4.1 Dark Matter in Ellipticals
For ellipticals, it has been much more difficult to establish that they too are imbedded in a dark matter halo. This is due to the fact that their luminosity distribution drops off very steeply with radius, and that they, ordinarily, do not contain neutral hydrogen. Moreover, the interpretation of the stellar velocity data in the outer parts depends also on the anisotropy of the velocity dispersion tensor. For a few systems which do have neutral hydrogen, the data show generally a flat rotation curve, hence a dark halo is inferred. The best studied system this way is IC 2006, which has an outer ring of HI. Indications are that the halo around this system is close to axisymmetric (cf. Franx et al. 1994).
Recent efforts in detailed modelling of high quality stellar radial velocity and velocity dispersion data have resulted in the demonstration that dark matter is a necessary ingredient to get good fits of the models to the data. This was done for NGC 2434 (Rix et al. 1997) and NGC 6703 (Gerhard et al. 1998).
Other tracers can be used. A fruitful one comes from the kinematics of planetary nebulae. The detection of such objects around individual galaxies have now become routine and for a few ellipticals the data indicate the presence of dark mass at large radii. This has been shown in particular for NGC 3384 (Tremblay et al. 1995) and NGC 5128, also known as Centaurus A (Hui et al. 1995). For this last system, the dynamics of the outer shells detected in neutral hydrogen also indicate the presence of a dark halo (Schiminovich et al. 1994).
The detection of X-ray gas in and around elliptical galaxies in clusters has lead early on to the detection of large dark matter halos around ellipticals like M87 and NGC 1399, which are in the center of clusters. For normal ellipticals, only recent data confirm the presence of extended dark matter halos around them. In particular, Buote & Canizares (1998) use a geometrical test to show the presence of dark matter around three field ellipticals.