Characterizing the Early Enrichment Population - A Potential Solution to the Missing Metal Conundrum

Rich and poor galaxy clusters have the same measured halo metallicity, 0.35-0.4 Z_solar, even though they are an order of magnitude apart in stellar fraction, M_*/M_gas. The measured intracluster medium (ICM) metallicity in high-mass (low M_*/M_gas) clusters cannot be explained by the visible stellar population, as stars typically make up 3%-20% of the total baryon mass. The independence of ICM metallicity (Z_ICM) and M_*/M_gas suggests an external and universal source of metals such as an early enrichment population (EEP). We posit the EEP to be a high-mass population of stars made up of primarily Population II stars that dominated during the reionization era ~6<z<10. Here we present the base theory around the EEP, and provide further constraints on the population. The trend of Z_ICM vs M_*/M_gas, as determined in our previous work using 27 galaxy groups and clusters, may be used to predict the total amount of metals needed from the EEP (Z_EEP), if Z_star vs M_*/M_gas is known. The contribution of the EEP to the observed ICM metallicity may then be used to predict the number and trend of supernova Type-Ia in a single cluster as a function of redshift. These SNe might be visible in observations of high-redshift clusters and protoclusters with the James Webb Space Telescope. Further constraints may be placed on the initial mass function (IMF) of the EEP. It is currently assumed the IMF must be top-heavy in order to produce a high amount of metals, however the exact mass cutoffs and slope(s) of the IMF are not known. Any remnants of the population likely exist in dwarf elliptical galaxies in clusters; the luminosity of these systems in a galaxy cluster will constrain the low-mass end of the EEP IMF. The high-mass end of the IMF is constrained by the ICM metal abundance found in galaxy groups (high M_*/M_gas) as Z_EEP may not exceed observations.