We present the vertical kinematics of stars in the Milky Way ’ s stellar disk inferred from SDSS/SEGUE G-dwarf data , deriving the vertical velocity dispersion , \sigma _ { 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 [ \alpha \mathrm { / Fe } ] and [ \mathrm { 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 : \sigma _ { z } \bigl ( z,R | [ \alpha \mathrm { / Fe } ] , [ \mathrm { Fe / H } ] \bigr ) \approx% \sigma _ { z } \bigl ( [ \alpha \mathrm { / Fe } ] , [ \mathrm { Fe / H } ] \bigr ) \times \exp \bigl ( - ( R - % R _ { 0 } ) / 7 \mathrm { kpc } \bigr ) .
The characteristic velocity dispersions of these components vary from \sim 15 km s ^ { -1 } for chemically young , metal-rich stars with solar [ \alpha \mathrm { / Fe } ] , to \gtrsim 50 km s ^ { -1 } for metal-poor stars that are strongly [ \alpha \mathrm { / Fe } ] -enhanced , and hence presumably very old .
The mean \sigma _ { z } gradient ( \mathrm { d } \sigma _ { z } / \mathrm { d } z ) away from the mid-plane is only 0.3 \pm 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 ( \propto \sigma ^ { 2 } _ { z } ) as a function of \bigl ( [ \alpha \mathrm { / Fe } ] , [ \mathrm { Fe / H } ] \bigr ) , which contribute to the total stellar surface-mass density approximately as \Sigma _ { R _ { 0 } } ( \sigma ^ { 2 } _ { z } ) \propto \exp ( - \sigma ^ { 2 } _ { z } ) .
This and the existence of isothermal mono-abundance populations with intermediate dispersions ( 30 to 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 } \bigl ( [ \alpha \mathrm { / Fe } ] , [ \mathrm { Fe / H } ] \bigr ) 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 \mathrm { / Fe } ] } \approx 0.07 dex and \delta _ { [ \mathrm { Fe / H } ] } \approx 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 .