The discovery of Segue 2: A prototype of the population of satellites of satellites

V. Belokurov, M. G. Walker, N. W. Evans, G. Gilmore, M. J. Irwin, M. Mateo, L. Mayer, E. Olszewski, J. Bechtold, T. Pickering

Research output: Contribution to journalArticlepeer-review

155 Scopus citations


We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of Mv = -2.5, a half-light radius of 34 pc and a systemic velocity of ∼ -40 km s-1. Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s-1 and the possible stream as ∼7 km s-1. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites - survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch.

Original languageEnglish (US)
Pages (from-to)1748-1755
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Aug 2009


  • Galaxies: dwarf
  • Galaxies: individual: Segue 2
  • Local Group

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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