FR0 jets and recollimation-induced instabilities

Context. The recently discovered population of faint Fanaroff-Riley type 0 (FR0) radio galaxies has been interpreted as the extension to low power of the classical FRI sources. Their radio emission appears to be concentrated in very compact parsec scale cores, any extended emission is very weak or absent, and very-long-baseline interferometry (VLBI) observations show that jets are already mildly or sub-relativistic at parsec scales. Based on these observational properties, we propose here that the jets of FR0s are strongly decelerated and disturbed at the parsec scale by hydrodynamical instabilities. Aims. With the above scenario in mind, we studied the dynamics of a low-power relativistic jet propagating into a confining external medium, focusing on the effects of entrainment and mixing promoted by the instabilities developing at the jet-environment interface downstream of a recollimation shock. Methods. We performed a 3D relativistic hydrodynamical simulation of a recollimated jet by means of the state-of-the-art code PLUTO. The jet was initially conical, relativistic (with an initial Lorentz factor Γ = 5), cold, and light with respect to the confining medium, whose pressure is assumed to slowly decline with distance. The magnetic field is assumed to be dynamically unimportant. Results. The 3D simulation shows that, after the first recollimation and reflection shock system, a rapidly growing instability develops, as a result of the interplay between recollimation-induced instabilities and Richtmyer-Meshkov modes. In turn, the instabilities promote strong mixing and entrainment that rapidly lead to the deceleration of the jet and spread its momentum to slowly moving, highly turbulent external gas. We argue that this mechanism could account for the peculiarities of the low-power FR0 jets. For outflows with a higher power, Lorentz factor, or magnetic field, we expect that the destabilizing effects are less effective, allowing the survival of the jet up to the kiloparsec scale, as observed in FRIs.