Abstract
WATER-VAPOUR maser emission at 22.23508 GHz has been observed in star-forming regions of our own galaxy and in the active nuclei of other galaxies1–5. Maser emission from active galactic nuclei is typically 105 times more luminous than in our galaxy, and the active galaxies concerned always exhibit radio or optical jets ejected from the nuclei. The superluminous masers are not well understood, but appear to be related to the mass outflow6. Using 16,000-channel spectrometers at the Nobeyama 45-m radio telescope, we have found H2O maser emission in the nucleus of the galaxy NGC4258, offset by ± 1,000 km s−1 from the motion of the nucleus itself. This is much higher than the typical velocities of galactic masers (less than a few hundred kilometres per second). We suggest that the high-velocity maser emission in NGC4258 might be from masers orbiting a massive central black hole, or ejected in a bipolar outflow. More intriguingly, the features may not be Doppler in origin, but due to up- and down-shifted frequencies by Raman scattering in a dense plasma. Further high-resolution observations will be needed to distinguish between these possibilities.
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