| Summary: | Thesis (Ph.D.)--Boston University === Astronomers have a limited understanding of the large-scale structure of the Galactic magnetic field and its role in the evolution of the interstellar medium (ISM). This understanding derives primarily from Faraday rotation and synchrotron observations which do not probe the cool, dusty ISM. To advance our knowledge of the Galactic magnetic field, this dissertation reports on the application of a different method, near-infrared (NIR) polarization of background starlight, to place new observational constraints on the nature of the Galactic magnetic field and to study the field's role in the evolution of interstellar material.
A radiative transfer computer code was developed to predict all-sky starlight polarization observations. Starlight polarimetry predictions were made for several different dynamo-driven magnetic field geometries, assuming that magnetically-aligned interstellar dust grains polarize background starlight. New NIR starlight polarimetry measurements in the outer Galaxy were tested against these predictions. These observations favor disk-symmetric magnetic fields while rejecting disk-antisymmetric magnetic fields. This result contradicts some previous interpretations of all-sky, radio Faraday rotation measurements. The Galactic magnetic pitch angle is constrained to p = -6 ± 2°.
The physical orientations of Galactic HII regions, traced by mid-infrared emission, are compared to the large-scale, disk-symmetric Galactic magnetic field geometry derived above. Hydrogen recombination line spectra towards these same objects revealed that many possessed turbulent linewidths. If fluid turbulence decays with time, then it may be used as a relative age indicator. A trend is seen between magnetic alignment and the degree of turbulence in the HII region. This result leads to the development of an observationally-driven HII region magnetic evolutionary sequence.
Resolved polarimetry across the face of the galaxy M51 was measured for comparison with the internal, edge-on view of the Milky Way seen from Earth. Strong upper limits ( < 0.05% at a resolution of 0.6 arcseconds) were placed on the degree of NIR polarization across the face of M51. These results were combined with resolved optical polarimetry measurements from the literature. Normal polarization mechanisms cannot explain the observed polarization dependence on wavelength.
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