Far Ultraviolet Spectroscopic Explorer spectroscopy of absorption and emission lines from the narrow-line Seyfert 1 galaxy NGC 4051

We present three Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the narrow-line Seyfert 1 galaxy NGC 4051. The most prominent features in the far-ultraviolet (FUV) spectrum are the O VI emission and absorption lines and the H I Lyman series absorption lines, which are detected up to the Lyman edge. We also identify weak emission from N III, C III, and He II. The C III line shows absorption, while none is detected in the N III and He II lines. In H I and C III we detect two main absorption systems at outflow velocities of -50 ± 30 and -240 ± 40 km s ^-1, as well as a possible third one at about -450 km s ^-1. These systems are consistent in velocity with the 10 absorption systems found previously in C IV, N V, and Si IV, though the individual systems are blended together in the FUV spectrum. We estimate column densities of the two main absorption systems and find that the H I column density is lower for systems with larger outflow velocity. We detect no flux or spectral variations of NGC 4051 at FUV wavelengths during three epochs spanning 1 yr. This is consistent with the optical light curve, which shows no variations between the three epochs. It is also consistent with the X-ray light curve, which shows consistent flux levels at the three epochs of the FUSE observations, although the X-ray light curve shows strong variations on much shorter timescales.