The far-ultraviolet wavelength range spans the peak of the intrinsic spectral energy distribution of active galactic nuclei (AGN). Thus, it is important for understanding the energy generation mechanism and the processes that govern accretion onto massive black holes. At low to moderate redshift, the low-energy portion of the ionizing continuum enters the far-UV spectral range, and it can be observed with spectrographs on board the Hubble Space Telescope (HST) and instruments such as the Hopkins Ultraviolet Telescope (HUT) (Davidsen et al. 1992) and the Far Ultraviolet Spectroscopic Explorer (FUSE) (Moos et al. 2000). Since this portion of the spectrum determines the radiative input to the broad-line region (BLR) and the narrow-line region (NLR) in AGN, as well as the surrounding host galaxy and the intergalactic medium (IGM), determining the spectral shape in individual objects as well as on average is a crucial input for understanding the physical conditions of the gas in surrounding regions.
The 900-1200 Å spectral range contains numerous diagnostic spectral features that can be applied to AGN physics. The most prominent emission line is the O VI resonance doublet 1032,1038, which, as shown in Fig. 1, is particularly strong in the spectra of low-redshift, low-luminosity AGN. This line can serve as a diagnostic of the energy input from the extreme ultraviolet to soft X-ray portions of the ionizing continuum. Likewise, the O VI doublet is a crucial diagnostic for the warm absorbing gas commonly seen as O VII and O VIII absorption in X-ray spectra of AGN (Reynolds 1997; George et al. 1998). The high-order Lyman lines and the Lyman limit provide additional diagnostics of absorbing gas. In some cases (e.g., NGC 4151 or NGC 3516) the neutral hydrogen can be optically thick and thereby play a significant role in collimating the ionizing radiation that illuminates the NLR (Kriss et al. 1997). Finally, numerous ground-state transitions of molecular hydrogen in the Lyman and Werner bands provide a sensitive tracer of molecular gas. Under the right circumstances, one might expect to see H2 associated with the obscuring torus in AGN in absorption against the continuum and broad emission lines.
Figure 1. Low-luminosity, low-redshift AGN have the relatively strongest O VI emission seen in AGN. (Updated from Zheng, Kriss, & Davidsen 1995).