A study of cores in a complete sample of radio sources

The high resolution provided by the Parkes-Tidbinbilla real-time interferometer (PTI) has been used to measure the core flux density for a complete sample of radio sources. Lower resolution maps are already available for most of these objects together with optical (spectroscopic) data. The new data show that an inverted spectral index alpha similar to -0.3 (S proportional to nu(-alpha)) could be characteristic of all the nuclei, going from low-luminosity radio galaxies to powerful quasars. Taking this spectral index into account, the measured flux density does not change very much, going from a scale of tens of kpc (corresponding to the low-resolution observations) to the sub-kpc scale of the new observations. Thus, most of the flux observed in the central region originates in a sub-kpc area. With the new PTI data we obtain a better estimate of the radio core dominance (R), i.e. the ratio between the core and the extended radio flux. This parameter is claimed to be a good indicator of the orientation of the beamed radiation with respect to the line of sight, and hence a very important parameter for testing 'unified schemes' for active galactic nuclei (AGN). Using this parameter, together with optical spectroscopic information, we find that the radio core dominance shows different distributions for different radio and optical characteristics. A statistically significant difference in the distribution of R is observed between Fanaroff-Riley (FR) I and FR II radio galaxies, supporting the idea that low-power sources are less affected by beaming because they have, on average, a lower Lorentz factor. Among the FR II radio galaxies, narrow-line radio galaxies (NLRGs) show lower values of R while the broad-line radio galaxies (BLRGs) have the largest R. Moreover, the median value of R for BLRGs is lower than for steep-spectrum quasars (SSQ) even after a number of selection effects are taken into account. This result can be explained in the framework of uni tied schemes for AGN assuming that in the BLRGs we are seeing more directly into the nucleus, although not as much as in SSQs.