Annu. Rev. Astron. Astrophys. 1989. 27: 87-138 Copyright © 1989 by Annual Reviews. All rights reserved

6. THE FUTURE

What we have learned so far about the X-ray properties of normal galaxies has been the result of limited exploratory observations. Future X-ray satellites, with increased sensitivity and higher spatial and spectral resolution, will be essential for answering the many open questions resulting from the present work and for expanding and deepening our knowledge of these systems. The German X-ray satellite ROSAT, which is scheduled to be launched in 1990, will increase the number of galaxies mapped in X rays with a good sensitivity to low-surface-brightness features, and the Japanese ASTRO-D (to be launched in 1993) will allow the study of galactic spectral properties with a tenfold increased spectral resolution (but only 2' spatial resolution). The next major US X-ray astronomy endeavor, AXAF, with its subarcsecond spatial resolution and good spectral resolution, will allow the study of the luminosity function of X-ray sources in nearby spiral galaxies down to limiting luminosities at least 100 times smaller than present ones, and it will be able to detect single early-type stars in the Magellanic Clouds. Spectral parameters or X-ray colors will be measured for these sources and should help in establishing their nature (e.g. black hole candidates vs. X-ray pulsars; see White & Marshall 1984). With AXAF and the European XMM, with its larger collective area and sensitivity to low-surface-brightness features, the astronomical community will be able to address some of the outstanding questions on the X-ray properties of elliptical galaxies. These include firmly establishing the nature of the X-ray emission in the less X-ray-luminous galaxies, i.e. if it comes from a collection of binary X-ray sources or from a hot gaseous halo; measuring temperatures, temperature gradients, and metallicities of these halos; studying their interaction with the surrounding medium; and, finally, measuring with good accuracy the mass of these galaxies. It will also be possible to establish the luminosity functions and the spectral characteristics of individual X-ray sources in different spiral galaxies and thus to investigate the nature and evolution of these sources in different environments. With the plenitude of X-ray data to come from these missions, this may well be the last review that can cover the entire topic of X rays from normal galaxies.

ACKNOWLEDGMENTS

I thank P. Biermann, C. Canizares, M. Elvis, J. Grindlay, D. Helfand, W. Mathews, A. Renzini, C. Sarazin, D. Schwartz, F. Seward, P. Slane, H. Tananbaum, and G. Trinchieri for sending me preprints of their work and for discussions and comments on the manuscript. This work was supported by NASA contract NAS8-30751.