3. DUST AND FR CLASSIFICATION

More than three decades ago Kotanyi & Ekers (1979) reported the perpendicularity of dust distribution and radio axes for a small sample of radio galaxies. At that time the sources involved only the nearby galaxies with prominent dust distributions. It was also the case that the radio sources were mostly FR-I radio galaxies. With the HST making possible the high resolution imaging of galaxies, dust on much smaller scales was revealed and in particular dust distributed in the nuclear regions.

van Dokkum & Franx (1995) and Verdoes Kleijn & de Zeeuw (2005) used archival HST data to study dust in elliptical galaxies. They reported a high prevalence of dust among the hosts of radio galaxies as compared to normal ellipticals that suggested a link between dust and nuclear activity. Differences in properties were seen for dust on small (< 250 pc) and large scales (> 250 pc). The only dust features to show kinematic coupling with the stars were those on small scales (4 out of 7; Fig. 7; van Dokkum & Franx 1995) where as large scale dust showed no such kinematic coupling. Large scale dust (and at least some small-scale dust) therefore carry signatures of angular momentum different from that of the host ellipticals and indicate external origin, perhaps in mergers. Mostly consistent with the differences are the properties of appearance and location of the dust on large and small scales. The dust on large scales generally had an irregular appearance and was distributed uniformly with respect to the major axis where as the dust on smaller scales was mostly of regular, relaxed appearance and was located on the projected major axis of the host galaxy (van Dokkum & Franx 1995, Verdoes Kleijn & de Zeeuw 2005).

de Koff et al. (2000) and de Ruiter et al. (2002) used HST observations of two well-known and well-imaged (both in radio and the optical) radio galaxy samples (the 3CRR and B2 samples respectively) to study the dust characteristics of the radio galaxy host ellipticals. These observations revealed the very different dust characteristics of the two FR types.

While both de Koff et al as well as de Ruiter et al reported the trend for the dust to be distributed perpendicular to the radio axis it was found to be the case predominantly for the FR-I radio sources. The FR-IIs on the other hand showed less or no tendency for perpendicularity. The dust in FR-Is was mostly seen in the form of small regular circumnuclear dust lanes or disks where as the dust in FR-IIs had varied morphology and extent. Both also reported the lower dust masses among FR-Is as compared to FR-IIs. Interestingly, de Ruiter et al. (2002) noted that the perpendicularity between the radio axis and dust is confined to lower power FR-Is, becoming weak or absent in stronger FR-Is. de Koff et al. (2000) noted that in FR-IIs dust-radio perpendicularity was seen only when dust was 'concentrated close to the nucleus'.

While the dust-radio axis relation and dust characteristics such as appearance and size have been examined in all the above works, the location of the dust (perpendicular to the radio axis) with respect to the host major axis got little attention except in the work on normal and FR-I ellipticals by Verdoes Kleijn & de Zeeuw (2005). Their Fig. 4 shows that in several cases where the dust is perpendicular to the radio axis it lies within 25 degrees of the host major axis (8 out of 14 sources). The coincidence with the major axis is even more impressive when only smooth, regular dust ellipses are considered (6 out of 7 sources). The persistence of dust-radio perpendicularity even when dust does not lie on the host major axis, as seen in some of the cases, is also reported by van Dokkum & Franx (1995) (in Fig. 8) where although the dust is perpendicular to the radio axis in 6 galaxies (admittedly, a rather small sample) it lies on the host major axis in only half the sources.

We point out that the detection as well as appearance of the dust features will depend on distance to the source. Since FR-Is and FR-IIs are generally selected from different redshift regimes (FR-Is near and FR-IIs far) could the differences in dust characteristics of the FR-types be attributed to distance related effects?

The dust characteristics of radio galaxies being discussed here have been sourced from mainly three previous works, de Koff et al. (2000), de Ruiter et al. (2002) and Verdoes Kleijn & de Zeeuw (2005). All three authors have endeavored to study the effect of distance on their findings. Little effects of distance are found for radio power-dust mass relation or dust morphologies, classification, or position angles.

To collect together the points relevant for the dust-radio axis perpendicularity relationship, it appears that the relation is strongest for lower power FR-Is with jets rather than for FR-IIs and that dust need not always lie on the host major axis.

As for the dust in elliptical galaxies, without concerning ourselves with whether it hosts a radio galaxy or not, it seems that dust can exist on large scales and small scales and the morphologies of the dust on the two scales is different. The dust on large scales is mostly unsettled where as dust on small scales appears mostly regular and settled. The division of large-scale dust and small-scale dust seems to largely adhere to the division of the FR type with FR-II radio galaxies associated with dust on large scales and FR-I radio galaxies with dust on small scales.

Dust however is clearly found to be important for the AGN activity and the dust mass is clearly found to be related to the radio power with increasing dust mass for higher power radio galaxies going from low power FR-Is, high power FR-Is and to the FR-IIs.

Table 1 puts together the dust properties of FR-I and FR-II sources.