Origin of companion galaxies in QSO hosts

Recent morphological studies of QSO host galaxies by the Hubble Space Telescope (HST) have revealed that a sizable fraction of QSO host galaxies possess close small companion galaxies. It is however not clear why and how these companion galaxies are physically associated with the activation of QSO nucleus and the formation of QSO hosts. We here demonstrate that QSO companion galaxies are tidally formed by a gas-rich major galaxy merger and then orbit the merger remnant, based on the results of numerical simulations which investigate both gas fueling to the central seed massive black holes in the merger and global morphological evolution of the merger. We furthermore suggest that some of companion galaxies tidally formed in a major merger can evolve eventually into small compact elliptical galaxies observed frequently in the present-day bright massive galaxies. Thus our numerical simulations not only clarify the origin of companion galaxies observed in low and intermediate redshift QSOs but also provide an evolutionary link between QSO companion galaxies and the present-day compact elliptical galaxies located in the vicinity of giant elliptical galaxies.