The spatial structure of mono-abundance sub-populations of the Milky Way disk

Details The spatial structure of mono-abundance sub-populations of the Milky Way disk The spatial structure of mono-abundance sub-populations of the Milky Way disk

2011-11-07

arxiv.org/abs/1111.1724v2

Astronomy and Astrophysics

Astrophysics

Galaxy Astrophysics

submitted to ApJ

Scientific paper

The spatial, kinematic, and elemental-abundance structure of the Milky Way's stellar disk is complex and has been difficult to dissect with local spectroscopic (few 100 pc) or global photometric data. Here, we investigate the global spatial structure of stellar sub-populations in narrow bins of [\alpha/Fe] and [Fe/H], using 30,353 G dwarfs from SDSS/SEGUE. For each such mono-abundance component of stars we fit models for their vertical and radial number density, properly accounting for the complex spectroscopic SEGUE sampling of the underlying stellar population and for the metallicity and color distributions of the samples. We find that each mono-abundance sub-population has a simple spatial structure that can be described by a single exponential in both the vertical and radial direction. We find a continuous change of this abundance-dependent disk structure, with increasing scale heights (~200 pc to 1 kpc) and decreasing scale lengths (>4.5 kpc to 2 kpc) for increasingly older sub-populations, as indicated by their lower metallicities and [\alpha/Fe] enhancements. The fact that the abundance-selected sub-component with the largest scale height has the shortest scale length is in sharp contrast with purely geometric `thick--thin disk' decompositions. To the extent that [\alpha/Fe] is an adequate proxy for age, our results directly show that older disk sub-populations are more centrally concentrated, which implies inside-out formation of galactic disks. The fact that the largest scale-height sub-components are most centrally concentrated in the Milky Way is an almost inevitable consequence of radial migration. Whether the simple spatial structure of the mono-abundance sub-components, and the striking correlations between age, scale length, and scale height can be plausibly explained by satellite accretion or other external heating remains to be seen. [abridged]

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