Context :

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 \approx 16 kpc and |z| \approx 2 kpc , where a significant deviation from a null average proper motion ( \sim 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 \pm 5 ( stat . ) \pm 58 ( syst . )
km/s ( assuming a solar Galactocentric distance of 8 kpc , and a circular/azimuthal velocity of 250 km/s for the Sun and of 238 km/s 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 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 can not 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 .