A LARGE NUCLEAR ACCRETION DISK IN THE ACTIVE GALAXY NGC4261

THE powerful emissions from the nuclei of active galaxies and quasars are thought to arise from the accretion of matter onto a massive black hole. Angular momentum will prevent matter from falling directly onto the central mass; instead, an 'accretion disk' should form, within which the gravitationally bound material will lose angular momentum and gradually spiral inwards. Accretion disks in active galactic nuclei have not hitherto been observed directly, but theoretical models1 suggest that they are comparable in size to the gravitational radius of the central black hole (about 10(13) cm) and are heated to high temperatures by the energy released from the accreting matter. Using the Planetary Camera on the Hubble Space Telescope, we have discovered an unexpectedly large (2 x 10(20) cm radius) disk of cool dust and gas surrounding a bright unresolved nucleus in the active galaxy NGC4261. We suggest that the bright point corresponds to thermal emission from the hot disk favoured by theoreticians, and that this is fuelled by material flowing from the cool 'outer' accretion disk. The spin axis of the accretion disk appears to determine the direction of the galaxy's radio jets.