The vertical motions of mono-abundance sub-populations in the milky way disk

Abstract

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, σz, 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: σz(z, R | [α/Fe], [Fe/H]) σz([α/Fe], [Fe/H]) × exp (- (R - R 0)/7 kpc). The characteristic velocity dispersions of these components vary from 15kms-1 for chemically young, metal-rich stars with solar [α/Fe], to ≳ 50kms-1 for metal-poor stars that are strongly [α/Fe]-enhanced, and hence presumably very old. The mean σz gradient (dσz/dz) away from the mid-plane is only 0.3 ± 0.2kms-1kpc-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 (σ2z) as a function of ([α/Fe], [Fe/H]), which contribute to the total stellar surface-mass density approximately as . This and the existence of isothermal mono-abundance populations with intermediate dispersions (30-40kms-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: δ[α/Fe] 0.07dex and δ[Fe/H] 0.15dex. 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. © 2012. The American Astronomical Society. All rights reserved.