Faranoff-Riley type I jet deceleration at density discontinuities Relativistic hydrodynamics with realistic equation of state
نویسندگان
چکیده
Context. The deceleration mechanisms for relativistic jets in active galactic nuclei remain an open question, and in this paper we propose a model which could explain sudden jet deceleration, invoking density discontinuities. This is particularly motivated by recent indications from HYbrid MOrphology Radio Sources, suggesting that in some case Fanaroff-Riley classification is induced by variations in density of the external medium. Aims. Exploiting high resolution, numerical simulations, we demonstrate that for both high and low energy jets, always at high Lorentz factor, a transition to a higher density environment can cause a significant fraction of the directed jet energy to be lost on reflection. This can explain how one-sided jet deceleration and a transition to FR I type can occur in HYbrid MOrphology Radio Sources, which start as FR II (and remain so on the other side). Methods. For that purpose, we implemented in the relativistic hydrodynamic grid-adaptive AMRVAC code, the Syngetype equation of state introduced in the general polytropic case by Meliani et al. (2004). To demonstrate its accuracy, we set up various test problems in appendix, which we compare to exact solutions that we calculate as well. We use the code to analyse the deceleration of jets in FR II/FR I radio galaxies, following them at high resolution across several hundreds of jet beam radii. Results. We present results for 10 model computations, varying the inlet Lorentz factor from 10 to 20, including uniform or decreasing density profiles, and allowing for cylindrical versus conical jet models. As long as the jet propagates through uniform media, we find that the density contrast sets most of the propagation characteristics, fully consistent with previous modeling efforts. When the jet runs into a denser medium, we find a clear distinction in the decelaration of high energy jets depending on the encountered density jump. For fairly high density contrast, the jet becomes destabilised and compressed, decelerates strongly (up to subrelativistic speeds) and can form knots. If the density contrast is too weak, the high energy jets continue with FR II characteristics. The trend is similar for the low energy jet models, which start as underdense jets from the outset, and decelerate by entrainment in the lower region as well. We point out differences that are found between cylindrical and conical jet models, together with dynamical details like the Richtmyer-Meshkov instabilities developing at the original contact interface.
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