Context. Relativistic jets emerging from active galactic nuclei (AGN) cores transfer energy from the core of the AGN to their surrounding interstellar/intergalactic medium through shock-related and hydrodynamic instability mechanisms. Because jets are observed to have finite opening angles, one needs to quantify the role of conical versus cylindrical jet propagation in this energy transfer.
Aims. We adopt parameters representative for Faranoff-Riley class II AGN jets with finite opening angles. We study how such an opening angle affects the overall dynamics of the jet and its interaction with its surrounding medium and therefore how it influences the energy transfer between the AGN and the external medium. We also point out how the characteristics of this external medium, such as its density profile, play a role in the dynamics.
Methods. This study exploits our parallel adaptive mesh refinement code MPI-AMRVAC with its special relativistic hydrodynamic model, incorporating an equation of state with varying effective polytropic index. We initially studied mildly underdense jets up to opening angles of 10 degrees, at Lorentz factors of about 10, inspired by input parameters derived from observations. Instantaneous quantifications of the various interstellar medium (ISM) volumes affected by jet injection and their energy content allows one to quantify the role of mixing versus shock-heated cocoon regions over the simulated time intervals.
Results. We show that a wider opening angle jet results in a faster deceleration of the jet and leads to a wider radial expansion zone dominated by Kelvin-Helmholtz and Rayleigh-Taylor instabilities. The energy transfer mainly occurs in the shocked ISM region by both the frontal bow shock and cocoon-traversing shock waves, in a roughly 3 to 1 ratio to the energy transfer of the mixing zone, for a 5 degree opening angle jet. The formation of knots along the jet may be related to X-ray emission blobs known from observations. A rarefaction wave induces a dynamically formed layered structure of the jet beam.
Conclusions. Finite opening angle jets can efficiently transfer significant fractions (25% up to 70%) of their injected energy over a growing region of shocked ISM matter. The role of the ISM stratification is prominent for determining the overall volume that is affected by relativistic jet injection. While our current 2D simulations give us clear insights into the propagation characteristics of finite opening angle, hydrodynamic relativistic jets, we need to expand this work to 3D.