Spatially Resolved Far-Ultraviolet Spectroscopy of the Nuclear Region of NGC 1068

Abstract

We carry out high-resolution FUSE spectroscopy of the nuclear region of NGC 1068. The first set of spectra was obtained with a 30'' square aperture that collected all emission from the narrow-line region. The data reveal a strong broad O VI component of FWHM ~3500 km s^-1 and two narrow O VI λλ1031, 1037 components of ~350 km s^-1. The C III λ977 and N III λ991 emission lines in this spectrum can be fitted with a narrow component of FWHM ~1000 km s^-1 and a broad one of ~2500 km s^-1. Another set of seven spatially resolved spectra was made using a long slit of 1.25'' × 20'' at steps of ~1'' along the axis of the emission-line cone. We find the following: (1) Major emission lines in the FUSE wavelength range consist of a broad and a narrow component. (2) There is a gradient in the velocity field for the narrow O VI component of ~200 km s^-1 from ~2^'' southwest of the nucleus to ~4'' northeast. A similar pattern is also observed with the broad O VI component, with a gradient of ~3000 km s^-1. These are consistent with the HST STIS findings and suggest a biconical structure in which the velocity field is mainly radial outflow. (3) A major portion of the C III and N III line flux is produced in the compact core. They are therefore not effective temperature diagnostics for the conical region. (4) The best-fit UV continuum suggests virtually no reddening, and the He II I(λ1640)/I(λ1085) ratio suggests a consistently low extinction factor across the cone. At ~2'' northeast of the nucleus there is a region characterized by (a) a strong Lyα flux but normal C IV flux, (b) a broad O VI line, and (c) a significantly enhanced C III flux.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE), which is operated for NASA by The Johns Hopkins University under NASA contract NAS5-32985, and observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.