3D kinematics through the X-shaped Milky Way bulge (Vasquez et al.)

Vol. 555
In section 5. Galactic structure; stellar clusters and populations

3D kinematics through the X-shaped Milky Way bulge

by S. Vasquez, M. Zoccali, V. Hill, et al. A&A 555, A91

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The Galactic bulge is X-shaped, caused by the two southern arms of the X both crossing the lines of sight, thereby producing a double red clump (one bright and one faint) in the color magnitude diagram. In this paper the authors measure the radial velocities and proper motions for a sample of 454 individual bulge giant stars, almost equally distributed between the two red clumps to determine how such a structure could be formed from bar instabilities. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the more distant overdensity, which is traced by faint red clump stars. This can be explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-­shaped bulge. Proper motions for these stars are consistent with qualitative predictions from (OR BY) dynamical models of peanut-­shaped bulges. Surprisingly, stars on elongated orbits mainly have metal-­poor (subsolar) metallicities, while the metal-rich ones, in both overdensities, are usually found in more axisymmetric orbits.