UNDERSTANDING THE KILOPARSEC-SCALE STRUCTURE OF M87

Motivated by the determination of proper motions in the M87 jet by Bir etta, Zhou, & Owen, we have investigated the relationship between the dynamics of the jet in M87 and the kiloparsec-scale radio and optical structure. Our results show that relativistic effects play an importan t role in the appearance of shocks in relativistic jets. We argue that knot A in the M87 jet, which appears to be almost transverse to the f low, is in fact a highly oblique shock, and that the direction of its normal with respect to the flow is within about 10 degrees of the Mach angle (the angle beyond which no shock is possible). A modest pressur e jump less than or similar to 5 at knot A is consistent with the obse rved small jet deflection for bulk Lorentz factors of the order of 3-5 . We suggest that helical modes of the Kelvin-Helmholtz instability ar e responsible for the development of oblique shocks in the jet, provid ed that the medium external to the jet is no more than about 10-100 ti mes denser than the jet. This implies that the kiloparsec-scale radio lobes are much less dense than the interstellar medium in the central regions of the M87 cooling flow. The radio jets in M87 are therefore d riving high-pressure, low-density bubbles into the surrounding ISM. We show that the radii of the lobes are consistent with this interpretat ion and the inferred energy flux in the jet if the age of the inner lo bes is similar to 10(6) yr, in agreement with that estimated by Turlan d. The resultant expansion of the lobes can comfortably power the exci tation of the surrounding optical filaments via radiative shocks. The initial conical expansion of the M87 jet may be due to mass injection by stars along its trajectory and the thermal density of the gas withi n the radio lobes may be the result of a combination of mass loss from stars over the similar to 10(6) yr lifetime of this region of the rad io source and the sweeping up of clouds that have condensed out of the interstellar medium. Since the knots in the M87 jet appear to be regi ons of transient overpressure inside an overpressured bubble, it is no t necessary to invoke magnetic confinement of the jet. The fact that t he inner lobes of M87 appear to be comparatively young regions immerse d in a much larger and older structure, evident at low frequency, sugg ests that the output from the black hole in M87 fluctuates on a timesc ale of 10(6)-10(7) yr. Intermittency may result from the ejected radio plasma ''choking off'' the mass accretion into the nucleus via the co oling flow.