We report proper motion dispersions for stars in the direction of two fields of the Galactic bulge , using HST/WFPC2 images taken six years apart .
Our two fields are Baade ’ s Window ( l,b ) = ( 1.13 ^ { \circ } , -3.77 ^ { \circ } ) and Sgr I ( l,b ) = ( 1.25 ^ { \circ } , -2.65 ^ { \circ } ) .
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 studies .
In Baade ’ s Window , we find the l and b proper motion dispersions are 2.9 and 2.5 mas/yr , while in Sgr I , they are 3.3 and 2.7 mas/yr , 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 non-disk 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 .