brought forward to create such an array of objects. In the failed massive halo scenario, possible mechanisms that could remove the gas and prevent the formation of a normal stellar population include extreme feedback from supernovae and young stars [@agertz2016; @dicintio2017; @chan2018], ram pressure stripping [@yozin2015; @jiang2019; @tremmel2019] or AGN feedback [@reines2013]. With these likely being extreme examples however, other mechanisms may also be in play to form UDGs within lower mass dark matter halos. If UDGs are “inflated dwarfs", both dark matter halos with anonymously high spins [@amorisco2016; @rong2017; @liao2019] and tidal interactions [@yozin2015; @jiang2019; @liao2019; @martin2019; @sales2019] may be responsible for their large sizes. Rare UDGs without dark matter may be the result of high-velocity collisions of dwarfs in protogroup environments [@silk2019]. Furthermore, the near-linear relation between the abundance of UDGs and cluster halo mass [@vdb2016; @roman2017b; @vdb2017; @janssens2017; @mancerapina2018], in addition to their existence in low density environments, suggests an ‘internal’ mechanism of UDG formation that is independent of environment, and that UDGs are a consistent fraction of the galaxy population in all environments [@amorisco2018].

At the other extreme of low stellar mass galaxies lie the ‘ultra-compact dwarfs’ (UCDs). With characteristic luminosities $\gtrsim 10^7~L_{\odot}$ and radii $r_h
