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We report the discovery of extremely broad 21-cm HI absorption (FWZI ~1600km/s) detected with the Westerbork Synthesis Radio Telescope in the radiosource 4C37.11 (B2 0402+379). This object has been claimed to host asuper-massive binary black hole (Rodriguez et al. 2006). The main features inthe absorption profile are two components, separated by ~1100 km/s. The HIabsorption in 4C37.11 is unusual because it is the first case where such broadabsorption is found to be centred on the systemic velocity of the host galaxyand not asymmetric and blueshifted as is seen in all other galaxies with broadHI absorption. Given the large width of the absorption, we suggest that apossible explanation for the extreme properties of the HI absorption is that itis the kinematic signature of a binary black hole. If this interpretation iscorrect, the combined black hole mass derived from the absorption profile isconsistent with that derived from the luminosity of the spheroid. If the broadabsorption is indeed due to a binary black hole, this finding confirms theimportance of the gaseous component in the merging process of supermassiveblack holes.

Broad HI absorption in the candidate binary black-hole 4C37.11 (B2 0402+379)