Tracing the evolution in the iron content of the intra-cluster medium
Tozzi, P.; Balestra, I.; Mainieri, V.; Rosati, P.; Ettori, S.; Norman, C.; Borgani, S.; Viola, M.
Germany, Italy, United States
Abstract
Context: 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.
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⪉ z ⪉ 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 2 higher than in hotter clusters, following Z(T)≃ 0.88 T-0.47 Z_⊙, which confirms the trend seen in local samples. We also find a constant average iron abundance Z_Fe≃ 0.25 Z_⊙ as a function of redshift, but only for clusters at z⪆0.5. The emission-weighted iron abundance is significantly higher (Z_Fe≃0.4 Z_⊙) in the redshift range z≃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<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.
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 z≃ 1.2. We confirm that the ICM is already significantly enriched (Z_Fe≃0.25 Z_⊙) 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.