reignited by the discovery of the enormous abundance of such ‘extreme’ low surface brightness systems in the richest environments, most notably the discovery of very large numbers of such systems in the Coma cluster [c.f @vandokkum2015a; @koda2015], coupled with the remarkable properties of the small number of such objects that have been investigated in detail by follow-up investigations [e.g. @beasley2016a; @vandokkum2016; @vandokkum2018a; @danieli2019a; @vandokkum2019a; @martin-navarro2019]. At least a subset of such objects appear to host very extensive globular cluster systems and exhibit anomalous dynamical mass-to-light ratios (described in greater detail below). As a result of this, and following a suggestion by [@vandokkum2015a], very large low-surface brightness galaxies in clusters have come to be known as ‘ultra-diffuse galaxies’ (UDGs).

There is no universally accepted definition of a UDG in the literature. Slight differences arise from the variety of instruments and techniques used [@martin2019]. However, they are typically defined in morphological terms as very extended stellar systems with large effective radii ($R_e \gtrsim 1.5~\mathrm{kpc}$), low Sérsic indices ($n \lesssim 1.5$), and characteristic surface brightnesses fainter than ${\sim}24~\mathrm{mag}~\mathrm{arcsec}^{-2}$, i.e.roughly Milky Way sized with 1/100 to 1/1000 the stellar mass. [@vandokkum2015a] chose to adopt the central surface brightness of UDGs as the characteristic surface 