The usual Fanaroff-Riley (FR) type-II structure of a radio galaxy can be substantially modified by cessation and a subsequent re-ignition of the activity. The signature of such an event is most convincing if it takes the form of a smaller double embedded by a larger, double-lobed structure. If both doubles share the common centre and are well aligned the whole object is labelled a double–double radio galaxy (DDRG). It can be, however, that we observe a DDRG at an early stage of the development of the inner double so that it is not resolved in the images showing the overall structure. Thus, “at first sight”, the source of such kind appears as a core-dominated triple (CDT). We carried out a systematic search through the FIRST catalogue for CDTs by means of an automated procedure and we selected several sources for a follow-up observations with MERLIN at 5 GHz. They revealed one source of particular interest: 0818+214.
In the FIRST image, 0818+214 appears as a CDT (Fig. 1, left panel). To explain the nature of its bright “core”, i.e. to answer the question if 0818+214 is actually a DDRG or not, we carried out an EVN+MERLIN observation at 18 cm. The resulting image of the “core” of 0818+214 (Fig. 1, right panel) shows it as a double located at position angle of −40 degrees . Given that the two components are asymmetric – the northwestern one is appreciably more distinguished than the southeastern one – it could be that the subarcsecond-scale structure is of a core-jet type. However, after combining the 18-cm EVN+MERLIN image with our earlier 6-cm MERLIN image and calculating the spectral indices, the inner structure of 0818+214 proves to be a mini-FR II, and the object as a whole turns out to be the most compact (in terms of the span of the outer lobes) DDRG to date. Given that DDRGs are known to be giant (Mpc-sized) radio sources, the case of 0818+214, which is clearly less than 1 Mpc, poses a challenge for the theory of DDRGs.
Marecki A., Szablewski M. (2009) Astronomy & Astrophysics 506, L33
Fig.1 0818+214 as seen by the VLA (left) and the EVN+MERLIN (right). The right panel shows a 100x magnified view of the component seen in the centre of the VLA image.