We discuss the effects that the synchrotron emitting relativistic electrons, which are known to exist throughout many distance scales in active nuclei, could have on the emission-line regions that characterize these objects. Detailed models of both the inner, dense, broad-line region and the outer, lower density, narrow-line region are presented, together with the first models of the optically emitting gas often found within extended radio lobes. We show that, in all cases, if the relativistic gas that produces the synchrotron radio emission is mixed with the gas in the emission-line region, then significant changes in the emission-line spectrum will result. For small cosmic-ray densities the main effect is to strengthen lines that are formed in neutral regions, where photoelectric heating rates are lowest. As the cosmic-ray density increases, so do both the temperature and the ionization, until the cooling peak near 105 K is reached and a thermal runaway occurs.
The implications of our results for correlations between radio and optical properties of nuclei are discussed. We find that the addition of a flux of cosmic rays to a standard model of the broad-line region can effectively quench Fe II emission, in agreement with Grandi and Osterbrock's discovery that radio-loud objects tend to be weak Fe II emitters. The models do not reproduce their correlation between radio properties and Balmer decrements. Models of low-density gas in the narrow-line region show that relativistic particles can raise the temperature in the [O III] zone by an amount sufficient to explain some observed λ5007/λ4363 ratios. Although the addition of a flux of cosmic rays is probably not the only way to explain these anomalies in the emission spectra of broad- and narrow-line regions, it does provide a mechanism that is both simple and natural for powering the optically emitting extended regions that lie in the extended radio lobes far from the central engine.
The effects of the synchrotron emitting electrons on filaments in the Crab Nebula are discussed in an appendix, along with a comparison between our calculations, which employ the mean escape probability formalism, and recent Hubbard and Puetter models.
Digital Object Identifier (DOI)
Ferland, Gary J. and Mushotzky, R. F., "Cosmic Rays and the Emission-Line Regions of Active Galactic Nuclei" (1984). Physics and Astronomy Faculty Publications. 177.