Jh Vangorkom

Evidence for axisymmetric 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, and it can detect deviations from axisymmetry at the level of a few percent. The velocity field of the ring or disk is expanded into harmonics, and we present analytic expressions which relate these harmonic terms to the intrinsic parameters, and the viewing angles. We show that both the velocity field and the geometry of the ring are necessary to give complete information on the shape of the potential in the plane of the ring. The velocity field alone gives incomplete information for small ellipticities. We present new neutral hydrogen data on the H I ring around the early-type galaxy IC 2006, which was discovered by Schweizer, van Gorkom, and Seltzer (1989). The new data show that the ring is filled and has a remarkably regular velocity field. Application of our method to this gas ring shows that the halo must be close to perfectly axisymmetric. We detect a nonsignificant ellipticity of the potential 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 nearly 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 component probably has a strong disk. The data demonstrate that galaxies other than spiral galaxies have massive halos. The inferred shape of the halo can be contrasted to the strongly triaxial halos found in simulations of dissipationless halo formation. As suggested by Katz and Gunn (1991), the inclusion of baryonic matter in the simulations may be necessary to resolve this issue.

RECOMBINATION-LINE OBSERVATIONS OF THE GALACTIC-CENTER

Aperture-synthesis observations of the H76α and H11Oα recombination lines are presented for the inner (3pc) region of the Galactic Nucleus. The large line width measured with single dishes (Pauls et al., 1974) is caused by well-ordered large-scale motions of the ionized gas. The velocities of the NeII clumps (Lacy et al., 1980) fit well into our smooth velocity field and smooth intensity distribution. We suggest therefore that the cloud picture of the Ne11 gas is (at least partly) invalid.

The Nature, Location and Environment of SgrA East

We have observed SgrA at 332 MHz (92 cm) with a resolution of 12 arcsec (0.6 pc) using the four configurations of the VLA. These results illustrate the dramatic and almost unique variation of radio spectral index within the central 3–4 arcmin of the galactic center. SgrA East is a non-thermal shell source that could be a supernova remnant or a very low-luminosity example of a radio component associated with the active nucleus of a spiral galaxy. The most dramatic aspect of the new 332 MHz observations is the appearance of the the SgrA West spiral features in absorption against SgrA East. Based on these results, SgrA East is situated behind SgrA West, the center of the galaxy. The halo is in front of or surrounds the former sources. The HII regions to the east of SgrA East (1 = -0°.02, b = -0°.07) are probably associated with the 50 km/s molecular cloud. The 7 arcmin halo (20 pc) has a non-thermal spectrum with turn-over below 1 GHz.