THE MILKY WAY HAS NO DISTINCT THICK DISK
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 [α/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 hz. Here, we work out how to determine the stellar surface-mass density contributions at the solar radius R0 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 , the surface-mass contributions of stellar populations with scale height hz. 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 , from hz ≈ 200 pc to hz ≈ 1 kpc. As hz 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  pc−2.