A Short Scale Length for the α-enhanced Thick Disk of the Milky Way: Evidence from Low-latitude SEGUE Data
Abstract
We examine the α-element abundance ratio, [α/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc <R < 16 kpc, 0.15 kpc <|Z| < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z|. Our results show that the high-α thick-disk population has a short scale length (L thick ~ 1.8 kpc) compared to the low-α population, which is typically associated with the thin disk. We find that the fraction of high-α stars in the inner disk increases at large |Z| and that high-α stars lag in rotation compared to low-α stars. In contrast, the fraction of high-α stars in the outer disk is low at all |Z|, and high- and low-α stars have similar rotational velocities up to 1.5 kpc from the plane. We interpret these results to indicate that different processes were responsible for the high-α populations in the inner and outer disk. The high-α population in the inner disk has a short scale length and large scale height, consistent with a scenario in which the thick disk forms during an early gas-rich accretion phase. Stars far from the plane in the outer disk may have reached their current locations through heating by minor mergers. The lack of high-α stars at large R and |Z| also places strict constraints on the strength of radial migration via transient spiral structure.
- Publication:
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The Astrophysical Journal
- Pub Date:
- June 2012
- DOI:
- 10.1088/0004-637X/752/1/51
- arXiv:
- arXiv:1204.5179
- Bibcode:
- 2012ApJ...752...51C
- Keywords:
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- Galaxy: abundances;
- Galaxy: disk;
- Galaxy: evolution;
- Galaxy: formation;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 14 pages, 7 figures, accepted to The Astrophysical Journal