In medio stat virtus: enrichment history in poor galaxy clusters

TL;DR

This study targets the chemical enrichment history of the intracluster medium by measuring iron-abundance profiles in three intermediate-mass, nearby clusters (MKW3s, A2589, Hydra A) using azimuthally complete XMM-Newton data. A two-step spectral analysis explicitly models the soft X-ray background and instrumental background, with azimuthal background sampling to mitigate systematics that bias Fe L-shell measurements in the outskirts. After accounting for XRB systematics, the clusters exhibit flat iron profiles out to ~0.8 $R_{500}$ at $Z_{Fe} \sim 0.3\,Z_{\odot}$, consistent with enrichment histories observed in more massive systems and supporting a common, early enrichment scenario across mass scales. The authors combine X-ray iron masses with optical stellar masses to derive iron yields, finding intermediate values that bridge the group and cluster regimes, but highlighting substantial systematic uncertainties, particularly in stellar masses. The work underscores the necessity of full azimuthal coverage and careful background treatment to obtain reliable outskirts abundances and lays groundwork for expanding the intermediate-mass regime with future facilities like XRISM and Athena.

Abstract

The enrichment history of galaxy clusters and groups remains far from being fully understood. Recent measurements in massive clusters have revealed remarkably flat iron abundance profiles out to the outskirts, suggesting that similar enrichment processes have occurred for all systems. In contrast, abundance profiles in galaxy groups have sometimes been measured to decline with radius, challenging our understanding of the physical processes at these scales. In this paper, we present a pilot study aimed at accurately measuring the iron abundance profiles of MKW3s, A2589, and Hydra A, three poor clusters with total masses of $M_{500} \simeq 2.0-2.5 \times 10^{14}$ M$_\odot$, intermediate between the scales of galaxy groups and massive clusters. Using XMM-Newton to obtain nearly complete azimuthal coverage of the outer regions of these systems, we show that abundance measurements in the outskirts are more likely to be limited by systematics than by statistical errors. In particular, inaccurate modelling of the soft X-ray background can significantly bias metallicity estimates in regions where the cluster emission is faint. Once these systematics are properly accounted for, the abundance profiles of all three clusters appear to be flat at $Z \sim 0.3$ Z$_{\odot}$, in agreement with values observed in massive clusters. Using available stellar mass estimates, we also computed their iron yields, thereby beginning to probe a largely unexplored mass range. We find $Y_{Fe,500} = 2.68\pm0.34$, $2.54\pm0.64$, and $7.51\pm1.47$ Z$_{\odot}$ for MKW3s, A2589, and Hydra A, respectively, spanning the transition regime between galaxy groups and massive clusters. Future observations of systems with temperatures of $2-4$ keV will be essential to further populate this intermediate-mass regime and to draw firmer conclusions on the chemical enrichment history of galaxy systems across the full mass scale.

Figures (14)

• Figure 1: XMM-Newton/EPIC count-rate images of MKW3s (top), A2589 (centre), and Hydra A (bottom) in the $0.7-1.2$ keV band. The red dashed circles mark $R_{500}$ for each cluster. The green circles are the regions used to estimate the contamination from the X-ray sky background. The white dashed region in the bottom panel has been masked to remove residual thermal emission associated with LEDA 87445. A Gaussian filter smooths the images for visual purposes only.
• Figure 2: Relative contribution between source emission and background components, in the outskirts of MKW3s. Black data are the XMM-Newton/MOS 2 spectrum, extracted from the ring with $r = 10' - 12'$ (i.e. $0.57-0.68~R_{500}$). Yellow is used to highlight the best-fitting model for the source emission; the Fe L-shell blend is visible at $\sim 1$ keV. Red, green, light blue, and purple are used to mark the LHB, the GH, the NPS, and the CXB, estimated from the region named bkg1. Dotted lines are the model adopted to describe the particle-induced background, which includes power laws and Gaussian lines. The solid black lines show the resulting model for the X-ray sky components (i.e. the source plus the XRB) and the particle-induced background.
• Figure 3: Temperature (left) and iron abundance (right) profiles for MKW3s, A2589 and Hydra A. Blue dots, orange squares, and green stars represent measurements obtained with azimuthally averaged XRB parameters. The effect of modelling the XRB contamination from individual regions is shown by the shaded areas, which capture the maximum dispersion of the measured profiles shown in Fig. \ref{['individual_prof']}. As mentioned in the text, the measurements derived from individual XRB modelling for both A2589 and Hydra A do not extend to the outermost radial bins due to the very low source signal which makes them difficult to constrain.
• Figure 4: Top panels: Soft X-ray background models obtained from individual offset regions around each cluster (see Fig. \ref{['clusters']}). From left to right: MKW3s, A2589, and Hydra A. Each model results from the combination of the parameters shown in Figs. \ref{['XRB_mkw3s']}, \ref{['XRB_a2589']}, and \ref{['XRB_hydra']}, and is colour-coded according to the metal abundance derived in the outermost bin for each cluster, using that particular parameterisation. Bottom panels: ratio of each individual XRB model to the azimuthally averaged model for each cluster. Models between 1.2 and 1.9 keV are indicated with dotted lines, as this energy range was excluded from the fitting procedure.
• Figure 5: Same as Fig. \ref{['fg_models']}, but for the reconstructed source models.