Jet-Driven Motions in the Narrow-Line Region of NGC 1068

Abstract

We have obtained Hubble Space Telescope Faint Object Camera (FOC) f/48 long-slit spectroscopy of the inner 4" of the narrow-line region (NLR) of NGC 1068 between 3500 and 5400 A with a spectral resolution of 1.78 A pixel-1. At a spatial scale of 0."0287 pixel-1 these data provide an order-of-magnitude improvement in resolution over previous ground-based spectra and allow us to trace the interaction between the radio jet and the gas in the NLR. Our results show that within +/-0."5 of the radio jet the emission lines are kinematically disturbed and split into two components whose velocity separation is 1500 km s-1. The filaments associated with the radio lobe also show a redshifted kinematic disturbance on the order of 300 km s-1, which is probably a consequence of the expansion of the radio plasma. Furthermore, the material enveloping the radio jet is in a much higher ionization state than that of the surrounding NLR gas. The highest excitation is coincident with the jet axis, where emission in the coronal line of [Fe VII] lambda 3769 is detected and the He II lambda 4686 is strong but where [O II] lambda 3727 is depressed. This large localized increase in ionization on the jet axis is accompanied by the presence of an excess continuum. Because the electron density is substantially larger in the jet compared with the surrounding NLR, these results can be explained only if there is a more intense ionizing continuum associated with the jet. This can be accomplished in a variety of ways, which include an intrinsically anisotropic nuclear radiation field, a reduced gas covering factor, or the presence of a local ionization source. The morphology, kinematics, and, possibly, the ionization structure of the NLR in the vicinity of the jet of NGC 1068 are a direct consequence of the interaction with the radio outflow.