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Active galactic nuclei

Unstable by design

Nature volume 397, pages 476–479 (1999)Cite this article

The Wright brothers made their first aircraft manoeuvrable by making it unstable. Now it seems that quasars — the most luminous type of active galactic nuclei — also have to be unstable to make them work. This finding may resolve a long-standing problem with the quasars' fuel supply and allow studies into the distribution of massive black holes in the Universe.

Flattened disks are common in astronomy (recall Saturn'srings and newly forming planetary systems). When a disk forms deep in a gravitational potential well, it can become one of the most efficient radiation engines possible, and so can power the most prolific of radiation sources, the quasar. But if quasars were continuously radiating, they would grow too large and run out of fuel. On the other hand, if quasar disks were unstable, then quasars would be intermittent1, and mostly non-radiating.

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Figure 1: Relationship between local surface density and accretion rate in active galactic nuclei.
Figure 2: The time history (light curve) of the output of a quasar accretion disk, with the ionization instability included, shows large luminosity outbursts (from ref. 11).

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  1. the Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, 02138, Massachusetts, USA

    Aneta Siemiginowska & Martin Elvis

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  1. Aneta Siemiginowska
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  2. Martin Elvis
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Siemiginowska, A., Elvis, M. Unstable by design. Nature 397, 476–479 (1999). https://doi.org/10.1038/17223

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