The Chemical Signature of a Relic Star Cluster in the Sextans Dwarf
  Spheroidal Galaxy - Implications for Near-Field Cosmology
  We present tentative evidence for the existence of a dissolved star cluster
at [Fe/H]=-2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of
chemical tagging to identify stars that are highly clustered in a
multi-dimensional chemical abundance space (C-space). In a sample of six stars,
three, possibly four stars are identified as potential cluster stars. The
initial stellar mass of the parent cluster is estimated from two independent
observations to M*,init=1.9^{+1.5}_{-0.9} (1.6^{+1.2}_{-0.8}) x 10^5 M_sol,
assuming a Salpeter (Kroupa) initial mass function (IMF). If corroborated by
follow-up spectroscopy, this star cluster is the most metal-poor system
identified to date. Chemical signatures of remnant clusters in dwarf galaxies
like Sextans provide us with a very powerful probe to the high-redshift
Universe. From available observational data, we argue that the average star
cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies
was notably lower than it is in the Galaxy today and possibly lower than in the
more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean
cumulative metallicity function of the dwarf spheroidals falls below that of
the ultra-faints, which increases with increasing metallicity as predicted from
our stochastic chemical evolution model. These two findings, together with a
possible difference in the <[Mg/Fe]> ratio suggest that the ultra-faint dwarf
galaxy population, or a significant fraction thereof, and the dwarf spheroidal
population, were formed in different environments and would thus be distinct in
origin.
