Coma's Bluing Gradient: The Intracluster Colour Profile as a Metallicity Gradient and a Changing Donor Mix

The intracluster light of the Coma cluster becomes bluer with radius, and that gradient is a record of where its stars came from. Adopting published g-r colours of 0.82 at 10 kpc, 0.74 at 50 kpc, and 0.63 at 150 kpc, the total colour change is -0.19 magnitudes. Read with standard simple-stellar-population response slopes, the gradient corresponds to a metallicity decline of -0.63 dex if age is constant, or an age decline of -0.86 dex if metallicity is constant; the age-only reading would require the outer population to be younger than the inner by a factor near seven, which no plausible assembly history supplies, so the gradient is principally a metallicity gradient of -0.54 dex per decade of radius. Read instead as a mixture of two donor populations, dwarf donors of colour 0.58 and massive donors of colour 0.80, the colour at 150 kpc requires dwarfs to supply 0.77 of the light and 0.64 of the mass there, against only 0.17 of the mass at 50 kpc. The outskirts of Coma's diffuse light are thus predominantly dwarf debris by mass, while its inner parts are predominantly the stripped envelopes of massive galaxies, a radial changing of the guard that matches how the cluster's tidal field sorts its donors.