In medio stat virtus: Enrichment history in poor galaxy clusters

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano via A.Corti 12 20133 Milano, Italy
² Universitá degli Studi di Milano via G. Celoria 16 20133 Milano, Italy
³ Dipartimento di Fisica e Astronomia DIFA – Università di Bologna via Gobetti 93/2 40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Brera via E. Bianchi 46 23807 Merate (LC), Italy
⁵ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 July 2025
Accepted: 22 November 2025

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, the situation for galaxy groups is less clear, as their abundance profiles sometimes appear 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 M₅₀₀ ≃ 2.0 − 2.5 × 10¹⁴ M_⊙, placing them in an intermediate category between the scales of galaxy groups and massive clusters. For these systems, we also obtained nearly complete azimuthal coverage of their outer regions with XMM-Newton, thus allowing for detailed characterisation of their chemical properties at large radii. We show that, in the outskirts, abundance measurements are more likely to be limited by systematic uncertainties 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. However, once these systematics are properly accounted for, the abundance profiles of all three clusters are found to be consistent with being flat, at a level of Z ∼ 0.3 Z_⊙, in agreement with values observed in massive clusters. Using the available stellar mass estimates for the three systems, we also computed their iron yields, thereby beginning to probe a mass range that remains largely unexplored. We find 𝒴_Fe, 500 = 2.68 ± 0.34, 2.54 ± 0.64, and 7.51 ± 1.47 Z_⊙ for MKW3s, A2589, and Hydra A, respectively, values that span the transition regime between galaxy groups and massive clusters. Future observations of systems with temperatures in the 2–4 keV range will be essential to further populate this intermediate-mass regime and to draw firmer conclusions about the chemical enrichment history of galaxy systems across the full mass scale.