Conclusion and Outlook

Our knowledge on the astrophysical and cosmological relevance of galaxy clusters has much increased in the last 20-30 yrs, mostly due to the spectacular results of X-ray astronomical observations. With the launch of the Chandra and XMM-Newton observatories the X-ray astrophysics of galaxy clusters has now entered a stage of precision previously only obtained in stellar or galaxy astronomy. This is due to the detailed X-ray spectroscopic capability of the observatories, Chandra's sub-arcsec angular resolution and the high photon collecting power of XMM-

Newton. Many of the results shown in this review nicely demonstrate the progress that has been made possible by these capabilities.

The tremendous impact made by the ROSAT All-Sky Survey, the so far only all-sky survey made with an imaging X-ray telescope, is spuring the dreams of a new, much improved X-ray survey effort. A less costly mission as compared to ROSAT with state-of-the art technology would provide an increased energy band, increased angular resolution, and a sensitivity increase of at least an order of magnitude. Already several proposals have been made for such a mission, but they have so far not been successful. It is interesting to note here that all the recent proposals for such a new X-ray survey had galaxy cluster cosmology as the main driver, underlining the importance that this field has currently gained. The last of these mission studies, for the planned "Dark Universe Observatory (DUO)," has made it through NASA phase A study, demonstrating the cosmological progress that could be made with such an X-ray mission [75,91]. Thus we can be confident that such a survey will be carried out in some form in the near future.

The studies for the next generation of large X-ray observatories following the heritage of Chandra and XMM-Newton involve the NASA plans for Constellation-X and the ESA plans for XEUS [110]. The plans for the latter mission are particularly ambitious in terms of gaining a photon collecting power equivalent to an effective area of ~10m2. With such a large observatory we would be able to perform structural studies and element abundance measurements of galaxy clusters at redshifts as large as z = 2-2.5, beyond which we should only find very poor clusters or X-ray luminous groups. Thus with some luck in the funding of such projects, the success story of X-ray astronomy with galaxy clusters will continue.

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