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, sigma(z), as a function of vertical height vertical bar z vertical bar and Galactocentric radius R for a set of mono-abundance sub-populations of stars with very similar elemental abundances [alpha/Fe] and [Fe/H]. We find that all mono-abundance components exhibit nearly isothermal kinematics in vertical bar z vertical bar, and a slow outward decrease of the vertical velocity dispersion: sigma(z) (z, R vertical bar [alpha/Fe], [Fe/H]) approximate to sigma(z) ([alpha/Fe], [Fe/H]) x exp(-(R-R-0)/7 kpc). The characteristic velocity dispersions of these components vary from similar to 15 km s(-1) for chemically young, metal-rich stars with solar [alpha/Fe], to greater than or similar to 50 km s(-1) for metal-poor stars that are strongly [alpha/Fe]-enhanced, and hence presumably very old. The mean sigma(z) gradient (d sigma(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 (proportional to sigma(2)(z)) as a function of ([alpha/Fe], [Fe/H]), which contribute to the total stellar surface-mass density approximately as Sigma(R0) (sigma(2)(z)) proportional to exp(-sigma(2)(z)). 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 sigma(z) ([alpha/Fe], [Fe/H]) combined with a constant sigma(z) (z) for each abundance bin provides an independent check on the precision of the SEGUE-derived abundances: delta([alpha/Fe]) approximate to 0.07 dex and delta([Fe/H]) approximate to 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.