The interstellar medium of a galaxy containing an active nucleus may be profoundly affected by the high energy (X-ray, EUV) continuum flux emanating from the central source. The energetic source may photoionize the interstellar medium out to several kiloparsecs, thereby creating a global H II region. The International Ultraviolet Explorer (IUE) satellite has attempted to observe in several Seyfert galaxies (NGC 3516, NGC 4151, NGC 1068, 3C 120) the narrow absorption lines expected from such global H II regions. Instead, in two of the galaxies (NGC 3516, NGC 4151) broad, variable absorption lines at C IV lambda 1550, N V lambda 1240, and Si IV lambda 1400 were found, as well as weaker absorption features at O I lambda 1302 and C II lambda 1335. These features swamp any possible global H II region absorption. Such broad absorption features have previously been observed in IUE data, but their origin is still not well understood.
NASA. Ames Research Center Summer School on Interstellar Processes: Abstracts of Contributed Papers; p 77-78