EMISSION
Some LINERs are known to exhibit broad
H emission (FWHM of a few
thousand km s-1), reminiscent of the broad emission lines
that define type 1 Seyfert nuclei
(Khachikian & Weedman
1974).
This subset of LINERs suffers
from the least degree of ambiguity in physical origin and can be most safely
regarded as representing genuine low-luminosity AGNs. These objects
are analogous to the so-called intermediate Seyferts (types 1.8 and 1.9)
in the terminology of
Osterbrock (1981),
except that their narrow-line spectra
have low ionization and satisfy the definition of LINERs. The luminosities
of broad H can be orders of
magnitude fainter than those in
classical Seyfert 1 nuclei. The well-known example of the nucleus of
M81
(Peimbert &
Torres-Peimbert 1981;
Shuder & Osterbrock 1981;
Filippenko & Sargent 1988),
for example, has a broad
H luminosity of only
1.8 x 1039 ergs s-1
(Ho et al. 1996b),
and a number of other even less
conspicuous cases have been recognized by
Filippenko & Sargent
(1985).
Searching for broad H emission
in nearby galaxy nuclei is a nontrivial
business, because it entails measurement of a (generally) weak, low-contrast,
broad emission feature superposed on a complicated stellar background. Thus,
the importance of careful starlight subtraction cannot be overemphasized.
Moreover, even if one were able to perfectly remove the starlight, one still
has to contend with deblending the H + [N II]
6548, 6583
complex. The narrow lines in this complex are often heavily blended together,
and rarely do the lines have simple profiles (see
Section 6.4).
The strategy adopted by
Ho et al. (1996f)
for the Palomar survey makes use of
the line profile of the [S II]
6716, 6731 doublet to model [N II]
and the narrow component of H.
Some examples of the line decomposition are shown in
Figure 10. As was already known from previous studies
(Heckman 1980b;
Blackman, Wilson, & Ward
1983;
Keel 1983b;
Filippenko & Sargent 1985),
broad H is
unmistakably present in the LINER NGC 3998
(Fig. 10a); the line has
FWHM 
2150 km s-1
and FWZI 
5000 km s-1
(Ho et al. 1996f).
Though much weaker than the component in NGC 3998, a broad component of
H also
seems necessary in order to adequately model the
H + [N II] complex in
NGC 4036
(Fig. 10b). Broad
H emission has
long been known to exist in NGC 4579
(Stauffer 1982b;
Keel 1983b;
Filippenko & Sargent 1985),
but its strength is substantially weaker than that deduced
by assuming that the narrow lines can be represented by single Gaussians. The
extended, asymmetric bases of the [N II] lines, visible in the
[S II] doublet, largely account for most of the broad wings in the
H +
[N II] blend (Fig. 10c). Finally, I pick
NGC 4594 (the Sombrero galaxy;
Fig. 10d) to illustrate the pitfalls
that can potentially afflict
data of insufficient S/N or spectral resolution. Judging by the
similarity of its H + [N II]
blend to that of NGC 4579, one might be
led to believe that NGC 4594 also has broad
H emission. However, careful
inspection of the line profiles indicates that the [S II] lines have
large widths (FWHM 500 km
s-1) and extended wings
(FWZI
3000 km s-1), and if one assumes that all the narrow lines
have identical
profiles, no broad H component
is required to achieve a satisfactory fit
in this object. Note that such subtleties would easily have escaped notice
in previous surveys.
|
Figure 10. Examples of LINERs with
(NGC 3998, 4036, 4579) and without (NGC 4594)
broad H |
Faint broad H emission has been
discovered or confirmed for the first time
in numerous nuclei. The overall statistics of the survey can be summarized as
follows: of the 223 emission-line nuclei classified as LINERs, transition
objects, and Seyferts, 33 (15%) definitely have broad
H, and an
additional 16 (7%) probably do. Questionable detections were found in
another 8 objects (4%). Thus, approximately 20%-25% of all nearby
AGNs, corresponding to ~ 10% of all nearby, bright (BT
12.5 mag) galaxies, can be considered ``Seyfert 1'' nuclei, if one adopts the
definition that a Seyfert 1 nucleus contains a visible BLR (see
Table 3). These numbers, of course, are merely
lower limits, since undoubtedly
there must exist AGNs with even weaker broad-line emission to which we are
insensitive. The fraction of galaxies hosting Seyfert 1 nuclei, therefore,
is much higher than previously thought
(Weedman 1977;
Huchra & Burg 1992;
Maiolino & Rieke 1995).
Of the 33 objects with definite detections of broad
H, only 9 are well-known
Seyfert 1 nuclei; the majority have substantially
lower H luminosities and can
truly be regarded as ``dwarf'' Seyfert 1 nuclei.
Excluding previously classified
(Véron-Cetty &
Véron 1993)
Seyfert 1 nuclei (retaining only NGC 4395;
Filippenko & Sargent
1989), the
broad H line of the remaining
40 objects has a median luminosity of
~ 1.2 x 1039 ergs s-1 and FWHM = 2150 km s-1
(Ho et al. 1996f).
Five of them
have broad H luminosities as
low as (1-3) x 1038 ergs s-1, and the probable
detection in NGC 4565, if real, has a luminosity of only 8 x
1037 ergs s-1!
It is illuminating to consider the detection rate of broad
H emission as
a function of spectral class (Table 3). Among
objects formally classified as
Seyferts (according to their narrow-line spectrum), approximately 40% are
Seyfert 1s. The implied ratio of Seyfert 1s to Seyfert 2s (1:1.6) has
important consequences for several models concerning the evolution and
small-scale geometry of AGNs (e.g.,
Osterbrock & Shaw 1988;
Lawrence 1991),
but such a discussion is beyond the scope of this paper and will be
considered elsewhere. In the present context, of greatest interest is the
fraction of LINERs showing broad
H emission. If we first consider
``pure'' LINERs, nearly 25% of them have a BLR. The detection rate among
transition objects, however, drops drastically. The cause for this dramatic
change is unclear, but a likely explanation is that the broad-line component
is simply too weak to be detected in the presence of substantial contamination
from the H II region component. Supposing for the moment that the ratio of
LINERs with and without BLRs is similar to that in Seyferts, and furthermore
that the statistics of the presence of broad
H in all LINERs (i.e.,
``pure'' LINERs + transition objects) are intrinsically the same as those of
``pure'' LINERs, one would conclude that at least 60% of all LINERs are
genuine AGNs.