2019MNRAS.486.1225T

Immensely bright quasars and radio-loud active galactic nuclei (AGNs) provide an enticing opportunity to construct standard candles detectable up to the very early universe. An analytic theory is proposed to measure the distance to powerful Fanaroff & Riley type-II (FR-II) radio sources based on their integrated flux density across a broad range of radio frequencies, and the angular size and axial ratio of their synchrotron-emitting lobes. This technique can be used at low redshift to construct absolute standard candles in conjunction with X-ray observations of the host cluster, or at high redshift to measure the relative distances of objects and constrain the curvature of our Universe. Distances calculated with this method are consistent for dissimilar objects at the same redshift; the two lobes of Cygnus A have flux densities, linear sizes, and spectral break frequencies varying between 15 and 35 per cent yet their fitted distances are the same to within 7 per cent. These distance estimates together yield a transverse comoving distance to Cygnus A of 261_–55⁺⁷⁰ Mpc corresponding to a Hubble constant of H₀=₆₄-13_⁺¹⁷ km s^–1 Mpc^–1. Large samples of suitable FR-II sources could provide a measure of the Hubble constant independent of existing techniques such as the cosmic microwave background, baryon acoustic oscillations, and type 1a supernovae.