In this section we list some of the well-known and well-used findings reported for the two classes of radio galaxies. Right in the seminal paper recognizing the two radio source morphologies, Fanaroff & Riley (1974) pointed out the correlation of the morphologies with the total radio power: the two morphologies are divided in their total radio power with the edge-darkened FR-I type radio galaxies having lower radio powers and the edge-brightened FR-II type radio galaxies having higher radio powers. The two radio source types were found to be divided at the so-called dividing power of 1025 W Hz-1 at 1.4 GHz.
FR-Is and FR-IIs also compare interestingly with respect to their environments. At nearby redshifts (z < 0.5) while the FR-Is are found to be located in dense environments (galaxy clusters) the FR-IIs are often found to be hosted by field galaxies. At higher redshifts though both FR-I and FR-II type radio sources are found in rich environments (Hill & Lilly 1991). The elliptical galaxies that host the two FR types were also found to show differences. Broad band imaging of the host galaxies showed that hosts of FR-IIs were found to be bluer than hosts of FR-Is and often showed signatures of mergers (Heckman et al. 1986, Smith & Heckman 1989, Baldi & Capetti 2008, Ramos-Almeida et al. 2012). Then again the host galaxies of FR-Is were found to be more massive than the FR-II hosts (Owen & Laing 1989, Govoni et al. 2000). While AGN optical spectra of some of the FR-II hosts showed emission lines this was almost never the case in FR-I optical spectra. We will return to this topic of AGN spectra later in this section.
One of the important developments in this area has been the Owen-Ledlow diagram (Owen 1993, Owen & Ledlow 1994) where the two source types divide about a line with slope of about 1.8 in the total radio power-absolute optical magnitude plane. The dividing power, hiterto believed to be fixed was instead found to be increasing with the host optical luminosity. If FR-I structures are a result of jets that are affected by entrainment, instabilities and turbulence then one could understand the increasing dividing power with absolute host optical magnitude as galaxies with jets that found it increasingly difficult to remain collimated and supersonic as the ambient medium became denser (Bicknell 1995). This dependence of the FR-I/II dividing power on the host absolute magnitude put the spotlight on the role of environment in producing the two different radio source morphologies.
The finding of a hybrid morphology in some radio galaxies, with one lobe being edge brightened and another edge darkened (Gopal-Krishna & Wiita 2000) strengthened the view that environment in which the jets propagated were ultimetely responsible for the kind of structure that developed on large scales.
With the increasing availability of high resolution optical imaging as well as more complete optical spectroscopic observations, details with respect to the AGN as well as host galaxy characteristics on more global scales emerged that had to be understood.
The study noting differences in optical spectral properties of FR-I and FR-II hosts reported by Hine & Longair (1979) was followed by works that used classifications based on spectral line ratios (Baum et al. 1992, Laing et al. 1994, Tadhunter et al. 1998, Chiaberge et al. 2002, Buttiglione et al. 2010) which clearly showed that the two morphological types exhibited different behaviour. While FR-I radio galaxy hosts always exhibit optical spectra with only absorption lines or O[III] / H < 0.2 (low ionization emission line radio galaxies, LEGs, following the definition of Laing et al.), the FR-II hosts were of mixed category. Some FR-II hosts were like FR-Is with either only absorption lines or prevalence of low ionization emission lines with low O[III] / H ratios but some others showed spectra with strong high ionization emission lines with O[III] / H > 0.2 (high ionization emission line radio galaxies, HEGs).
Baum et al. (1992) found clear differences in the characteristics of the emission line gas in the two FR types. Most FR-IIs appeared to have rotating disks of line-emitting gas on large scales up to 15 kpc that also sometimes included disks with chaotic and turbulent motions which contrasts with those in FR-I type sources. These findings were linked with two different modes of gas acquisition for fueling the AGN in the two FR types. The quite different emission line characteristics of FR-I and FR-II radio galaxies led Baum et al. (1995) to explore different ways of producing the two morphologies. A preferred model was that the differences arose in the different accretion rates: low accretion rates in FR-Is and high accretion rates in FR-IIs. Differences in the black hole spin too was suggested for the two classes with FR-Is having a lower black hole spin. Marchesini et al. (2004) also found that the accretion rates needed in FR-Is were very low, less than ∼ 0.001 in Eddington units. However again interestingly the FR-IIs appeared to span two regions, one that had similarly low accretion rates as FR-Is and a small fraction that required higher accretion rates.
X-ray and IR observations (Hardcastle et al. 2007 and references therein and Best & Heckman 2012) and the optical spectroscopic studies (Buttiglione et al. 2010, Mahony et al. 2011) further support and highlight the division based on line ratios (LEG and HEG type) and the link with accretion mode and source of fuel. Increasingly it is being suggested that the two optical spectral classes are powered by two different accretion modes and fuel sources: radiatively inefficient low accretion rates sourced from hot gas accretion that powers the low and high power LEGs where as radiatively efficient high accretion rates sourced from cold gas accretion powering the HEG sources. The hot gas is suggested as originating from the hot coronae of the hosts and from the stellar mass loss from the stars in the galaxy. On the other hand the cold gas is suggested as originating from a gas-rich merger with another galaxy.
High resolution optical imaging with HST revealed the prevalence of nuclear optical cores in both FR-Is and FR-IIs (Chiaberge et al. 1999, Chiaberge et al. 2000, Chiaberge et al. 2002). However there were clear differences between the two FR types, with a clear correlation seen between the optical core powers and the radio core powers for the FR-Is and a more complex behaviour for the FR-IIs. While the FR-1s were inferred to have unobscured, radiatively inefficient accretion disks the FR-IIs once again divided into two distinct types: a population that showed similar properties as the FR-Is (constituted by the LEG FR-IIs) and a population constituted by the HEG FR-IIs where presence of dust torii and radiatively efficient accretion disks were inferred.
In the following sections we look at some additional properties of the two FR types where they show differences and we try to develop a framework inclusive of these findings. We first consider the dust characteristics of the hosts of the two FR types.