Galactic bulge has an X-shaped structure (Nataf et al., 2010, McWiliam and Zoccali, 2010). It manifests as a double red clump on the color-magnitude diagrams in fields with |b|>5 deg. In these fields the stars belonging to the two different arms are observed. We have investigated the proper motions of these stars in order to derive the properties of the X-shaped structure kinematics.
We selected three fields which fulfill two criteria: there is large enough collection of the OGLE-III images to allow measuring proper motions with an accuracy better than 1 mas/yr and the double red clump is well pronounced. The I- and V-band brightness of each star was corrected for extinction and luminosity function was constructed for each field separately. We fitted the luminosity functions with a model, which included red giants, red clump stars and red giant branch bump stars for each arm of the X-shaped structure separately. This allowed estimating the probability that given star belongs to the brighter or fainter arm of the X-shaped structure. The proper motions were measured and combined with these probabilities what resulted in intrinsic proper motion differences between the two arms. Figure below presents them as a function of Galactic longitude.
We found that the proper motion difference between the two arms is a linear function of the Galactic longitude for -0.1 deg < l < 0.5 deg. We interpret this as a signature of the streaming motion of stars in the X-shaped structure. The proper motion difference is constant for -0.8 deg < l < -0.1 deg. We used its longitudinal component to estimate the buge rotation speed of 87.9±8.2 km/s/kpc.
PLEASE cite the following paper when using the data or referring to these OGLE results:
Poleski et al. 2013, ApJ, 776, 76; arXiv:1304.6084
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