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Title: Farthest Neighbor: The Distant Milky Way Satellite Eridanus II

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 $$v_{\rm hel} = 75.6 \pm 1.3~\mbox{(stat.)} \pm 2.0~\mbox{(sys.)}~\mathrm{km\,s^{-1}}$$ and a velocity dispersion of $$6.9^{+1.2}_{-0.9}~\mathrm{km\,s^{-1}}$$. Assuming that Eri~II is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of Eri II is $$1.2^{+0.4}_{-0.3} \times 10^{7}~\mathrm{M_\odot}$$, indicating a mass-to-light ratio of $$420^{+210}_{-140}~\mathrm{M_\odot}/\mathrm{L_\odot}$$ and confirming that it is a dark matter-dominated dwarf galaxy. From the equivalent width measurements of the CaT lines of 16 red giant member stars, we derive a mean metallicity of $${\rm [Fe/H]} = -2.38 \pm 0.13$$ and a metallicity dispersion of $$\sigma_{\rm [Fe/H]} = 0.47 ^{+0.12}_{-0.09}$$. The velocity of Eri II in the Galactic Standard of Rest frame is $$v_{\rm GSR} = -66.6~\mathrm{km\,s^{-1}}$$, indicating that either Eri II is falling into the Milky Way potential for the first time or it has passed the apocenter of its orbit on a subsequent passage. At a Galactocentric distance of $$\sim$$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 MACHO dark matter with mass $$\gtrsim10~\mathrm{M_\odot}$$.
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Publication Date:
Report Number(s):
FERMILAB-PUB-16-479-AE-PPD; arXiv:1611.05052
Journal ID: ISSN 1538-4357; 1498275
Grant/Contract Number:
AC02-07CH11359; AC05-00OR22725; AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 838; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Research Org:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States); Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Orgs:
DES Collaboration
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; dark matter; galaxies: dwarf; galaxies: individual (Eridanus II); galaxies: stellar content; Local Group; stars: abundances
OSTI Identifier:
1349627
Alternate Identifier(s):
OSTI ID: 1345629; OSTI ID: 1368580