V-band luminosity as a function of line-of-sight velocity dispersion in each model galaxy is compared to the observational data of Milky Way satellites in gray squares. In the ΛCDM setting, increasing the magnetic field strength results in fainter systems (illustrated by colored circles) and can even completely prevent star formation in ultra-faint dwarf galaxies. In order to include halos which do not form any stars in the plot, their luminosities are represented at L_V ∼ 10³ L_⊙. In the ΛCDM+PMFs setting, despite lower luminosities, velocity dispersion of galaxies (illustrated by colored empty diamonds) increases due to a higher frequency of mergers.