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The question of the collimation of relativistic jets is the subject of alively debate in the community. We numerically compute the apparent velocityand the Doppler factor of a non homokinetic jet using different velocityprofile, to study the effect of collimation on the appearance of relativisticjets (apparent velocity and Doppler factor). We argue that if the motion isrelativistic, the high superluminal velocities are possible only if thegeometrical collimation is smaller than the relativistic beaming angle$\gamma^{-1}$. In the opposite case, the apparent image will be dominated bythe part of the jet traveling directly towards the observer resulting in asmaller apparent velocity. Furthermore, getting rid of the homokinetichypothesis yields a complex relation between the observing angle and theDoppler factor, resulting in important consequences for the numericalcomputation of AGN population and unification scheme model.

The influence of collimation on the appearance of relativistic jets