The extended narrow-line region of 3C 299

We present results of Hubble Space Telescope (HST) observations of the radi o galaxy 3C 299. The broadband F702W (R) and F555W (V) images (WFPC2/PC) sh ow an elliptical galaxy with a comet-like structure extending to the northe ast in the radio jet direction. The [O III] lambda 5007 emission-line map s hows a biconical structure centered on the nucleus, which overlaps the stru cture found in the broadband filters. The radio core coincides with the cen ter of the biconical structure and the radio axes are aligned with the dire ction of the cones. These data show clear evidence of a strong interaction between the radio jet and the northeastern morphology of the galaxy. We sho w evidence that this northeast region is an extended narrow-line region (EN LR); the line-ratio diagnostics show that models involving gas shocked by t he radio jet plus ionization from a precursor H II region-itself produced b y the ionizing photons of the postshocked gas on the preshocked gas-provide a good match to the observations. We investigate the spatial behavior of t he ionizing parameter U, by determining the [O III]/[O II] line ratio, whic h 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 father it is approximat ely constant over a large range of distance from the nucleus, thus requirin g a local source of ionization, which seems to be compatible with the shock models driven by the radio jet.