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

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Using numerical simulations of a 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 disk kinematics. The underlying mechanism consists of resonant interactions between halo particles and the stellar bar, facilitated by a chaotic phase space of the system. The cooling mechanism acts both on dynamical and secular timescales, from ∼0.5 Gyr to a few Gyr. The stellar bar acts to absorb the kinetic energy of the vertical motions. Using a Milky Way (MW)–type stellar halo, we estimate the population of metal-poor disk stars trapped by the MW disk and analyze its kinematics. We find that the population of metal-poor MW disk stars with ∣z∣ ≲ 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 retrograde ones. Next, we 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 the 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 the chemical evolution of disk galaxies in general.