Spectroscopic Analysis of Milky Way Outer Halo Satellites: Aquarius II and Boötes II

Abstract

In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s ^−1 and a metallicity of [Fe/H] = $-{2.57}_{-0.17}^{+0.17}$ . When compared with previous measurements, these values display a ∼6 km s ^−1 difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of $-{130.4}_{-1.1}^{+1.4}$ km s ^−1 , more than 10 km s ^−1 different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = $-{2.71}_{-0.10}^{+0.11}$ , and a velocity dispersion 3 times smaller at ${\sigma }_{{v}_{\mathrm{hel}}}={2.9}_{-1.2}^{+1.6}$ km s ^−1 . Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density.

Keywords

• Dwarf galaxies
• Dark matter
• Stellar kinematics
• Milky Way Galaxy
• Spectroscopy
• Binary stars