THE ORIGIN OF THE OPTICAL-EMISSION LINES ASSOCIATED WITH EXTRAGALACTIC RADIO-SOURCES

We investigate the observed extended emission-line regions (EELRs) associated with the jets and lobes of low-redshift extragalactic radio sources with moderate power (3C 227, 3C 277.3, 3C 305, Cen A [NGC 5128], NGC 7385, PKS 0349-27, PKS 2152-69, and Minkowski's Object). Using hybrid models which take into account the coupled effect of both the photoionization by a nonthermal radiation and the thermal collisions due to shocks, we calculated the line intensity ratio diagrams that best fit the observations. We found that the main energy sources of the EELR are: (1) a power-law radiation, with a flux at 1 Ryd F(H) congruent-to (10(8)-10(10)) cm-2 s-1 eV-1 and a spectral index alpha = 2.2-2.8 (equivalent to 1.2-1.8 for a flux given in units of ergs cm-2 s-1 eV-1), which probably originates at the central region of the galaxy, and (2) shocks with velocities v0 congruent-to 300-1000 km s-1 and preshock densities n0 congruent-to 5-100 cm-3, resulting from the interaction of the relativistic plasma flow with the ambient medium. In all cases investigated, a [N/O] underabundance by a factor 4 relative to the cosmic value is required in order to explain the observed [N II]/[O II] line ratios. The ability of radio jets to interact with the ambient medium and stimulate star formation has been suggested by a number of observational works and demonstrated by a number of authors. Considering both the estimated lifetimes of the radio sources and the stellar evolution time scales, the [N/O] underabundance could be explained by a burst of star formation where only the more massive stars (M greater-than-or-equal-to 8 M.) have already evolved.