Seminar by Zoe Hackshaw, UT Austin
The chemodynamic structure of the Milky Way encodes its full assembly history, providing a test bed for galaxy formation and evolution. Employing the chemodynamics of stars as puzzle pieces, we can characterize large-scale patterns across the Galactic disk that inform the dominant processes driving galaxy formation, as well as investigate edge-case systems that lie at the boundaries of the Milky Way's assembly. Using giant stars from APOGEE DR17, we confirm a radial metallicity gradient in the Galactic disk of Δ[Fe/H]/ΔR ∼ −0.068 dex/kpc and identify azimuthal variations of ±0.1 dex superimposed on this gradient. These variations span both young and old stellar populations, implicating dynamical rather than natal processes as the dominant driver. We find correlations with kinematic ridges and dynamical parameters that suggest non-axisymmetric features such as the bar, spiral arms, or a Sgr-like interaction are responsible for these chemical signatures. At the boundary of this puzzle sits anomalous systems Gaia BH3, a 33 solar mass stellar-origin black hole orbiting a metal-poor halo giant. A comprehensive 29-element spectroscopic analysis of the stellar companion reveals a meta-poor, r-I neutron-capture star with no chemical peculiarities relative to halo red giants, broadly consistent with a dynamical capture formation scenario, though a larger census of dormant black hole-star binaries anticipated from Gaia DR4 will be needed to robustly constrain the formation pathway of these systems. Together, these results highlight that a complete picture of Galactic chemical evolution spans from the disk interior to the rare, chemically extreme systems at its edges.
Host: Jennifer Mead