Kelvin-Helmholtz instabilities in radio jets

Models in which the lobes of radio galaxies are powered by continuous beams from the underlying optical galaxy have proved very successful. An important question with these theories is the possibly disruptive effects of Kelvin–Helmholtz instabilities occurring in the vortex sheet between the beam and the external medium. We consider beams with and without axial magnetic fields. If there is no field it is shown that, when Mi (the beam Mach number) ∼ O (1), pinching modes, peculiar to cylindrical geometry, can lead to the breaking up of the beam. When Mi is in the range 3–10, pinching modes can be disregarded, although helical ones can still cause disruption. We then consider the effects of an axial field. It is found that magnetic energy densities in equipartition with the bulk kinetic energy of the beam do not prevent the growth of pinching modes, but can prevent the helical modes from growing. (It is emphasized that ‘equipartition’ in this paper usually refers to equipartition in the above sense.) As an example, the twin tail radio galaxy NGC 1265 (3C83.1B) is examined and it is shown that its internal field could play a very significant role in limiting the growth of the above instabilities.