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Context: There is a possibility that the optically unidentified radio sourceJ1218+2953 may act as a gravitational lens, producing an optical arc ~4" awayfrom the radio position. Until now, the nature of the lensing object has beenuncertain since it is not detected in any waveband other than the radio. Theestimated high mass-to-light ratio could even allow the total mass of thisgalaxy to be primarily in the form of dark matter. In this case, J1218+2953could be the first known example of a "dark lens".  Aims: We investigate the nature of J1218+2953 by means of high-resolutionradio imaging observations to determine whether there is a radio-loud activegalactic nucleus (AGN) in the position of the lensing object.  Methods: We report on Very Long Baseline Interferometry (VLBI) observationswith the European VLBI Network (EVN) at 1.6 and 5 GHz.  Results: Our images, having angular resolutions of ~1 to ~10 milli-arcseconds(mas), reveal a rich and complex radio structure extending to almost 1". Basedon its radio spectrum and structure, J1218+2953 can be classified as a compactsteep-spectrum (CSS) source, and as a medium-size symmetric object (MSO). Theobject harbours an AGN. It is also found as an X-ray source in the XMM-NewtonEPIC (European Photon Imaging Cameras) instrument serendipitous sourcecatalogue.  Conclusions: Rather than being a dark lens, J1218+2953 is most likely amassive, heavily obscured galaxy in which the nuclear activity is currently inan early evolutionary stage.

The radio-loud active nucleus in the "dark lens" galaxy J1218+2953