Order in Chaos: Decoding the Age-Metallicity Structure of the Milky Way disk

Abstract

In the Milky Way, billion-star observations have opened avenues to study the complex interplay between processes that build disk galaxies. The Sun's position in the Galactic disk allows us to observe a large number of stars, but these observations only cover a part of the disk. Additionally, the data are noisy, high-dimensional, and heterogeneous, making it challenging to model processes underlying the Milky Way. This thesis tackles these challenges by bridging the gap between astrophysics and statistics, and developing novel data-driven tools that guide models of formation and evolution of the Galactic disk. Stars encode information about their origins in their age and chemical properties. Estimating the age-metallicity (or age-abundance) structure of stars across the Galactic disk thus allows us to decode its history of star formation, chemical enrichment, dynamical evolution, and accretion. To obtain this structure, I develop two innovative methods that improve the accuracy and/or precision of current chemical abundance and age estimates of red giant stars. First, I extract the low-dimensional component of APOGEE spectroscopic data that is intrinsic to stars from that due to systematics. Combining this intrinsic component with simulation-based Bayesian inference, I efficiently and accurately estimate chemical abundances of red giants. Second, I develop a novel frequency analysis method for asteroseismic data that tackles statistical issues faced by current methods, and helps estimate high precision ages of Kepler giants with efficiency greater than the state-of-the-art. Using the chemical and age information of stars, I study the origin of the chemical bimodality of the two-component disk of the Milky Way, traditionally referred to as the thick and thin disks. The results suggest that the two disk components form sequentially, with the high-α disk forming in the early turbulent phase of the Galaxy and the low-α disk in the later quiescent phase. Further, I estimate the correlation between stellar age and metallicity across the two-component disk using distances from the Gaia satellite, and find evidence for strong spiral-driven radial migration in the low-α disk.