THE DISTRIBUTION OF DARK-MATTER IN A RINGED GALAXY

Outer rings are located at the greatest distance from the galaxy cente r of any feature resonant, with a bar. Because of their large scale, t heir morphology is sensitive to the distribution of the dark matter in the galaxy. We introduce here how study of these rings can constrain the mass-to-light ratio of the bar, and so the percentage of dark matt er in the center of these galaxies. We compare periodic orbits integra ted in the ringed galaxy NGC 6782 near the outer Lindblad resonance to the shape of the outer ring. The nonaxisymmetric component of the pot ential resulting from the bar is derived from a near-infrared image of the galaxy. The axisymmetric component is derived assuming a flat rot ation curve. We find that the pinched non-self-intersecting, periodic orbits are more elongated for higher bar mass-to-light ratios and fast er bars. The inferred mass-to-light ratio of the bar depends on the as sumed inclination of the galaxy. With an assumed galaxy inclination of i = 41 degrees, for the orbits to be consistent with the observed rin g morphology, the mass-to-light ratio of the bar must be high, greater than 70% of a maximal disk value. For i = 45 degrees, the mass-to-lig ht ratio of the bar is 75% +/- 15% of the maximal disk value. Since th e velocity field of these rings can be used to constrain the galaxy in clination as well as the periodic orbit that is represented in the rin g, further study will yield tighter constraints on the mass-to-light r atio of the bar. If a near-maximal disk value for the bar is required, then there would either be little dark matter within the bar or the d ark matter contained in the disk of the galaxy would be nonaxisymmetri c and would rotate with the bar.