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Large-scale photodissociation regions in nearby spiral galaxies. Atomic hydrogen as a tracer for molecular gas

06 April 2009

PhD ceremony: J.S. Heiner, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Large-scale photodissociation regions in nearby spiral galaxies. Atomic hydrogen as a tracer for molecular gas

Promotor(s): prof. P.C. van der Kruit, prof. R.J. Allen

Faculty: Mathematics and Natural Sciences


Hydrogen is thought to be the most common 'ordinary' matter in the universe. It can combine to form molecular hydrogen and be destroyed again to revert to atomic hydrogen (HI). This process happens in photodissociation regions (PDRs), under the influence of ultraviolet light and dust. The atomic hydrogen can then be used to trace the molecular gas.

In my thesis, I refine and expand the method to do this, and look for indications that the method works. I investigated PDRs at a scale of several hundred lightyears, in the nearby grand design spiral galaxies M33, M81, and M83. I looked for indications of the actual existence of these 'large-scale' PDRs, and calculated the gas densities. Higher cloud densities are often accompanied by carbon monoxide emission, a commonly used tracer of molecular gas. Also, more complex molecules (PAHs) are found near the atomic hydrogen of these PDRs. The distance of the galaxies does not influence the results noticeably. The total gas density does not seem to change much further away from the center of the galaxies either. Hints of cloud structure can be found in M33, and we can predict the way the HI behaves in the PDRs. Finally, it is confirmed in M33 that the rate of star formation seems to be related directly to the hydrogen density.


Last modified:15 September 2017 3.39 p.m.
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