Revisiting the Influence of Unidentified Binaries on Velocity Dispersion Measurements in Ultra-Faint Stellar Systems

Abstract

Velocity dispersion measurements of recently discovered Milky Way satellites with M V −7 imply that they posses high mass-to-light ratios. The expected velocity dispersions due to their baryonic mass are ∼0.2 km s −1 , but values 3 km s −1 are measured. We perform Monte Carlo simulations of mock radial velocity measurements of these systems assuming that they have mass-to-light ratios similar to globular clusters and posses an unidentified binary star population, to determine if these stars could boost the velocity dispersion to the observed values. We find that this hypothesis is unlikely to produce dispersions much in excess of ∼4.5 km s −1 , in agreement with previous work. However, for the systems with the potentially smallest velocity dispersions, values consistent with observations are produced in 5%-40% of our simulations for binary fractions in excess of f bin (P 10 yr) ∼ 5%. This sample includes the dwarf galaxy candidates that lie closest to classical globular clusters in M V −r h space. Considered as a population, it is unlikely that all of these dwarf galaxy candidates have mass-to-light ratios typical of globular clusters, but boosting of the observed dispersion by binaries from near-zero values cannot be ruled out at high confidence for several individual dwarf galaxy candidates. Given the importance of obtaining accurate velocity dispersions and dynamical masses for the faintest satellites, it is clearly desirable to directly exclude the possible effect of binaries on these systems. This requires multi-epoch radial velocity measurements with individual uncertainties of 1 km s −1 to identify spectroscopic binaries with orbital velocities of the order of the observed velocity dispersion.

A Megacam Survey of Outer Halo Satellites. Vi. The Spatially Resolved Star-Formation History of the Carina Dwarf Spheroidal Galaxy*

“…After this, we chose a binary fraction value 8 for Carina of 0.4, because this value produced the Hess diagram models that most resembled the data. This binary fraction is consistent with values obtained in other dwarf galaxies (e.g., McConnachie & Côté 2010;Geha et al 2013). Figure 7 shows the Hess diagram of the data (left panels) and the best-fit model (right panels), as well as the residuals for each Carina region.…”

Dark matter annihilation and decay from non-spherical dark halos in galactic dwarf satellites

“…For classical dSphs, the above works in the literature investigated the effect of binary systems on velocity second moments and concluded that the influence of binary systems in these dSphs is, in fact, negligible because their intrinsic velocity second moments are much larger than the velocity distributions inflated by binaries. Although not all of the UFD galaxies were investigated for this effect, some authors considered that a binary star is unlikely to make the measured velocity second moments dramatically inflated (see Simon & Geha 2007;Simon et al 2011Simon et al , 2015Koposov et al 2011;Kirby et al 2013;McConnachie & Côté 2010). Therefore, we suppose that the velocity data of each dSph is not affected by the presence of binary stars.…”

One Law to Rule Them All: The Radial Acceleration Relation of Galaxies

“…Here we stress that velocity dispersions (σ ) are derived from high-resolution individualstar spectroscopy and their errors are indicative. The reliability of these measurements depends on the available number of stars, the possible contaminations from foreground objects and binary stars, and the dynamical state of the galaxy (e.g., Walker et al 2009c;McConnachie & Côté 2010;McGaugh & Wolf 2010;Minor et al 2010).…”