Kinematic Map of the Galactic Bulge Using HST Proper Motions

Abstract

Constraining the Galactic bulge's non-axisymmetric dynamics requires proper motions in deep fields across many sightlines, yet crowding and extinction limit such measurements. Using multi-epoch observations of 27 fields in the southern Galactic bulge (-6$^\circ$ < l < 4$^\circ$, -5$^\circ$ < b < -1$^\circ$) from the ultraviolet-visible (UVIS) channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) with time baselines of 3 to 13 years, and combining them with the absolute proper motion reference frame provided by Gaia Data Release 3 (DR3), we measure absolute proper motions for ~ 400,000 stars. Colour-magnitude diagrams are used to separate bulge stars from foreground disc stars, identify red clump stars and correct for interstellar extinction and reddening. Statistical analysis shows that the mean proper motion in Galactic latitude is consistent with that of the central supermassive black hole Sgr A*, while the mean proper motion in Galactic longitude is affected by disc stars but agrees with Sgr A* after extrapolation to the Galactic plane. Proper motion dispersions increase towards the Galactic centre and are asymmetric across Galactic longitude, with a steeper gradient for l < 0$^\circ$, revealing the bulge's non-axisymmetric bar structure. The velocity distribution shows clear anisotropy ($σ_l$/$σ_b$ = 1.131 $\pm$ 0.005). The normalised covariance is negative in the l > 0$^\circ$ region (~ -0.085) and close to zero in the l < 0$^\circ$ region. The proper motion distributions are non-Gaussian in both Galactic longitude and latitude, mainly due to contamination by foreground disc stars.