Galaxy collisions and mergers - The genesis of very powerful radio sources?

The authors discuss a program of deep optical broad- and narrow-band imaging of 43 radio galaxies, supplemented by optical long-slit spectroscopy and VLA radio mapping. The principal result is that between one-fourth and one-third of very powerful radio galaxies are strongly peculiar in optical morphology at relatively high levels of surface brightness. This fraction is much greater than for less powerful radio galaxies and for radio-quiet ellipticals. The morphological peculiarities take the form of tails, fans, bridges, dust features, shells, etc. They probably arise from the collision or merger of galaxy pairs, at least one member of which is a disk galaxy. Two idealized classes of radio galaxies are proposed. Class A galaxies have strong optical emission lines and frequently peculiar optical morphologies. These may be radio sources fueled by collisions or mergers involving at least one gas-rich system. Class B galaxies have weak or no optical emission lines and are morphologically normal giant elliptical or cD galaxies. These radio sources may be powered by the stored rotational energy of a central black hole or by accretion of an intracluster medium.