Authors: Alister W. Graham
Abstract:It is explained why ultra-diffuse galaxies (UDGs), a subset of (IC 3475)-type galaxies, do not have unexpectedly large sizes but large sizes that are in line with expectations from the curved size-luminosity relation defined by brighter early-type galaxies (ETGs). UDGs extend the faint end of the (absolute magnitude, M)-log(Sérsic index, n) and M-(central surface brightness, μ0) relations defined by brighter ETGs, leading to the large effective half-light radii, Re, in UDGs. It is detailed how the scatter in μ0, at a given M, relates to variations in the galaxies' values of n and effective surface brightness, μe. These variations map into changes in Re and produce the scatter about the M-Re relation at fixed M. Similarly, the scatter in M, at fixed μ0 and n, can be mapped into changes in Re. The increased scatter about the faint end of the M-Re relation and the smaller scatter about M-(isophotal radii, Riso) relations are explained. Artificial and potentially misleading size-luminosity relations for UDGs are also addressed. The suggestion that there may be two types of UDG appears ill-founded, arising from the scatter about the M-μ0 relation, which persists at all magnitudes. Hopefully, the understanding presented here will prove helpful for interpreting the many low surface brightness galaxies that the Large Synoptic Survey Telescope will detect.