Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Axisymmetric Flows

We introduce a simple and economical but effective method for includingrelativistic electron transport in multi-dimensional simulations of radiogalaxies. The method is designed to follow explicitly diffusive acceleration atshocks, and, in smooth flows 2nd order Fermi acceleration plus adiabatic andsynchrotron cooling. We are able to follow both the spatial and energydistributions of the electrons, so that direct synchrotron emission propertiescan be modeled in time-dependent flows for the first time. Here we present first results in the form of some axis-symmetric MHDsimulations of Mach 20 light jet flows. These show clearly the importance ofnonsteady terminal shocks that develop in such flows even when the jet inflowis steady. As a result of this and other consequences of the fundamentallydriven character of jets, we find complex patterns of emissivities andsynchrotron spectra, including steep spectral gradients in hot spots, islandsof distinct spectra electrons within the lobes and spectral gradients comingfrom the dynamical histories of a given flow element rather than fromsynchrotron aging of the embedded electrons. In addition, spectral aging in thelobes tends to proceed more slowly than one would estimate from regions of highemissivity.Comment: 30 pages of Latex generated text plus 7 figures in gif format. Accepted for publication in the Astrophysical Journal. High resolution postscript figures available through anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/RGje