Spectroscopic analysis of Milky Way outer halo satellites: Aquarius II and Bootes II

Abstract

In this paper we present a chemical and kinematic analysis of two ultra-faint dwarf galaxies (UFDs), Aquarius II (Aqu~II) and \text{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 (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 $-130.4_{-1.1}^{+1.4}$ 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.

arXiv comment21 pages, 7 figures, 5 tables. Submitted to AAS Journal

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