CHEMICAL ENRICHMENT IN THE FAINTEST GALAXIES: THE CARBON AND IRON ABUNDANCE SPREADS IN THE BOOTES I DWARF SPHEROIDAL GALAXY AND THE SEGUE 1 SYSTEM

We present an AAOmega spectroscopic study of red giants in the ultra-faint dwarf galaxy Bootes I (M-V similar to -6) and the Segue 1 system (M-V similar to -1.5), either an extremely low luminosity dwarf galaxy or an unusually extended globular cluster. Both Bootes I and Segue 1 have significant abundance dispersions in iron and carbon. Bootes I has a mean abundance of [Fe/H] = -2.55 +/- 0.11 with an [Fe/H] dispersion of sigma = 0.37 +/- 0.08, and abundance spreads of Delta[Fe/H] = 1.7 and Delta[C/H] = 1.5. Segue 1 has a mean of [Fe/H]= -2.7 +/- 0.4 with [Fe/H] dispersion of sigma = 0.7 +/- 0.3, and abundances spreads of Delta[Fe/H]= 1.6 and Delta[C/H]= 1.2. Moreover, Segue 1 has a radial-velocity member at four half-light radii that is extremely metal-poor and carbon-rich, with [Fe/H] = -3.5, and [C/Fe] = +2.3. Modulo an unlikely non-member contamination, the [Fe/H] abundance dispersion confirms Segue 1 as the least-luminous ultra-faint dwarf galaxy known. For [Fe/H] < -3.0, stars in the Milky Way's dwarf galaxy satellites exhibit a dependence of [C/Fe] on [Fe/H] similar to that in Galactic field halo stars. Thus, chemical evolution proceeded similarly in the formation sites of the Galaxy's extremely metal-poor halo stars and in the ultra-faint dwarf galaxies. We confirm the correlation between (decreasing) luminosity and both (decreasing) mean metallicity and (increasing) abundance dispersion in the Milky Way dwarf galaxies at least as faint as MV = -5. The very low mean iron abundances and the high carbon and iron abundance dispersions in Segue 1 and Bootes I are consistent with highly inhomogeneous chemical evolution starting in near zero-abundance gas. These ultra-faint dwarf galaxies are apparently surviving examples of the very first bound systems.