* Astronomy

Title: The Chemical Signature of a Relic Star Cluster in the Sextans Dwarf Spheroidal Galaxy - Implications for Near-Field Cosmology
Authors: Torgny Karlsson, Joss Bland-Hawthorn, Ken Freeman, Joe Silk

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 solar masses, 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 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.

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