Iron abundance distribution in the hot gas of merging galaxy clusters

Abstract

We present XMM-Newton/EPIC observations of six merging galaxy clusters and study the distributions of their temperature, iron (Fe) abundance and pseudo-entropy along the merging axis. For the first time, we focus simultaneously, and in a comprehensive way, on the chemical and thermodynamic properties of the freshly collided intracluster medium (ICM). The Fe distribution of these clusters along the merging axis is found to be in good agreement with the azimuthally-averaged Fe abundance profile in typical non-cool-core clusters out to $r_{500}$. In addition to showing a moderate central abundance peak, though less pronounced than in relaxed systems, the Fe abundance flattens at large radii towards $\sim$0.2-0.3 $Z_\odot$. Although this shallow metal distribution is in line with the idea that disturbed, non-cool-core clusters originate from the merging of relaxed, cool-core clusters, we find that in some cases, remnants of metal-rich and low entropy cool cores can persist after major mergers. While we obtain a mild anti-correlation between the Fe abundance and the pseudo-entropy in the (lower entropy, $K$ = 200-500 keV cm$^2$) inner regions, no clear correlation is found at (higher entropy, $K$ = 500-2300 keV cm$^2$) outer radii. The apparent spatial abundance uniformity that we find at large radii is difficult to explain through an efficient mixing of freshly injected metals, particularly in systems for which the time since the merger is short. Instead, our results provide important additional evidence in favour of the early enrichment scenario - in which the bulk of the metals are released outside galaxies at $z$ > 2-3 - and extend it from cool-core and (moderate) non-cool-core clusters to a few of the most disturbed merging clusters as well. These results constitute a first step towards a deeper understanding of the chemical history of merging clusters.