Chemical Enrichment in the Faintest Galaxies: the Carbon and Iron Abundance Spreads in the Bo\"otes I Dwarf Spheroidal Galaxy and the Segue 1 System

TL;DR

This study analyzes the chemical abundance spreads of carbon and iron in ultra-faint dwarf galaxies Bootes I and Segue 1, revealing large dispersions that suggest inhomogeneous early chemical evolution and potential links to the first bound systems.

Contribution

It provides the first detailed measurement of carbon and iron abundance spreads in Bootes I and Segue 1, highlighting their significance in understanding early galaxy formation.

Findings

Large carbon abundance spreads in both galaxies.

Iron abundance ranges of approximately 1.6 dex.

Evidence of inhomogeneous chemical evolution in ultra-faint systems.

Abstract

We present an AAOmega spectroscopic study of red giant stars in Bootes I, which is an ultra-faint dwarf galaxy, and Segue 1, suggested to be either an extremely low-luminosity dwarf galaxy or a star cluster. Our focus is quantifying the mean abundance and abundance dispersion in iron and carbon, and searching for distant radial-velocity members, in these systems. The primary conclusion of our investigation is that the spread of carbon abundance in both Bootes I and Segue 1 is large. For Bootes I, 4 of our 16 velocity members have [C/H] < ~-3.1, while 2 have [C/H] > ~-2.3, suggesting a range of Delta[C/H] ~ 0.8. For Segue 1 there exists a range Delta[C/H] ~ 1.0, including our discovery of a star with [Fe/H] = -3.5 and [C/Fe] = +2.3, which is a radial velocity member at a distance of 4 half-light radii from the system center. The accompanying ranges in iron abundance are Delta[Fe/H] ~ 1.6 for both Bootes I and Segue 1. For [Fe/H] < -3.0, the Galaxy's dwarf galaxy satellites exhibit a dependence of [C/Fe] on [Fe/H] which is very similar to that observed in its halo populations. We find [C/Fe] ~ 0.3 for stars in the dwarf systems that we believe are the counterpart of the Spite et al. (2005) ``unmixed'' giants of the Galactic halo and for which they report [C/Fe] ~ 0.2, and which presumably represents the natal relative abundance of carbon for material with [Fe/H] = -3.0 to -4.0. We confirm the correlation between luminosity and both mean metallicity and abundance dispersion in the Galaxy's dwarf satellites, which extends to at least as faint as Mv = -5. The very low mean metallicity of Segue 1, and the high carbon dispersion in Bootes I, consistent with inhomogeneous chemical evolution in near zero-abundance gas, suggest these ultra-faint systems could be surviving examples of the very first bound systems.