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Farthest neighbor: the distant Milky Way satellite Eridanus II*

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journal contribution
posted on 2023-06-09, 06:14 authored by Kathy RomerKathy Romer, The DES Collaboration
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 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 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 times 10^7 Mo, indicating a mass-to-light ratio of 420 -140+210 Mo Lo 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 v 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 ~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 >~Mo.

Funding

Astronomy Centre Rolling Grant; G0679; STFC-SCIENCE AND TECHNOLOGY FACILITIES COUNCIL; ST/F002858/1

History

Publication status

  • Published

File Version

  • Published version

Journal

Astrophysical Journal

ISSN

0004-637X

Publisher

Institute of Physics

Issue

1

Volume

838

Article number

a8

Department affiliated with

  • Physics and Astronomy Publications

Research groups affiliated with

  • Astronomy Centre Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2017-05-10

First Open Access (FOA) Date

2017-05-10

First Compliant Deposit (FCD) Date

2017-05-10

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