Hydra II: a faint and compact milky way dwarf galaxy found in the survey of the magellanic stellar history

Nicolas F. Martin, David L. Nidever, Gurtina Besla, Knut Olsen, Alistair R. Walker, A. Katherina Vivas, Robert A. Gruendl, Catherine C. Kaleida, Ricardo R. Muñoz, Robert D. Blum, Abhijit Saha, Blair C. Conn, Eric F. Bell, You Hua Chu, M-R.L. Cioni, Thomas J. L. De Boer, Carme Gallart, Shoko Jin, Andrea Kunder, Steven R. MajewskiDavid Martinez-Delgado, Antonela Monachesi, Matteo Monelli, Lara Monteagudo, Noelia E.D. Noël, Edward W. Olszewski, Guy S. Stringfellow, Roeland P. Van Der Marel, Dennis Zaritsky

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We present the discovery of a new dwarf galaxy, Hydra II, found serendipitously within the data from the ongoing Survey of the Magellanic Stellar History conducted with the Dark Energy Camera on the Blanco 4 m Telescope. The new satellite is compact (r_h = 68 ± 11 pc) and faint (M_V = -4.8 ± 0.3), but well within the realm of dwarf galaxies. The stellar distribution of Hydra II in the color-magnitude diagram is well-described by a metal-poor ([Fe/H] = -2.2) and old (13 Gyr) isochrone and shows a distinct blue horizontal branch, some possible red clump stars, and faint stars that are suggestive of blue stragglers. At a heliocentric distance of 134 ± 10 kpc, Hydra II is located in a region of the Galactic halo that models have suggested may host material from the leading arm of the Magellanic Stream. A comparison with N-body simulations hints that the new dwarf galaxy could be or could have been a satellite of the Magellanic Clouds.
Original languageEnglish
Article numberL5
JournalAstrophysical Journal Letters
Issue number1
Early online date23 Apr 2015
Publication statusPublished - 1 May 2015


  • galaxies: individual (Hydra II)
  • Local Group
  • Magellanic Clouds


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