Simulating electron transport and synchrotron emission in radio galaxies: Shock acceleration and synchrotron aging in three-dimensional flows

Abstract

We present the first three-dimensional MHD radio galaxy simulations that explicitly model transport of relativistic electrons, including diffusive acceleration at shocks as well as radiative and adiabatic cooling in smooth flows. We discuss here three simulations of light Mach 8 jets, designed to explore the effects of shock acceleration and radiative aging on the nonthermal particle populations that give rise to synchrotron and inverse-Compton radiations. Because our goal is to explore the connection between the large-scale flow dynamics and the small-scale physics underlying the observed emissions from real radio galaxies, we combine the magnetic field and relativistic electron momentum distribution information to compute an approximate but self-consistent synchrotron emissivity and produce detailed synthetic radio telescope observations. We have gained several key insights from this approach: (1) The jet head in these multidimensional simulations is an extremely complex environment. The classical jet termination shock is often absent, but motions of the jet terminus spin a "shock-web complex" within the backflowing jet material of the head. (2) Correct interpretation of the spectral distribution of energetic electrons in these simulations relies partly upon understanding the shock-web complex, for it can give rise to distributions that confound interpretation in terms of the standard model for radiative aging of radio galaxies. (3) The magnetic field outside of the jet itself becomes very intermittent and filamentary in these simulations, yet adiabatic expansion causes most of the cocoon volume to be occupied by field strengths considerably diminished below the nominal jet value. Radiative aging is very slow in these volumes, so population aging rates vary considerably from point to point. (4) Overall, the intricate dynamical behaviors in these models make it difficult to capture the histories of the nonthermal particles in broad generalizations. Understanding even the simplest of these models requires attention to details of the flow