The Host Galaxy and the Extended Emission‐Line Region of the Radio Galaxy 3C 79

Abstract

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-lineYfree HST R-band image reveals that the host galaxy is a massive elliptical with a compact companion 0.800 away and 4 mag 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 (k 1⁄4 1:8); the stellar populations are consistent with supersolar metallicities, with the best fit given by the 2.5 Z models. We derive a dynamical mass of 4 ; 1011 M 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 subsolar (0.3Y0.7 Z), significantly lower than the supersolar 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 from 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.