Annu. Rev. Astron. Astrophys. 1991. 29: 239-274 Copyright © 1991 by Annual Reviews. All rights reserved

5.2 Observations

It is crucial to obtain better limits on the presence of dark halos around elliptical galaxies. We need to know the typical masses and velocity dispersions of these halos, as it is important to distinguish between galaxy-size halos and dark matter in clusters. Some of the tracers (globular clusters, planetary nebulae) that have been used for this purpose also give very interesting information on the rotation of the outer regions, which may provide unique tests for some formation scenarios. The detection of black holes in the nuclei of ellipticals remains a pressing issue, and awaits high-resolution imaging and spectroscopy. The presence of substructure in elliptical galaxies should be further investigated. Typical masses, and dynamical alignments/anti-alignments can provide useful constraints on their formation. The division between elliptical galaxies and S0 galaxies has become fuzzier, as many ellipticals are now known to have weak disks (193). Detailed studies of S0's will provide interesting information on the similarity of these types of systems. The dependence of the global properties on environment is an area of research that has not been explored yet.