brightness of the class, and required this to be fainter than $\mu_{0,g} \approx 24~\mathrm{mag}~\mathrm{arcsec}^{-2}$. On the other hand, [@koda2015] adopted the mean surface-brightness inside $R_e$ in the $R$-band as the characteristic surface brightness, and defined their sample using a cut of $\langle\mu\rangle_{e,R} >
24~\mathrm{mag}~\mathrm{arcsec}^{-2}$. Concentrated, likely background, systems were removed by requiring $\langle\mu\rangle_e$ to not significantly deviate from the surface brightness at $R_e$. Similarly, [@vdb2016] used a $r$-band surface brightness cut of $\langle\mu\rangle_{e,r} \geq 24~\mathrm{mag}~\mathrm{arcsec}^{-2}$ and removed concentrated systems with a Sérsic index cut of $n \leq 4$.

UDGs appear to be quenched systems and occupy the red sequence in clusters [@vandokkum2015a; @vdb2016], although UDG-like systems in the field are typically bluer [e.g. @leisman2017; @roman2017b]. The axial ratios of UDGs in Coma are consistent with being prolate shaped, with a mean axial ratio of ${\sim}0.7$ and very few with axial ratios less than 0.4 [@burkert2017, but see; @rong2019]. This, along with their low $V_\mathrm{rot}/\sigma$ [@vandokkum2019b], suggests that UDGs are dispersion-dominated systems and not rotationally supported thick disks, and are perhaps related to the lower mass low surface brightness dwarf spheroidals in the Local Group [@burkert2017].

It was first proposed that UDGs are “failed" $L^*$ galaxies after having lost their gas 