X-ray clusters: towards a new determination of the density parameter of the universe

We use a self–consistent modeling of X-ray cluster properties to constrain cosmological scenarios of structure formation in the case of open cosmological models. We first show that an unbiased open model can reproduce present day observations, provided that the density parameter is in the range 0.15−0.4. Although this estimate is derived in a rather different way, it is very close to dynamical measurements.We also obtain constraints on the shape of the initial fluctuation power spectrum: we find that the local spectral index n falls in the range −1.1 ≤ n ≤ −1.6 on scales between 10 and 30h−1Mpc. We focus on differences between open models and the Einstein–de Sitter model, and we exhibit a test which depends only on the cosmological parameter and is insensitive to the shape of the initial fluctuations power spectrum: we show that, contrary to what is generally admitted, the redshift distribution of X-ray clusters selected on the basis of their apparent temperature is almost independent of the power spectrum and that it is only sensitive to the value of Ω0. Since the relation between the virial mass and the X-ray temperature seems rather robust, we expect very little theoretical uncertainty in the modeling. This allows us to show that the combined knowledge of the redshift distribution of Xray flux selected clusters and that of the luminosity–temperature correlation at high redshifts offers a new and efficient way to measure the mean density of the universe. This appears to be a clean test of Ω0, not suffering of the problems attached to local density estimators. We have tentatively applied our test to the redshift distribution of clusters detected within the Einstein medium sensitivity X-ray survey. As temperature information is not available for these clusters, no definitive conclusion can be reached. Taking into account the selection function, we find that the observed redshift distribution can be equallywell fittedwith a non– evolving LX − T correlation in the case of the critical universe or a substantial negative evolution in the case of a low–density universe. Our test appears then to be able to provide a definitive answer to the question concerning the density of the universe, provided that the LX − T relation is measured at a redshift of Send offprint requests to: J. Oukbir the order of 0.4 − 0.5 and that temperature measurements can be achieved with an accuracy of the order of 20%.