EVN plus MERLIN observations of radio-intermediate quasars: Evidence for boosted radio-weak quasars

We present VLBI (EVN+MERLIN) observations of a sample of three low-redshift radio-intermediate PG quasars (RIQ) with hat and variable radio spectrum (III Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is slightly lower than the average R for steep-spectrum quasars, but their radio spectral properties are those of core-dominated quasars. It was proposed previously that these sources might be relativistically boosted jets in radio-weak quasars. Our VLBI observations now indeed confirm the presence of a high brightness temperature core in all three of these objects-two of them have lower limits on T-B well in excess of 10(10) K. Moreover, we find no ''missing flux,'' which means that basically all the flux of these quasars is concentrated in the compact radio core. As the total radio flux is already at the low end for radio-loud quasars, we can place a strong limit on the presence of any extended emission. This limit is consistent with the extended emission in radio-weak quasars but excludes that the flat-spectrum RIQ reside in typical radio-loud quasars. The observations therefore strongly support the idea that relativistic jets are present in radio-weak quasars and, hence, that radio-loud and radio-weak quasars have very similar central engines.