Investigating orientation effects considering angular resolution for a sample of radio-loud quasars using VLA observations

TL;DR

This study demonstrates that low-resolution radio surveys overestimate core flux in quasars due to unresolved extended emission, emphasizing the need for high-resolution data for accurate orientation analysis.

Contribution

The paper provides empirical evidence on resolution effects in radio core measurements of quasars and introduces high-resolution VLA observations to improve orientation studies.

Findings

FIRST survey overestimates core flux due to limited resolution.

High-resolution VLA images reveal extended features missed by FIRST.

Resolution effects are more significant for quasars with smaller angular sizes.

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.