DEFICIT OF DISTANT X-RAY-EMITTING GALAXY CLUSTERS AND IMPLICATIONS FOR CLUSTER EVOLUTION

The ROSAT International X-ray Optical Survey (RIXOS) was aimed at the optical identification of a complete sample of ~400 serendipitous X-ray sources found in 81 northern ROSAT fields, achieved using an International Time award on the Canarian Telescopes. Fields at high Galactic latitude (b>+28°) were selected with exposure times longer than 8000 seconds achieving a limiting flux optimized for wide-area optical follow-up. In total, 385 X-ray sources were catalogued over 20.4 deg² to a limiting flux of f_X>=3e+10^–14 erg/s/cm² in the 0.5–2.0 keV energy band.

An overview of the various stages of data preparation and acquisition for RIXOS included: source searching and positional calibration of the X-ray images, the construction of finding charts around each of the sources using digitised sky-survey plates, a search for previously-known catalogued sources from on-line services, deep imaging of the optically empty fields using the Nordic Optical Telescope and the INT, spectroscopic observation of the brighter sources with the INT and of the fainter ones with the WHT, and, finally, multicolour imaging photometry of extended or interesting objects using the JKT. The results of the RIXOS survey provided a sample which is complete over 15 deg2 of sky, including 319 X-ray sources of which the largest population is of Active Galactic Nuclei (AGN), followed by stars, clusters of galaxies, Emission Line Galaxies (ELG), and finally, just one 'normal' galaxy.

The most significant scientific result from the survey was the deficit of distant X-ray-emitting galaxy clusters found. Clusters of galaxies are the largest gravitationally bound systems in the Universe and therefore provide important constraints on the formation and evolution of large-scale structure. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution, and the intracluster gas is compressed and shock-heated by the gravitational collapse. If the gas is in hydrostatic equilibrium the resulting X-ray emission is related in a simple way to the evolving gravitational potential. But if processes such as radiative cooling or pre-collapse heating of the gas are also important, the X-ray evolution will be strongly influenced by the thermal history of the gas. In the RIXOS project very few distant clusters were identified, and their redshift distribution seems to be inconsistent with simple models based on the evolution of the gravitational potential. These results thus suggest that radiative cooling or non-gravitational heating of intracluster gas must be important in the evolution of clusters.