Direct Distance Measurements to Superluminal Radio Sources

We present a new technique for directly measuring the distances tosuperluminal radio sources. By comparing the observed proper motions ofcomponents in a parsec scale radio jet to their measured Doppler factors, wecan deduce the distance to the radio source independent of the standard rungsin the cosmological distance ladder. This technique requires that the jet angleto the line of sight and the ratio of pattern to flow velocities aresufficiently constrained. We evaluate a number of possibilities forconstraining these parameters and demonstrate the technique on a well definedcomponent in the parsec scale jet of the quasar 3C279 (z = 0.536). We find anangular size distance to 3C279 of greater than 1.8 (+0.5,-0.3) n^{1/8} Gpc,where n is the ratio of the energy density in the magnetic field to the energydensity in the radiating particles in that jet component. For an Einstein-deSitter Universe, this measurement would constrain the Hubble constant to be H <65 n^{-1/8} km/s/Mpc at the two sigma level. Similar measurements on higherredshift sources may help discriminate between cosmological models.Comment: 18 pages, 8 figures, to be published in The Astrophysical Journa