Binary galaxies. II - Dynamics and mass-to-light ratios

With particular attention to the removal of selection effects, extensive statistical and dynamical analyses of previously presented data lead to several conclusions. The ratio of the mass-to-light ratio in early-type galaxies to that in late types is 2.0 plus or minus 0.5. The distribution of spatial separations r between binary galaxies is approximately proportional to the inverse square root of r. A variety of dynamical models for binary-galaxy systems are viable; however, they all require total mass-to-light ratios for spirals far larger than conventional (rotation curve) values. The most plausible interpretation of the binary-galaxy data requires that spiral galaxies possess halos containing about 10 times the disk mass and have total mass-to-light ratios of about 65 times the solar ratio for a Hubble constant of 50 km/s per Mpc. It is shown that previous studies of binary galaxies have probably underestimated masses by a factor of at least 10, primarily because selection biases (particularly those toward small projected separations) were underestimated. There is some evidence against halos containing significant mass on scales very large compared to 100 kpc. Orbits of moderate eccentricity are more consistent with the present data than either purely circular or purely radial (probably excluded) ones.