Signatures of restarted activity in core-dominated, triple radio sources selected from the FIRST surveyA. Marecki1, P. Thomasson2, K.-H. Mack3 and M. Kunert-Bajraszewska1
1 Torun Centre for Astronomy, N. Copernicus University, 87-100 Torun, Poland
2 Jodrell Bank Observatory, The University of Manchester, Macclesfield, Cheshire, SK11 9DL, UK
3 INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
(Received 5 August 2005 / Accepted 21 October 2005)
Signatures of the re-occurrence of activity in radio-loud AGNs, indicated either by the so-called double-double or X-shaped structures, have been observed in a number of radio sources. All such objects known to date have linear sizes of the order of a megaparsec. A number of the sources that are appreciably more compact than this, but that exhibit hints of a past phase of activity, were found in the VLA FIRST survey. Their structures show symmetric relic lobes straddling relatively bright, unresolved cores. Observations of the cores of 15 such structures with MERLIN at 5 GHz have shown that four of them are doubles or core-jets on the subarcsecond scale. Misalignments of between the axis of the inner structure and the line connecting the fitted maxima of the arcminute-scale relic lobes are clearly visible in three of the four sources. From these results, we can infer that a rapid repositioning of the central engine in each of these three radio sources is the most plausible interpretation of the observed morphology and that a merger is most likely the original cause of such a repositioning. In the case of TXS 1033+026, the optical image extracted from the SDSS archives clearly suggests that two objects separated by only 2.7 kpc (projected onto the sky plane) are indeed merging. The inner parts of TXS 0818+214 and TXS 1312+563 could be interpreted as double-lobed, and consequently, these sources could be of the double-double type; but further multifrequency observations are necessary to provide support for such an interpretation.
Key words: radio continuum: galaxies -- galaxies: active
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