Satellites as Probes of the Masses of Spiral Galaxies

Abstract

We present H I observations and analyses of the kinematics of 24 satellite-primary galaxy pairs with projected separations between 4.9 and 240 kpc. The satellites have masses of less than 3% of their primary spirals. Two estimates for the masses of the primaries are available, one from their rotation curves and one from the orbital properties of the satellites. Defining χ as the ratio of these two mass estimates, it is a measure of the presence, or absence, of a significant halo. The χ-distribution for these 24 pairs is presented and the selection effects are discussed. Moreover, we show that the χ-distribution of more numerous pairs, with projected separations of less than 200 kpc, identified by Zaritsky et al., after adopting selection criteria quite different from ours, is similar to our χ-distribution. We show that the observational biases have a negligible effect; the biased and unbiased distributions are essentially identical. In order to understand this distribution, N-body calculations were executed to simulate the dynamical behavior of relatively low mass satellites orbiting primary disk galaxies with and without extended halos. The models and the real galaxies were in the same fashion. In addition, we made a partially analytical analysis of the behavior of orbits in a logarithmic potential. We find that a model, characterized by a single disk/halo combination, cannot reproduce the observed P(χ) distribution. However, a simple two-component population of galaxies, composed of not more than 60% with halos and 40% without halos, is successful, if galaxies have dimensions of order 200 kpc. If galaxies are considerably larger with sizes extending to 400 kpc or more, the constraints become more onerous. No generic model can describe the full range of the observed P(χ), particularly if the distribution for rp 200 kpc. Regardless of the mix of orbital eccentricities, neither pure halo, nor canonical (disk and halo masses are comparable within the disk radius) models will work. A multicomponent approximation to reality can be constructed for which the canonical model must be mixed with a small fraction of systems essentially devoid of a massive dark halo. Only by including these complexities can the full range of P(χ) be modeled with any degree of success over all radial extents. We show that dynamical friction cannot be ignored in these explorations and that the average mass of a galaxy is in the range of (1-5)×1012 M☉, with a mass-to-luminosity ratio of at most a few hundred. This is insufficient to close the universe.