Hubble Space Telescope photometry of multiple stellar populations in the
  inner parts of NGC 2419
  We present new deep imaging of the central regions of the remote globular
cluster NGC 2419, obtained with the F343N and F336W filters of HST/WFC3. The
new data are combined with archival imaging to constrain nitrogen and helium
abundance variations within the cluster. We find a clearly bimodal distribution
of the nitrogen-sensitive F336W-F343N colours of red giants, from which we
estimate that about 55% of the giants belong to a population with about normal
(field-like) nitrogen abundances (P1), while the remaining 45% belong to a
nitrogen-rich population (P2). On average, the P2 stars are more He-rich than
the P1 stars, with an estimated mean difference of Delta Y = 0.05, but the P2
stars exhibit a significant spread in He content and some may reach Delta Y =
0.13. A smaller He spread may be present also for the P1 stars. Additionally,
stars with spectroscopically determined low [Mg/Fe] ratios ([Mg/Fe]<0) are
generally associated with P2. We find the P2 stars to be slightly more
centrally concentrated in NGC 2419 with a projected half-number radius of about
10% less than for the P1 stars, but the difference is not highly significant
(p=0.05). We find evidence of rotation for the P1 stars, whereas the results
are inconclusive for the P2 stars, which are consistent with no rotation as
well as the same average rotation found for the P1 stars. Because of the long
relaxation time scale of NGC 2419, the radial trends and kinematic properties
of the populations are expected to be relatively unaffected by dynamical
evolution. Hence, they provide constraints on formation scenarios for multiple
populations, which must account not only for the presence of He spreads within
sub-populations identified via CNO variations, but also for the relatively
modest differences in the spatial distributions and kinematics of the
populations.
