FR0 jets and recollimation-induced instabilities. (arXiv:2312.08767v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Costa_A/0/1/0/all/0/1">A. Costa</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bodo_G/0/1/0/all/0/1">G. Bodo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tavecchio_F/0/1/0/all/0/1">F. Tavecchio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rossi_P/0/1/0/all/0/1">P. Rossi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Capetti_A/0/1/0/all/0/1">A. Capetti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Massaglia_S/0/1/0/all/0/1">S. Massaglia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sciaccaluga_A/0/1/0/all/0/1">A. Sciaccaluga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Baldi_R/0/1/0/all/0/1">R. D. Baldi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Giovannini_G/0/1/0/all/0/1">G. Giovannini</a>
The recently discovered population of faint FR0 radiogalaxies has been
interpreted as the extension to low power of the classical FRI sources. Their
radio emission appears to be concentrated in very compact (pc-scale) cores, any
extended emission is very weak or absent and VLBI observations show that jets
are already mildly or sub-relativistic at pc scales. Based on these
observational properties we propose here that the jets of FR0s are strongly
decelerated and disturbed at pc scale by hydrodynamical instabilities. With the
above scenario in mind, we study 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. We perform a 3D
relativistic hydrodynamical simulation of a recollimated jet by means of the
state-of-the-art code PLUTO. The jet is initially conical, relativistic (with
initial Lorentz Factor $Gamma$=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.The 3D simulation
shows that, after the first recollimation/reflection shock system, a rapidly
growing instability develops, as a result of the interplay between
Kelvin-Helmholtz and Richtmyer-Meshkov modes. In turn, the instability promotes
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 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 kpc scale, as observed in FRIs.
The recently discovered population of faint FR0 radiogalaxies has been
interpreted as the extension to low power of the classical FRI sources. Their
radio emission appears to be concentrated in very compact (pc-scale) cores, any
extended emission is very weak or absent and VLBI observations show that jets
are already mildly or sub-relativistic at pc scales. Based on these
observational properties we propose here that the jets of FR0s are strongly
decelerated and disturbed at pc scale by hydrodynamical instabilities. With the
above scenario in mind, we study 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. We perform a 3D
relativistic hydrodynamical simulation of a recollimated jet by means of the
state-of-the-art code PLUTO. The jet is initially conical, relativistic (with
initial Lorentz Factor $Gamma$=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.The 3D simulation
shows that, after the first recollimation/reflection shock system, a rapidly
growing instability develops, as a result of the interplay between
Kelvin-Helmholtz and Richtmyer-Meshkov modes. In turn, the instability promotes
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 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 kpc scale, as observed in FRIs.
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