2006MNRAS.370..435K

We present a proper motion mini-survey of 35 fields in the vicinity of Baade window, (l, b) = (1°, -4°), sampling roughly a 5x2.5-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 15863 stars were determined in the reference frame defined by the mean motion of stars with magnitudes between I_F814W_ = 16.5 and 21.5 along the line of sight. We clearly detect small gradients in proper motion dispersions (σ_l, σ_b_∼ (3.0, 2.5) mas.yr^–1, and in the amount of anisotropy (σ_l_/σb_∼ 1.2). Both the longitude dispersion σ_l_ and its ratio to the vertical motion σ_b increase towards the Galactic plane. The decline of the anisotropy ratio σ_l/σ_b_ towards the minor axis of the bulge is mostly due to increasing σ_b_. We also find, for the first time, a significant negative covariance term in the transverse velocity field σ_lb/(σ_lσ_b_) ≃ -0.10. Our results extend by a factor of ∼15 the number of the Galactic bar fields with good proper motion dispersions.