A SHORT SCALE LENGTH FOR THE α-ENHANCED THICK DISK OF THE MILKY WAY: EVIDENCE FROM LOW-LATITUDE SEGUE DATA

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.