The Vertical Motions of Mono-Abundance Sub-Populations in the Milky Way Disk

We present the vertical kinematics of stars in the Milky Way's stellar disk inferred from Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration (SDSS/SEGUE) G-dwarf data, deriving the vertical velocity dispersion, σ z, as a function of vertical height |z| and Galactocentric radius R for a set of "mono-abundance" sub-populations of stars with very similar elemental abundances α/Fe and Fe/H. We find that all mono-abundance components exhibit nearly isothermal kinematics in |z|, and a slow outward decrease of the vertical velocity dispersion: σ z (z, R | α/Fe, Fe/H) ≈ σ z (α/Fe, Fe/H) × exp (- (R - R 0) /7 kpc). The characteristic velocity dispersions of these components vary from ~15 km s-1 for chemically young, metal-rich stars with solar α/Fe, to >~ 50 km s-1 for metal-poor stars that are strongly α/Fe-enhanced, and hence presumably very old. The mean σ z gradient (dσ z /dz) away from the mid-plane is only 0. 3 ± 0. 2 km s-1 kpc-1. This kinematic simplicity of the mono-abundance components mirrors their geometric simplicity; we have recently found their density distribution to be simple exponentials in both the z- and R-directions. We find a continuum of vertical kinetic temperatures (vpropσ2 z) as a function of (α/Fe, Fe/H), which contribute to the total stellar surface-mass density approximately as Σₑ䃐 (σ²ᵦ) ∝ \ (-σ²ᵦ). This and the existence of isothermal mono-abundance populations with intermediate dispersions (30-40 km s-1) reject the notion of a thin-thick-disk dichotomy. This continuum of disk components, ranging from old, "hot, " and centrally concentrated ones to younger, cooler, and radially extended ones, argues against models where the thicker disk portions arise from massive satellite infall or heating; scenarios where either the oldest disk portion was born hot, or where internal evolution plays a major role, seem the most viable. In addition, the wide range of σ z (α/Fe, Fe/H) combined with a constant σ z (z) for each abundance bin provides an independent check on the precision of the SEGUE-derived abundances: δα/Fe ≈ 0. 07 dex and δFe/H ≈ 0. 15 dex. The slow radial decline of the vertical dispersion presumably reflects the decrease in disk surface-mass density. This measurement constitutes a first step toward a purely dynamical estimate of the mass profile of the stellar and gaseous disk in our Galaxy.