The Cosmic Hunt for members in the outskirts of ultra-faint dwarf galaxies: Ursa Major I, Coma Berenices, and Bootes I

Gaia EDR3 data were used to identify potential members in the outskirts of three ultra-faint dwarf (UFD) galaxies: Coma Berenices (>2R(h)), Ursa Major I (similar to 4R(h)), and Bootes I (similar to 4R(h)), as well as a new member in the central region of Ursa Major I. These targets were observed with the Gemini GRACES spectrograph, which was used to determine precision radial velocities and metallicities that confirm their associations with the UFD galaxies. The spectra were also used to measure absorption lines for 10 elements (Na, Mg, K, Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of the outermost stars are in good agreement with stars in the central regions. The abundance ratios and chemical patterns of the stars in Coma Berenices are consistent with contributions from SN Ia, which is unusual for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through tidal stripping and/or supernova feedback. In Bootes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.

Automated summary

Utilizing Gaia EDR3 data, this study identified potential members in the outskirts of three ultra-faint dwarf galaxies (Coma Berenices, Ursa Major I, and Bootes I), as well as a new member in the central region of Ursa Major I. The Gemini GRACES spectrograph was used to observe these targets and determine their precision radial velocities, metallicities, and absorption lines for 10 elements. The results confirm the associations between the identified stars and their respective UFD galaxies, providing insights into their formation and evolution processes.