A STUDY OF CORES IN A COMPLETE SAMPLE OF RADIO-SOURCES

The high resolution provided by the Parkes-Tidbinbilla real-time inter ferometer (PTI) has been used to measure the core flux density for a c omplete sample of radio sources. Lower resolution maps are already ava ilable 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 quas ars. Taking this spectral index into account, the measured flux densit y does not change very much, going from a scale of tens of kpc (corres ponding to the low-resolution observations) to the sub-kpc scale of th e new observations. Thus, most of the flux observed in the central reg ion originates in a sub-kpc area. With the new PTI data we obtain a be tter 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 respe ct to the line of sight, and hence a very important parameter for test ing 'unified schemes' for active galactic nuclei (AGN). Using this par ameter, together with optical spectroscopic information, we find that the radio core dominance shows different distributions for different r adio and optical characteristics. A statistically significant differen ce in the distribution of R is observed between Fanaroff-Riley (FR) I and FR II radio galaxies, supporting the idea that low-power sources a re less affected by beaming because they have, on average, a lower Lor entz factor. Among the FR II radio galaxies, narrow-line radio galaxie s (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 i s lower than for steep-spectrum quasars (SSQ) even after a number of s election effects are taken into account. This result can be explained in the framework of uni tied schemes for AGN assuming that in the BLRG s we are seeing more directly into the nucleus, although not as much a s in SSQs.