Context : We present the analysis of the baryonic content of 52 X-ray luminous galaxy clusters observed with Chandra in the redshift range 0.3 – 1.273 .

Aims : Our study aims at resolving the gas mass fraction in these objects to place constraints on the cosmological parameters \Omega _ { m } , \Omega _ { \Lambda } and the ratio between the pressure and density of the dark energy , w .

Methods : We deproject the X-ray surface brightness profiles to recover the gas mass profiles and fit a single thermal component to the spectrum extracted from a region around the cluster that maximizes the signal-to-noise ratios in the observation .
The measured values of the gas temperature are used to evaluate the temperature profile with a given functional form and to estimate the total gravitating mass in combination with the gas density profiles .
These measured quantities are then used to statistically estimate the gas fraction and the fraction of mass in stars .
By assuming that galaxy clusters are representative of the cosmic baryon budget , the distribution of the cluster baryon fraction in the hottest ( T _ { gas } > 4 keV ) systems as a function of redshift is used to constrain the cosmological parameters .
We discuss how our constraints are affected by several systematic effects , namely the isothermality , the assumed baryon fraction in stars , the depletion parameter and the sample selection .

Results : By using only the cluster baryon fraction as a proxy for the cosmological parameters , we obtain that \Omega _ { m } is very well constrained at the value of 0.35 with a relative statistical uncertainty of 11 % ( 1 \sigma level ; w = -1 ) and a further systematic error of about ( -6 , +7 ) % .
On the other hand , constraints on \Omega _ { \Lambda } ( without the prior of flat geometry ) and w ( using the prior of flat geometry ) are definitely weaker due to the presence of greater statistical and systematic uncertainties ( of the order of 40 per cent on \Omega _ { \Lambda } and greater than 50 per cent on w ) .
If the WMAP 5-year best-fit results are assumed to fix the cosmological parameters , we limit the contributions expected from non-thermal pressure support and ICM clumpiness to be lower than about 10 per cent , also leaving room to accommodate baryons not accounted for either in the X-ray emitting plasma or in stars of the order of 18 per cent of the total cluster baryon budget .
This value is lowered to zero for a no-flat Universe with \Omega _ { \Lambda } > 0.7 .

Conclusions :