Farthest Neighbor: The Distant Milky Way Satellite Eridanus II

Li, T S and Simon, J D and Drlica-Wagner, A and Bechtol, K and Wang, M Y and García-Bellido, J and Frieman, J and Marshall, J L and James, D J and Strigari, L and Pace, A B and Balbinot, E and Zhang, Y and Abbott, T M C and Allam, S and Benoit-Lévy, A and Bernstein, G M and Bertin, E and Brooks, D and Burke, D L and Rosell, A C and Kind, M C and Carretero, J and Cunha, C E and D’Andrea, C B and Costa, L N da and DePoy, D L and Desai, Shantanu and Diehl, H T and Eifler, T F and Flaugher, B and Goldstein, D A and Gruen, D and Gruendl, R A and Gschwend, J and Gutierrez, G and Krause, E and Kuehn, K and Lin, H and Maia, M A G and March, M and Menanteau, F and Miquel, R and Plazas, A A and Romer, A K and Sanchez, E and Santiago, B and Schubnell, M and Sevilla-Noarbe, I and Smith, R C and Sobreira, F and Suchyta, E and Tarle, G and Thomas, D E and Tucker, D L and Walker, A R and Wechsler, R H and Wester, W and Yanny, B (2017) Farthest Neighbor: The Distant Milky Way Satellite Eridanus II. Astrophysical Journal, 838 (1). p. 8. ISSN 1538-4357

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Abstract

We present Magellan/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 nu(hel)= 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 Eri. II is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of 1.2(-0.3)(+0.4) x 10(7) M-circle dot, indicating a mass-tolight ratio of 420(-140)(+210) M-circle dot/L-circle dot 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 sigma[Fe/H]= 0.47(-0.09)(+0.12). The velocity of Eri. II in the Galactic standard of rest frame is nu(GSR) = -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 similar to 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 Eri. II. The lack of gas and recent star formation in Eri II 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 greater than or similar to 10 M-circle dot.

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