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J. P. Leahy (University of Manchester)
Relativistic jets from radio-loud AGN are among the most spectacular features of both the radio and X-ray skies. But they are not just a beautiful demonstration of special relativity in action. Jets and DRAGNs, the radio `debris' they produce, can be up to several Mpc across, and so provide a record of the activity of the AGN over periods of up to 100 Myr. A major priority at present is to use this record to reconstruct the life-cycles of DRAGNs. I will review the current status of this effort, showing how multi-frequency radio imaging can greatly clarify the picture suggested by morphology alone, both by revealing hidden structures and by allowing quantitative, if model-dependent, age estimates. In addition, Chandra and XMM-Newton are now revealing the impact of the jets on the thermal plasma in and around the host galaxy.
The structures of DRAGNs reveal that jets are episodic on all time-scales. Expansion ages in the smallest ones suggest that jets "turn on" in a few centuries or less. For larger and older DRAGNs, statistical evidence points to long-term stability of the jets, but many contain evidence, sometimes incontrovertible and sometimes ambiguous, for discrete outbursts of high-power activity. The direction of the jets, while often remarkably stable, can also change by large angles, on time-scales which are short in the context of galactic dynamics, but explicable in terms of the interaction of a super-massive black hole with its accretion disk. The lack of DRAGNs bigger than a few Mpc implies that jet lifetimes are finite, but this leads to a long-standing puzzle: the shortage of `dead' DRAGNs with spectra characteristic of radiative losses without any energy input. The recent discovery of radio-silent cavities in clusters of galaxies, which may be `ghost' DRAGNs, only sharpens the problem.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.