Abstract
Radio core dominance measurements, an indicator of jet orientation, sometimes
rely on core flux density measurements from large-area surveys like Faint
Images of the Radio Sky at Twenty cm (FIRST) that have an angular resolution of
only 5''. Such low-resolution surveys often fail to resolve cores from the
extended emission resulting in an erroneous measurement. We focus on
investigating this resolution effect for a sample of 119 radio-loud quasars. We
obtained continuum observations from NSF's Karl G. Jansky Very Large Array
(VLA) at 10 GHz in A-array with a 0.2'' resolution. Our measurements show that
at FIRST spatial resolution, core flux measurements are indeed systematically
high even after considering the core-variability. For a handful of quasars, 10
GHz images reveal extended features, whereas the FIRST image shows a point
source. We found that the resolution effect is more prominent for quasars with
smaller angular sizes. We further computed two radio core dominance parameters
R & R5100 for use in statistical orientation investigations with this sample.
We also present the spectral energy distributions between 74 MHz and 1.4 GHz,
which we used to measure the spectral index of the extended emission of these
quasars. Our results empirically confirm that determination of radio core
dominance requires high-spatial resolution data. We highlight the practical
issues associated with the choice of frequency and resolution in the
measurement of core and extended flux densities.
