EVIDENCE FOR AXISYMMETRICAL HALOS - THE CASE OF IC-2006

We present a new method to derive the shape of the potential from the velocity field of a gas ring, or a gas disk with a flat rotation curve . The method is an extension of previous work by Binney and Teuben, an d it can detect deviations from axisymmetry at the level of a few perc ent. The velocity field of the ring or disk is expanded into harmonics , and we present analytic expressions which relate these harmonic term s to the intrinsic parameters, and the viewing angles. We show that bo th the velocity field and the geometry of the ring are necessary to gi ve complete information on the shape of the potential in the plane of the ring. The velocity field alone gives incomplete information for sm all ellipticities. We present new neutral hydrogen data on the H I rin g around the early-type galaxy IC 2006, which was discovered by Schwei zer, van Gorkom, and Seltzer (1989). The new data show that the ring i s filled and has a remarkably regular velocity field. Application of o ur method to this gas ring shows that the halo must be close to perfec tly axisymmetric. We detect a nonsignificant ellipticity of the potent ial of 0.012 +/- 0.026. The 95% confidence limit on the ellipticity is 0.05. This implies that the potential is nearly circular in the plane of the ring. The analysis indicates that the circular velocity is nea rly constant from 0.5R(e) to 6.5R(e). We confirm that the MIL ratio in the outer parts increases (Schweizer et al. 1989). The stellar compon ent probably has a strong disk. The data demonstrate that galaxies oth er than spiral galaxies have massive halos. The inferred shape of the halo can be contrasted to the strongly triaxial halos found in simulat ions of dissipationless halo formation. As suggested by Katz and Gunn (1991), the inclusion of baryonic matter in the simulations may be nec essary to resolve this issue.