How Many Galaxies Built the Coma Cluster's Intracluster Light? A Tidal-Stripping Donor Budget

A rich cluster's intracluster light has a definite mass, and that mass had previous owners. This paper asks, for a Coma-scale diffuse mass of 1.50e+12 solar masses, how many galaxies of each kind must have been stripped to supply it. A massive donor of 6.0e10 solar masses surrendering 35 percent of its stars yields 2.1e+10 solar masses per event, so 71 such donors suffice alone. An intermediate donor of 1.0e10 solar masses surrendering half its stars requires 300 donors, and a dwarf of 5.0e8 solar masses surrendering 80 percent requires 3750. No single class is plausible alone: the massive-only count exceeds the supply of massive galaxies a cluster ever holds, and the dwarf-only count exceeds the dwarf population's total mass budget. A mixed model in which massive, intermediate, and dwarf donors supply 50, 35, and 15 percent of the diffuse mass requires 36, 105, and 562 donors respectively, 703 in total, which is 0.66 of the cluster's present catalogued membership. The mean deposition rate over eight billion years is 188 solar masses per year. The diffuse component is therefore the integrated loss of a substantial fraction of all the galaxies the cluster has ever processed, not the debris of a few spectacular disruptions.