  Extragalactic jets are formed close to supermassive black-holes in the center
of galaxies. Large amounts of gas, dust, and stars cluster in the galaxy
nucleus, and interactions between this ambient material and the jet base should
be frequent, having dynamical as well as radiative consequences. This work
studies the dynamical interaction of an obstacle, a clump of matter or the
atmosphere of an evolved star, with the innermost region of an extragalactic
jet. Jet mass-loading and the high-energy outcome of this interaction are
briefly discussed. Relativistic hydrodynamical simulations with axial symmetry
have been carried out for homogeneous and inhomogeneous obstacles inside a
relativistic jet. These obstacles may represent a medium inhomogeneity or the
disrupted atmosphere of a red giant star. Once inside the jet, an homogeneous
obstacle expands and gets disrupted after few dynamical timescales, whereas in
the inhomogeneous case, a solid core can smoothen the process, with the
obstacle mass-loss dominated by a dense and narrow tail pointing in the
direction of the jet. In either case, matter is expected to accelerate and
eventually get incorporated to the jet. Particles can be accelerated in the
interaction region, and produce variable gamma-rays in the ambient matter,
magnetic and photon fields. The presence of matter clumps or red giants into
the base of an extragalactic jet likely implies significant jet mass-loading
and slowing down. Fast flare-like gamma-ray events, and some level of
persistent emission, are expected due to these interactions.
