We present extensive ground-based spectroscopy and HST imaging of 3C 79 , an FR II radio galaxy associated with a luminous extended emission-line region ( EELR ) .
Surface brightness modeling of an emission-line-free HST R -band image reveals that the host galaxy is a massive elliptical with a compact companion 0 \farcs 8 away and 4 magnitudes fainter .
The host galaxy spectrum is best described by an intermediate-age ( 1.3 Gyr ) stellar population ( 4 % by mass ) , superimposed on a 10 Gyr old population and a power law ( \alpha _ { \lambda } = -1.8 ) ; the stellar populations are consistent with super-solar metallicities , with the best fit given by the 2.5 Z _ { \odot } models .
We derive a dynamical mass of 4 \times 10 ^ { 11 } M _ { \odot } within the effective radius from the velocity dispersion .
The EELR spectra clearly indicate that the EELR is photoionized by the hidden central engine .
Photoionization modeling shows evidence that the gas metallicity in both the EELR and the nuclear narrow-line region is mildly sub-solar ( 0.3 - 0.7 Z _ { \odot } ) — significantly lower than the super-solar metallicities deduced from typical active galactic nuclei in the Sloan Digital Sky Survey .
The more luminous filaments in the EELR exhibit a velocity field consistent with a common disk rotation .
Fainter clouds , however , show high approaching velocities that are uncoupled with this apparent disk rotation .
The striking similarities between this EELR and the EELRs around steep-spectrum radio-loud quasars provide further evidence for the orientation-dependent unification schemes .
The metal-poor gas is almost certainly not native to the massive host galaxy .
We suggest that the close companion galaxy could be the tidally stripped bulge of a late-type galaxy that is merging with the host galaxy .
The interstellar medium of such a galaxy is probably the source for the low-metallicity gas in 3C 79 .