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 [alpha/Fe]-[Fe/H] space, are well described by single-exponential spatialdensity 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(R0) (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(R0) (h(z)) alpha exp(-h(z)), from h(z) approximate to 200 pc to h(z) approximate to 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*(R0) = 30 +/- 1M(circle dot)pc(-2).