Tracing Early Cosmic Chemical Enrichment: A Uniform XMM-Newton Survey of Metallicity in Galaxy Groups and Clusters

TL;DR

The paper tests whether the metals in the intracluster medium ($Z_{ICM}$) can be produced by present-day stellar populations within a closed-box framework, and finds a systematic shortfall that indicates an early enrichment population (EEP). Using a uniform XMM-Newton survey of 26 groups and clusters, the authors measure radial $Z_{ICM}$ profiles, derive $Z_*$ from updated supernova yields with remnant corrections, and compute the residual $Z_{EEP} = Z_{ICM} - Z_*$. They report a robust linear relation between $Z_{ICM}$ and $\log(M_*/M_{gas})$ with an intrinsic scatter, and show that $Z_{EEP}$ remains necessary even when testing the closed-box assumption by excluding groups; three anomalous systems are discussed as late-forming with distinct metallicity gradients. The results quantify the need for an early enrichment population and provide a framework to test cluster chemical evolution models, linking the metal budget to formation history and IMF assumptions via $Z_* = (1.14 \pm 0.52) \log\left(1 + \frac{M_*}{M_{gas}}\right)$. Overall, the study constrains the origin and properties of early enrichment and informs models of cosmic chemical evolution in large-scale structures.

Abstract

Observed metal abundances in the intracluster medium (ICM) of galaxy groups and clusters, $Z_{ICM}$, exceed what is expected from present-day stellar populations alone. Galaxy clusters are presumed to be near closed-box systems, allowing constraints to be placed on the origins of metals and stellar populations responsible for $Z_{ICM}$. We present a uniform XMM-Newton survey of 26 galaxy groups and clusters, measuring radial metallicity profiles and relating $Z_{ICM}$ with the stellar fraction $M_*/M_{gas}$. We determine $Z_{ICM}$ via spectral fitting across multiple annuli finding a best fit of $Z_{ICM} = -0.08^{+0.07}_{-0.07}\, log\left(\frac{M_*}{M_{gas}}\right) + 0.30^{+0.06}_{-0.06}$ with intrinsic scatter $σ_p = 0.09^{+0.02}_{-0.01}$. We use closed-box chemical evolution models to estimate the metallicity yield from observable stellar populations, incorporating updated supernova yields and corrections for metals locked in remnants, $Z_* = (1.14 \pm 0.52) \, log\left(1 + \frac{M_*}{M_{gas}}\right)$. Our results demonstrate that present-day stellar populations systematically underpredict $Z_{ICM}$, with an inferred excess component increasing in systems with low $M_*/M_{gas}$. This trend supports the need for an early enrichment population (EEP) distinct from visible stars, $Z_{EEP}$. We find this necessity holds when reconsidering the closed-box assumption by removing all galaxy groups, potential leaky systems, deriving $Z_{EEP}$ within $1σ$ when including and excluding groups. Three systems (NGC1132, NGC5098, and NGC4325) deviate from the survey trend, exhibiting steep negative radial metallicity gradients and unusually low $Z_{ICM}$. We posit these systems to be late-forming whose ICM enrichment reflects only recent stellar populations. Our analysis quantifies the necessity of an EEP and provides trends for testing cluster chemical evolution models.

Figures (8)

• Figure 1: Surface brightness profiles of AWM4 (left) and NGC4104 (right). The blue highlighted region shows where the background region was defined.
• Figure 2: Radial trend of the fit metallicity for all clusters. We show the weighted average (black points) and median (blue points) within each bin. The median trend flattens out to a mean metallicity of $0.37 Z_\odot$ in the outer radial bins. The weighted average trend shows a negative trend of $Z_{ICM}$ with radius best described by a line with slope $-0.70 \pm 0.04$ and intercept $0.57 \pm 0.01$.
• Figure 3: Left: We present the fit relationship of $Z_{ICM}$ vs $M_*/M_{gas}$ for all 26 groups and clusters. We find $Z_{ICM} = -0.08^{+0.07}_{-0.07}\, log(\frac{M_*}{M_{gas}}) + 0.30^{+0.06}_{-0.06}$ with an intrinsic scatter of $\sigma_p = 0.09^{+0.02}_{-0.01}$ (red solid line). This fit was done using techniques described in Sharma2017. The derived trend of $Z_*$ (green dash-dot, Equation \ref{['eq:z_star_yield']}) is subtracted from $Z_{ICM}$ to find $Z_{EEP} = log_{10}\left[ \left(\frac{M_*}{M_{gas}}\right)^{-0.08^{+0.07}_{-0.07}} \left(1 + \frac{M_*}{M_{gas}}\right)^{1.14 \pm 0.52} \right] + 0.30^{+0.06}_{-0.06}$ (blue dotted line). All shaded regions show the $1\sigma$ confidence intervals. Right: Total fit excluding NGC5098, NGC4325, and NGC1132 to determine a fit of $Z_{ICM}$ versus $M_*/M_{gas}$ as $Z_{ICM} = 0.06^{+0.06}_{-0.05}\, log(\frac{M_*}{M_{gas}}) + 0.46^{+0.05}_{-0.05}$ and $\sigma_p = 0.05^{+0.02}_{-0.01}$, shown as the red solid. We subtract off the derived $Z_*$ (green dash-dot line) to derive $Z_{EEP} = log_{10}\left[ \left(\frac{M_*}{M_{gas}}\right)^{0.06^{+0.06}_{-0.05}} \left(1 + \frac{M_*}{M_{gas}}\right)^{1.14 \pm 0.52} \right] + 0.46^{+0.05}_{-0.05}$ (blue dotted line). The derived reduced $\chi^2$ is $\sim 0.90$ for all four fits, however the required $\sigma_p$ (printed in the top right corner of all plots) to keep the $\chi^2$ at an acceptable value decreases as the number of potential outliers excluded increases.
• Figure 4: Posterior distribution of $\eta^{SN}$. The black vertical line shows the mean value of $0.0035$, with 1$\sigma$ shown by the dashed red lines.
• Figure 5: Relationship between the derived $y$, the yield from SNe when accounting for metals locked up in stars and remnants, as a function of assumed mean BH remnant mass, $M_{BH}^{rem}$. All resulting $y$ values are within $1\sigma$ indicating only a weak dependence on $M_{BH}^{rem}$.