Superluminal motion in the double-lobed quasar 3C263

Abstract

Apparent faster-than-light motion has been detected with very long baseline interferometry (VLBI) in the central regions of ∼12 extragalactic radio sources^1,2. These are typically strong, core-dominated objects and, so far the only weak-cored classical double radio source to show the effect was 3C179 (ref. 3). Unified source models^4–6, which invoke relativistic motion at a small angle to the observer's line of sight, predict that large superluminal motion Should be rarely seen in a randomly oriented sample of such objects. These models have motivated statistical studies of quasar samples^7–9, designed to detect superluminal objects and, subsequently, to compare the distribution of velocities with model predictions. Here we report the detection of superluminal motion in the quasar 3C263. The milliarc second structure of this source consists of two nearly unresolved components, whose separation is increasing at a rate of 0.06 ±0.02 marc s yr⁻¹. This corresponds to an expansion speed of 2.7±0.9 c (H₀=55 kms⁻¹ Mpc⁻¹, q₀=0.05). This quasar is the weakest superluminal source found so far, and there are indications that superluminal motion occurs frequently in this class of object.