Milky Way rotation curve from proper motions of red clump giants

Aims. We derive the stellar rotation curve of the Galaxy in the range of Galactocentric radii of R = 4−16 kpc at different vertical heights from the Galactic plane of z between –2 and +2 kpc. With this we reach high Galactocentric distances in which the kinematics is poorly known due mainly to uncertainties in the distances to the sources. Methods. We used the PPMXL survey, which contains the USNO-B1 proper motions catalog cross–correlated with the astrometry and near-infrared photometry of the 2MASS Point Source Catalog. To improve the accuracy of the proper motions, we calculated the average proper motions of quasars to know their systematic shift from zero in this PPMXL survey, and we applied the corresponding correction to the proper motions of the whole survey, which reduces the systematic error. We selected from the color–magnitude diagram K vs. (J − K) the standard candles corresponding to red clump giants and used the information of their proper motions to build a map of the rotation speed of our Galaxy. Results. We obtain an almost flat rotation curve with a slight decrease for higher values of R or |z|. The most puzzling result is obtained for the farthest removed and most off-plane regions, that is, at R ≈ 16 kpc and |z| ≈ 2 kpc, where a significant deviation from a null average proper motion (~4 mas/yr) in the Galactic longitude direction for the anticenter regions can be directly translated into a rotation speed much lower than at the solar Galactocentric radius. In particular, we obtain an average speed of 82 ± 5(stat.) ± 58(syst.) km s-1 (assuming a solar Galactocentric distance of 8 kpc, and a circular/azimuthal velocity of 250 km s-1 for the Sun and of 238 km s-1 for the Local Standard of Rest), where the high systematic error bar is due mainly to the highest possible contamination of non-red clump giants and the proper motion systematic uncertainty. Conclusions. A scenario with a rotation speed lower than 150 km s-1 in these farthest removed and most off-plane regions of our explored zone is intriguing, and invites one to reconsider different possibilities for the dark matter distribution. However, given the high systematic errors, we cannot conclude about this. Hence, more measurements of the proper motions at high R and |z| are necessary to validate the exotic scenario that would arise if this low speed were confirmed