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Active galaxies

Light jets near light speed

Nature volume 395, pages 440–441 (1998)Cite this article

Any galaxy whose central emission does not appear to be coming from stars is said to have an active galactic nucleus (AGN). AGNs often emit radiation at a very wide range of wavelengths, from radio waves to γ-rays, and some of them are the most luminous objects in the Universe. Although they are diverse in their spectrum and luminosity, we can explain most of their properties using a single, fairly simple theoretical picture. But this picture has two superficially contradictory central elements.

We believe that in all these objects, gas falls onto a very massive black hole in the galactic nucleus3 — sometimes called the ‘beast’. Because it has some angular momentum, the gas forms an accretion disk4,5. Friction dissipates the angular momentum, allowing the gas to move inwards and heating it up, and thermal emission from this hot gas is thought to be responsible for most of the energy released by a typical AGN, especially between the infrared and X-ray bands. That much of the picture is fairly clear, both observationally and theoretically. We even find similar accretion disks in other compact astronomical objects, such as newborn stars surrounded by remnants of their birth, and white dwarfs and neutron stars collecting mass from close binary companions, so AGN disks are only the most extreme cases of a common natural phenomenon.

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Figure 1: A jet in the quasar 3C273.

D. C. HOMAN

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  1. the School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, 55455, Minnesota, USA

    Tom Jones

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Jones, T. Light jets near light speed. Nature 395, 440–441 (1998). https://doi.org/10.1038/26623

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