We investigate the dynamics and morphology of jets propagating into the interstellar medium using twodimensional relativistic hydrodynamic simulations. The calculations are performed assuming axisymmetric geometry and follow jet propagation over a long distance. The jets are assumed to be "light," with the density ratio between the beam to the ambient gas much less than unity. We examine the mechanism for the appearance of vortices at the head of jets in the hot spot. Such vortices are known as a trigger of a deceleration phase, which appears after a short phase in which the jet propagation follows the results from one-dimensional analysis. We find that an oblique shock at the boundary rim near the end of the beam strongly affects the flow structure in and around the hot spot. Weakly shocked gas passes through this oblique shock and becomes a trigger for the generation of vortices. We also find the parameter dependence of these effects for the propagation and dynamics of the jets. The jet with slower propagation velocity is weakly pinched, has large vortices, and shows very complex structure at the head of the jets and extended synchrotron emissivity.
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