A&A 407, L33-L36 (2003)
DOI: 10.1051/0004-6361:20031062

Letter

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

A. Tarchi^{1, 2}, C. Henkel³, M. Chiaberge^{1, 4} and K. M. Menten<sup>3

1</sup>  Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129 Bologna, Italy
²  Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
³  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

(Received 26 May 2003 / Accepted 3 July 2003)

Abstract
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 . With a recessional velocity of c z ~ 17 680 km s ^-1 it is the most distant H ₂O 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.

Key words: galaxies: individual: 3C 403 -- galaxies: active -- galaxies: ISM -- masers -- radio lines: ISM -- radio lines: galaxies

Offprint request: A. Tarchi, a.tarchi@ira.cnr.it

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