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.
Highlights
López-Corredoira [1] has reminded us that there are still many quasar/QSO observations that remain difficult to explain
The large apparent superluminal motions observed in the jets of many radio-loud quasars can be explained by assuming either, 1) that the objects are at their cosmological redshift (CR) distance and almost all of their radio flux density comes from ejected material that is relativistically beamed towards us in a highly collimated jet at near light speed and with a small inclination angle, i, close to the line-of-sight [2,3]; or 2) that the objects are much closer than their redshifts imply so the observed angular motions in their jets lead to only subluminal linear speeds [4,5]
It is concluded here that Doppler boosting could not have played a significant role in finding radio-loud, high redshift quasars because the component of their radiation that comes from material being ejected outwards at relativistic speeds, and in a tightly confined jet, is insignificant compared to the total flux obtained in the low-frequency finding surveys
Summary
López-Corredoira [1] has reminded us that there are still many quasar/QSO observations that remain difficult to explain. It has been claimed that the former model explains the apparent superluminal motions, but that it can through Doppler boosting, explain why most of the detected sources would naturally have very small inclination angles. For this model to work, one of its main requirements is that, in the finding survey, the Doppler boosted component of the source flux density must be the dominant one. Whether or not this requirement is met needs to be examined closely. It will be demonstrated below that with the existing observational evidence it may no longer be possible to use the relativistic beaming model to explain the high percentage of radio loud quasars exhibiting superluminal motion
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