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, four show evidence for almost circular, presumably face-on, dust disks. This is strong support for the twofold idea that (1) jets emerge close to perpendicular to inner gas disks and (2) optical nonthermal synchrotron emission is seen only when the jet points toward the observer. The implied critical angle to the line of sight is approximately 30^deg-40^deg i.e., if the angle of the jet to the line of sight is less than about 40^deg we see an optical jet. The corresponding relativistic γ factor is ~1.5, which is consistent with current observations of jet proper motion that show a range up to γ~6 for M87. The relatively low speeds implied by γ~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. Based on observations obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5-26555.