2012A&A...545A..65P

The remarkable stability of extragalactic jets is surprising, given the reasonable possibility of the growth of instabilities. In addition, much work in the literature has invoked this possibility to explain observed jet structures and obtain information about the jet from these structures. For example, it has been shown that the observed helical structures in the jet in S5 0836+710 could be associated with helical pressure waves generated by a Kelvin-Helmholtz instability. Our aim is to resolve the arc-second structure of the jet in the quasar S5 0836+710 and confirm the lack of a hot-spot (reverse jet-shock) found by present observing arrays, as this lack implies a loss of jet collimation before interaction with the intergalactic medium. We use an observation performed in 2008 using EVN (European Very Long Baseline Interferometry Network) and MERLIN (Multi-Element Radio Linked Interferometer Network). The resultant combined image after data reduction has provided a complete image of the object on arc-second scales. The lack of a hot-spot in the arc-second radio structure is taken as evidence that the jet loses its collimation between the VLBI (Very Long Baseline Interferometry) region and the region of interaction with the ambient medium. This result and the previous identification of the helical structures in the jet with helical pressure waves that grow in amplitude with distance allow us to conclude that the jet is probably disrupted by the growth of Kelvin-Helmholtz instability. This observational evidence confirms that the physical parameters of jets can be extracted using the assumption that instability is present in jets and can be the reason for many observed structures. Interestingly, the observed jet is classified as a FRII (Fanaroff-Riley type II) object in terms of its luminosity, but its large-scale morphology does not correspond to this classification. The implications of this finding are discussed.