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Companions around the nearest luminous galaxies: segregation and selection effects

Using the "Updated Nearby Galaxy Catalog", we consider different properties of companion galaxies around luminous hosts in the Local Volume. The data on stellar masses, linear diameters,surface brightnesses, HI-richness, specific star formation rate (sSFR), and morphological types are discussed for members of the nearest groups, including the Milky Way and M 31 groups, as a function of their separation from the hosts. Companion galaxies in groups tend to have lower stellar masses, smaller linear diameters and fainter mean surface brightnesses as the distance to their host decreases. The hydrogen-to-stellar mass ratio of the companions increases with their linear projected separation from the dominant luminous galaxy. This tendency is more expressed around the bulge-dominated hosts. While linear separation of the companions decreases, their mean sSFR becomes lower, accompanied with the increasing sSFR scatter. Typical linear projected separation of dSphs around the bulge-dominated hosts, 350 kpc, is substantially larger than that around the disk-dominated ones, 130 kpc. This difference probably indicates the presence of larger hot/warm gas haloes around the early-type host galaxies. The mean fraction of dSph (quenched) companions in 11 the nearest groups as a function of their projected separation R_p can be expressed as f(E) = 0.55 - 0.69 x R_p. The fraction of dSphs around the Milky Way and M 31 looks much higher than in other nearby groups because the quenching efficiency dramatically increases towards the ultra-low mass companions. We emphasize that the observed properties of the Local Group are not typical for other groups in the Local Volume due to the role of selection effects caused by our location inside the Local Group.