NGC 4151 is not the only AGN to show prominent narrow emission lines in its FUSE spectrum. So far, strong narrow O VI emission lines have been detected in 4 out of 28 objects with z < 0.15: NGC 4151, NGC 3516, NGC 5548, and IRAS 1143-1483. Such a high percentage is a bit surprising since strong narrow-line emission in lower-excitation UV lines such as C IV 1549 is rarely seen in HST spectra. The one well-known exception is the HST/FOS spectrum of NGC 5548 in a low state (Crenshaw, Boggess, & Wu 1993). The FUSE observations suggest that there is a very high ionization component of the NLR present in many AGN.
Figure 8. The FUSE spectrum of IRAS 1143-1483 shows strong, narrow O VI emission superposed on a broad O VI emission line. Broad C III 977 is also prominent.
The physical conditions in this region are unclear. While in three of the present cases they are easily seen because the galaxy is in a low flux state (NGC 4151, NGC 3516, and NGC 5548), as shown in Fig. 8 for IRAS 1143-1483, the narrow O VI components are quite visible superposed on a strong broad O VI profile. In three of these cases the narrow lines also have the usual 2:1 flux ratios indicative of optically thin gas. However, as shown in Fig. 9, the lines in NGC 3516 have a 1:1 ratio more indicative of optically thick gas.
Figure 9. The FUSE observation of NGC 3516 shows strong, narrow O VI emission lines. As evidenced by the 1:1 intensity ratio, these lines are optically thick.