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

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.