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

TL;DR

This study reveals a previously unreported streaming motion within the X-shaped structure of the Galactic bulge by analyzing proper motions, providing new insights into the bulge's kinematic properties and rotation speed.

Contribution

It introduces the first measurement of streaming motion in the Galactic bulge's X-shaped structure using proper motions, enhancing understanding of bulge dynamics.

Findings

Proper motion difference varies linearly with Galactic longitude near the center.

Detected streaming motion within the X-shaped structure.

Estimated bulge rotation speed as 87.9 +/- 8.2 km/s/kpc.

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.