Although radio galaxies come in a variety of morphologies a basic division in structural types has stood the test of time: the Fanaroff and Riley classification (Fanaroff & Riley 1974). It was based on the recognition that radio sources often came in two flavours, those having edge-darkened (FR-I) morphologies and those having edge-brightened (FR-II) morphologies. The division was based entirely on radio structures that exist on large scales from few to several hundred kiloparsec to several megaparsec. That the two radio morphologies also divide on the basis of several other properties both on large scale and small points to more fundamental processes at work that can have profound effect on what type of structures form on large scales.
Since the time of this simple classification scheme there have been several attempts to understand how these two basic morphologies arise. Building on the basic models of Blandford & Rees (1974), Scheuer (1974), and Falle (1991) there are detailed models today that are able to reproduce with fair consistency data gathered on these source populations (Kaiser et al. 1997, Blundell et al. 1999). At the heart of any model that seeks to reproduce the large scale radio structures is the basic tenet of interaction between a jet and its ambient medium. The interplay between the two under varying characteristics of the jet and the external medium through which it propagates is shown to produce the two types of structures. Although other source morphological types have been discovered they have usually been understood within the Fanaroff-Riley classification.
While the two types of radio structures are understood as arising out of the different interactions that low and high power jets have with the environment (Scheuer 1974, Bicknell 1986) a basic question remained however as to whether the two source types were intrinsically the same or different: could the FR-Is and FR-IIs be the consequence as well of very different central engines and host galaxies or the consequence entirely of the different interactions with the environments?
Over the several decades a number of multi-wavelength studies on FR-I and FR-II radio sources has gathered a variety of data on these sources testifying to the importance of the question lying at the focus of these efforts. Several important findings related to the two categories of sources were reported. A picture is emerging which highlights the intimate connection between the elliptical galaxy and the radio galaxy it hosts, where there is a dynamic relationship between them and where the history of the host galaxy has an important role.
In this paper we develop a framework that brings together some key research in this area that was not considered earlier. There have been efforts in the past, particularly the work of Baum et al. (1995) who have put forward a scenario for forming the two types of radio galaxies. We make use of their research as well as more recent findings in our effort to present a coherent picture in which we may understand the conditions in which the two basic morphological types form. We summarize some of the key observations of FR-Is and FR-IIs in section 2 after which in sections 3 - 6 we introduce and examine properties of FR-Is and FR-IIs not considered earlier, which we now incorporate within our framework. In section 7 we present and discuss the framework where due attention is given to the insightful Owen-Ledlow diagram (Owen 1993, Owen & Ledlow 1994) and the crucial role it has in the effort to understand radio galaxies. This is followed by the summary.