5. FUTURE PROGRESS

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:

• measurements of T(r) - combined with surface brightness profiles (which can be used to determine the gas density profile), the temperature profiles will give precise estimates for the gravitating mass as a function of radius. Assuming hydrostatic equilibrium and spherical symmetry, the gravitating mass is given by

  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 T_gas(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.

• heavy element abundance determinations - these should provide constraints on some cosmological models, e.g., cold dark matter.

• radial distribution of iron - since the stellar light (and therefore mass) declines more rapidly than the gas mass as a function of radius, we might expect to find a radial decline in the iron abundance if the iron originated from now dead stellar systems in the galaxies we see today. Other enrichment scenarios could produce differing radial dependences.

• Determine abundances in clusters at a wide range of redshifts - if infall is important, then we might expect abundances to increase with redshift. The dependence of abundance with redshift will provide important information on the epoch of enrichment of the ICM and on the evolution of galaxies which are the most likely source of the enriched material.

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.