Face-on dust discs in galaxies with optical jets

The presence of optical synchrotron jets in radio galaxies is relatively unusual. We show that of the nearest five FRI 3CR radio galaxies showing optical jets, four display evidence for almost circular, presumably face-on, dust discs. None of the other twenty nearby FRIs in our sample show circular dust discs, although dust is found in 19/20 cases. This is strong support for the two-fold idea that (1) jets emerge close to perpendicular to inner gas discs and (2) optical non-thermal synchrotron emission is seen only when the jet points towards the observer. The implied critical angle to the line-of-sight is approximately 30-40 degrees, i.e. if the angle of the jet to the line-of-sight is less than about 35 degrees we see an optical jet. The corresponding Lorentz factor is gamma approximate to 1.5, which is consistent with current observations of jet proper motion that show an apparent velocity range from; approximate to 0.6c to 6c for M 87 in the optical (Biretta, Sparks & Macchetto 1999). The relatively low speeds implied by the dust discs may be due to a global deceleration of the jet as in unified theories, or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disc sizes are typically several hundred parsecs, to kiloparsec size. The galaxy with an optical jet that does not show a face-on disc, M 87, instead has more complex radial dust and ionized gas filaments.