A numerical simulation of the evolution and fate of a Fanaroff-Riley type I jet. The case of 3C 31

The evolution of Fanaroff-Riley type I (FRI) jets has long been studied in the framework of the FRI-FRII dichotomy. The present paradigm consists of the expansion of overpressured jets in the ambient medium and the generation of standing recollimation shocks, followed by mass entrainment from the external medium that decelerates the jets to subsonic speeds. In this paper, we test the present theoretical and observational models via a relativistic numerical simulation of the jets in the radio galaxy 3C 31. We use the parameters derived from the modelling presented by Laing & Bridle as input parameters for the simulation of the evolution of the source, thus assuming that they have not varied over the lifetime of the source. We simulate about 10 per cent of the total lifetime of the jets in 3C 31. Realistic density and pressure gradients for the atmosphere are used. The simulation includes an equation of state for a two-component relativistic gas that allows a separate treatment of leptonic and baryonic matter. We compare our results with the modelling of the observational data of the source. Our results show that the bow shock evolves self-similarly at a quasi-constant speed, with slight deceleration by the end of the simulation, in agreement with recent X-ray observations that show the presence of bow shocks in FRI sources. The jet expands until it becomes underpressured with respect to the ambient medium, and then recollimates. Subsequent oscillations around pressure equilibrium and generation of standing shocks lead to the mass-loading and disruption of the jet flow. We derive an estimate for the minimum age of the source of t > 1 × 10⁸yr, which may imply continuous activity of 3C 31 since the triggering of its activity. The simulation shows that weak compact steep spectrum sources may be the young counterparts of FRI sources. We conclude that the observed properties of the jets in 3C 31 are basically recovered by the standing shock scenario.