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Introduction to active galaxies
Introduction to active galaxies

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7.5 Outstanding issues

  • Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes.

  • Quasars were most abundant at redshifts of 2-3 and have been declining in number for the last 10 billion years.

  • It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenated as a result of galactic collisions or mergers.

  • Supermassive black holes found in the nuclei of the Milky Way and other galaxies may be the remnants of extinct AGNs.