Exploring the properties of milliarcsecond radio sources

Cosmological applications of the "redshift—angular size" test require knowledge of the linear size of the "standard rod" used. In this paper, we study the properties of a large sample of 140 mas compact radio sources with flux densities measured at 6 and 20 cm, compiled by Gurvits et al. Using the best-fitted cosmological parameters given by Planck/WMAP9 observations, we investigate the characteristic length l_m, as well as its dependence on the source luminosity L and redshift l_m = lL^β (1 + z)ⁿ. For the full sample, measurements of the angular size θ provide a tight constraint on the linear size parameters. We find that cosmological evolution of the linear size is small $(|n|\simeq {10}^{-2})$ and consistent with previous analysis. However, a substantial evolution of linear sizes with luminosity is still required (β ≃ 0.17). Furthermore, similar analysis done on sub-samples defined by different source optical counterparts and different redshift ranges seems to support the scheme of treating radio galaxies and quasars with distinct strategies. Finally, a cosmological-model-independent method is discussed to probe the properties of the angular size of milliarcsecond radio quasars. Using the corrected redshift—angular size relation for the quasar sample, we obtained a value of the matter density parameter, ${\Omega }_m={0.292}_{-0.090}^{+0.065}$, in the spatially flat ΛCDM cosmology.