A short scale length for the α-enhanced thick disk of the milky way: Evidence from low-latitude segue data

Judy Y. Cheng, Constance M. Rockosi, Heather L. Morrison, Young Sun Lee, Timothy C. Beers, Dmitry Bizyaev, Paul Harding, Elena Malanushenko, Viktor Malanushenko, Daniel Oravetz, Kaike Pan, Katharine J. Schlesinger, Donald P. Schneider, Audrey Simmons, Benjamin A. Weaver

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99 Scopus citations


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 6kpc<R < 16kpc, 0.15kpc<|Z| < 1.5kpc, 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 1.8kpc) 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 1.5kpc 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.

Original languageEnglish (US)
Article number51
JournalAstrophysical Journal
Issue number1
StatePublished - Jun 10 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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