Kinematic Study of White Dwarfs with BASE-9 Ages and Gaia Astrometry

The detailed processes by which spiral galaxies form remains an open question in modern cosmology. Observations of the current configuration of spiral galaxies including the Milky Way reveal thin and thick disk and halo populations which must all be accounted for in formation theories and likely have distinct ages. Using the Milky Way as an example to probe this question, we are studying the formation history of these structures. This work details our approach to age-dating the galaxy, velocity-selecting targets from a sample of white dwarfs from the Gaia DR3 catalog that have also been age-analysed using BASE-9. BASE-9 uses photometry, as well as prior measurements and values from ancillary data, such as trigonometric parallaxes, fitting up to nine stellar parameters for a given target. White dwarfs provide a valuable target type, as they are long–lived and stable objects which remain from the earliest epochs of star formation in each Galactic population. Gaia astrometry provides us with proper motions of these stars, though not radial velocities. We have developed a scheme to complete the velocity vector by estimating radial velocities in terms of the proper motion, and assigning stars by probability of membership to the thin disk, thick disk, or halo population. We then obtain an overall age-probability distribution for each component by the use of a hierarchical analysis of the BASE-9 age posterior distributions of the individual stars. This method using the described sample has thus far given a probable age for the halo less than 10 Gyr, too young for most probable existing models. This suggests that the sample of high–velocity white dwarfs in the solar neighbourhood is contaminated by stars from a later merger event, supernovae within prior binaries, or gravitational scattering due to molecular clouds or galactic features such as transient spiral structures or a bar. We have completed a hierarchical analysis for a subset of stars identified as thick disk objects, and analysis is underway. Initial results show an age posterior distribution which is singly-peaked near 6 Gyr.