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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 661058.0, MPI für Astronomie / Publikationen_mpia
The vertical motions of mono-abundance sub-populations in the Milky Way disk
Authors:Bovy, J.; Rix, H.-W.; Hogg, D. W.; Beers, T. C.; Lee, Y. S.; Zhang, L.
Date of Publication (YYYY-MM-DD):2012
Journal Abbrev.:The Astrophysical Journal
Issue / Number:2
Start Page:id. 115
Audience:Not Specified
Abstract / Description: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 Σ_{R_0}(σ^2_z)∝ \exp (-σ^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 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] ≈ 0.07 dex and δ[Fe/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.
Free Keywords:Galaxy: abundances; Galaxy: disk; Galaxy: evolution; Galaxy: formation; Galaxy: kinematics and dynamics; Galaxy: structure
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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