We now attempt to put the dust properties of FR-I and FR-II sources in perspective. First, the scale lengths that we are dealing with regarding the dust on one hand and the location of the jet origin and hence the black hole spin axis are entirely different. As noted by van Dokkum & Franx (1995) one does not expect there to be any correspondence between the jet direction and the inflowing material considering that in the vicinity of the black hole frame dragging precesses the orbit of the incoming fuel. Even with the resolution of the HST observations the seen dust is on scales nearly 7 orders of magnitude larger than the accretion disk scale. Yet it is with this large scale dust disk that a close relationship exists between it and the jet axis and that too predominantly only in FR-Is. Now, dust as we have seen is important for AGN activity. Dust exists also in FR-IIs and in fact dust masses in FR-IIs are found to be larger than in FR-Is. Although, as seen in section 5, the host ellipticals of FR-IIs and FR-Is are similar, having similar range of absolute magnitudes and ellipticities yet the dust in FR-Is can appear different and also behave differently in its relationship with the radio axis: we need to understand why FR-Is are being singled out for the dust-radio perpendicularity relation and why the dust appears different in FR-Is and FR-IIs. The reasons as to why the dust-radio axis perpendicularity exists were speculated upon by Verdoes Kleijn & de Zeeuw (2005). In attempting to understand the reason for the particular relationship preferentially shown by the FR-I radio galaxies and in particular the lower power FR-Is we will also need to understand why the correlation is weaker or not seen among the FR-IIs. We will address this in Section 7.
Then again, with regard to the radio-host major axis perpendicularity relation it is the FR-Is that show this and not the FR-IIs. If the host galaxies of the two FR types are similar sharing the same range of optical luminosities and ellipticities we need to understand why FR-Is are being singled out for the radio-major axis perpendicularity.
We may express in other words to say that predominantly in FR-Is there is some kind of equilibrium configuration set up between the host galaxy, the dust and the black hole at the centre. Whereas although hosted by ellipticals of similar type such an 'equilibrium' evades the FR-IIs. It appears that more powerful the beams less are the equilibrium relations with the dust axes and host major axes respected.
Below we will attempt to understand the conditions that may be causing these differences between FR-Is and FR-IIs.