The analysis of a large sample of X-ray selected nearby galaxy clusters,
allow us to
perform an accurate determination of the fraction of baryon in stars
as a function of cluster mass, to shows that clusters display a scatter
(in the stellar, gas and baryon fraction), showing that the regions
from which clusters and groups collect matter are not yet
representative of the mean Universe content. Stellar and gas mass fractions
at a given cluster mass are compared with theoretical predictions, showing
that X-ray cluster properties can be matched to theoretical models for
parameter choices in major disagreement with stellar fraction measurements.
We also found
that X-ray luminosity and richness (n200, the number of bright red
galaxies) perform identically as mass proxy: both allow to predict
mass with a 0.3 dex accuracy. However, richness is observationally
easier to collect, with obvious cosmological consequences.
We performed an X-ray follow-up of an optically (n200) selected sample of local clusters, showing that all them are real systems, with potential deep enough to hot and retain an intracluster medium. From these observations we infer that at most 6 % (at 90 % confidece) of colour-selected clusters are fake systems, and that X-ray and colour searches sample the same cluster population and thus that we safely use clusters selected with any of the two methods for cosmological purposes.
Finally, we adress the measurement of the evolution of the ICM (Lx-T relation), by observing two, carefully selected, clusters at z about 2, hence maximizing the redshift leverage, finding them about 5 times less luminous than expected if evolution were self-similar in the last three quarter of the Universe age.
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