Jet-Induced Emission-Line Nebulosity and Star Formation in the High-Redshift Radio Galaxy 4C 41.17

Abstract

The high-redshift radio galaxy 4C 41.17 has been shown in earlier work to consist of a powerful radio source in which there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds, and we construct a jet-cloud interaction model to explain the major features revealed by the detailed radio, optical, and spectroscopic data of 4C 41.17. The interaction of a high-powered (∼ 1046 ergs s-1) jet with a dense cloud in the halo of 4C 41.17 produces shock-excited emission-line nebulosity through ∼ 1000 km s-1 shocks and induces star formation. The C IV luminosity emanating from the shock implies that the preshock density in the line-emitting cloud is high enough (hydrogen density ∼ 1-10 cm-3) that shock-initiated star formation could proceed on a timescale (∼a few × 106 yr) well within the estimated dynamical age (∼ 3 × 107 yr) of the radio source. Broad (FWHM ≈ 1100-1400 km s-1) emission lines are attributed to the disturbance of the gas cloud by a partial bow shock, and narrow emission lines (FWHM ≈ 500-650 km s-1; in particular, C IV λλ1548, 1550) arise in precursor emission in relatively low-metallicity gas or in shocked line emission in the lateral regions of the bow shock. The implied baryonic mass ∼ 8 × 1010 M⊙ of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line, and UV continuum images in some of the highest redshift radio galaxies, and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high-redshift radio galaxies.