In the work presented here we have argued for a framework within which to understand the two basic morphological classes, the FR-I and FR-II type radio galaxies. We have made a connection between host galaxy properties and the FR classes that has crucially included observations of dust and the relative orientations of dust, host major axis and radio axis. This contributes to a picture of FR classification that now encompasses a broader range of phenomena. With the backdrop of the Owen-Ledlow diagram we have attempted to connect various observations into a coherrent picture for understanding the FR dichotomy.
Noting the quite different dust properties and relative orientations between dust, radio and major axes of the two FR-classes as well as differences in many other respects and also considering their relationship with the host galaxy (as displayed in the Owen-Ledlow diagram) we have tried to explore the physical conditions that could give rise to the seen differences. We are able to provide a qualitative framework for understanding the many characteristics of FR-I and FR-II radio galaxies by considering that although massive elliptical galaxies may generate sufficient fuel from internal sources to power the AGN given their high threshold jet energy flux they would more commonly host FR-I type source morphologies and also that for lower mass ellipticals on the other hand the lower threshold jet energy fluxes may more easily be breached and FR-IIs more easily generated as a result of any additional external sources of fuel (coming in through mergers or interactions).
(a) The frequently seen alignments between dust, major axis and radio axis in FR-Is and their frequent association with more massive ellipticals suggest stable fueling conditions without need for mergers. The alignments suggest that they may have been left relatively unperturbed for a long enough time to have their black holes realign with the minor axes. However, for the FR-IIs, such undisturbed conditions may have eluded them.
(b) Mergers appear crucial for the formation of FR-IIs where as FR-Is form in more benign conditions.
(c) The observation of mostly aligned representations of multiple activity epochs in restarted radio galaxies suggests that while mergers could have perturbed black hole axes in FR-II hosts the cause for the interruption and re-triggering of the AGN is less perturbing and may not be due to a new merger event. The aligned structures demand that the black hole axis be steady within a merger event. A possible cause has been identified in the form of an assocation of the interruption and re-triggering to the fueling as each tranche of fuel is exhausted. Moreover, the timescale for the entire beam activity (including the quiescent phase and renewed activity) is likely small compared to the realignment timescale.
(d) LEG FR-IIs, a class that shares with the FR-Is a state of the AGN characterized by a weak central ionizing source may include sources that have waned in AGN activity. At least some LEG FR-IIs are the transition FR-IIs (whether dying or restarting). A bigger compilation of LEG FR-IIs properties is needed to investigate their nature.
In the framework the association of an FR-type with an elliptical galaxy is flexible depending on the power of the jets that are created in the prevailing conditions, the type of environment they encounter as well as the age of the activity; mass and history of the host elliptical galaxy play a key role. A combination of host galaxy mass, its environment, the merger history, dust acquisition and distribution, accretion rates, ambient environment of the jets, black hole realignments may all need to be considered in understanding the FR-I and FR-II characteristics that we observe. We have tried to elucidate how these could influence the AGN and the radio source it generates.
Radio galaxies with powers at the extremes and hosted by ellipticals of a fixed absolute magnitude will be interesting to study to look at factors responsible for the different radio powers and morphologies.