The evolution of extragalactic radio sources

Abstract

A model for the evolution of low-luminosity radio galaxies is presented. In the model, the lobes inflated by low-power jets are assumed to expand in near pressure-balance against the external medium. Both cases of constant external pressure and decreasing external pressure are considered. Evolution of an individual source is described by the power-size track. The source appears as its lobe is inflated and radio luminosity increases to above the detection level; the source then moves along the track and eventually disappears as its luminosity drops below the detection limit. The power-size tracks are calculated including the combined energy losses due to synchrotron radiation, adiabatic expansion, and inverse Compton scattering. It is shown that in general, the constant-pressure model predicts an excess number of luminous, small-size sources while underpredicting large-size sources in the power-size diagram. The predicted spectra are steep for most sources, which is inconsistent with observations. By comparison, the pressure-limiting model fits observations better. In this model, low-luminosity sources undergo substantial expansion losses in the initial phase and as a result, it predicts fewer luminous, small-size sources. The resultant spectra are flat for most sources except for the oldest ones, which seems consistent with observations. The power-size tracks, in contrast to that of high-luminosity radio galaxies, are characterized by a slow increase in luminosity for most of the source's life, followed by a rapid decline when the synchrotron or inverse Compton scattering losses set in.