The dust-radio connection in 3CR radio galaxies

We investigate dust in the nuclear regions of radio galaxies on the basis of 120 images of 3CR radio galaxies in the redshift range 0 < z < 0.5, taken with the WFPC-2 on the Hubble Space Telescope. At least 40 of these radio galaxies show evidence for absorption due to clumpy dust features; a large range of morphologies is observed for these dust features, including disklike and filamentary structures. No difference in host galaxy properties was found between the radio galaxies with clumpy dust and those without. Where dust absorption is detected, the morphological properties of the dust depend strongly upon the Fanaroff & Riley (FR) classification of the source. The dust in FR I host galaxies is generally situated in sharply defined disks on small (<2.5 kpc) scales, and for eight out of the nine FR I galaxies with dust disks, the radio source lies nearly perpendicular to the dust disk. In contrast, FR II host galaxies have dust that varies both in morphology and in linear size, and the correlation between the dust and radio orientation only exists (and less strongly) for the sources that have a significant mass of dust in disks within a distance of 2.5 kpc of the center of the galaxy. The derived dust masses also correlate with the FR type of the host galaxy: high masses of clumpy dust are not seen in FR I radio galaxies, while they are present in FR II radio galaxies. Further, FR I galaxies have derived dust masses that are typically larger than the dust masses found in a matched sample of radio-quiet ellipticals. We suggest that the observed differences between FR I and FR II radio galaxies are produced due to differences in their formation.