Authors: Alister W. Graham
Abstract:It is explained why relatively gas-poor ultra-diffuse galaxies (UDGs), a subset of IC 3475 galaxy types, 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). These 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 these 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 suggestion that there may be two types of relatively gas-poor UDGs appears ill-founded, arising from the scatter about the M-μ0 relation. 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. Finally, expected trends with dynamical mass, and evolutionary pathways towards relatively gas-rich galaxies, are briefly discussed. Hopefully, the understanding presented here will prove helpful for interpreting the many low surface brightness galaxies that the Legacy Survey of Space and Time will detect.