Two distinct red giant branch populations in the globular cluster NGC
  2419 as tracers of a merger event in the Milky Way
  Recent spectroscopic observations of the outer halo globular cluster (GC) NGC
2419 show that it is unique among GCs, in terms of chemical abundance patterns,
and some suggest that it was originated in the nucleus of a dwarf galaxy. Here
we show, from the Subaru narrow-band photometry employing a calcium filter,
that the red giant-branch (RGB) of this GC is split into two distinct
subpopulations. Comparison with spectroscopy has confirmed that the redder RGB
stars in the $hk$[=(Ca$-b)-(b-y)$] index are enhanced in [Ca/H] by $\sim$0.2
dex compared to the bluer RGB stars. Our population model further indicates
that the calcium-rich second generation stars are also enhanced in helium
abundance by a large amount ($\Delta$Y = 0.19). Our photometry, together with
the results for other massive GCs (e.g., $\omega$ Cen, M22, and NGC 1851),
suggests that the discrete distribution of RGB stars in the $hk$ index might be
a universal characteristic of this growing group of peculiar GCs. The planned
narrow-band calcium photometry for the Local Group dwarf galaxies would help to
establish an empirical connection between these GCs and the primordial building
blocks in the hierarchical merging paradigm of galaxy formation.
