Spatially Resolved Far-ultraviolet Spectroscopy Of The Nuclear Region Of Ngc 1068

摘要

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 λ911 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(λ 1640)/I(λ 1085) 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.