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Published September 1, 2015 | Published + Submitted
Journal Article Open

Spectroscopic Confirmation of the Dwarf Galaxies Hydra II and Pisces II and the Globular Cluster Laevens 1


We present Keck/DEIMOS spectroscopy of stars in the recently discovered Milky Way satellites Hydra II, Pisces II, and Laevens 1. We measured a velocity dispersion of 5.4^(+3.6)_(-2.4) km s^(−1) for Pisces II, but we did not resolve the velocity dispersions of Hydra II or Laevens 1. We marginally resolved the metallicity dispersions of Hydra II and Pisces II but not Laevens 1. Furthermore, Hydra II and Pisces II obey the luminosity–metallicity relation for Milky Way dwarf galaxies (〈[Fe/H] 〉 = -2.02 ± 0.08 and -2.45 ± 0.07, respectively), whereas Laevens 1 does not (〈[Fe/H]〉= -1.68 ± 0.05). The kinematic and chemical properties suggest that Hydra II and Pisces II are dwarf galaxies, and Laevens 1 is a globular cluster. We determined that two of the previously observed blue stars near the center of Laevens 1 are not members of the cluster. A third blue star has ambiguous membership. Hydra II has a radial velocity 〈ν_(helio) 〉 = 303.1 ± 1.4 km s^(−1), similar to the leading arm of the Magellanic stream. The mass-to-light ratio for Pisces II is 370^(+310)_(-240) M⊙/L⊙. It is not among the most dark matter-dominated dwarf galaxies, but it is still worthy of inclusion in the search for gamma-rays from dark matter self-annihilation.

Additional Information

© 2015 The American Astronomical Society. Received 2015 June 2; accepted 2015 July 28; published 2015 August 28. We thank D. Perley for obtaining LRIS images of Laevens 1, J.A. Newman for assistance with the DEIMOS wavelength solution, E. Tollerud for the spectrum of the radial velocity template star HD 38230, and M. de los Reyes for pointing out that tidal dwarf galaxies do not obey the LZR. We also thank B. Sesar and the Pan-STARRS team for information regarding star 399 in Laevens 1. The referee's helpful comments significantly improved this article. We are grateful to the many people who have worked to make the Keck Telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. Facility: Keck:I (LRIS), Keck:II (DEIMOS).

Attached Files

Published - Kirby_2015.pdf

Submitted - 1506.01021v1.pdf


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August 20, 2023
October 23, 2023