Kinematics and chemical properties of the old disk of the Galaxy

Abstract

About 600 old disk K giants in selected Galactic low-absorption windows in the plane of the disk have been observed with both broadband photometry and medium-resolution spectroscopy. The observations are used to obtain accurate radial velocities and to estimate each star's chemical abundance and photometric parallax. The results are used to measure the radial variation of the mean motions and velocity dispersion in the plane of the old disk and of (Fe/H) from about 1 to 17 kpc in Galactocentric radius. The observed exponential decrease with radius of the radial component of the velocity dispersion gives an estimate of the disk's radial scale length that is consistent with other recent structural determinations. This agreement indicate that the anisotropy of the velocity dispersion of the old disk is uniform through the disk. The inner parts of the old disk are hot, and the random motions can stabilize the disk against global bisymmetric modes. Possible mechanisms to produce such large random motions are discussed.