Rₑ. II. Understanding the IC 3475 galaxy type, including ultra-diffuse galaxy, structural scaling relations

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 all 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 Large Synoptic Survey Telescope will detect.