Doppler Boosting May Have Played No Significant Role in the Finding Surveys of Radio-Loud Quasars

TL;DR

This paper challenges the assumption that Doppler boosting significantly influences the observed properties of radio-loud quasars, suggesting that most observed flux is not strongly boosted and that alternative explanations for superluminal motions are needed.

Contribution

It demonstrates that Doppler boosting likely plays a minimal role in the observed flux of radio-loud quasars, questioning existing models of relativistic jets and superluminal motion explanations.

Findings

Only a small percentage of flux is Doppler boosted.

Most sources are not preferentially selected by small jet angles.

A new model may be needed for high superluminal motions.

Abstract

There appears to be a fundamental problem facing Active Galactic Nuclei jet models that require highly relativistic ejection speeds and small jet viewing angles to explain the large apparent superluminal motions seen in so many of the radio-loud quasars with high redshift. When the data are looked at closely it is found that only a small percentage of the observed radio frequency flux density from these sources can be Doppler boosted. Without a highly directed, Doppler boosted component that dominates the observed flux, radio sources found in low-frequency finding surveys cannot be preferentially selected with small jet viewing angles. The distribution of jet orientations will then follow the sin$i$ curve associated with a random distribution, where only a very few sources (~1%) will have the small viewing angles (<8 deg) required to explain apparent superluminal motions v{app > 10c, and this makes it difficult to explain how around 33% of the radio-loud AGNs with high redshift can exhibit such highly superluminal motions. When the boosted component is the dominant one it can be argued that in a flux limited sample only those members with small viewing angles would be picked up while those with larger viewing angles (the un-boosted ones) would be missed. However, this is not the case when the boosted component is small and a new model to explain the high apparent superluminal motions may be needed if the redshifts of high-redshift quasars are to remain entirely cosmological.