Metal-Poor Stars in the MW Disk: Resonant Cooling of Vertical Oscillations of Halo Stars in Barred Galaxies

TL;DR

This paper introduces a resonant cooling mechanism driven by stellar bars in barred galaxies, which can explain the presence and kinematics of metal-poor halo stars in the Milky Way disk through numerical simulations.

Contribution

It presents a novel resonant interaction mechanism that cools halo star oscillations and accounts for their migration and distribution in the galactic disk, supported by simulation and observational analysis.

Findings

Halo stars can be trapped and cooled into the disk via bar interactions.

The ratio of prograde to retrograde orbits varies with radius and halo spin.

Cooled halo stars explain observed metal-poor populations in the Milky Way disk.

Abstract

Using numerical simulations of barred disk galaxy embedded in nonspinning and spinning dark matter (DM) halos, we present a novel mechanism of `cooling' the vertical oscillations of DM particles, which acquire the disk kinematics. The underlying mechanism consists of resonant interactions between halo particles and the stellar bar, facilitated by chaotic phase space of the system. The cooling mechanism acts both on dynamical and secular timescales, from $\sim 0.5$\,Gyr to few Gyr. The stellar bar acts to absorb kinetic energy of the vertical motions. Using Milky Way-type stellar halo, we estimate the population of metal-poor disk stars trapped by the MW disk and analyze its kinematics. We find that population of metal-poor MW disk stars with $|z|\ltorder 3$\,kpc detected by the Gaia DR3 and other surveys can have their origin in the stellar halo. The cooled population also migrates radially outwards by exchanging energy and angular momentum with the spinning bar, and prograde-moving stars have a different distribution from the retrograde ones. Next, we have calculated the ratio of the prograde-to-retrograde orbits of the cooled population and found that this ratio varies radially, with the fast-spinning stellar halo resulting in the shallower radial increase of this ratio outside of the corotation. The nonspinning stellar halo shows a monotonic increase of this ratio with radius outside the corotation. Together with analyzed radial migration of these halo stars, the cooling phenomenon of halo metal-poor stars can explain their current disk population, and has corollaries for chemical evolution of disk galaxies in general.