The Making of FR Is I. Numerical Hydrodynamic 3D Simulations of Low Power Jets

Extragalactic radiosources have been classified in two classes, Fanaroff-Riley I and II, which differ in morphology and radio power. Strongly emitting sources belong to the edge brightened FR II class while the weak ones to the edge darkened FR I class. origin of this dichotomy is not yet fully understood. Numerical simulations are successful in generating FR~II morphologies but they fail to reproduce the diffuse structure of FR Is. By means of hydro-dynamical 3D simulations of supersonic jets, we investigate how the displayed morphologies depend on the jet parameters. Bow shocks and Mach disks at the jetu0027s head, likely responsible for the presence of hot spots in the FR II sources, disappear for a jet kinetic power less than 10^43 erg/s. This threshold compares favorably with the luminosity at which the FR~I/FR~II transition is observed. The problem is addressed by numerical means carrying out three-dimensional HD simulations of supersonic jets that propagate in a non homogeneous medium with the ambient temperature that increases with distance from the jet origin, maintaining pressure constant. In the lower power sources, the jet energy, instead of being deposited at the terminal shock, is gradually dissipated by the turbulence. jets spreads out while propagating, it smoothly decelerates while mixing with the ambient medium and produces the plumes characteristic of FR I objects. Three-dimensionality is an essential ingredient to explore the FR I evolution, because the properties of turbulence in two and three dimensions are very different, since in two dimensions there is no energy cascade to small scales and two-dimensional simulations with the same parameters, lead to FRII like behavior.