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

TL;DR

High-resolution radio imaging reveals that the 'dark lens' galaxy J1218+2953 hosts an active galactic nucleus, indicating it is a heavily obscured galaxy with early-stage nuclear activity rather than a dark matter-dominated lens.

Contribution

This study provides the first high-resolution radio imaging evidence of an AGN in J1218+2953, challenging its classification as a dark lens and suggesting it is a heavily obscured galaxy.

Findings

J1218+2953 hosts a compact steep-spectrum radio source

The galaxy contains an active galactic nucleus (AGN)

It is likely a heavily obscured galaxy in early evolutionary stage

Abstract

Context: There is a possibility that the optically unidentified radio source J1218+2953 may act as a gravitational lens, producing an optical arc ~4" away from the radio position. Until now, the nature of the lensing object has been uncertain since it is not detected in any waveband other than the radio. The estimated high mass-to-light ratio could even allow the total mass of this galaxy to be primarily in the form of dark matter. In this case, J1218+2953 could be the first known example of a "dark lens". Aims: We investigate the nature of J1218+2953 by means of high-resolution radio imaging observations to determine whether there is a radio-loud active galactic nucleus (AGN) in the position of the lensing object. Methods: We report on Very Long Baseline Interferometry (VLBI) observations with 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". Based on its radio spectrum and structure, J1218+2953 can be classified as a compact steep-spectrum (CSS) source, and as a medium-size symmetric object (MSO). The object harbours an AGN. It is also found as an X-ray source in the XMM-Newton EPIC (European Photon Imaging Cameras) instrument serendipitous source catalogue. Conclusions: Rather than being a dark lens, J1218+2953 is most likely a massive, heavily obscured galaxy in which the nuclear activity is currently in an early evolutionary stage.