Fluid Flow and Spectral Aging in Tailed Radio Galaxies

Two-frequency surface brightness measurements of radio tails can be used to infer the evolution of velocity, magnetic field and relativisitic particle density in these tails. This information can then be used to determine the age of plasma as a function of distance down the tail. I apply these methods to a set of 12 radio tails from O'Donoghue's Wide-Angle Tailed source survey. I find a characteristic velocity law, which is not easily explained by current subsonic jet models. I also that the plasma in the tails evolves away from equipartion, in that it becomes more magnetized farther down the tail. I also find that the rate of decay of the cutoff frequency is much slower than predicted by standard synchrotron aging theory. This means that the cutoff frequency cannot be used to measure the source age: the sources are older than they look.

¹ NMT

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