Jet‐induced Shocks in 3C 171: An Intermediate‐Redshift Analog of High‐Redshift Radio Galaxies

Abstract

We present spectroscopic observations of the extended emission-line region (EELR) aligned with the radio axis of the powerful radio galaxy 3C 171. The results obtained from a detailed analysis of the observational data show that shocks induced by jet-cloud interactions have a dramatic effect on the extended line-emitting gas. The evidence for shocks includes (1) close radio-optical associations; (2) ionization minima coincident with both radio hot spots; (3) high-velocity (~500 km s-1) line splitting displaced by ~2'' (10 kpc) (a Hubble constant of H0 = 50 km s-1 and a decleration parameter of q0 are assumed throughout this paper) behind the hot spots on either side of the nucleus, spatially associated with the two inner radio knots; (4) large line widths in the extended gas (FWHM ~ 1300 km s-1); and (5) an anticorrelation between line width and ionization state in the extended gas. Given that shocks clearly determine the morphology, kinematics, and physical conditions of the extended line-emitting gas, shock ionization is a possible alternative to AGN photoionization in the EELR. The high [O III] temperatures and low He II/Hβ ratios in the EELR provide evidence that local ionization by shocks is indeed important.