NGC 5824: a luminous outer halo globular cluster with an intrinsic
  abundance spread
  We present a detailed study of the strengths of the calcium triplet
absorption lines in the spectra of a large sample of red giant members of the
luminous outer Galactic halo globular cluster NGC 5824. The spectra were
obtained with the FORS2 and GMOS-S multi-object spectrographs at the VLT and
the Gemini-S telescope, respectively. By comparing the line strengths of the
NGC 5824 stars with those for red giants in clusters with well established
abundances, we conclude that there is an intrinsic abundance dispersion in NGC
5824 characterized by an inter-quartile range in [Fe/H] of 0.10 dex and a total
range of ~0.3 dex. As for omega-Cen and M22, the abundance distribution shows a
steep rise on the metal-poor side and a shallower decline on the metal-rich
side. There is also some indication that the distribution is not unimodal with
perhaps 3 distinct abundance groupings present. NGC 5824 has a further unusual
characteristic: the outer surface density profile shows no signs of a tidal
cutoff. Instead the profile has a power-law distribution with cluster stars
detected to a radius exceeding 400 pc. We postulate that NGC 5824 may be the
remnant nuclear star cluster of a now disrupted dwarf galaxy accreted during
the formation of the Galaxy's halo. We further speculate that the presence of
an intrinsic [Fe/H] spread is the characteristic that distinguishes former
nuclear star clusters from other globular clusters.
