The ``angular size - redshift" relation for compact radio structures in quasars and radio galaxies

We discuss the ``angular size - redshift'' relation for compact radio sources distributed over a wide range of redshifts 0.011 <= z <= 4.72. Our study is based on a sample of 330 5 GHz VLBI contour maps taken from the literature. Unlike extended source samples, the ``angular size - redshift'' relation for compact radio sources appears consistent with the predictions of standard Friedmann world models with q_deg =~ 0.5 without the need to consider evolutionary or selection effects due to a ``linear size - luminosity'' dependence. By confining our analysis to sources having a spectral index, -0.38 <= alpha <= 0.18, and a total radio luminosity, Lh(2) >= 10(26) W/Hz (H<sub>deg</sub> = 100 h km s(-1) Mpc(-1) , q<sub>deg</sub> = 0.5 used as a numerical example), we are able to restrict the dispersion in the ``angular size - redshift'' relation. The best fitting regression analysis in the framework of the Friedmann-Robertson-Walker model gives the value of the deceleration parameter q_deg = 0.21 +/- 0.30 if there are no evolutionary or selection effects due to a ``linear size - luminosity'', ``linear size - redshift'' or ``linear size - spectral index'' dependence. Full version of Table~\ref{TSou1} is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or http://cdsweb.u-strasbg.fr/Abstract.html