In this paper we present a chemical and kinematic analysis of two ultra-faint dwarf galaxies (UFDs), Aquarius II (Aqu~II) and \text{ Bo\"{ o}tes 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 (8). 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 $-65.3 \pm 1.8$ km s$^{ -1}$ and a metallicity of [Fe/H] = $-2.57^{ +0.17}_{ -0.17}$. When compared with previous measurements, these values display a $\sim 6$ 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 $-130.4^{ +1.4}_{ -1.1}$ km s$^{ -1}$, more than 10 km s$^{ -1}$ different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = $-2.71^{ +0.11}_{ -0.10}$, and a velocity dispersion 3 times smaller at $\sigma_{ v_{ \rm hel}} = 2.9^{ +1.6}_{ -1.2}$ 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.
Categories: astro-ph.GA