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The dynamic role of magnetic fields in galaxy evolution


Magnetic fields are present in a variety of astrophysical environments, where they can affect the motions and structure of matter. Observations of magnetic fields in the Milky Way show that magnetic fields are an important force acting on gas in the interstellar medium. Observing magnetic fields outside of the Milky Way is challenging, and the most reliable observations of extragalactic magnetic fields are in galaxies similar to the Milky Way.

This dissertation focuses on results from observations of magnetic fields in galaxies at a very different stage of their evolution than the Milky Way. There are two main types of galaxies described here: starburst galaxies and early-type galaxies. Starburst galaxies are galaxies that are very rapidly forming stars as a result of a merger, and represent a short lived, but dramatic, phase during galaxy evolution. Early-type galaxies have limited star formation, and typically have little remaining gas. The three main approaches I use to study extragalactic magnetic fields are: (i) directly measuring magnetic fields via Zeeman splitting, targeting starburst galaxies that host such strong OH maser lines that they are called OH megamasers (OHMs); (ii) exploring the excitation of OH in both starburst galaxies and early-type galaxies, in order to understand how and where OHMs are produced; and (iii) assessing the quality of an alternative method of measuring magnetic fields in galaxies based on measuring synchrotron fluxes, looking in starburst galaxies and one early-type galaxy.

Using both single dish and interferometric observations of Zeeman splitting in OHMs, I find that magnetic fields are dynamically important within starburst galaxies, both on scales relevant to star formation (roughly a parsec), and on larger scales with the interstellar medium of starburst galaxies. I discuss observations of OH lines in starburst galaxies and in early-type galaxies, and conclude that they support models in which OHMs are excited radiatively through far-infrared emission. I present evidence that estimates of magnetic fields from applying the minimum energy argument to observations of synchrotron fluxes are wrong in two very different environments: the interstellar medium of starburst galaxies and a radio jet in an early-type galaxy.

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