The Cosmic Hunt for Members in the Outskirts of Ultra Faint Dwarf Galaxies: Ursa Major I, Coma Berenices, and Bo\"otes I

TL;DR

This study uses Gaia EDR3 data and Gemini spectroscopy to identify and analyze the chemical properties of stars in the outskirts of three ultra faint dwarf galaxies, revealing insights into their formation and evolutionary history.

Contribution

It provides the first detailed chemical abundance analysis of outermost stars in these UFDs, challenging previous assumptions about their chemical evolution and suggesting complex formation scenarios.

Findings

Outer stars' chemical abundances match central regions.

Coma Berenices shows SN Ia contributions, unusual for its history.

Bo"otes I's outer stars suggest possible dwarf galaxy merger.

Abstract

Gaia EDR3 data was used to identify potential members in the outskirts of three ultra faint dwarf (UFD) galaxies; Coma Berenices (> 2Rh), Ursa Major I ($\sim$ 4Rh), and Bo\"otes I ($\sim$ 4Rh), 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 Bo\"otes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.