Present observations do provide some precise quantitative information. For example, we have good estimates of the gas mass as a function of radius. We have a good estimates for the mean iron abundance in clusters with the best value around 20% for Coma. We have high quality images of a large sample of clusters and good integrated temperatures for the brighter clusters.
The most poorly known cluster properties are those which require spatially resolved spectroscopic information. For example, the radial distribution of the heavy elements is very poorly known and better observations could provide information relating to the enrichment mechanism and the epoch of enrichment. Similarly, temperature profiles are not known for many systems and hence virial mass determinations (based on gas density and gas temperature profiles) still have large uncertainties. Optical estimates are subject to contamination and sub-structure and X-ray observations seem to provide the most direct method for obtaining detailed mass distributions.
Future missions will provide the missing information to expand our understanding of clusters. Questions we can address with future missions like ASTRO-D, AXAF, XSPECT, and XMM, include:
where is the gas density and r is the radial distance from the cluster center. Only M87 and Centaurus (NGC4696) have measured density and temperature gradients (Fabricant and Gorenstein 1983, Matilsky et al. 1985). For most clusters Tgas(r) remains unknown (or very uncertain). Thus, ASTRO-D, AXAF, and XMM can add immeasurably to our present knowledge of the gravitating mass distributions which can be directly compared to formation models of clusters.
In conclusion, while considerable advances have been made in understanding clusters through their X-ray emission, the potential of X-ray observations has yet to be fully achieved and awaits the application of spatially resolved spectroscopic observations and studies of clusters at high redshifts.