Impact of ultraviolet radiation from giant spirals on the evolution of dwarf galaxies

We show that ultraviolet (UV) radiation, with wavelengths shorter than 2000 Angstrom, escaping from the discs of giant spirals could be one of the principal factors affecting the evolution of low-mass satellite galaxies. We demonstrate, using an analytical approach, that the Lyman continuum part of the radiation field can lead to the ionization of the interstellar medium (ISM) of dwarf galaxies through the process of photoevaporation, making the ISM virtually unobservable. The far-ultraviolet part (912<2000 Angstrom) is shown to dominate over the internal sources of radiation for most of the Galactic dwarf spheroidals. The proposed environmental factor could be at least partially responsible for the bifurcation of the low-mass protogalaxies into two sequences: dwarf irregulars and dwarf spheroidals. We discuss many peculiarities of the Local Group early-type dwarfs, which can be accounted for by the impact of the UV radiation from the host spiral galaxy (Milky Way or M31).