2025/04/23 by Alister W. Graham, Graham, Alister W.
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies
paper · pdf · doi:10.48550/arxiv.2504.16593
openalex publication_date 2025/04/23 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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, \mathfrakM)-log(Sérsic index, n) and \mathfrakM-(central surface brightness, μ\rm 0) relations defined by all ETGs, leading to the large effective half-light radii, R\rm e, in these UDGs. It is detailed how the scatter in μ\rm 0, at a given \mathfrakM, relates to variations in the galaxies' values of n and effective surface brightness, μ\rm e. These variations map into changes in R\rm e and produce the scatter about the \mathfrakM-R\rm e relation at fixed \mathfrakM. Similarly, the scatter in \mathfrakM, at fixed μ\rm 0 and n, can be mapped into changes in R\rm e. The suggestion that there may be two types of relatively gas-poor UDGs appears ill-founded, arising from the scatter about the \mathfrakM-μ\rm 0 relation. The increased scatter about the faint end of the \mathfrakM-R\rm e relation and the smaller scatter about \mathfrakM-(isophotal radii, R\rm iso) 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.