to be answered and a comprehensive theory that explains and predicts how, where and why stars and their clusters form proves elusive. One such open question is how does the environment, on both local scales and galactic scales, influence star formation? The enormous radiative and mechanical outputs of high-mass stars (M > 8M_sol ) are known to have a strong impact on their surroundings and are able to erode their natal molecular clouds via their stellar winds, ionizing radiation and supernovae. It has been proposed that the shock fronts at the edges of expanding HII regions might trigger subsequent generations of star formation (e.g. Elmegreen & Lada, 1977; Bertoldi, 1989), and there are observational studies to support this (e.g. Thompson et al., 2012). It has also been proposed that large-scale effects such as the spiral structure of galaxies like the Milky Way might trigger the formation of stars in otherwise quiescent gas (e.g. Dobbs et al., 2008), though observations within the Galaxy appear to suggest that spiral arms are playing only a minor role, if any, in the triggering of star formation (e.g. Moore et al., 2012; Eden et al., 2015). To answer this question, and others concerning star formation, large samples of imminently and currently star-forming regions are required, and surveys of the plane of the Milky Way in various tracers are providing the data to acquire these. Molecular clouds are the initial conditions for star formation, and a complete theory of star formation must necessarily involve a detailed understanding of molecular clouds. In this thesis a survey of molecular gas in the Inner Galaxy known as CHIMPS is presented; these data provide measurements of denser and more optically thin molecular gas at a higher angular resolution than preceding surveys and over a significant area of the first quadrant of the Galactic plane. The combination of CHIMPS data with data from other surveys, such as Hi-GAL, allows the star-forming content of clumps of dense molecular gas to be studied. The clumps of molecular emission identified within CHIMPS appear to be highly turbulent in nature, and are over-pressurized with respect to the encompassing neutral gas. This would appear to suggest that they are transient features in a highly dynamic interstellar medium. The efficiency of star formation within the CHIMPS clumps is not found to vary significantly on kiloparsec scales between the spiral arms and their inter-arm regions, with the exception of the Scutum-Centaurus arm, within which the current level of star formation per unit gas mass appears to be somewhat suppressed. On a clump-to-clump basis, the distribution of star formation efficiency is log-normal, indicating that the efficiency is determined by many random processes, with no single dominant agent. The conclusion is that it is turbulence that controls the star formation efficiency, which is powered on a wide range of scales from the feedback of high-mass stars to the shear induced by the rotation of the entire Galaxy.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: star formation; molecular clouds; Milky Way; spiral arm; Galaxy
Subjects: Q Science > QB Astronomy
Divisions: Astrophysics Research Institute
Date Deposited: 26 Sep 2016 08:56
Last Modified: 26 Sep 2016 08:56
Supervisors: Moore, TJT
URI: http://researchonline.ljmu.ac.uk/id/eprint/4172

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