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 less than or equal to z less than or equal to 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(o) similar or equal to 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 less than or equal to alpha less than or equal to 0.18, and a total radio luminosity, Lh(2) greater than or equal to 10(26) W/Hz (H-o = 100 h km s(-1) Mpc(-1), q(o) = 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(o) = 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.