1996A&A...307..347F

We develop a model in which mildly-relativistic jets propagate through the interstellar medium in active galaxies and collide with dense gaseous obstacles (supernova remnants, winds from giant stars or line-emitting clouds for example). Shock waves result from these interactions. Particles are accelerated at the shocks and subsequently emit synchrotron emission. This model can explain the properties of radio/optical jets similar to the one observed in M 87 (knotty structure, radiation spectra, energetics etc). We show that supernova remnants and red giant stars give at most a minor contribution to the jets' structure. Line-emitting clouds are the most probable obstacle candidates.