Faber-Jackson relation and dark matter content in LEWIS sample. Left panel: Velocity dispersion σ_eff as a function of stellar masses of UDGs from the literature (green diamonds) and the LEWIS sample. The blue and red circles represent rotating and non-rotating LEWIS galaxies, respectively. The shaded light blue region represents the luminosity Faber-Jackson relation, extrapolated to the low-mass regime (L ∼ σ^2.2, Kourkchi et al. 2012). The luminosities are converted into stellar masses using the Into & Portinari (2013) colour – M/L relation. UDG7 and UDG12 are marked with triangles since their values of σ_eff might be overestimated due to their non-negligible rotation. Right panel: Dynamical mass (M_dyn) as a function of the V-band total luminosity (L_V, eff) computed within 1R_eff of UDGs from the literature (green diamonds) and the LEWIS sample. The points are colour-coded as in the left panel. The solid grey lines mark the loci in the plane where M_dyn/L_V, eff is constant and equal to 1, 10, 100, 1000, and 10 000, respectively. The magenta points represent the dwarf galaxies in the Local Group analysed by Battaglia & Nipoti (2022).