The linear sizes of quasars and radio galaxies in the unified scheme

A key argument in favor of orientation-based unification schemes is the finding that among the most powerful 3CRR radio sources the (apparent) median linear size of quasars is smaller than that of radio galaxies, which supports the idea that quasars are a subset of radio galaxies, distinguished by being viewed at smaller angles to the line of sight. Recent measurements of radio sizes for a few other low-frequency samples are, however, not in accord with this trend, leading to the claim that orientation may not be the main difference between radio galaxies and quasars. We point out that this ''inconsistency'' can be removed by making allowance for the temporal evolution of sources in both size and luminosity, as inferred from independent observations. This approach can also readily explain the other claimed ''major discrepancy'' with the unified scheme, namely, the difference between the radio luminosity-size correlations for quasars and radio galaxies.