CHEERS The chemical evolution RGS sample

Context. The chemical yields of supernovae and the metal enrichment ofthe intra-cluster medium (ICM) are not well understood. The hot gas inclusters of galaxies has been enriched with metals originating frombillions of supernovae and provides a fair sample of large-scale metalenrichment in the Universe. High-resolution X-ray spectra of clusters ofgalaxies provide a unique way of measuring abundances in the hotintracluster medium (ICM). The abundance measurements can provideconstraints on the supernova explosion mechanism and the initial-massfunction of the stellar population. This paper introduces the CHEmicalEnrichment RGS Sample (CHEERS), which is a sample of 44 bright localgiant ellipticals, groups, and clusters of galaxies observed withXMM-Newton.
Aims: The CHEERS project aims to provide the mostaccurate set of cluster abundances measured in X-rays using this sample.This paper focuses specifically on the abundance measurements of O andFe using the reflection grating spectrometer (RGS) on board XMM-Newton.We aim to thoroughly discuss the cluster to cluster abundance variationsand the robustness of the measurements. Methods: We have selectedthe CHEERS sample such that the oxygen abundance in each cluster isdetected at a level of at least 5σ in the RGS. The dispersivenature of the RGS limits the sample to clusters with sharp surfacebrightness peaks. The deep exposures and the size of the sample allow usto quantify the intrinsic scatter and the systematic uncertainties inthe abundances using spectral modeling techniques.
Results: Wereport the oxygen and iron abundances as measured with RGS in the coreregions of all 44 clusters in the sample. We do not find a significanttrend of O/Fe as a function of cluster temperature, but we do find anintrinsic scatter in the O and Fe abundances from cluster to cluster.The level of systematic uncertainties in the O/Fe ratio is estimated tobe around 20-30%, while the systematic uncertainties in the absolute Oand Fe abundances can be as high as 50% in extreme cases. Thanks to thehigh statistics of the observations, we were able to identify andcorrect a systematic bias in the oxygen abundance determination that wasdue to an inaccuracy in the spectral model.
Conclusions: The lackof dependence of O/Fe on temperature suggests that the enrichment of theICM does not depend on cluster mass and that most of the enrichmentlikely took place before the ICM was formed. We find that the observedscatter in the O/Fe ratio is due to a combination of intrinsic scatterin the source and systematic uncertainties in the spectral fitting,which we are unable to separate. The astrophysical source of intrinsicscatter could be due to differences in active galactic nucleus activityand ongoing star formation in the brightest cluster galaxy. Thesystematic scatter is due to uncertainties in the spatial linebroadening, absorption column, multi-temperature structure, and thethermal plasma models.