Annu. Rev. Astron. Astrophys. 2002. 40: 539-577
Copyright © 2002 by . All rights reserved

Next Contents Previous

6. OUTLOOK AND FUTURE WORK

Considerable observational progress has been made in tracing the evolution of global physical properties of galaxy clusters as revealed by X-ray observations. The ROSAT satellite has significantly contributed to providing the statistical samples necessary to compute the space density of clusters in the local Universe and its evolution. A great deal of optical spectroscopic studies of these samples has consolidated the evidence that the bulk of the cluster population has not evolved significantly since z ~ 1. However, the most X-ray luminous, massive systems do evolve. Similarly, the thermodynamical properties of clusters as indicated by statistical correlations, such as the LX - TX relation, do not show any strong evolution. Moreover, the Chandra satellite has given us the first view of the gas distribution in clusters at z > 1; their X-ray morphologies and temperatures show that they are already in an advanced stage of formation at these large lookback times.

These observations can be understood in the framework of hierarchical formation of cosmic structures, with a low density parameter, Omegam ~ 1/3, dominated by cold dark matter: structure formation started at early cosmic epochs and a sizable population of massive clusters was in place already at redshifts of unity. In addition, detailed X-ray observation of the intra-cluster gas show that the physics of the ICM needs to be regulated by additional non-gravitational processes.

With Chandra and Newton-XMM, we now realize that physical processes in the ICM are rather complex. Our physical models and numerical simulations are challenged to explain the new level of spatial details in the density and temperature distribution of the gas, and the interplay between heating and cooling mechanisms. Such complexities need to be well understood physically before we can use clusters as high-precision cosmological tools, particularly at the beginning of an era in which cosmological parameters can be derived rather accurately by combining methods that measure the global geometry of the Universe (the CMB spectrum, type Ia Supernovae (e.g. Leibungut 2001)), and the large-scale distribution of galaxies (e.g. Peacock et al. 2001). It remains remarkable that the evolution of the cluster abundance, the CMB fluctuations, the type Ia Supernovae and large scale structure - all completely independent methods - converge toward Omegam appeq 0.3 in a spatially flat Universe (Omegam + Omegalambda = 1). Further studies with the current new X-ray facilities will help considerably in addressing the issue of systematics discussed above, although some details of the ICM in z gtapprox 1 clusters, such as temperature profiles or metallicity, will remain out of reach until the next generation of X-ray telescopes. Direct measurements of cluster masses at z gtapprox 1 via gravitational lensing techniques will soon be possible with the Advanced Camera for Surveys (Ford et al. 1998) on-board the Hubble Space Telescope, which offers an unprecedented combination of sensitivity, angular resolution and field of view.

The fundamental question remains as to the mode and epoch of formation of the ICM. When and how was the gas pre-heated and polluted with metals? What is the epoch when the first X-ray clusters formed, i.e. the epoch when the accreted gas thermalizes to the point at which they would lie on the LX-T relation (Figure 14)? Are the prominent concentrations of star forming galaxies discovered at redshift z ~ 3 (Steidel et al. 1998) the progenitors of the X-ray clusters we observed at z ltapprox 1 ? If so, cluster formation should have occurred in the redshift range 1.5-2.5. Although the redshift boundary for X-ray clusters has receded from z = 0.8 to z = 1.3 recently, a census of clusters at z appeq 1 has just begun and the search for clusters at z > 1.3 remains a serious observational challenge. Using high-z radio galaxies as signposts for proto-clusters has been the only viable method so far to break this redshift barrier. These searches have also lead to the discovery of extended Lyalpha nebulae around distant radio galaxies (e.g., Venemans et al. 2002), very similar to those discovered by Steidel et al. (2000) in correspondence with large scale structures at z appeq 3. The nature of such nebulae is still not completely understood, however they could represent the early phase of collapse of cool gas through mergers and cooling flows.

In this review we have not treated the formation and evolution of the galaxies in clusters. This must be linked to the evolution of the ICM and the fact that we are still treating the two aspects as separate points to the difficulty in drawing a comprehensive unified picture of the history of cosmic baryons in their cold and hot phase. Multiwavelength studies are undoubtedly essential to reach such a unified picture. When surveys exploiting the Sunyaev-Zeldovich effect (e.g. Carlstrom et al. 2001) over large solid angles become available, one will be able to observe very large volumes at z > 1. In combination with a deep large area X-ray survey (e.g. Wide Field X-ray Telescope, Burrows et al. 1992) and an equivalent deep near-IR survey (e.g. the Primordial Explorer (PRIME), Zheng et al. 2002), this could reveal the evolutionary trends in a number of independent physical parameters, including: the cluster mass, the gas density and temperature, the underlying galactic mass and star formation rates. Advances in instrumentation and observational technique will make this approach possible and will provide vital input for models of structure formation and tight constraints on the underlying cosmological parameters.


ACKNOWLEDGMENTS

We acknowledge useful discussions with Hans Böhringer, Alfonso Cavaliere, Guido Chincarini, Roberto Della Ceca, Stefano Ettori, Gus Evrard, Isabella Gioia, Luigi Guzzo, Brad Holden, Silvano Molendi, Chris Mullis, and Adam Stanford. We thank Paolo Tozzi for his help in producing Figure 10. PR thanks Riccardo Giacconi for continuous encouragement of this work. PR is grateful for the hospitality of the Astronomical Observatory of Trieste. SB and CN acknowledge the hospitality and support of ESO in Garching.

Next Contents Previous