Turning to a more detailed discussion of individual objects observed by FUSE, let me start with the spectrum of a recurrent favorite: NGC 4151. This FUSE observation was obtained as part of a coordinated set that included Chandra grating spectra and HST/STIS echelle spectra. While our primary intent was to study the rich absorption-line spectrum of NGC 4151, we found to our surprise that NGC 4151 was ~ 20 x fainter than in recent HUT (Kriss et al. 1995) and ORFEUS (Espey et al. 1998) observations. As shown in Fig. 6, the FUSE spectrum is dominated by narrow-line emission, rather than the usual bright continuum and broad O VI seen in this spectral range. The usual broad absorption lines, including the opaque Lyman limit, are not readily apparent. The narrow lines have a full-width-half-maximum of ~ 450 km s-1, and they are at the systemic velocity of the galaxy. Some residual broad O VI emission can be seen at the base of the narrow lines.
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Figure 6. The FUSE spectrum of NGC 4151 shows a continuum flux 20 x lower than that observed with HUT in 1995 (Kriss et al. 1995). The FUSE spectrum is dominated by high excitation narrow-line emission, and the optically thick blanket of broad Lyman-line absorption is no longer apparent. |
The HUT and ORFEUS observations of NGC 4151 showed that the Lyman absorption
lines were broad and saturated, yet their relative equivalent widths
suggested that they only partially covered the broad-line and continuum
emission, with 10-20% of the flux leaking through or around (due to
scattering) the absorbing material.
The broad absorption that is present in the O VI region is not
very deep in the FUSE spectrum, and its profile never drops below the current
continuum flux level.
This suggests that the little continuum emission we do see
(~ 3 x 10-14 erg cm-2 s-1
Å-1 at 1030 Å)
may be completely dominated by scattered light, and that the current
low flux state of NGC 4151 is not due to intrinsic dimming of the
central engine, but rather heavier-than-usual obscuration that is completely
opaque at UV wavelengths.
Note that the red and blue-shifted "satellite" lines often visible around
C IV 
1549 in earlier low
states of NGC 4151
(Ulrich et al. 1985;
Clavel et al. 1987)
do not appear to have any counterparts in the
current O VI profile. (See the paper by M. Crenshaw in this
volume for a possible explanation of the C IV satellite lines.)
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Figure 7. If the FUSE spectrum of NGC 4151 is smoothed to ~ 3 Å resolution, it is nearly identical to the HUT spectrum of NGC 1068 (Kriss et al. 1992b). |
The prominent narrow lines we see in the spectrum are very reminiscent of
the HUT spectrum of NGC 1068, where the high excitation lines of
C III 977 and N III
991 were particularly
prominent
(Kriss et al. 1992b;
Grimes, Kriss, & Espey
1999).
In Fig. 7 we have used a Gaussian kernel to
smooth the FUSE spectrum
of NGC 4151 down to the ~ 3 Å resolution of HUT
and overlayed
it directly on the HUT spectrum of NGC 1068.
The correspondence is quite remarkable.
Whether one thinks the narrow-line emission is powered predominantly by
kinetic energy in shocks or photoionization by the central engine, it
is clear that the same mechanism is at work in the NLRs of both NGC 1068
and NGC 4151.
This provides even more support for unified models of Seyfert 1s and
Seyfert 2s based on the obscuration, orientation, and reflection paradigm
(Antonucci 1993).