How Binary Stars Complicate the Dark Matter Mystery in Tiny Galaxies
In their new study, Gration and collaborators explore how binary stars, the many pairs of stars orbiting one another, affect the way astronomers measure the mass of some of the faintest galaxies in the universe: ultrafaint dwarf (UFD) galaxies.
Introduction: Why Study UFDs?
UFD galaxies are among the dimmest known, yet they are dominated by dark matter. Because they are so close to us as satellites of the Milky Way, astronomers can observe individual stars within them. That makes UFDs special testbeds for theories of how dark matter is distributed. To estimate the mass of a galaxy, astronomers often measure the velocity dispersion, the spread in the speeds of its stars. The virial theorem links this dispersion directly to the galaxy’s total mass. However, binary stars complicate the picture: the orbital motion of two stars around each other can artificially boost the measured velocity dispersion, leading us to overestimate a galaxy’s mass.
Methods: Modeling Velocity Distributions
Gration and collaborators write down a mathematical description of the line-of-sight (LOS) velocity distribution for a mixed population of stars: singles, visual binaries (separately resolved), and spectroscopic binaries (unresolved). They then use Monte Carlo simulations to model how different binary properties, like mass ratio, orbital period, and eccentricity, affect the observed velocities. They also consider two kinds of stellar populations: stars on the main sequence (ZAMS) and stars that have evolved into the red giant branch (RGB). For the host systems, they adopt different models for the gravitational potential: Hernquist for UFDs and Plummer for globular clusters.
Results: The Impact of Binary Stars
The team finds that whether binaries are resolved or not has a large effect. If all binaries were resolved, the extra velocity dispersion could be extremely high. However, UFDs are so far away that nearly all binaries are unresolved, and the boost is much smaller but still important. For a typical binary fraction of about 30%, the additional dispersion is roughly 12–15 km²/s². This translates to a significant overestimation of the galaxy’s mass if binaries are ignored.
The results also show that globular clusters, dense stellar systems that sometimes resemble UFDs, can be especially tricky. In some cases, binaries can increase their apparent velocity dispersion by factors of up to 100. This means that without accounting for binaries, we could mistakenly classify a cluster as a galaxy.
The Role of the Stellar Mass Function
The authors further test what happens if UFD galaxies have a “bottom-light” initial mass function (IMF), meaning they form fewer low-mass stars than expected. In this case, the contribution from binaries increases by about half a dex (a factor of ~3). That makes binaries an even more serious concern in studies of faint galaxies.
Discussion and Comparisons
The paper compares these findings with earlier work, which often assumed all binaries were spectroscopic and focused only on RGB stars. Gration and colleagues’ approach is more general, applying to both UFDs and globular clusters and allowing for more realistic mixtures of binary types. They caution, however, that some of their assumptions (like the independence of binary orbits and galactic environment) may not hold in every case, especially for older globular clusters that experience mass segregation.
Conclusion: Getting Mass Right
The key takeaway is that binary stars can substantially alter the observed velocity dispersions in UFDs and globular clusters. For UFDs, the fractional increase is typically of order one, while for globular clusters it can reach orders of magnitude larger. Accounting for binaries is therefore essential if astronomers want accurate mass measurements and to avoid confusing clusters with galaxies.
Source: Gration
