Letter to the Editor

Evidence of a thick disk rotation–metallicity correlation

¹ INAF-Osservatorio Astronomico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy e-mail: spagna@oato.inaf.it
² University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, SLO-1000 Ljubljana, Slovenia

Received: 23 October 2009
Accepted: 13 January 2010

Abstract

We analyze a new kinematic survey that includes accurate proper motions derived from SDSS DR7 positions, combined with multi-epoch measurements from the GSC-II database. By means of the SDSS spectro-photometric data (effective temperature, surface gravity, metallicity, and radial velocities), we estimate photometric parallaxes for a sample of 27 000 FGK (sub)dwarfs with [Fe/H] < -0.5, which we adopted as tracers of the seven-dimensional space distribution (kinematic phase distribution plus chemical abundance) of the thick disk and inner halo within a few kiloparsecs of the Sun.
We find evidence of a kinematics-metallicity correlation,  km s^-1 dex^-1, amongst thick disk stars located between one and three kiloparsecs from the plane and with abundance -1 < [Fe/H] < -0.5, while no significant correlation is present for [Fe/H] -0.5. In addition, we estimate a shallow vertical rotation velocity gradient,  km s^-1 kpc^-1, for the thick disk between 1 kpc < |z| < 3 kpc, and a low prograde rotation, 37 ± 3 km s^-1 for the inner halo up to 4 kpc. 
Finally, we briefly discuss the implications of these findings for the thick disk formation scenarios in the context of CDM hierarchical galaxy formation mechanisms and of secular evolutionary processes in galactic disks.