The Binary Fraction of Stars in Dwarf Galaxies: The Case of Leo II

Meghin E. Spencer, Mario Mateo, Matthew G. Walker, Edward W. Olszewski, Alan W. McConnachie, Evan N. Kirby, Andreas Koch

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


We combine precision radial velocity data from four different published works of the stars in the Leo II dwarf spheroidal galaxy. This yields a data set that spans 19 years, has 14 different epochs of observation, and contains 372 unique red giant branch stars, 196 of which have repeat observations. Using this multi-epoch data set, we constrain the binary fraction for Leo II. We generate a suite of Monte Carlo simulations that test different binary fractions using Bayesian analysis and determine that the binary fraction for Leo II ranges from 0.30-+0.10 0.09 to 0.34-+0.11 0.11, depending on the distributions of binary orbital parameters assumed. This value is smaller than what has been found for the solar neighborhood (~0.4-0.6) but falls within the wide range of values that have been inferred for other dwarf spheroidals (0.14-0.69). The distribution of orbital periods has the greatest impact on the binary fraction results. If the fraction we find in Leo II is present in low-mass ultra-faints, it can artificially inflate the velocity dispersion of those systems and cause them to appear more dark matter rich than in actuality. For a galaxy with an intrinsic dispersion of 1 km s-1 and an observational sample of 100 stars, the dispersion can be increased by a factor of 1.5-2 for Leo II-like binary fractions or by a factor of three-for binary fractions on the higher end of what has been seen in other dwarf spheroidals.

Original languageEnglish (US)
Article number254
JournalAstronomical Journal
Issue number6
StatePublished - Jun 2017


  • binaries: general
  • galaxies: dwarf
  • galaxies: individual (Leo II)
  • galaxies: kinematics and dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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