One Structure, Two Names: The Halo of NGC 1399 Against the Diffuse Light of the Fornax Cluster

The Fornax cluster is the nearest system where a central galaxy's halo and a cluster's diffuse light can be studied at Virgo-like proximity in a poorer, quieter environment, and NGC 1399's outer halo is its dominant diffuse structure. Adopting a total cluster light of 2.4e11 solar luminosities within 700 kpc, an NGC 1399 galaxy-plus-halo system of 9.5e10, and an exponential halo profile of central brightness 24.0 magnitudes per square arcsecond and scale length 30 kpc, the halo beyond 30 kpc carries 3.75e+10 solar luminosities. That is 0.39 of the central system and 0.156 of all the cluster's light. The halo stands at 25.81 magnitudes per square arcsecond at 50 kpc and 27.62 at 100 kpc. Against the cluster's total diffuse light at the published fraction of 0.15, 3.60e+10 solar luminosities, the central galaxy's outer halo alone amounts to 1.04 times the nominal cluster component, a ratio greater than one that exposes the bookkeeping truth of poor clusters: in Fornax, the diffuse light and the central galaxy's halo are the same structure counted under two names, and the cluster's diffuse fraction depends on which name the accountant chooses.