The Missing Metal Problem in Galaxy Clusters: Characterizing the Early Enrichment Population

Abstract

Rich and poor galaxy clusters have the same measured halo metallicity, 0.35–0.4 Z ⊙, even though they are an order of magnitude apart in stellar fraction, M */M gas. The measured intracluster medium (ICM) metallicity in high-mass clusters cannot be explained by the visible stellar population as stars typically make up 3%–20% of the total baryon mass. The independence of metallicity of M */M gas suggests an external and universal source of metals such as an early enrichment population (EEP). Galaxy cluster RX J1416.4+2315, classified as a fossil system, has a stellar fraction of M */M gas = 0.054 ± 0.018, and here we improve the halo metallicity determination using archival Chandra and XMM-Newton observations. We determine the ICM metallicity of RXJ1416 to be 0.303 ± 0.053 Z ⊙ within 0.3 < R/R 500 < 1, excluding the central galaxy. We combine this measurement with other clusters with a wider range of M */M gas, resulting in the fit of Z tot = (0.36 ± 0.01) + (0.10 ± 0.17)(M */M gas). This fit is largely independent of M */M gas and shows that for a low M */M gas system, the observed stellar population can make only 10%–20% of the total metals. We quantify the Fe contribution of the EEP further by adopting a standard Fe yield for visible stellar populations, and find that Z EEP = (0.36 ± 0.01) − − (0.96 ± 0.17)(M */M gas). To account for the observed Fe mass, a supernova (SN) rate of 10 ± 5 SNe yr−1 (Type Ia) and 40 ± 19 SNe yr−1 (core collapse) is required over the redshift range 3 < z < 10 for a single galaxy cluster with mass ∼3 × 1014 M ⊙ at z = 0. These SNe might be visible in observations of high-redshift clusters and protoclusters with the James Webb Space Telescope.