The Extended Narrow‐Line Region of 3C 299

Abstract

We present results of Hubble Space Telescope (HST) observations of the radio galaxy 3C 299. The broadband F702W (R) and F555W (V) images (WFPC2/PC) show an elliptical galaxy with a comet-like structure extending to the northeast in the radio jet direction. The [O III] λ5007 emission-line map shows a biconical structure centered on the nucleus, which overlaps the structure found in the broadband filters. The radio core coincides with the center of the biconical structure and the radio axes are aligned with the direction of the cones. These data show clear evidence of a strong interaction between the radio jet and the northeastern morphology of the galaxy. We show evidence that this northeast region is an extended narrow-line region (ENLR); the line-ratio diagnostics show that models involving gas shocked by the radio jet plus ionization from a precursor H II region—itself produced by the ionizing photons of the postshocked gas on the preshocked gas—provide a good match to the observations. We investigate the spatial behavior of the ionizing parameter U, by determining the [O III]/[O II] line ratio, which is sensitive to the change of the ionization parameter, and we trace its behavior over the ENLR along the radio jet direction. We find that [O III]/[O II] does not follow a simple dilution model, but rather it is approximately constant over a large range of distance from the nucleus, thus requiring a local source of ionization, which seems to be compatible with the shock models driven by the radio jet.