3.7. Thrust and Energy Balance
If it is supposed that radio jets are bent by ram pressure of motion of the parent galaxy through the intergalactic medium, then the relation:
applies, where j and ext are the jet and external densities, vj is the jet velocity, vg is the velocity of the galaxy through the IGM, R is the radius of curvature of the bend and h is a characteristic scale height. h may be the width of the jet or the scale of the parent galaxy's interstellar medium. For example, O'Dea (1985) estimates that the jet thrust for NGC 1265 is between 1028 and 1029 N, the limits corresponding to extreme assumptions about h. In FRII sources, a similar argument can be applied to ram-pressure balance at the hot-spots.
Estimates of the total energy supplied to the source and its lifetime can be used to derive the energy flux through the jet. There are several possible methods (Leahy 1991), none of any great accuracy. Energy fluxes in the range 1036.5 - 1039.5W are estimated for FRII sources (Rawlings & Saunders 1991; Leahy 1991); values for FRI sources are < 1036W.
The theme of this section has been that diagnostics of physical conditions in extended extragalactic radio sources are extremely unreliable and that almost all of the methods used to derive velocities for jets are seriously flawed. For this reason, the next section considers a completely different approach to the velocity problem.