An observational study of the dynamics of binary galaxies

Abstract

We present high quality radial velocities for all galaxies in Turner's (1976a) catalogue of 156 binary systems. We estimate the rms error of a single velocity measurement in our data set to be 29 km s−1. The observed velocity differences depend significantly on the environment of pairs and on the morphological type of their member galaxies; pairs which are associated with larger galaxy groupings have systematically higher velocity differences than more isolated systems, and pairs containing an elliptical or S0 galaxy tend to have higher velocity differences than pairs of spiral or irregular objects. Typical velocity differences are very small; half of the 55 systems in the subsample of isolated spiral–spiral pairs which is our most reliable set of ‘pure’ binaries have measured velocity differences of less than 75 km s−1. Velocity difference correlates extremely weakly with either luminosity or projected separation, and our data are consistent with it being independent of these observables. Using Monte Carlo simulations of scale-free ensembles of binary galaxies we have been able to show that this behaviour is inconsistent both with the hypothesis that the mass of a binary system increases linearly with its luminosity, and with the hypothesis that galaxies in pairs may effectively be treated as point masses. The data favour models where the galaxies interact through a quasi-isothermal potential which depends only weakly on their luminosity. While it turns out to be impossible to estimate either the orbital eccentricity or the mass-to-light ratio of these pairs directly, the velocity differences we have measured are too low to be consistent with the much larger characteristic velocities seen in the outer parts of individual galaxies unless most of our pairs are on near circular orbits. If this is the case their mass-to-light ratio is similar to that predicted by an extrapolation of the flat rotation curves seen in many spiral galaxies.