Face-on Dust Disks in Galaxies with Optical Jets

Abstract

The presence of optical synchrotron jets in radio galaxies is relatively rare. Here, we show that of the nearest five FR-I 3CR radio galaxies showing optical jets, {\it four} show evidence for almost circular, presumably face-on, dust disks. This is strong support for the two-fold idea that (\romannumeral1) jets emerge close to perpendicular to inner gas disks and (\romannumeral2) 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 - $40^{\circ}$, i.e. if the angle of the jet to the line-of-sight is less than about $40^{\circ}$ we see an optical jet. The corresponding relativisitic $γ$ factor is $\approx 1.5$ which is consistent with current observations of jet proper motion that show a range up to $γ\sim 6$ for M87. The relatively low speeds implied by $γ\approx 1.5$ 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 disk sizes are typically several hundred parsecs, to kiloparsec size. The galaxy with an optical jet that does not show a face-on disk, M87, instead has more complex radial dust and ionized gas filaments.