Superluminal sources

Abstract

This review of the superluminal motion phenomenon in Active Galaxies takes a largely statistical approach. Predictions for the apparent velocity distribution under simple beaming models are presented and compared to the observations. The potential applications for tests of unification models and for cosmology (source counts, measurements of H ₀ and q ₀) are discussed. First results from a large homogeneous survey are presented. The data do not show compelling evidence for the existence of intrinsically different populations of galaxies, BL Lac objects, or quasars. β_app in the range (1−−5) h ⁻¹ occur with roughly equal frequency; higher values, up to β_app=10 h ⁻¹ are rather more scarce than appeared to be the case from earlier work, which evidently concentrated on sources which are not representative of the general population. The β_app distribution suggests that there might be a skewed distribution of Lorentz factors over the sample, with a peak at γ_b≈2 h ⁻¹ and a tail up to at least γ_b≈10 h ⁻¹. There appears to be a clearly rising upper envelope to the β_app distribution when plotted as a function of observed 5 GHz luminosity; a combination of source counts and the apparent velocity statistics in a larger sample could provide much insight into the properties of radio jet sources.