Jo Bovy, Hans-Walter Rix, Chao Liu, David W. Hogg, Timothy C. Beers, Young Sun Lee
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]
View original:
http://arxiv.org/abs/1111.1724
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