Torgny Karlsson, Joss Bland-Hawthorn, Ken Freeman, Joe Silk
(ABRIDGED) We present tentative evidence for the existence of a dissolved
star cluster in the Sextans dwarf spheroidal galaxy. In a sample of six stars,
three (possibly four) stars around [Fe/H] = -2.7 are identified as potential
cluster stars by the technique of chemical tagging. This finding, together with
the recognition of an apparent excess of stars in the metallicity distribution
function (MDF) of Sextans at a similar metallicity as the cluster stars, is
used to estimate the initial stellar mass of the parent cluster 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 at [Fe/H] = -2.7 is the most metal-poor system
identified to date. In an era of extremely large telescopes, we anticipate that
chemical tagging will be a powerful technique, in particular for tracing the
star formation process and the evolution of the initial cluster mass function
in dwarf galaxies, and for putting firm constraints on the dwarf-galaxy origin
of the Milky Way's stellar halo. From available observational data, we also
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. Moreover, the slope of the cumulative metallicity function
(below [Fe/H] = -2.5) in 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.
View original:
http://arxiv.org/abs/1201.5376
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