Farthest neighbor : the distant Milky

We presentMagellan/IMACS spectroscopy of the recently discovered Milky Way satellite EridanusII (Eri II). We identify 28 member stars in EriII, from which we measure a systemic radial velocity of ( ) =   v 75.6 1.3 stat. hel ( ) 2.0 sys. km s 1 and a velocity dispersion of 6.9 0.9 1.2 km s 1. Assuming that EriII is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of ́ + 1.2 10 0.3 0.4 7  M , indicating a mass-tolight ratio of + 420 140 210  M /  L and confirming that it is a dark matter-dominated dwarf galaxy. From the equivalent width measurements of the Ca triplet lines of 16 red giant member stars, we derive a mean metallicity of [Fe/ H]=−2.38±0.13 and a metallicity dispersion of [ ] / s = + 0.47 Fe H 0.09 . The velocity of EriII in the Galactic standard of rest frame is vGSR=−66.6 km s 1, indicating that either EriII is falling into the Milky Way potential for the first time or that it has passed the apocenter of its orbit on a subsequent passage. At a Galactocentric distance of ∼370 kpc, Eri II is one of the Milky Way’s most distant satellites known. Additionally, we show that The Astrophysical Journal, 838:8 (15pp), 2017 March 20 https://doi.org/10.3847/1538-4357/aa6113 © 2017. The American Astronomical Society. All rights reserved. * This paper includes data gathered with the 6.5 meter Magellan Telescopes located at Las Campanas Observatory, Chile.