Introduction

In Chapters 1 through 4, we saw that the outer radio lobes associated with active galaxies receive their energy from a bulk hydrodynamic flow which emanates from the galactic nucleus. Bisymmetric outflow occurs on a wide range of scales in less energetic objects as well, as will be shown in Chapter 10.

Perhaps the most astonishing feature of cosmic jets is their ability to stay together over a very large range of distance scales. On their way from the black hole in the galactic nucleus to a radio lobe, cosmic jets cover a stupendous factor 109 in length scale. That is as if, exhaling forcefully at my desk in Leiden, I could blow about the papers on the desk of a colleague in Minneapolis. This should be a caution against off-the-cuff comparison between jets in radio galaxies and such comparatively easily understood items as laboratory jets, rocket exhausts, and numerical simulations.

The most natural explanation of the coherence of jets is that they are not jets at all, but gaseous cannonballs with a density that is much higher than that of their surroundings. What we perceive as jets would be a mixture of gas ablated from these “tracer bullets” and surrounding gas set aglow by their passage.