Abstract
We have mapped the extragalactic radio source Cygnus A at 81.5 MHz using a two-element interferometer with variable orientation and spacing (maximum 45km). This is the lowest frequency at which the radio source has been mapped with sufficient resolution (16 arc s) to resolve the bridge of emission joining the two outer lobes, and our data provide new information about the radiation spectrum. By comparing our map with one made at 2.7 GHz, (ref. 1) we have measured the two-point spectral index as a function of distance along the source axis. We estimate the age of the radio source by using a model in which two oppositely directed beams ram their way out through the surrounding medium, depositing relativistic plasma as they do so2,3. Synchrotron radiation losses are more severe at high frequencies, causing a break in the spectrum which occurs at progressively lower frequencies as the plasma ages, and our analysis suggests that Cyg A is ∼3 Myr old.
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Spinks, M., Rees, W. & Duffett-Smith, P. How old is Cygnus A?. Nature 319, 471–473 (1986). https://doi.org/10.1038/319471a0
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DOI: https://doi.org/10.1038/319471a0
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