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°, -4°), sampling roughly a 5 × 2.5-deg2 region of the Galactic bar. Our second epoch observations collected with the Advanced Camera for Surveys/High Resolution Channel instrument onboard the Hubble Space Telescope were combined with the archival Wide Field Planetary Camera 2/PC images. The resulting time baselines are in the range of 4 to 8 yr. Precise proper motions of 15 863 stars were determined in the reference frame defined by the mean motion of stars with magnitudes between IF814W = 16.5 and 21.5 along the line of sight. We clearly detect small gradients in proper motion dispersions (σl, σb ∼ (3.0, 2.5) mas yr-1, and in the amount of anisotropy (σl/σb, ∼ 1.2). Both the longitude dispersion σl and its ratio to the vertical motion σb increase towards the Galactic plane. The decline of the anisotropy ratio σl/σb towards the minor axis of the bulge is mostly due to increasing σb. We also find, for the first time, a significant negative covariance term in the transverse velocity field σlb/(σlσb) ≃ -0.10. Our results extend by a factor of ∼15 the number of the Galactic bar fields with good proper motion dispersions. © 2006 RAS.