AN ASYMMETRIC STREAMING MOTION IN THE GALACTIC BULGE X-SHAPED STRUCTURE REVEALED BY OGLE-III PROPER MOTIONS

The Galactic bulge shows a double red clump in sightlines at |b| ≳ 5° and −3° ≲ l ≲ 4°. This dump is interpreted as the 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 arms 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°, we find that the proper motion difference between the two arms is a linear function of Galactic longitude for −0.°1 < l < 0.°5, which we interpret as a streaming motion of the stars within the X-shaped structure. A streaming motion was previously reported based on radial velocity data, not the proper motions. The proper motion difference in longitude is constant for −0.°8 < l < −0.°1, which provides an estimate of the bulge rotational speed of 87.9 ± 8.2 km s−1 kpc−1.