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Astron. Astrophys. 336, L21-L24 (1998)

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4. Discussion

Read and Ponman (1997) investigated in detail X-ray properties of a larger sample of nearby spiral galaxies. The integrated X-ray luminosity of all their sample galaxies was less than log [FORMULA]=40.78. In a further paper (Read & Ponman,1998) they determined the X-ray luminosities of prominent merging galaxies in different evolutionary stages: the integrated X-ray luminosities amounted to log [FORMULA]=41.18 at the most. In contrast, the X-ray luminosity of the northern jet in the Mrk 266 system (log [FORMULA]=41.49) is higher than the integrated X-ray luminosities of all the merging galaxies. Furthermore, the X-ray luminosity in the Mrk 266 jet is a factor of ten higher than that of known X-ray jets in galaxies e.g. M87 (Biretta et al.,1991) and of typical galactic super-winds in e.g. M82 (Read and Ponman, 1997). There might be known one extragalactic system showing similar X-ray properties: the interacting galaxy group Stephan's Quintet (Pietsch et al., 1997). But the opt./X-ray morphology is slightly different.

No data have been published so far about interacting/merging galaxies with tidal arms showing comparable V-R colours and/or X-ray fluxes. We are investigating a larger sample of interacting galaxies with active nuclei: none of these shows as extreme properties.

We discuss four possibilities to explain the extra-nuclear X-ray emission in the northern region: starburst in an extra-nuclear region; excitation from one of the active nuclei; `central' starburst with super-wind; photoionisation by a fast shock.

A starburst in a tidal arm region is extremely unlikely: there is no indication for an underlying stellar/starburst component in the continuum flux. The X-ray flux is too high to be explained by a reasonable number of SN (log [FORMULA] =33.-36.) and X-ray binaries (log [FORMULA]=37.5-38.5). Furthermore, the optical/ X-ray morphology and the gas velocities do not support this picture.

Photoionisation by the southern Seyfert nucleus is very unlikely since the excitation of the optical spectra is lower in the intermediate region between the Seyfert nucleus and the jet.

The explanation of the X-ray jet by a super-wind model caused by a central starburst is unlikely, too, because of the non-radial geometry. The opening angle of the X-ray/ [OIII] emission region in Mrk 266 is much smaller in comparison to other known bipolar wind outflows (Suchkov et al.,1994). Additionally, one would expect a radial velocity gradient; this is not to be seen in the emission region of the jet. Wang et al.(1997) preferred the super-wind model to explain their observations of Mrk 266; but their X-ray image was not deep enough to detect the jet and they had no information about spectra and kinematics in the X-ray jet. Furthermore, the X-ray intensity in the jet is too strong to be explained by a super-wind in comparison to other galaxies showing this phenomenon.

Taking into account the optical line intensity ratios, the optical and X-ray morphology, and the X-ray luminosity in Mrk 266, the most plausible mechanism explaining the X-ray jet is excitation by hot postshock gas caused by fast shocks emitting EUV/soft X-ray radiation (Bremsstrahlung) and photoionising the gas (Dopita & Sutherland, 1996). The X-ray jet is redshifted by 300 km [FORMULA] with respect to the innermost galactic region. On the other hand the velocity structure within the jet in south-north direction shows no further outflow signs but solid body rotation of the outer filamentary regions. This might be explained by a helical motion of the jet. The morphology seen on the HST image supports this idea. On spectra taken along the major axis of the jet it turns out that the gaseous velocity field is retarded at the end of the jet. The large scale curved structure of the jet seen in the HST and X-ray images is likely to be caused by precessional motions of the merging system.

It is not clear where the energy source generating the jet is located. The HST image supports the idea that the jet originates at the central radio source between the two optical nuclei. On the other hand no optical or near-infrared K-band counterpart (McLeod and Rieke, 1995) has been found so far. Mazzarella et al. (1988) proposed that the central radio emission is produced by synchrotron emission stimulated by the collision of two merging galaxies. But the question regarding the energy source of the jet needs further detailed investigations. The jet might be generated in the outer parts of the prograde interacting disks of system. With time more energy is added and eventually elongates the direction of the steepest density and pressure gradients. During the break out process the disk might play an important role in focusing the flow in the direction of the steepest density and pressure gradients out of the merging system. The high-velocity jet drives a shock into the halo gas leading to X-ray emission at the shock interaction region.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998