Very light magnetized jets on large scales - I. Evolution and magnetic fields

V. Gaibler, M. Krause, M. Camenzind

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Magnetic fields, which are undoubtedly present in extragalactic jets and responsible for the
observed synchrotron radiation, can affect the morphology and dynamics of the jets and their
interaction with the ambient cluster medium. We examine the jet propagation, morphology
and magnetic field structure for a wide range of density contrasts, using a globally consistent
setup for both the jet interaction and the magnetic field. The magnetohydrodynamic code
NIRVANA is used to evolve the simulation, using the constrained transport method. The density
contrasts are varied between η = 10−1 and 10−4 with constant sonic Mach number 6. The jets
are supermagnetosonic and simulated bipolarly due to the low jet densities and their strong
backflows. The helical magnetic field is largely confined to the jet, leaving the ambient medium
non-magnetic. We find magnetic fields with plasma β ∼ 10 already stabilize and widen the jet
head. Furthermore, they are efficiently amplified by a shearing mechanism in the jet head and
are strong enough to damp Kelvin–Helmholtz instabilities of the contact discontinuity. The
cocoon magnetic fields are found to be stronger than expected from simple flux conservation
and capable to produce smoother lobes, as found observationally. The bow shocks and jet
lengths evolve self-similarly. The radio cocoon aspect ratios are generally higher for heavier
jets and grow only slowly (roughly self-similar) while overpressured, but much faster when
they approach pressure balance with the ambient medium. In this regime, self-similar models
can no longer be applied. Bow shocks are found to be of low eccentricity for very light jets
and have low Mach numbers. Cocoon turbulence and a dissolving bow shock create and excite
waves and ripples in the ambient gas. Thermalization is found to be very efficient for low jet
Original languageEnglish
Pages (from-to)1785-1802
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date8 Dec 2009
Publication statusPublished - 21 Dec 2009


  • magnetic fields , MHD , methods: numerical , galaxies: clusters: general , galaxies: jets , radio continuum: galaxies


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