Satellites as Probes of the Masses of Spiral Galaxies

Details Satellites as Probes of the Masses of Spiral Galaxies Satellites as Probes of the Masses of Spiral Galaxies

Apr 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...515..153e&link_type=abstract

The Astrophysical Journal, Volume 515, Issue 1, pp. 153-168.

Astronomy and Astrophysics

Astronomy

14

Galaxies: Fundamental Parameters, Galaxies: General, Galaxies: Kinematics And Dynamics

Scientific paper

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 chi as the ratio of these two mass estimates, it is a measure of the presence, or absence, of a significant halo. The chi-distribution for these 24 pairs is presented and the selection effects are discussed. Moreover, we show that the chi-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 chi-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 ``observed'' 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 ``generic'' model, characterized by a single disk/halo combination, cannot reproduce the observed P(chi) 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(chi), particularly if the distribution for r_p<200 kpc is compared with that for r_p>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(chi) 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)x10^12 M_solar, with a mass-to-luminosity ratio of at most a few hundred. This is insufficient to close the universe.

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