  We investigate the dynamics and morphology of jets propagating into the
interstellar medium using 2D relativistic hydrodynamic simulations. The
calculations are performed assuming axisymmetric geometry and trace a long
propagation of jets. 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 one dimensional analysis is dominant. 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 passed
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.
