HELICAL JETS AND THE MISALIGNMENT DISTRIBUTION FOR CORE-DOMINATED RADIO-SOURCES

The distribution of apparent misalignment between the position angle o f VLBI and VLA scale jets in core-dominated radio sources shows an une xpected bimodal form with one peak close to 0-degrees and the other ne ar 90-degrees. We find that this misalignment distribution implies tha t there are two populations of core-dominated radio sources. The align ed population can be explained by sources with straight VLBI jets and small, randomly oriented, intrinsic bends between parsec and kiloparse c scales. In contrast the misaligned sources must have parsec-scale je ts which are curved into low-pitch (i.e., gently curving) helices. Suc h a geometry when combined with the selection effects due to relativis tic beaming can, under certain conditions, naturally explain these ort hogonal sources. Such a helical distortion might be caused either by K elvin-Helmholtz instabilities or precession of the jet ejection axis. In order to fit the presently observed misalignment distribution the s implest such helical model requires that misaligned sources must have core bulk Lorentz factors gamma > 20 (15-degrees/zeta), where zeta is the half-cone opening angle of the helix. Observations of lobe-dominat ed sources suggest that zeta < 15-degrees; implying gamma > 20. Once t he effects of jet curvature are taken into account, such large Lorentz factors can be made consistent with the observed proper motions provi ded the Hubble constant is toward the lower end of its plausible range (i.e., H-0 almost-equal-to 40 km s-1 Mpc-1). If the Hubble constant i s larger, then either the jet decelerates between core and VLBI scales or the bulk flow and pattern gamma are decoupled.