Global cluster morphology and its evolution:: X-ray data versus CDM, ΛCDM and CHDM models

The global structure of galaxy clusters and its evolution are tested within a large set of TREESPH simulations aimed to allow a systematic statistical comparison with available X-ray data. Structure tests are based on the so-called power ratios introduced by Buote & Tsai (1995)[ApJ, 452, 522]. The cosmological models considered are flat CDM, Lambda CDM (Omega(Lambda) = 0.7) and CHDM (Omega(h) = 0.2, 1 massive nu). All models are normalized so to provide a fair number density of clusters. For each model we perform a P3M cosmological simulation in a large box, where we select the volumes where the most massive 40 clusters form. Then we go back to the initial redshift and run a hydrodynamical TREESPH simulation for each of them. In this way we can perform a statistical comparison of the global morphology of clusters, expected in each cosmological model, with ROSAT data, using the Student t-test, the F-test and the Kolmogorov-Smirnov test. The last test and its generalization to 2-dimensional distributions are also used to compare the joint distributions of 2 or 3 power ratios. We find that using DM distribution, instead of gas, as was done in some of previous analyses, leads to systematically biased results, as the baryon distribution is substantially less structured than DM distribution. We also find that the cosmological models considered have different behaviours in respect to these tests: Lambda CDM has the worst performance. CDM and the CHDM mixture considered here have similar scores. Although the general trend of power ratio distributions is already fit by these models, a further improvement is expected either from a different DM mix or a non-flat CDM model. (C) 1999 Elsevier Science B.V. All rights reserved.