HST proper motions on the far side of the Galactic bar-data
Clarkson, William I.; Rich, R. Michael; Kunder, Andrea; Johnson, Christian I.; Kuijken, Konrad; Fernández-Trincado, José G.; Vieira, Katherine; Simion, Iulia T.; Alonso, M. Victoria; Soto, Mario; Baravalle, Laura D.; Nilo Castellón, José Luis; Contreras Ramos, Rodrigo
Chile, Netherlands, United States, Argentina, China
Abstract
This is the third paper in a series that attempts to observe a clear signature of the Galactic bar/bulge using kinematic observations of the bulge stellar populations in low foreground extinction windows. We report on the detection of ~100 000 new proper motions in four fields covering the far side of the Galactic bar/bulge, at negative longitudes. Our proper motions have been obtained using observations from the Advance Camera for Surveys (ACS), on board of the Hubble Space Telescope (HST), with a time-baseline of 8-9 years, which has produced accuracies better than 0.5 mas yr-1 for a significant fraction of the stellar populations with F814W < 23 mag. Interestingly, as shown in previous works, the Hess diagrams show a strikingly similar proper motion distribution to fields closer to the Galactic center and consistent with an old stellar population. The observed kinematics point to a significant bulge rotation, which seems to predominate even in fields as far as l ≃ -8°, and is also reflected in the changes of the velocity ellipsoid in the l, b plane as a function of distance.