Magnetic Fields in the Milky Way and the Magellanic Clouds

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Magnetic Fields in the Milky Way and the Magellanic Clouds

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Title: Magnetic Fields in the Milky Way and the Magellanic Clouds
Author: Mao, Sui Ann
Citation: Mao, Sui Ann. 2011. Magnetic Fields in the Milky Way and the Magellanic Clouds. Doctoral dissertation, Harvard University.
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Abstract: This thesis is an observational investigation into the origin of large-scale coherent magnetic fields in the Milky Way and the Magellanic Clouds. I use Faraday rotation measure of both polarized extragalactic sources and that derived from diffuse polarized synchrotron emission as probes of interstellar magnetism. Rotation measure uniquely provides the magnetic field direction in the diffuse interstellar medium, which cannot be achieved using other observing techniques. Using extragalactic source rotation measures behind the Small Magellanic Cloud (SMC), I find a large-scale coherent magnetic field in this irregular galaxy, where the classic dynamo effect is expected to be weak. I suggest that the cosmic-ray driven dynamo, which relies heavily on cosmic ray pressure to generate poloidal fields could be in operation. By analyzing rotation measures derived from diffuse polarized emission from the Large Magellanic Cloud (LMC) simultaneously with rotation measures of 100 extragalactic sources behind it, I conclude that the LMC has a quadrupolar-type magnetic field structure, in accord with predictions from dynamo theories. However, the field amplification time scale of a classic dynamo is too long compared to frequent tidal-triggered star forming episodes experienced by the LMC which can prevent any large-scale coherent field from building up. I propose that the cosmic-ray driven dynamo, which has a much shorter amplification time scale than the classic dynamo, could be the origin of the observed coherent fields in the LMC. The dynamo theory predicts a symmetric vertical magnetic field across the galactic disk for Milky Way-type galaxies. However, based on rotation measures of 1,000 extragalactic sources toward the Galactic poles, I find a lack of vertical field symmetry across the Galactic mid-plane. I suggest that the observed RMs could be the superposition of a symmetric disk field and an anti-symmetric field produced by a separate dynamo effect in the Galactic halo. Furthermore, I demonstrate that existing Galactic halo magnetic field models cannot successfully reproduce extragalactic rotation measures at mid-Galactic latitudes in the second Galactic quadrant. I propose that halo fields consist of magnetic spirals could potentially account for the observed RM pattern.
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