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Astron. Astrophys. 329, 845-852 (1998)

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6. Conclusion

Saikia et al. (1995) have found evidence for a more pronounced intrinsic asymmetry in steep spectrum sources with small linear size. This could be related to the non-homogeneities of the medium in which the jets propagate. In this respect, and if this trend is observed down to the scales of 10-100 pc of the compact GPS sources that we are dealing with, one can speculate whether it has to do with the possibly different number of clouds with which the two jets will collide in their way out from the central object. The asymmetry parameter should be proportional to the fractional fluctuation in the number of clouds [FORMULA] that is equal to [FORMULA]. In Fig. 5 we have plotted [FORMULA] as a function of R for [FORMULA], [FORMULA] pc and [FORMULA] and 2. We see that for small R the fluctuation is large and decreases as R increases until it reaches an asymptotic value for [FORMULA], that is, outside the cloud region. This is expected since the number of clouds the jet encounters is small near the nucleus and increases progressively as it advances through the cloudy medium. We conclude that the model predicts that sources of small size ([FORMULA] pc), being confined by a cloudy medium, should present a more accentuated asymmetry than the bigger ones.

[FIGURE] Fig. 5. Fractional fluctuation in the number of clouds with which the jet collides ([FORMULA]) as a function of the jet size. Labels 0, 1 and 2 are the values of the parameter [FORMULA].

The scenario presented here is one in which a subclass of the GPS radio sources can be confined near the nucleus of the host galaxy after an increasing of the gas density around the central engine has taken place, mainly in the form of clouds. The diffuse outer regions observed around some of them are the remains of the extended radio lobes that fade as their energy supply has been interrupted.

We have proposed three models for the motion of the jet near the galactic nucleus. The jet would propagate either by scattering dense clouds (Model A) or by drilling through the clouds (Model B). These turn out to be quite efficient ways to confine the jet with the result that the age of GPS sources smothered in a dense clumpy medium is of the same order as the age of the extended sources. A dense but otherwise uniform medium (Model C) will confine the et less efficiently. We have also discussed how these results are affected by the various aspects of the model like source power, jet opening angle, mass of the central object and radius and distribution of clouds. Although Models A and B are very schematic, they should represent limiting cases and we expect the real situation will lie between these two extremes. They predict that a subclass of GPS object are confined by a cloudy medium and are as old as the extended classical double, having ages of the order of [FORMULA] yr.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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