Farthest neighbor : the distant Milky Way satellite Eridanus II

Abstract

We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite EridanusII (Eri II). We identify 28 member stars in EriII, from which we measure a systemic radial velocity of vhel = 75.6 1.3 (stat.) 2.0 (sys.) km s-1 and a velocity dispersion of - 6.9+0.9 1.2 km s-1. Assuming that EriII 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 0.12. The velocity of EriII in the Galactic standard of rest frame is vGSR=−66.6 km s-1, indicating that either EriII 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 bright blue stars previously suggested to be a young stellar population are not associated with EriII. The lack of gas and recent star formation in EriII is surprising given its mass and distance from the Milky Way, and may place constraints on models of quenching in dwarf galaxies and on the distribution of hot gas in the Milky Way halo. Furthermore, the large velocity dispersion of Eri II can be combined with the existence of a central star cluster to constrain massive compact halo object dark matter with mass 10 M.