Tracing the evolution in the iron content of the intra-cluster medium

Context. We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z greater than or similar to 0.3, which cover a temperature range of 3 less than or similar to kT less than or similar to 15 keV. Aims. Our analysis is aimed at measuring the iron abundance in the intra-cluster medium (ICM) out to the highest redshift probed to date. Methods. We made use of combined spectral analysis performed over five redshift bins at 0.3 less than or similar to z less than or similar to 1.3 to estimate the average emission weighted iron abundance. We applied non-parametric statistics to assess correlations between temperature, metallicity, and redshift. Results. 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 similar to 2 higher than in hotter clusters, following Z(T) similar or equal to 0.88 T-0.47 Z(circle dot), which confirms the trend seen in local samples. We also find a constant average iron abundance Z(Fe) similar or equal to 0.25 Z(circle dot) as a function of redshift, but only for clusters at z greater than or similar to 0.5. The emission-weighted iron abundance is significantly higher (Z(Fe) similar or equal to 0.4 Z(circle dot)) in the redshift range z similar or equal to 0.3-0.5, approaching the value measured locally in the inner 0.15 R-vir radii for a mix of cool-core and non cool-core clusters in the redshift range 0.1 less than or similar to z less than or similar to 0.3. The decrease in metallicity with redshift can be parametrized by a power law of the form similar to(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. Conclusions. The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of similar to 2 larger than at z similar or equal to 1.2. We confirm that the ICM is already significantly enriched (ZFe similar or equal to 0.25 Z(circle dot)) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.