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

¹ Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: balestra@mpe.mpg.de
² INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ INFN, National Institute for Nuclear Physics, Trieste, Italy
⁴ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁶ Dipartimento di Astronomia dell'Università di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
⁷ Department of Physics and Astronomy, Johns Hopkins University, MD 21218 Baltimore, USA

Received: 8 May 2006
Accepted: 20 September 2006

Abstract

Context.We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at , which cover a temperature range of  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 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 in clusters below 5 keV is, on average, a factor of ~2 higher than in hotter clusters, following , which confirms the trend seen in local samples. We also find a constant average iron abundance as a function of redshift, but only for clusters at . The emission-weighted iron abundance is significantly higher () in the redshift range , approaching the value measured locally in the inner radii for a mix of cool-core and non cool-core clusters in the redshift range . The decrease in metallicity with redshift can be parametrized by a power law of the form ~. 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 ~2 larger than at . We confirm that the ICM is already significantly enriched () 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.