### Abstract

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 language | English (US) |
---|---|

Article number | 254 |

Journal | Astronomical Journal |

Volume | 153 |

Issue number | 6 |

DOIs | |

State | Published - Jun 1 2017 |

### Fingerprint

### Keywords

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

### ASJC Scopus subject areas

- Astronomy and Astrophysics
- Space and Planetary Science

### Cite this

*Astronomical Journal*,

*153*(6), [254]. https://doi.org/10.3847/1538-3881/aa6d51

**The Binary Fraction of Stars in Dwarf Galaxies : The Case of Leo II.** / Spencer, Meghin E.; Mateo, Mario; Walker, Matthew G.; Olszewski, Edward W; McConnachie, Alan W.; Kirby, Evan N.; Koch, Andreas.

Research output: Contribution to journal › Article

*Astronomical Journal*, vol. 153, no. 6, 254. https://doi.org/10.3847/1538-3881/aa6d51

}

TY - JOUR

T1 - The Binary Fraction of Stars in Dwarf Galaxies

T2 - The Case of Leo II

AU - Spencer, Meghin E.

AU - Mateo, Mario

AU - Walker, Matthew G.

AU - Olszewski, Edward W

AU - McConnachie, Alan W.

AU - Kirby, Evan N.

AU - Koch, Andreas

PY - 2017/6/1

Y1 - 2017/6/1

N2 - 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.

AB - 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.

KW - binaries: general

KW - galaxies: dwarf

KW - galaxies: individual (Leo II)

KW - galaxies: kinematics and dynamics

UR - http://www.scopus.com/inward/record.url?scp=85020757848&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85020757848&partnerID=8YFLogxK

U2 - 10.3847/1538-3881/aa6d51

DO - 10.3847/1538-3881/aa6d51

M3 - Article

AN - SCOPUS:85020757848

VL - 153

JO - Astronomical Journal

JF - Astronomical Journal

SN - 0004-6256

IS - 6

M1 - 254

ER -