Milky Way rotation curve from proper motions of red clump giants
  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. 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 CM
diagram K vs. (J-K) the red clump giants and used the information of their
proper motions to build a map of the rotation speed of our Galaxy.
  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, 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 in the solar Galactocentric radius: an average speed of
82+/-5(stat.)+/-58(syst.) km/s. A scenario with a rotation speed lower than 150
km/s in these 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.
