Observational Evidence from SDSS for a Merger Origin of the Milky Way's Thick Disk

Details Observational Evidence from SDSS for a Merger Origin of the Milky Way's Thick Disk Observational Evidence from SDSS for a Merger Origin of the Milky Way's Thick Disk

2010-09-08

arxiv.org/abs/1009.1616v1

Astronomy and Astrophysics

Astrophysics

Galaxy Astrophysics

10 pages, 2 figures, 2 tables (Table 2 in electronic form only, available from http://www.mpia-hd.mpg.de/homes/rix/Data/Dier

Scientific paper

We test competing models that aim at explaining the nature of stars in the Milky Way that are well away (|z|$\gtrsim$ 1kpc) from the midplane, the so-called thick disk: the stars may have gotten there through orbital migration, through satellite mergers and accretion, or through heating of pre-existing thin disk stars. Sales et al. (2009) proposed the eccentricity distribution of thick disk stars as a diagnostic to differentiate between these mechanisms. Drawing on SDSS DR7, we have assembled a sample of 34,223 G-dwarfs with 6-D phase-space information and metallicities, and have derived orbital eccentricities for them. Comparing the resulting eccentricity distributions, p(e|z), with the models, we find that: a) the observed p(e|z) is inconsistent with that predicted by orbital migration only, as there are more observed stars of high and of very low eccentricity; b) scenarios where the thick disk is made predominantly through abrupt heating of a pre-existing thin disk are also inconsistent, as they predict more high-eccentricity stars than observed; c) the observed p(e|z) fits well with a "gas-rich merger" scenario, where most thick disk stars were born from unsettled gas in situ.

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