Discovery of rotation axis alignments in Milky Way globular clusters
  There is an increasing number of recent observational results which show that
some globular clusters exhibit internal rotation while they travel along their
orbital trajectories around the Milky Way center. Based on these findings, we
looked for any relationship between the inclination angles of the globular
clusters' orbits with respect to the Milky Way plane and those of their
rotation. We discovered that the relative inclination, in the sense rotation
axis inclination - orbit axis inclination, is a function of the globular
cluster's orbit inclination. Rotation and orbit axes are aligned for an
inclination of ~ 56deg, while the rotation axis inclination is far from the
orbit's one between ~ 20deg and -20deg when the latter increases from 0deg up
to 90deg. We further investigated the origin of such a linear relationship and
found no correlation with the semimajor axes and eccentricities of the globular
clusters' orbits, nor with the internal rotation strength, the globular
clusters' sizes, actual and tidally disrupted masses, half-mass relaxation
times, among others. The uncovered relationship will impact on the development
of numerical simulations of the internal rotation of globular clusters, on our
understanding about the interaction of the globular clusters with the Milky Way
gravitational field, and on the observational campaigns for increasing the
number of studied globular clusters with detected internal rotation.
