Far Ultraviolet Spectroscopic Explorer Observations of the Seyfert 1.5 Galaxy NGC 5548 in a Low State

Abstract

We present far-ultraviolet spectra of the Seyfert 1.5 galaxy NGC 5548 obtained in 2000 June with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our data span the observed wavelength range 915-1185 Å at a resolution of ~20 km s^-1. The spectrum shows a weak continuum and emission from O VI λλ1032, 1038, C III λ977, and He II λ1085. The FUSE data were obtained when the active galactic nucleus was in a low state, which has revealed strong, narrow O VI emission lines. We also resolve intrinsic, associated absorption lines of O VI and the Lyman series. Several distinct kinematic components are present, spanning a velocity range of ~0 to -1300 km s ^-1 relative to the systemic velocity, with kinematic structure similar to that seen in previous observations of longer wavelength ultraviolet (UV) lines. We explore the relationships between the far-UV absorbers and those seen previously in the UV and X-rays. We find that the high-velocity UV absorption component is consistent with being low ionization, contrary to some previous claims, and is consistent with its nondetection in high-resolution X-ray spectra. The intermediate-velocity absorbers, at -300 to -400 km s ^-1, show H I and O VI column densities consistent with having contributions from both a high-ionization X-ray absorber and a low-ionization UV absorber. No single far-UV-absorbing component can be solely identified with the X-ray absorber.