| The goal of this dissertation is to use atomic and molecular spectroscopy to improve our understanding of the large-scale Galactic distribution of three phases of the interstellar medium associated with star formation: cold atomic gas, molecular cores, and H II regions. This is important because (1) the face-on spiral structure of the Galaxy is poorly known, and (2) understanding the spatial association between these three gas phases will help constrain models of star formation.; We determine the Galactocentric radial distribution of cold atomic gas opacity based on 21 cm H I line absorption spectra toward 54 extragalactic radio continuum sources. The opacity peaks at Galactocentric radii of 5, 6, and 8 kpc. According to previous studies, these radii are the locations of the 5 kpc molecular ring; and the Sagittarius and Perseus spiral arms.; In order to study the face-on structure of the Galaxy, distance information is crucially important. Because of our position in the plane of the Galaxy, however, distances to Galactic objects are difficult to obtain directly. Kinematically-derived distances are available for any source of spectral line radiation in the Galaxy, but there is a two-fold ambiguity in distance.; The distance ambiguity for 51 H II regions is resolved using 21 cm H I absorption spectra. In a face-on diagram of the Galaxy, these H II regions lie in three circular arcs which match the positions of the cold atomic gas features.; We designed an algorithm to resolve the distance ambiguity automatically for molecular cores based on the presence or absence of 21 cm H I self-absorption features. Based on several test core samples, we find that this algorithm correctly resolves the ambiguity for 85% of cores. Applying it to a sample of over 3000 molecular cores, we find that molecular core mass is concentrated in three circular features that correspond well with the Galactocentric radii of the cold atomic gas peaks and H II region arcs. |
