Hubble Space Telescope WFPC2 proper motions in two bulge fields: Kinematics and stellar population of the Galactic bulge

We report proper-motion dispersions for stars in the direction of two fields of the Galactic bulge, using Hubble Space Telescope Wide Field Planetary Camera 2 images taken 6 years apart. Our two fields are Baade's window [(l, b) = (1.degrees13, 3.degrees77)] and Sgr I [( l, b) = (1.degrees25, 2.degrees65)]. Our proper-motion dispersions are in good agreement with prior ground- and space-based proper-motion studies in bulge fields, but in contrast to some prior studies, we do not exclude any subset of stars from our sample. In Baade's window, we find the l and b proper-motion dispersions are 2.9 and 2.5 mas yr(-1), while in Sgr I, they are 3.3 and 2.7 mas yr(-1), respectively. For the first time, we can clearly separate the foreground disk stars out from the bulge because of their large mean apparent proper motion. The population with nondisk kinematics ( which we conclude to be the bulge) has an old main-sequence turnoff point, similar to those found in old, metal-rich bulge globular clusters, while those stars selected to have disk kinematics lie on a fully populated main sequence. Separating main-sequence stars by luminosity, we find strong evidence that the bulge population is rotating, largely explaining observations of proper-motion anisotropy in bulge fields. Because we have isolated such a pure sample of stars in the bulge, we have one of the clearest demonstrations that the old stellar population of the inner bulge/bar is in fact rotating.