Asymmetric streaming motion in the Galactic bulge X-shaped structure revealed by the OGLE-III proper motions

Abstract

The Galactic bulge shows a double red clump in sight lines with |b| > 5 deg and -3 deg < l < 4 deg. It is interpreted as a signature of an X-shaped structure seen almost edge-on. We measure the proper motions of the stars belonging to the closer and the further arm of the X-shaped structure. The intrinsic kinematic properties of the two arms are found by incorporating information taken from the luminosity function. At b = -5 deg we find 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, which we interpret as a streaming motion of the stars within the X-shaped structure. Such a streaming motion has not previously been reported. The proper motion difference is constant for -0.8 deg < l < -0.1 deg, which gives us an estimate of bulge rotation speed of 87.9 +/- 8.2 km/s/kpc.