Velocity dispersion and metallicity as a function of stellar mass for UDGs. Left panel: σ–mass relation for UDGs from the literature (coloured points) and UDG11 (blue square). The grey area corresponds to the observed baryonic Tully-Fisher relation (McGaugh 2012) obtained by scaling the rotation velocity by . Right panel: mass–metallicity relation from observations, including UDG11. The mass–metallicity relations found for dwarf galaxies by Kirby et al. (2013) and by Simon (2019) are marked in grey and purple regions, respectively. The average mass–metallicity relation for bright galaxies is marked by the black dashed line, and its range is shown by the two dotted lines (Gallazzi et al. 2005). The metallicities derived by fitting the spectral energy distribution by Buzzo et al. (2022) and Barbosa et al. (2020) are marked with grey and yellow triangles, respectively. In both panels, data points from Chilingarian et al. (2019) are UDG-like, because only two objects of the sample have μ₀ ≥ 24 mag arcsec⁻² and R_e ≥ 1.5 kpc.