4.2. Host galaxies of FR I and FR II radiosources
FR Is and FR IIs are associated with E galaxies (); no FR I or FR II radio galaxies are known to have any spiral structure (; see however ), while radio quiet Seyfert galaxies are nearly all hosted by spirals. FR I radio galaxies are optically indistinguishable from randomly selected radio-quiet cluster galaxies (). The host galaxies of FR IIs are generally gE galaxies, but not first rank cluster galaxies (; ).
The host galaxy of radio QSOs (which always have an FR II morphology) have the same absolute magnitude distribution as the FR II radio galaxies, suggesting that they are also gE galaxies; the radio quiet QSO host galaxies are somewhat fainter, leading to the conclusion that spiral galaxies host Seyferts with some higher luminosity extension among the radio quiet QSOs, while E galaxies host radio QSOs and a substantial part of the radio quiet QSOs (). HST images of a number of QSOs have confirmed these findings (; ; ).
The parent galaxies of FR IIs frequently exhibit peculiar optical morphologies (tails, fans, bridges, shells and dust lanes) suggesting that they probably arise from the collision or merger of galaxy pairs, at least one member of which is a disk galaxy (; ). Morphological studies of QSO host galaxies by the HST have revealed that a sizable fraction of them show twisted, asymmetric, or disturbed isophotes or possess close small companion galaxies (; ); numerical simulations show that they are tidally formed by a gas-rich major galaxy merger ().
A large fraction of FR I radio galaxies shows evidence of ongoing or past interaction or merging processes; these interactions seem not to involve gas-rich galaxies (; ; ).
FR I clustering environments are consistent with environments about as rich as Abell richness 0; the FR II galaxies are in environments which are significantly poorer (by a factor 2-3) (; ). FR IIs often inhabits richer cluster environments at z ~ 0.5, but almost totally avoid them at low redshift; FR Is show no change in environment between the two epochs ().
It results from these observations that the host galaxies of FR Is and FR IIs are different implying that FR IIs cannot evolve into FR Is or vice-versa (; ). However, for , the sharp division between FR Is and FR IIs suggests that FR IIs could decay into FR Is; he noted, in support of this assumption, the evidence indicating that the more powerful FR II radio galaxies, and perhaps radio-loud QSOs too, generally live in clusters.