MILKY WAY MORPHOLOGY PROBED BY 6D ASTROMETRIC DATA FROM THE GAIA SPACE TELESCOPE

At varying height above and below the plane of the Milky Way, I have used astrometric methods to classify stars of different galactic components of the Milky Way - the thin disk, thick disk, and stellar halo. This work complements prior study of Milky Way sub-structure - notably involving number density and/or pairwise correlations - which demonstrate non-steady state effects in the galaxy, such as axial/north-south symmetry breaking or more complex phenomena like the Gaia snail. This has motivated my exploration of stellar population changes with height about the Milky Way mid-plane, and the study of symmetry in such changes above and below the galactic mid-plane. Drawing from the second data release (DR2) of the Gaia space telescope, I have chosen a stellar sample with notably small parallax error and a 6D astrometric phase space, totaling 707,772 stars. Prior work in Hinkel 2020 was used as a reference for optimal sampling parameters. Selection cuts have been taken to avoid saturation from the Magellanic Clouds and the mid-plane of the Milky Way, in addition to controlling for Gaia\u27s astrometric uncertainties and sampling biases. After a conversion of astrometric to galactocentric coordinates, I have employed a statistical method to distinguish the populations of stars by their likelihood of belonging to the Milky Way\u27s thin/thick disk and stellar halo components, using stellar kinematic data. Kinematic criteria for galactic populations were chosen to avoid galactic component contamination, where stars are attributed to the incorrect galactic components due to phase space overlap of the different components - a common issue in population separation routines. To assess confidence in this statistical method, I used the bootstrapping method to construct error estimates within sub-slices in galactocentric z. My methods have yielded a notable, novel result: at low galactic z, the population fraction variation is asymmetric North and South of the galactic mid-plane