7.3 Gas Disks
Other methods to infer the mass distribution in ellipticals have been explored, although most give rather inconclusive results. Some ellipticals contain disks of ionized gas which can be exploited for this purpose (e.g. de Zeeuw 1991). Caldwell, Kirshner and Richstone (1986) studied such a disk in NGC 7097 and found that the mass-to-light ratio increased from less than unity in the galaxy center to over 3 at 3.5 kpc. This requires DM, but assumes that the gas in the disk is on circular orbits. Bertola et al. (1991) carried out a similar study of NGC 5077 and concluded that circular orbits required the presence of DM. However, they also showed that the motions could be understood without resorting to a dark halo if the assumption of circular motions was dropped.
Some ellipticals contain disks of cold HI gas (e.g. van Gorkom 1992). The HI typically extends much further than the ionized gas disks mentioned above and can also be used as a probe of DM (Lake, Schommer and van Gorkom 1987). For 5 such galaxies, Kent (1990) compared central mass-to-light ratios based on stellar velocity dispersions with values derived from the gas disks. Only IC 2006 showed an increase in M/L. This galaxy is surrounded by a counter-rotating ring of HI. Schweizer, van Gorkom and Seitzer (1989) find that (M/LB) rises from around 5 in the center to 16 at the ring radius, assuming the ring is circular. This suggests the presence of a dark halo with about twice as much mass within this radius as the stellar component. However, Lees (1991) finds the increase can be reduced if the orbits are elliptic and claims that such orbits along with a triaxial halo provide a better fit to the data.
The evidence for DM in ellipticals is compelling, although not as overwhelming as is the case for DM in spirals. The problem is that the techniques for probing DM in ellipticals tend to be model-dependent, so it is often possible to find gravitational potentials that are consistent with the data and which do not require dark halos. However, the above results suggest that it is extremely likely that elliptical galaxies are surrounded by significant amounts of DM.