Journal
ASTROPHYSICAL JOURNAL
Volume 752, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/752/1/51
Keywords
Galaxy: abundances; Galaxy: disk; Galaxy: evolution; Galaxy: formation
Categories
Funding
- David and Lucile Packard Foundation
- U.S. National Science Foundation [PHY 02-16783, PHY 08-22648]
- Alfred P. Sloan Foundation
- National Science Foundation
- U.S. Department of Energy Office of Science
- University of Arizona
- Brazilian Participation Group
- Brookhaven National Laboratory
- University of Cambridge
- Carnegie Mellon University
- University of Florida
- French Participation Group
- German Participation Group
- Harvard University
- Instituto de Astrofisica de Canarias
- Michigan State/Notre Dame/JINA Participation Group
- Johns Hopkins University
- Lawrence Berkeley National Laboratory
- Max Planck Institute for Astrophysics
- Max Planck Institute for Extraterrestrial Physics
- New Mexico State University
- New York University
- Ohio State University
- Pennsylvania State University
- University of Portsmouth
- Princeton University
- Spanish Participation Group
- University of Tokyo
- University of Utah
- Vanderbilt University
- University of Virginia
- University of Washington
- Yale University
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1009886] Funding Source: National Science Foundation
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We examine the alpha-element abundance ratio, [alpha/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 < vertical bar Z vertical bar < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane vertical bar Z vertical bar. Our results show that the high-alpha thick-disk population has a short scale length (L-thick similar to 1.8 kpc) compared to the low-alpha population, which is typically associated with the thin disk. We find that the fraction of high-a stars in the inner disk increases at large vertical bar Z vertical bar and that high-alpha stars lag in rotation compared to low-alpha stars. In contrast, the fraction of high-alpha stars in the outer disk is low at all vertical bar Z vertical bar, and high- and low-alpha 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-alpha populations in the inner and outer disk. The high-alpha 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-alpha stars at large R and vertical bar Z vertical bar also places strict constraints on the strength of radial migration via transient spiral structure.
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