Equipartition Brightness Temperature and the Inverse Compton Catastrophe

It is shown that the distribution of brightness temperatures in powerful extragalactic radio sources does not accord with the commonly accepted hypothesis that the observed upper limit in brightness temperatures is due to the "inverse Compton catastrophe." The "equipartition brightness temperature" is defined and is shown to be close to the observed cutoff in brightness temperature, which is significantly lower than the limiting brightness temperature predicted by the inverse Compton catastrophe. The equipartition brightness temperature is found to depend only weakly on observed quantities, in consequence of which it can be estimated with high precision. It is shown further that the observed brightness temperature and the equipartition brightness temperature may be used to make an independent estimate of the Doppler factor-defined here as the "equipartition Doppler factor." The evidence suggests that there is some mechanism which prevents radical departures from minimum energy and equipartition in these objects.