Chemical enrichment in the faintest galaxies: the carbon and iron abundance spreads in the Boötes I dwarf spheroidal galaxy and the Segue 1 system

We present an AAOmega spectroscopic study of red giants in the ultra-faint dwarf galaxy Boötes I (MV ∼ -6) and the Segue 1 system (M_V ∼ -1.5), either an extremely low luminosity dwarf galaxy or an unusually extended globular cluster. Both Boötes I and Segue 1 have significant abundance dispersions in iron and carbon. Boötes I has a mean abundance of [Fe/H] = -2.55 ±0.11 with an [Fe/H] dispersion of σ = 0.37 ± 0.08, and abundance spreads of Δ[Fe/H] = 1.7and Δ [C/H] = 1.5. Segue 1 has a mean of [Fe/H] = -2.7 ±0.4 with [Fe/H] dispersion of s = 0.7±0.3, and abundances spreads of Δ [Fe/H] = 1.6 and Δ[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 M_V = - 5. The very low mean iron abundances and the high carbon and iron abundance dispersions in Segue 1 and Boötes 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.