A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets

We compare results from a relativistic and a nonrelativistic set of 2Daxisymmetric jet simulations. For a set of five relativistic simulations thateither increase the Lorentz factor or decrease the adiabatic index we computenonrelativistic simulations with equal useful power or thrust. We examine thesesimulations for morphological and dynamical differences, focusing on thevelocity field, the width of the cocoon, the age of the jets, and the internalstructure of the jet itself. The primary result of these comparisons is thatthe velocity field of nonrelativistic jet simulations cannot be scaled up togive the spatial distribution of Lorentz factors seen in relativisticsimulations. Since the local Lorentz factor plays a major role in determiningthe total intensity for parsec scale extragalactic jets, this suggests that anonrelativistic simulation cannot yield the proper intensity distribution for arelativistic jet. Another general result is that each relativistic jet and itsnonrelativistic equivalents have similar ages (in dynamical time units, =R/a_a, where R is the initial radius of a cylindrical jet and a_a is the soundspeed in the ambient medium). In addition to these comparisons, we havecompleted four new relativistic simulations to investigate the effect ofvarying thermal pressure on relativistic jets. The simulations generallyconfirm that faster (larger Lorentz factor) and colder jets are more stable,with smaller amplitude and longer wavelength internal variations. The apparentstability of these jets does not follow from linear normal mode analysis, whichsuggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)