The Milky Way has no thick disk
Jo Bovy (IAS), Hans-Walter Rix (MPIA), David W. Hogg (NYU, MPIA)

TL;DR
This study uses spectroscopic data to analyze the vertical structure of the Milky Way's stellar disk, revealing a continuous distribution of disk thicknesses without a distinct thick disk component.
Contribution
It demonstrates that the Milky Way's disk does not have a separate thick disk but exhibits a smooth, exponential distribution of stellar scale heights based on spectroscopic survey data.
Findings
No evidence of a bi-modal thin-thick disk structure.
Surface-mass density decreases exponentially with scale height.
Total stellar surface-mass density at the Sun is approximately 30 M_sun/pc^2.
Abstract
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/Fe]-[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…
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