Mapping stellar kinematics across the Galactic bar: HST measurements of proper motions in 35 fields

Abstract

We present a proper motion mini-survey of 35 fields in the vicinity of Baade window, (l, b) = (1 deg, -4 deg), sampling roughly a 5 x 2.5 deg region of the Galactic bar. Our second epoch observations collected with the ACS/HRC instrument on board the Hubble Space Telescope were combined with the archival WFPC2/PC images. The resulting time baselines are in the range of 4 - 8 years. Precise proper motions of 15,863 stars were determined in the reference frame defined by the mean motion of stars with magnitudes between I_F814W = 16.5 - 21.5 along the line of sight. We clearly detect small gradients in proper motion dispersions (sigma_l, sigma_b) ~ (3.0, 2.5) mas/yr, and in the amount of anisotropy (sigma_l/sigma_b ~ 1.2). Both the longitude dispersion sigma_l and its ratio to the vertical motion sigma_b increase toward the Galactic plane. The decline of the anisotropy ratio sigma_l/sigma_b toward the minor axis of the bulge is mostly due to increasing sigma_b. We also find, for the first time, a significant negative covariance term in the transverse velocity field sigma_lb/(sigma_l*sigma_b) ~ -0.10. Our results extend by a factor of ~15 the number of the Galactic bar fields with good proper motion dispersions.