The Shape of the Galaxy

by Jeffrey Arthur Larsen

Under the supervision of Dr. Roberta M. Humphreys

ABSTRACT

Using star counts from the University of Minnesota's Automated Plate Scanner catalog of the Palomar Sky Survey Epoch I, this thesis determines the shape parameters of the Galaxy. Precise information on the shape is an important constraint in ongoing astronomical research involving the Galaxy's formation history, motion and composition. The power of the APS data is in its large count areas, individual photometric calibrations, and statistically significant sample sizes.
 
The application of more fields than previously possible has led to the resolution of two apparent conflicts in the literature. Star counts are used to show that the bulk of stars external to the disk exist in a flattened distribution in addition to those already ascribed to a thick disk. Additionally, APS star counts derived a value for the solar distance of Z = 20.5 pc, bringing to an end the apparent conflict between values derived from precious optical star counts and younger stellar populations.
 
The APS catalog is also a useful exploratory tool. A unique new methodology is developed which can be used to mine larger data sets. A fast, optimized galaxy model suited for multi-field work is presented and a robust global optimization routine (the genetic algorithm) is applied for the first time to Galactic structure studies. From these tools, a set of parameters matching the APS star counts better than extant models was self-consistently derived from the data.
 
Most importantly, a large and significant asymmetry in the number of stars in the first quadrant (l = 20° - 45°) of the Galaxy is discovered relative to longitudes in the fourth quadrant. Assuming these stars are subdwarfs, they are on average approximately 3 kpc away, in the direction and at the same distance as the stellar bar proposed by Weinberg (1992). However, the relatively high galactic latitudes involved place this asymmetry 1.7 kpc above the place. Possible explanations for this excess include an elliptical thick disk, disk heating from and interaction with the Sagittarius dwarf, or heating of the disk by the bar.