  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.
