Active galactic nuclei.
Active galactic nuclei are the most powerful, long-lived objects in the Universe. Recent data confirm the theoretical idea that the power source is accretion into a massive black hole. The common occurrence of obscuration and outflows probably means that the contribution of active galactic nuclei to the power density of the Universe has been generally underestimated.
Fabian
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Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdom.