Discovery of a luminous water megamaser in the FR IIradiogalaxy 3C 403

We report the first detection of a water megamaser in a radio-loud galaxy, 3C 403. This object has been observed as part of a small sample of FR IIs with evidence of nuclear obscuration. The isotropic luminosity of the maser is ~ 1200 \solum. With a recessional velocity of cz ~ 17 680 km s-1 it is the most distant H2O maser so far reported. The line arises from the densest interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are observed, likely bracketing the systemic velocity of the galaxy. These may either arise from the tangentially seen parts of a nuclear accretion disk or from dense warm gas interacting with the radio jets.