Different stellar sub-populations of the Milky Way ’ s stellar disk are known to have different vertical scale heights , their thickness increasing with age .
Using SEGUE spectroscopic survey data , we have recently shown that mono-abundance sub-populations , defined in the [ \alpha \mathrm { / Fe } ] - [ \mathrm { Fe / H } ] space , are well described by single exponential spatial-density profiles in both the radial and the vertical direction ; therefore any star of a given abundance is clearly associated with a sub-population of scale height h _ { z } .
Here , we work out how to determine the stellar surface-mass density contributions at the solar radius R _ { 0 } of each such sub-population , accounting for the survey selection function , and for the fraction of the stellar population mass that is reflected in the spectroscopic target stars given populations of different abundances and their presumed age distributions .
Taken together , this enables us to derive \Sigma _ { R _ { 0 } } ( h _ { z } ) , the surface-mass contributions of stellar populations with scale height h _ { z } .
Surprisingly , we find no hint of a thin-thick disk bi-modality in this mass-weighted scale-height distribution , but a smoothly decreasing function , approximately \Sigma _ { R _ { 0 } } ( h _ { z } ) \propto \exp ( - h _ { z } ) , from h _ { z } \approx 200 pc to h _ { z } \approx 1 kpc .
As h _ { z } is ultimately the structurally defining property of a thin or thick disk , this shows clearly that the Milky Way has a continuous and monotonic distribution of disk thicknesses : there is no ‘ thick disk ’ sensibly characterized as a distinct component .
We discuss how our result is consistent with evidence for seeming bi-modality in purely geometric disk decompositions , or chemical abundances analyses .
We constrain the total visible stellar surface-mass density at the Solar radius to be \Sigma ^ { { } ^ { * } } _ { R _ { 0 } } = 30 \pm 1 M _ { \odot } pc ^ { -2 } .