JET-INDUCED SHOCKS IN 3C-171 - AN INTERMEDIATE-REDSHIFT ANALOG OF HIGH-REDSHIFT RADIO GALAXIES

We present spectroscopic observations of the extended emission-line re gion (EELR) aligned with the radio axis of the powerful radio galaxy 3 C 171. The results obtained from a detailed analysis of the observatio nal data show that shocks induced by jet-cloud interactions have a dra matic effect on the extended line-emitting gas. The evidence for shock s includes (1) close radio-optical associations: (2) ionization minima coincident with both radio hot spots; (3) high-velocity (similar to 5 00 km s(-1)) line splitting displaced by similar to 2 '' (10 kpc) (a H ubble constant of H-0, = 50 km s(-1) and a decleration parameter of q( 0), are assumed throughout this paper) behind the hot spots on either side of the nucleus, spatially associated with the two inner radio kno ts: (4) large line widths in the extended gas (FWHM similar to 1300 km s(-1)); and (5) an anticorrelation between line width and ionization state in the extended gas. Given that shocks clearly determine the mor phology, kinematics, and physical conditions of the extended line-emit ting gas, shock ionization is a possible alternative to AGN photoioniz ation in the EELR. The high [O III] temperatures and low He II/H beta ratios in the EELR provide evidence that local ionization by shocks is indeed important. A pronounced UV continuum excess is also detected i n the extended line-emitting regions. From the measured strength of th e Balmer lines, it is found that the extended UV continuum is dominate d by nebular continuum emission from the warm gas. There is no evidenc e whatsoever for jet-induced star formation. The properties of 3C 171- in particular the alignment of the optical line and UV-continuum emiss ion with the radio axis, and the extreme emission-line kinematics in t he extended gas-are similar to those observed in high-redshift radio g alaxies. This supports the hypothesis that jet-induced shocks determin e the distribution, kinematics, physical conditions, and ionization of the EELR in the powerful radio galaxies observed at high redshifts.