An Etymological Dictionary of Astronomy and Astrophysics

A galaxy of low stellar density, defined to have low central → surface brightness (> 24 mag arcsec^-2) and an → effective radius (R_e) of over 1.5 kpc. The question of whether UDGs represent a separate class of galaxies is still under debate. Currently, known UDGs that have been discovered in clusters, in groups, and in the field can have R_e as large as 5 kpc which is comparable to that of giant Milky Way like galaxies. This fact has been used to suggest that UDGs are “failed” giants. As R_e captures (at most) the central parts of giant galaxies, whether this radius can be used to fairly compare the sizes of UDGs to the more massive galaxies is questionable (see, e.g., Chamba et al., 2020, A&A 633, L3).

See also: Term proposed by van Dokkum et al. (2015), arXiv: 1410.8141v2; → ultra-; → diffuse; → galaxy.