Infrared Extinction and Stellar Structures in the Milky Way Midplane

Our Milky Way (MW) Galaxy is a critical resource for the development of galaxy formation and chemodynamical evolution models because it istheonlymassivegalaxy in which we can resolve large numbers of individual stars. Though the recent availability of extensive IR stellar surveys has enabled us to peer fartherthroughthe interstellar dust than ever before, our ability to exploit the MW fully as a "galactic laboratory" is severely hampered by the fact that even IR observations of the midplane and central bulge - where most of the stellar mass lies - remain significantly affected by extinction. This dissertation describes the development of a method for reliably removing the effects of interstellar dust and the application of this method in studies of the interstellar medium (ISM) itself and the Galactic stellar populations it obscures. The Rayleigh-Jeans Color Excess (RJCE) extinction-correction method measures the foreground extinction toward individual stars independently of intrinsic stellar type, which in most cases enables the stellar type to be recovered. The RJCE method's ability to produce both cleaned color-magnitude diagrams and extinction maps at different distances in the heavily-reddened Galactic midplaneisdemonstrated. The IR extinction law as a function of Galactic position is measured to reveal spatial variations previously undetected in the diffuse ISM, thus establishing constraints for models of ISM dust grain formation and evolution. IR imaging is combined with RJCE extinction estimates and used to characterize previouslyunknown or poorly-studied open clusters in the outer MW disk. Thestellaroverdensities of the heavily-obscured stellar bar(s) and central bulge are mapped and their structural properties (orientation, scaleheight, degree of clumpiness) derived using a uniquely spatially contiguous set of IR data. Finally, a target selection scheme for the APOGEE survey is developed that incorporates RJCE extinction corrections to v produce a homogeneous sample of 100,000 RGB stars spanning all Galactic environments. The RJCE extinction-correction method, the APOGEE survey, and the new constraints on Galactic dust and stellar populations presented herecontributedatatoward both a full characterization of the MW within the generalgalacticcontextand its use as an effective galactic analogue.

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