2007A&A...462..429B

We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3, which cover a temperature range of 3≲kT≲15 keV. Our analysis is aimed at measuring the iron abundance in the intra-cluster medium (ICM) out to the highest redshift probed to date. We made use of combined spectral analysis performed over five redshift bins at 0.3≲z≲1.3 to estimate the average emission weighted iron abundance. We applied non-parametric statistics to assess correlations between temperature, metallicity, and redshift. We find that the emission-weighted iron abundance measured within (0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ∼2 higher than in hotter clusters, following Z(T)≃0.88T^–0.47Z_☉, which confirms the trend seen in local samples. We also find a constant average iron abundance Z_Fe≃0.25Z_☉ as a function of redshift, but only for clusters at z>0.5. The emission-weighted iron abundance is significantly higher (Z_Fe≃0.4Z_☉) in the redshift range z≃0.3-0.5, approaching the value measured locally in the inner 0.15R_vir radii for a mix of cool-core and non cool-core clusters in the redshift range 0.1<z<0.3. The decrease in metallicity with redshift can be parametrized by a power law of the form ~(1+z)^–1.25. We tested our results against selection effects and the possible evolution in the occurrence of metallicity and temperature gradients in our sample, and we do not find any evidence of a significant bias associated to these effects. The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of ∼2 larger than at z≃1.2. We confirm that the ICM is already significantly enriched (Z_Fe≃0.25Z_☉) at a look-back time of 9Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.