Like ellipticals, SO galaxies present problems in the detection of a dark matter halo. Like ellipticals, SO galaxies were considered to possess low amounts of gas, but new techniques are able to observe sufficient quantities of gas to determine the rotation curve and hence dark matter, under the standard interpretation for spirals. The general conclusion is that these galaxies also have dark matter halos, but that they may be relatively smaller, as these galaxies are bright.
Estimations of DM in SO galaxies are more promising in these exceptional lenticulars that have a large amount of gas. The gas is distributed in a large outer ring, at about twice the optical radius, often warped with respect to the optical plane, and is possibly of external origin. Van Driel and van Woerden (1991, 1997) have studied gas-rich lenticulars. NGC 2787, 4262 and 5084 have large M/L ratios at twice the optical radius (about 25-30). A problem encountered in this study was that no surface photometry was available to carry out standard analysis, except for NGC 4203. The large DM halos needed were surprising for these gas-rich lenticulars, compared with normal lenticulars. Magnetic fields could again introduce a different interpretation as gaseous rings are subject to magnetic centripetal forces.
NGC 404 has been measured in HI by del Rio et al. (1999), who found a large MHI/LB ratio of the order of 0.2, mainly contained in two rings. The most important fact under our present perspective is that this galaxy declines with a near perfect Keplerian profile, the Keplerian fit being characterized by a correlation coefficient of 0.9 (see Fig. 12 which was interpreted by these authors as due to a central mass concentration with no dark matter).
Figure 12. Observed rotation curve of NGC 404, uncorrected for inclination; crosses: observations, solid squares: Keplerian decline. From del Río et al. (1999). Courtesy of the Astronomical Society of the Pacific.
Pignatelli and Galletta (1997) obtained dynamical models of SO and Sa galaxies that do not need a dark matter halo.