Formation of dynamical structures in relativistic jets: the FRI case

Context. Strong observational evidence indicates that all extragalactic jets associated with AGNs move at relativistic speed up to 100 pc-1 kpc scales from the nucleus. At greater distances, reflecting the Fanaroff-Riley radio source classification, we observe an abrupt deceleration in FR-I jets while relativistic motions persist up to Mpc scale in FR-II. Moreover, VLBI observations of some object such as B2 1144+35, Mrk501 and M87 show limb brightening of the jet radio emission on the parsec scale. This effect is interpreted kinematically as due to the deboosted central spine at high Lorentz factor and of a weakly relativistic external layer. Aims. In this paper we investigate whether these effects can be interpreted by a braking of the collimated flow by external medium entrainment favored by shear instabilities, namely Kelvin-Helmholtz instabilities. We examine in details the physical conditions under which significant deceleration of a relativistic flow is produced. Methods. We investigated the phenomenon by means of high-resolution three-dimensional relativistic hydrodynamic simulations using the PLUTO code for computational astrophysics. Results. We find that the most important parameter in determining the instability evolution and the entrainment properties is the ambient/jet density contrast. We show that lighter jets suffer stronger slowing down in the external layer than in the central part and conserve a central spine with a high Lorentz factor. Conclusions. Our model is verified by constructing synthetic emission maps from the numerical simulations which compare reasonably well with VLBI observations of the inner part of FR-I sources.