Properties of the intra cluster medium at high redshift P. Tozzi a , S. Ettori b , P. Rosati b and S. Borgani c a INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I–34131, Trieste, Italy b ESO, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany c Dip. di Astronomia, Universit` a di Trieste, via G.B. Tiepolo 11, I-34131 Trieste, Italy We analyze the X–ray properties of the most distant clusters of galaxies (0.3 <z< 1.3) observed so far with the Chandra and XMM satellites. Our analysis is aimed at deriving the properties of the Intra Cluster Medium (ICM) out to the highest redshifts probed to date. In particular, we investigate the evolution of the iron abundance in the ICM with cosmic epoch. We find that the mean iron abundance at 〈z〉 =0.8 is ZFe =0.25 +0.04 −0.06 Z⊙, and at 〈z〉∼ 1.2 is ZFe =0.35 +0.06 −0.05 Z⊙ (5 clusters at z> 1), both measures consistent with the local value ZFe ≃ 0.3 Z⊙, within 1σ confidence level, thus suggesting no evolution of the mean iron abundance out to z ≃ 1.3. The X–ray scaling properties show hints of negative evolution in the L bol –T and Mgas –T relations, and a positive evolution in the Entropy–T relation. Together, these results can provide interesting constraints on the thermodynamics of the ICM at large lookback times, pointing towards a redshift z ∼ > 2 for the onset of non–gravitational processes. 1. X-RAY CLUSTERS OF GALAXIES AT HIGH Z During the last four years, the X–ray satel- lites Chandra and XMM provided several rele- vant novelties in the field of X–ray clusters of galaxies. Observations of nearby clusters allowed one to detect cold fronts in the ICM, hot bub- bles, lack of low–temperature gas (kT ∼ < 1 keV) in cool cores, and other unexpected features (see [1–4]). These observations show that the ther- modynamics of the ICM are strongly affected both by major merger and by non–gravitational processes (like heating by stellar winds, super- novae (SNae) explosions, interaction with jets from AGN, radiative cooling, thermal conduc- tion, magnetic fields). The observations of X–ray bright, nearby clusters, are complemented by the study of the global X–ray properties in distant clusters (z ∼ > 0.4), to investigate possible evolu- tion of the X–ray scaling relations. Such an evo- lution, if detected, would provide information on the epoch of the non–gravitational heating pro- cesses. However, the number of high–z clusters known from previous surveys is still small. In particular, only five bona-fide X–ray cluster of galaxies are known at z> 1, four from the RDCS survey [5–7] and one from the Lockman Hole sur- vey [8], while serendipitous surveys with Chan- dra and XMM are still in the early days. In this perspective, we started a systematic analy- sis of the highest–z clusters of galaxies observed with Chandra (and, for three at z > 1, also with XMM). In particular, we measured the aver- age iron abundance Z Fe [9], and investigated the X–ray scaling relations between luminosity L bol , temperature T , gas mass M gas , and entropy S [10]. In these Proceedings we briefly summarize the main results obtained so far, with the inclu- sion of RXJ1252 (z =1.237) that was recently analyzed [11]. 2. AVERAGE IRON ABUNDANCE IN THE ICM A number of observations concerning the X–ray scaling properties in clusters [12,13] suggests that non–gravitational processes that significantly af- fected the thermodynamics of the ICM, must have taken place in the past [14–18]. The dis- cussion on the nature and the timescale of such non–gravitational heating is still open. In princi- ple, the presence of metals in the ICM can be used to trace the history of star formation in cluster Nuclear Physics B (Proc. Suppl.) 132 (2004) 48–53 0920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved. www.elsevierphysics.com doi:10.1016/j.nuclphysbps.2004.04.006