Seyfert galaxies have been among the most intensively studied objects
in astronomy, primarily because they are thought to be nearby, low-luminosity versions of the same phenomenon observed in quasars. A
massive black hole in the nucleus of a galaxy, accreting gas from its
surrounding environment, is thought to power all these objects. Of
course, we do not see the black hole itself, but the UV continuum
radiation is generally presumed to be thermal emission from the hot
gas that forms an accretion disk surrounding the black hole. In addition,
very broad emission lines are observed, which are thought to come
from clouds somewhat farther away, moving at velocities of order
. These broad-line clouds are photoionized and
heated by the extreme-UV radiation from the central source, resulting
in the strong, broad resonance line emission observed from hydrogen
Lyman-
, CIV (1550 Å), and other elements. The
permitted lines also sometimes show narrower cores, and there are also
narrow forbidden lines, which are thought to arise from more distant,
lower density, photoionized gas in a narrow-line region.
The broad-line component dominates the spectra of quasars and type 1
Seyfert galaxies, while the narrow-line component dominates in type 2
Seyferts. It is widely believed that the objects may be basically
similar, but that obscuration of the central region as viewed from
certain directions may hide the continuum and broad-line regions in type 2
Seyfert galaxies, leaving a clear view of only the narrow-line
region. The luminosity of Seyferts is typically 
, where 
is the luminosity of the Sun,
making the tiny nuclear region as luminous as an entire galaxy of
stars, and the inferred mass of the central black hole is 
to 
, where is the mass of the Sun. The luminosity is proportional to the mass-accretion rate, which is 
1 yr
. Small values of
these parameters are associated with low-luminosity Seyferts, and
large values are thought to characterize the much rarer, high-luminosity
quasars.
The far-UV spectral region is of fundamental importance in determining the nature of all these active galactic nuclei. The UV continuum radiation may arise in an accretion disk very close to the black hole, while UV emission and absorption lines provide the best diagnostics of the surrounding material in the broad- and narrow-line regions. Consequently, observations of Seyfert galaxies and quasars were a goal of one of the major observational programs for HUT on Astro-1.
One of the brightest and best-studied Seyfert galaxies is NGC 4151. It
has been classified as type 1.5, showing the characteristic features of
both types 1 and 2 (Osterbrock & Koski 1976). We obtained a high quality spectrum of
NGC 4151 in a 2200 s observation with HUT (Figure 3).
Below 1200 Å, a region in which no Seyfert galaxy has previously been observed,
we find strong emission in the OVI doublet and a very
complex absorption-line spectrum. The Lyman-
line and the
OVI line are both found to have broad wings with full width at half maximum 
, identical to the CIV feature, but
overlying absorption by numerous lines tends to obscure this fact.
Kriss et al. found that the broad lines have relative intensities
similar to those seen in quasars (where the large redshift of quasar radiation makes this
spectral region accessible to other telescopes) and to theoretical
photoionization calculations. All of the permitted lines also have similar
cores, with 
.
The strongest absorption lines include the Lyman series of hydrogen, as
well as features due to CIII and NIII and higher
ionization states, up to NV and OVI. All of the
absorption lines are blueshifted, with respect to the galaxy rest
frame, by several hundred km s, and they appear to have
intrinsic widths of about 
. The UV continuum
disappears completely below 924 Å, owing to strong absorption by
overlapping Lyman lines. The ratio of the strengths of the CIII 977 Å line and the 
1176 Å line (which arises
in an excited state) indicates densities in the absorbing gas greater
than 
. Such high densities are characteristic of
the gas in the clouds that yield broad emission lines. Kriss et al. (1993) conclude that
the absorption may arise in the disintegrating remnants of outflowing,
radiatively accelerated, broad-line clouds. Furthermore, this same
material may be responsible for producing the narrow cores of the
permitted emission lines. Finally, this material may collimate the ionizing radiation from the central source, explaining the bipolar cone-like appearance of the narrow emission-line region (Kriss et al. 1993). The absorption lines seen by HUT in the
far-UV thus provide an important new means for studying conditions in active galactic nuclei.
