The chemical evolution of the Bootes I ultra-faint dwarf galaxy

TL;DR

This study analyzes chemical abundances in two metal-poor stars of Bootes I, revealing similarities to halo stars and suggesting Bootes I's status as an early galaxy with multiple stellar populations.

Contribution

First detailed elemental abundance measurements for Bootes I stars, providing insights into its chemical evolution and early galaxy formation.

Findings

Boo I-980 has [Fe/H]=-3.1, first elemental measurements for this star.

Approximately 21% of stars are carbon-enhanced metal-poor (CEMP).

Bootes I likely experienced complex assembly, involving accretion or merging with smaller systems.

Abstract

We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE high-resolution spectra. For Boo I-980, with [Fe/H]=-3.1, we present the first elemental abundance measurements while Boo I-127, with [Fe/H]=-2.0, shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Bootes I stars, as well as those of most other Boootes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend, and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions (MDF) for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Bootes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today.