Apparent high metallicity in 3-4 keV galaxy clusters: the inverse iron-bias in action in the case of the merging cluster Abell 2028

TL;DR

This study investigates the apparent high iron abundance in 3-4 keV galaxy clusters, revealing that a bias in spectral fitting, known as the inverse iron bias, can artificially inflate metallicity measurements in complex, multi-temperature ICM structures.

Contribution

The paper demonstrates how the inverse iron bias affects metallicity estimates in galaxy clusters and emphasizes the importance of accounting for multi-temperature structures in spectral analysis.

Findings

The inverse iron bias causes overestimation of metallicity in single-component spectral fits.

Multi-temperature ICM structures lead to biased high metallicity measurements.

Correcting for the bias aligns metallicity values with those typical of hotter clusters.

Abstract

Recent work based on a global measurement of the ICM properties find evidence for an increase of the iron abundance in galaxy clusters with temperature around 2-4 keV up to a value about 3 times larger than that typical of very hot clusters. We have started a study of the metal distribution in these objects from the sample of Baumgartner et al. (2005), aiming at resolving spatially the metal content of the ICM. We report here on a 42ks XMM observation of the first object of the sample, the cluster Abell 2028. The XMM observation reveals a complex structure of the cluster over scale of 300 kpc, showing an interaction between two sub-clusters in cometary-like configurations. At the leading edges of the two substructures cold fronts have been detected. The core of the main subcluster is likely hosting a cool corona. We show that a one-component fit for this region returns a biased high metallicity. This inverse iron bias is due to the behavior of the fitting code in shaping the Fe-L complex. In presence of a multi-temperature structure of the ICM, the best-fit metallicity is artificially higher when the projected spectrum is modeled with a single temperature component and it is not related to the presence of both Fe-L and Fe-K emission lines in the spectrum. After accounting for the bias, the overall abundance of the cluster is consistent with the one typical of hotter, more massive clusters. We caution the interpretation of high abundances inferred when fitting a single thermal component to spectra derived from relatively large apertures in 3-4 keV clusters, because the inverse iron bias can be present. Most of the inferences trying to relate high abundances in 3-4 keV clusters to fundamental physical processes will likely have to be revised.