THE SPATIAL STRUCTURE OF MONO-ABUNDANCE SUB-POPULATIONS OF THE MILKY WAY DISK

Abstract

The spatial, kinematic, and elemental-abundance structure of the Milky Way's\nstellar disk is complex, and has been difficult to dissect with local\nspectroscopic or global photometric data. Here, we develop and apply a rigorous\ndensity modeling approach for Galactic spectroscopic surveys that enables\ninvestigation of the global spatial structure of stellar sub-populations in\nnarrow bins of [\\alpha/Fe] and [Fe/H], using 23,767 G-type dwarfs from\nSDSS/SEGUE. We fit models for the number density of each such mono-abundance\ncomponent, properly accounting for the complex spectroscopic SEGUE sampling of\nthe underlying stellar population. We find that each mono-abundance\nsub-population has a simple spatial structure that can be described by a single\nexponential in both the vertical and radial direction, with continuously\nincreasing scale heights (~200 pc to 1 kpc) and decreasing scale lengths (>4.5\nkpc to 2 kpc) for increasingly older sub-populations, as indicated by their\nlower metallicities and [\\alpha/Fe] enhancements. That the abundance-selected\nsub-components with the largest scale heights have the shortest scale lengths\nis in sharp contrast with purely geometric `thick--thin disk' decompositions.\nTo the extent that [\\alpha/Fe] is an adequate proxy for age, our results\ndirectly show that older disk sub-populations are more centrally concentrated,\nwhich implies inside-out formation of galactic disks. The fact that the largest\nscale-height sub-components are most centrally concentrated in the Milky Way is\nan almost inevitable consequence of explaining the vertical structure of the\ndisk through internal evolution. Whether the simple spatial structure of the\nmono-abundance sub-components, and the striking correlations between age, scale\nlength, and scale height can be plausibly explained by satellite accretion or\nother external heating remains to be seen.\n