A comparison of the morphology and stability of relativistic and nonrelativistic jets

We compare results from a relativistic and a nonrelativistic set of two-dim ensional axisymmetric jet simulations. For a set of five relativistic simul ations that either increase the Lorentz factor or decrease the adiabatic in dex, we compute nonrelativistic simulations with equally useful power or th rust. We examine these simulations for morphological and dynamical differen ces, focusing on the velocity held, the width of the cocoon, the age of the jets, and the internal structure of the jet itself. The primary result of these comparisons is that the velocity held of nonrelativistic jet simulati ons cannot be scaled up to give the spatial distribution of Lorentz factors seen in relativistic simulations. Since the local Lorentz factor plays a m ajor role in determining the total intensity for parsec-scale extragalactic jets, this suggests that a nonrelativistic simulation cannot yield the pro per intensity distribution for a relativistic jet. Another general result i s that each relativistic jet and its nonrelativistic equivalents have simil ar ages (in dynamical time units, =R/a(a), where R is the initial radius of a cylindrical jet and a(a) is the sound speed in the ambient medium). Also , jets with a larger Lorentz factor have a smaller cocoon size. In addition to these comparisons, we have completed four new relativistic simulations to investigate the effect of varying thermal pressure on relativistic jets. The simulations confirm that faster (larger Lorentz factor) and colder jet s are more stable, with smaller amplitude and longer wavelength internal va riations. However, an exception to this occurs for the hottest jets, which appear the most stable. The apparent stability of these jets does not follo w from linear normal mode analysis, which suggests that there are available growing Kelvin-Helmholtz modes. However, these modes are not excited becau se of a lack of perturbations able to couple to them. As an example of how these simulations can be applied to the interpretation of observations, we use our results to estimate some parameters of Cygnus A. Although none of t hese estimates alone can determine if the jets in Cyg A are relativistic or nonrelativistic, estimates for the age and the jet to ambient density rati o confirm values for these parameters estimated by other means.