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AGN SPECTRA 1. INTRODUCTION
2. HISTORICAL BACKGROUND The observational study of
active galactic nuclei (AGNs) began with the work of E. A. Fath at Lick
observatory in 1908. Most of the nuclei of the brightest “spiral nebulae”,
now known to be galaxies, showed absorption-line spectra, which Fath
interpreted to result from the integrated light from large numbers of stars.
He recognized six emission lines in the spectrum of the nucleus of NGC 1068.
In 1926 E. Hubble in his monumental study of “extragalactic nebulae” noted
the planetary-nebula-type emission-line spectra of three galaxies: NGC 1068,
4051, 4151. Nearly two decades later C. K. Seyfert stated that a small
fraction of galaxies have nuclei with many high-ionization emission line
spectra. Very rapid advances in radio astronomy in the 50s led to the first optical identifications of the strong
radio sources. Among them was Cyg A, identified by W. Baade and R.
Minkowski with a faint galaxy with z = 0.057. Its rich emission-line spectrum
proved to be very similar to the spectra of Seyfert galaxies. In 1963 M. Schmidt broke the puzzle of the spectrum
of 3C 273 by identifying several well-known
nebular emission lines with the unusually large z = 0.158. Soon
afterwards J. Greenstein and T. Matthews identified similar lines in 3C 48
with z= 0.367. It was immediately clear these
quasistellar radio sources, called quasars, are highly luminous and can be
observed to very great distances. We understand most of them as AGNs,
so luminous and distant, that the host G could not be detected. Corresponding radio-quiet high-luminosity “quasistellar objects” –
QSOs, were found soon afterward. The
first known Seyfert galaxies (SyGs) were
discovered, or recognized, on slit spectra of individual galaxies taken
mostly in radial-velocity programmes. More were discovered when spectra of
“compact galaxies” were obtained, since many of them turned out to fit the
spectroscopic criteria for Seyfert galaxies. Objective-prism surveys with a
Schmidt camera by B. Markarian turned up many additional Seyfert galaxies. He
catalogued galaxies with strong UV continuous spectra. About 10 % turned out
to be Seyfert galaxies, most of the rest proved to be starburst
galaxies. A number of studies also
indicated that in many instances the spectra revealed abnormal line-intensity
ratios, most notably the unusually great strength of [N II] relative to H
(Burbidge & Burbidge 1962, 1965; Rubin & Ford 1971). That the optical
emission-line spectra of some nuclei show patterns of low ionization was
recognized from time to time, primarily by Osterbrock and his colleagues
(e.g., Osterbrock & Dufour 1973; Osterbrock & Miller 1975; Grandi
& Osterbrock 1978), but also by others (e.g., Disney & Cromwell 1971;
Stauffer & Spinrad 1979). Most of the activity in this
field culminated in the 1980s, beginning with the recognition (Heckman,
Balick, & Crane 1980) of LINERs as a
major constituent of the extragalactic population, and followed by further
systematic studies of larger samples of galaxies (Filippenko & Sargent
1985). 3. DEFINITIONS AND CLASSIFICATION OF EMISSION-LINE
NUCLEI The emission-line spectra in SyGs can be
classified in two types,
following a scheme, first proposed by Khachikian and Weedman. Seyfert 1 Gs (Sy1Gs) are those with very broad H I,
He I and He II emission lines (ELs) with FWHM ~ (1-5)x103km.s-1,
while the forbidden lines, like [OIII] λλ4959, 5007, [NII]
λλ6548, 6583 and [S II] λλ6716, 6731, typically have
FWHMs of order 5x102 km.s-1.
The forbidden ELs (FELs),
though narrower than the permitted ELs (PELs), nevertheless are broader than
the ELs in most starburst Gs. Besides their broad ELs, many Sy1Gs also show
broad permitted Fe II ELls, coming from several strong multiplets of Fe II.
They overlap in two broad “features” near λ4570 and λ5250. Seyfert 2 Gs
(Sy2Gs), on the other hand, have
permitted and forbidden lines with approximately the same FWHMs, typically
5x102 km.s-1, similar to the FWHMs of the forbidden
lines in Sy1Gs. This classification into two types may be furthur
subdivided. Some Sy1Gs have H I EL
profiles that can be described as composite, consisting of a broad component
(as in a Sy1G), on which a narrower one (as in a Sy2G) is superimposed. The SyGs with
intermediate-type H I profiles (Ps), in which both types can easily be
recognized, are classified as Seyfert 1.5 Gs. Those with strong narrow
components and very weak but still visible broad components of Hα and
Hβ, are called Seyfert 1.8 Gs; and those in which a weak broad component
may be seen at Hα but none at Hβ, Seyfert 1.9 Gs. In radio Gs (RGs) the synchrotron radio-frequency
emission typically comes from two large diametrically opposite lobes, far
outside the optical limits of the G. Strong optical ELs together with the
featureless continuum (FC) spectrum come from this AGN. The optical spectra of RG AGNs can be classified into two types, analogous to
the classification of Sy AGNs. One type is the
radio-loud equivalent of Sy1Gs, with broad H I, He I and He II Els, but
narrower FELs – the broad-line RGs (BLRGs). The other type, similar to Sy2Gs,
has narrower PELs and FELs (but broader than in typical starburst Gs) - the narrow-line RGs (NLRGs). There are some
differences between the spectra of RGs and SyGs, esp. BLRGs and Sy1Gs, despite
their general similarity:
These observed differences indicate that, on the average, radio-loud and radio-quiet broad-line AGNs differ in their optical properties, as well. Any differences between the optical spectra of NLRGs and Sy2Gs are much smaller, if they exist at all. There are also significant differnces
between the host galaxies of the Sy and Radio types of AGNs:
It is known that spiral Gs (SpGs) contain more interstellar matter than giant ellipticals (EGs) and that they are more condensd to their principal planes. Very probably a difference between SyGs and RGs may be more in the near-nuclear environment – the former flattened, rotating and rich in interstellar matter, the latter more nearly spherical and poor in interstellar matter – than in the structure of the nucleus itself. All AGNs have a FC in the optical band. It comes from a tiny unresolved object within the nucleus. It is the seat of energy release distinctive to AGNs. The FC in Sy1Gs is so much stonger than the integrated stellar absorption-line spectrum, that the latter is nearly invisible. The FC is much fainter in typical SyGs. The broad ELs are closely connected with the FC. As a result of their FC, the AGNs of Sy1Gs are generally more luminous than Sy2AGNs. This makes Sy1Gs as a whole more luminous than Sy2Gs. The best avilable luminosity function (LF) of Sy1Gs has its maximum near MB=-21 mag, while for SyGs it is near MB=-20 mag. Quasars and QSOs are simply the rarest and most luminous AGNs. QSOs are objects with MB<-23 mag. All Gs, more luminous than MB~-22 mag, are Seyferts. There are no known QSOs analogous to Sy2Gs – all known quasars and QSOs are of the BLRGs or Sy1G type. This is consistent with the observational data that if the FC is so bright that the light from the AGN completely dominates the total light of the G, broad PELs are almost certain to be present. Radio-loud quasars seem to be the extension of the BLRGs to high optical luminosity. Heckman (1980b) originally defined LINERs strictly using the optical forbidden
lines of oxygen: [O II] λ 3727 > [O
III] λ 5007 and [O I] λ 6300 >
0.33 [O III] λ 5007. Compared with the spectra of Seyfert
nuclei or H II regions, the low-ionization states of oxygen in the spectra of
LINERs are unusually strong relative to its high-ionization states.
Recognizing the arbitrariness of this definition, Heckman drew attention to a
group of “transition objects'' whose
spectra were intermediate between those
of “pure'' LINERs (as defined above) and classical Seyfert nuclei. As a consequence of the near coincidence between the
ionization potentials of hydrogen and neutral oxygen,
the collisionally-excited [O I] line in an ionization-bounded nebula arises
predominantly from the “partially-ionized zone,'' wherein both neutral
oxygen and free electrons coexist. In
addition to O0, the conditions of the partially-ionized zone are
also favorable for S+ and N+, whose ionization
potentials are 23.3 eV and 29.6 eV, respectively. Hence, in the absence of
abundance anomalies, [N II] λλ 6548,
6583 and [S II] λλ 6716, 6731 are strong (relative to, say,
Hα) whenever [O I] λλ 6300, 6363 are strong, and vice versa.
This theoretical expectation and the empirical evidence that extragalactic H
II regions rarely exhibit [N II] λ6583/Hα ≥ 0.6 (e.g., Searle
1971) have led some subsequent investigators to short-cut Heckman's original
definition of LINERs. For instance, it has become customary to classify emission-line objects solely on the basis
of the [N II]/Hα ratio (e.g., Keel 1983b; Keel et al. 1985; Phillips
et al. 1986; Véron-Cetty & Véron 1986). While this
convention does permit a convenient first-order
separation between nuclei photoionized by stars (small [N II]/Hα) and
those photoionized by a harder, AGN-like spectrum (large [N II]/Hα),
it provides no information on the excitation level of the AGN-like objects -
in other words, one cannot distinguish LINERs from Seyfert nuclei. Based on the
dereddened line-intensity ratios [O III] λ 5007/Hβ, [O I] λ
6300/Hα, [N II] λ 6583/Hα, and [S II] λλ 6716,
6731/Hα (Hα and Hβ refer only
to the narrow component of the line), the Veilleux-Osterbrock system is not
only relatively insensitive to extinction corrections, but also conveniently
falls within the spectral range of the optical survey. For concreteness, the following
definitions will be adopted: · H II nuclei ([O I] < 0.08 Hα, [N II] < 0.6 Hα, [S II] < 0.4 Hα), · Seyferts ([O I] ≥ 0.08 Hα, [N II] ≥ 0.6 Hα, [S II] ≥ 0.4 Hα, [O III]/Hβ ≥ 3), and · LINERs ([O I] ≥ 0.17 Hα, [N II] ≥ 0.6 Hα, [S II] ≥ 0.4 Hα, [O III]/Hβ < 3). Although the adopted
definition of LINERs differs from that of Heckman, inspection of the full
optical spectra of Ho, Filippenko, & Sargent (1993) reveals that
emission-line nuclei classified as LINERs based on the Veilleux &
Osterbrock diagrams almost invariably also satisfy Heckman's criteria. This
is a consequence of the inverse correlation between [O III]/Hβ and [O
II]/[O III] in photoionized gas with fairly low excitation ([O
III]/Hβ ≤ 3). In addition to these three categories of nuclei, Ho et
al. (1993) identified a class of “transition
objects'' whose [O I] strengths are
intermediate between those of H II nuclei and LINERs. Although O-star
models with an appropriate choice of parameters can account for their
line-intensity ratios of these objects (Filippenko & Terlevich 1992), an
alternative explanation is that these objects are composite systems having
both an H II region and a LINER component (Ho et al. 1993). We will define transition objects using the same criteria as for LINERs, except that 0.08
Hα ≤ [O I] < 0.17 Hα. It should be emphasized that the classification process is not always straightforward, since the three conditions involving the low-ionization lines do not hold simultaneously in all cases. In view of potential selective N enhancement in galactic nuclei, less weight is given to the [N II]/Hα ratio than to either [O I]/Hα or [S II]/Hα. [O I]/Hα, if reliably determined, deserves the most weight, since it is most sensitive to the shape of the ionizing spectrum. 4. DENSITIES AND TEMPERATURES IN THE IONIZED GAS The NELs observed in Sy2Gs and NLRGs are similar to these in H II regions and planetary nebulae, except that in AGNs the range of ionization is considerably greater. Not only [O II], [O III], [N II], [S II] and [Ne III] are observed, but also [O I], [N I], [Ne V], [Fe VII] and [Fe X]. The PELs of H I, He I and He II are moderately strong. The values of extinction, derived from the Balmer-line ratios are used to correct the observed line ratios. The already corrected line intensities may be used to derive diagnostic information on the physical conditions in the ionized gas. The [O III] intensity ratio (λ4959 + λ5007)/λ4363 gives a mean temperature in the [O III] emitting region in both low- and high-density Ne limit. The [S II] intensity ratio λ6716/λ6731 is a good electron-density Ne diagnostic in the [S II] emitting region in both low- and high-temperature limit, at least for the portion of the narrow-line region (NLR) characterized by densities not greatly in excess of the critical density of [S II] (~ 3 x 103 cm-3), above which the lines become collisionally de-excited. A range of densities, spanning nearly five orders of magnitude, exists in the NLRs of some LINERs and Seyferts. The [S II] densitometer strictly probes only the low-density regions. LINERs have smaller electron densities (median Ne = 175 cm-3) than Seyferts (median Ne = 290 cm-3), and the difference is highly significant. Transition objects have smaller densities than LINERs, most notably in a considerable excess of low-density members, as seen in a large fraction of H II nuclei. It is interesting to point out
that the electron densities among Seyfert
nuclei appear to decrease with decreasing nuclear luminosity. In a
sample of bright, mostly Markarian Seyfert 2 galaxies, Koski (1978) found
that the average density, again as determined from [S II], is ~ 2000 cm-3,
far greater than that encountered in a sample of low-luminosity Seyferts.
Although the systematic effect discussed above may also affect Koski's
measurements to some degree, it probably cannot account for the large
difference, especially in view of the much larger emission-line equivalent
widths in his sample. The relative abundances of the ions responsible for the observed lines may next be estimated. Schematic estimations show that a typical AGN has approximately the same composition that our Galaxy and other observed Gs with H II regions or starburst nuclei have. H is the most abundant element; He is about ten times less abundant; O, Ne, N and C are the most abundant heavy elements. 5. CONTINUUM Today,
the word “continuum” in the context of AGN might bring to mind anything from radio to gamma-ray frequencies. However, in the early days of QSO
studies, the term generally meant the optical continuum, extending to the
ultraviolet and infrared as observations in these bands became available.
Techniques of photoelectric photometry and spectrum scanning were becoming
established as QSO studies began. The variability of QSOs, including 3C 48
and 3C 273 (e.g., Sandage 1963), was known and no doubt contributed to
astronomers’ initial hesitation to interpret QSO spectra in terms of large
redshifts. In his contribution to the
four discovery papers on 3C 273, Oke (1963) presented spectrophotometry
showing a continuum slope Lν ~ ν0.3 in the
optical, becoming redder toward the near-infrared. He noted that the energy
distribution did not resemble a
blackbody, and inferred that there must be a substantial contribution
of synchrotron radiation. A key issue for continuum studies has been the relative importance of thermal and nonthermal
emission processes in various wavebands. Early work tended to assume
synchrotron radiation, or “nonthermal emission”, in the absence of strong
evidence to the contrary. The free-free and bound-free emission from the gas
producing the observed emission lines was generally a small contribution. The
possibility of thermal emission from very hot gas was considered or some
objects such as the flat blue continuum of 3C 273 (e.g., Oke 1966). The
energy distributions tend to slope up into the infrared; and for thermal
emission from optically thin gas, this would have required a rather low
temperature and an excessive Balmer continuum jump. This left the
possibilities of nonthermal emission or thermal emission from warm dust,
presumably heated by the ultraviolet continuum. Observational indicators of
thermal or nonthermal emission include broad features in the energy
distribution, variability, and polarization. For the infrared, one also has
correlations with reddening, the silicate absorption and emission features,
and possible angular resolution of the source. For
some objects, rapid optical variability implied
brightness temperatures that clearly required a nonthermal emission
mechanism. For many objects, the energy distributions were roughly
consistent with a power law of slope near ν-1.2. Power laws
of similar slopes were familiar from radio galaxies and the Crab Nebula,
where the emission extended through the optical band. These spectra were
interpreted in terms of synchrotron radiation
with power-law energy distributions for the radiating, relativistic
electrons. Such a power-law energy distribution was also familiar from
studies of cosmic rays, and thus power laws seemed natural in the context of
high-energy phenomena like AGN. In addition to simple synchrotron radiation,
there might be a hybrid process involving synchrotron emission in the
submillimeter and far-infrared, with some of these photons boosted to the
optical by “inverse” Compton scattering (Shklovskii
1965). The idea of a nonthermal continuum in
the optical, whose highfrequency extrapolation provided the ionizing
radiation for the emission-line regions, was widely held for many years. This
was invoked not only for QSOs but also for Seyfert galaxies, where techniques
such as polarization were used to separate the “nonthermal” and galaxy
components (e.g., Visvanathan & Oke 1968). From
an extensive survey of Seyfert galaxies, Rieke (1978) concluded that strong infrared emission was a “virtually universal”
feature and that the energy
distributions in general did not fit a simple power law. The amounts
of dust required were roughly consistent with the expected dust in the
emission-line gas of the active nucleus and the surrounding interstellar
medium. A consensus emerged that the infrared
emission of Seyfert 2 galaxies was thermal dust emission, but the
situation for Seyfert 1 galaxies was less clear (e.g., Neugebauer et al.
1976; Stein & Weedman 1976). The former group was consistent with a class
of objects known as “blazars” that are dominated at all wavelengths by a variable,
polarized nonthermal continuum. Blazars were found to be highly variable at all wavelengths, but most AGN appeared to be systematically less variable
in the far-infrared than at higher frequencies. This supported the
idea of thermal emission from dust in the
infrared. Bolometric luminosities ranged from 109 to 1014
LSun, dominated by the 1-100 km band. There was evidence for a
thermal infrared component in many of the less luminous objects and an ultraviolet continuum bump associated with the
presence of emission lines. When gamma
rays are observed from AGN (e.g.,
Swanenburg et al. 1978), they appear to be
associated with the beamed nonthermal continuum. The relationship of
blazars to “normal” AGN is a key question in the effort to unify the diverse
appearance of AGN. The infrared emission is thermal emission from dust, energized
in many cases by star formation but in some cases by an AGN. 6. DEMOGRAPHICS OF EMISSION-LINE GALAXIES Although the specific numbers
cited differ from one investigator to another, all the older surveys agree
that LINERs are extremely common in nearby galaxies. They also concur
that the detection rate of LINERs varies
strongly with Hubble type, with early-type systems being the preferred
hosts; this result essentially confirms what was already found by Burbidge
& Burbidge (1962), who noted that most of the galaxies showing enhanced
[N II]/Hα ratios tended to be of early type. The conclusions that can be drawn from the Palomar survey are the following. 1. At the limit of the survey, which is at least 4 times more sensitive to the detection of emission lines than any of the older surveys, most galaxies (86%) exhibit optical line emission in their central few hundred parsecs, implying that ionized gas is almost invariably present. This fraction, of course, represents a lower limit. Keel (1983a) detected emission in all the galaxies he surveyed, but his sample was restricted to spirals; (essentially all spirals have nuclear emission lines).
Theoretical studies (e.g., Heller & Shlosman 1994) suggest that large-scale stellar bars can be highly effective in delivering gas to the central few hundred parsecs of a spiral galaxy, which may then lead to rapid star formation. Further instabilities may result in additional inflow to smaller physical scales relevant for AGNs. Thus, provided that a reservoir of gas exists, the presence of a bar might be expected to influence the fueling rate, and hence the activity level. Ho, Filippenko, & Sargent (1996a, e) find that the presence of a bar does indeed enhance both the probability and rate of the formation of massive stars in galaxy nuclei, but only for spirals with types earlier than Sbc. By contrast, AGNs seem to be altogether unaffected. 7. STATISTICAL PROPERTIES 7.1. EMISSION-LINE LUMINOSITY In lieu of direct measurement
of the nonstellar featureless continuum at optical wavelengths, an almost impossible
feat for the low-luminosity sources in question, one might use, as a
substitute, an indirect measure such as the luminosity of a narrow emission
line powered by the continuum. In luminous AGNs, whose nonstellar optical
continuum generally overwhelms the stellar background,
the Hα luminosity scales linearly with the luminosity of the continuum (Searle
& Sargent 1968; Yee 1980; Shuder 1981). For
any given object, the amount of line emission sampled will depend on its
distance as well as on the physical extent of the line-emitting region.
Moreover, circumnuclear H II regions undoubtedly contaminate the line
emission at some level. Remarkably, the
distributions for LINERs and Seyferts appear very similar, both having a
median L(Hα) ~ 6 x 1038 ergs s-1; transition
objects tend to be somewhat less luminous, but the difference is
insignificant according to the Kolmogorov-Smirnov (K-S) test. The above
comparison is not obviously affected by known systematic biases, since all
three subclasses have virtually identical distance distributions, modest
reddening corrections were consistently applied, and the host galaxies of
LINERs and Seyferts are grossly similar. 7.2. INTERNAL REDDENING AND
INCLINATION EFFECTS Another parameter that can be easily examined is the internal reddening along the line of sight, as inferred from the relative intensities of the narrow Balmer emission lines. The conventional Balmer decrement method, unfortunately, assumes that the extinction arises from a uniform, foreground screen of dust, and it is unclear to what extent such an oversimplified geometry applies to the actual line-emitting regions in galaxy nuclei. The derived reddening values, therefore, should be strictly regarded as lower limits. With this caveat in mind, it is intriguing that LINERs are noticeably less reddened than Seyferts. That LINERs are also less reddened compared to transition objects is to be expected, since H II nuclei in general are much more heavily extinguished than LINERs [median E (B - V) = 0.21 and 0.47 mag for LINERs and H II nuclei, respectively]. These data constitute the first set of reliable reddening measurements for such faint nuclei. In the older surveys, the Balmer decrements were either completely unconstrained (e.g., because only the red part of the spectrum was surveyed) or otherwise very poorly determined because of the difficulties associated with starlight correction. 7.3. LINE PROFILES AND
KINEMATICS The kinematic information
contained in line profiles provides unique clues to the LINER puzzle. The FWHM of the forbidden lines in LINERs rarely
exceed 500 km s-1. Despite being blended with
Hα most of the time, we will use [N II]
λ 6583 as the fiducial probe of the velocity field of the NLR, since
it is usually the strongest line in the red
spectrum, and it is relatively
unaffected by stellar absorption. [O III] λ 5007 normally is more ideal for measurement of narrow-line
profiles, but, in our case, both the S/N and the resolution of the
blue spectra are lower than those of the red spectra. The line widths range from being unresolved ( 115 km s-1)
to 500-700 km s-1, with a median
value (excluding the first bin, whose values are very uncertain
because they are near the resolution limit) of
350, 230, and 290 km s-1, respectively, for LINERs, transition
objects, and Seyferts. Not surprisingly, transition
objects have narrower lines compared to LINERs; this is to be expected
because of the difference in their average Hubble types and the well-known
dependence of nebular line width on bulge prominence (e.g., Whittle 1992a,
b). What is unexpected is the clear difference evident between LINERs and
Seyferts: LINERs have wider forbidden lines
than Seyferts, significant at a level greater than 99.999% according
to the K-S test. Since it was first pointed out by Phillips et al. (1983), it
has been well established that the luminosities
of the forbidden lines in Seyfert nuclei are positively correlated with their
widths (Whittle 1985, 1992b). LINERs evidently also obey the
correlation, contrary to what Wilson & Heckman (1985) thought; the
shallower slope reflects the larger
line widths found in LINERs. Transition objects, on the other hand, appear
not to follow the correlation. The interpretation of the relation between
line luminosity and line width has been unclear, mainly because of the
existence of other mutual correlations between line width, line luminosity,
and radio power (Wilson & Heckman 1985). The recent analysis by Whittle
(1992b), however, shows quite convincingly that the
fundamental parameter underlying all these correlations is the bulge mass (or
central gravitational potential) of the host galaxy. In light of the dependence of
line width on luminosity, it is hardly surprising that the “typical'' Seyfert nucleus has much narrower lines than
conventionally assumed. Hence, the criterion for distinguishing
Seyfert 2 nuclei from “normal'' emission-line nuclei (i.e., H II nuclei) on
the basis of the widths of the narrow lines, either as originally proposed by
Weedman (1970, 1977), or as later modified by Balzano & Weedman (1981)
and Shuder & Osterbrock (1981), is clearly inappropriate for the majority
of the Seyfert galaxy population and should be abandoned. Of course, the FWHM is the crudest, first-order characterization
of the line profile. Actually, the shapes of the emission lines in
most emission-line nuclei, when examined with sufficient spectral resolution
(e.g., Heckman et al. 1981; Whittle 1985; Veilleux 1991; Ho et al. 1996f),
deviate far from simple analytic functions (such as a Gaussian), often
exhibiting weak extended wings and asymmetry. In fact, most Seyfert nuclei have asymmetric narrow lines, and
there seems to be a preponderance of blue wings,
usually interpreted as evidence of a
substantial radial component in the velocity field coupled with a source of
dust opacity. It would be highly instructive to see if this trend
extends to LINERs, as it could offer insights into possible differences
between the NLRs in the two types of objects. These subtleties have never
before been examined systematically in LINERs. Detailed studies of Seyferts
(e.g., De Robertis & Osterbrock 1984, 1986) and LINERs (Filippenko &
Halpern 1984; Filippenko 1985; Filippenko & Sargent 1988; Ho et al. 1993,
1996b) in the past have found that the widths
of the forbidden lines correlate positively with their critical densities. This
empirical trend has been interpreted as
evidence that the NLR contains a wide range of gas densities (102-107
cm-3), stratified such that the densest material is located
closest to the center. In such a picture, [O I]
λ 6300 (Ncrit ~ 106 cm-3) should be
broader than [S II] λ λ 6716, 6731 (Ncrit ~3x103
cm-3). Among the objects with securely
determined FWHM for [O I] and [S II], approximately 15%-20% of LINERs and 10%
of Seyferts show detectable evidence of density stratification in the sense
that FWHM([O I]) > FWHM([S II]). In no instance is [O I] ever observed to
be narrower than [S II]. Whittle (1985) finds that Seyfert 1 nuclei have a
greater likelihood of showing profile differences in their forbidden lines
than do Seyfert 2s. The implication is that somehow density
stratification in the NLR is directly related to the presence of a BLR.
8. SUMMARY 1) From a newly completed
spectroscopic survey of nearby galaxies, it is confirmed that LINERs are extremely common, being present in about
1/3 of all galaxies with BT ≤ 12.5 mag. If all LINERs are regarded as active nuclei, they constitute > 70% of the AGN population, and AGNs
altogether make up nearly half of all bright galaxies. These
statistics should be regarded strictly as lower limits, because very faint
AGNs can be hidden by brighter nuclear H II regions, while others deficient
in ionized gas may be completely invisible. 2) Approximately half of all LINERs (the so-called transition objects) show evidence in their
integrated spectra of contamination by
circumnuclear star formation (H II regions). It is argued that the
majority of transition objects are not powered exclusively by stellar
photoionization. 3)
AGNs (transition objects, LINERs, and Seyferts) preferentially occur in
early-type galaxies, mostly of Hubble types E-Sbc. The presence of a
bar has no visible effect on the probability of a galaxy hosting an AGN or on
the level of activity of the AGN, when present. 4)
LINERs share a number of similarities
with Seyferts, but there are several subtle
differences. The host galaxies of both classes of emission-line nuclei
have nearly identical distributions of Hubble types, absolute magnitudes, and
inclinations angles. The line luminosities and the general properties of the
bulk velocity field of their NLRs are also comparable. However, the NLRs of
LINERs differ from those of Seyferts in that the densities (in the low-density
region) are lower, the reddenings are lower, the line widths are larger, and
density stratification may be more common. 5) Based on the relative
intensities of the narrow emission lines, at
least 10% of all galaxies in the Palomar survey are classified as Seyfert nuclei (types 1 and 2). 6)
A BLR, as revealed by the presence of broad (FWHM ~ 2000 km s-1)
Hα emission, has been detected in
approximately 20%-25% of all nearby AGNs, or in ~ 10% of all galaxies,
implying that the space-density of broad-lined AGNs is much higher than
previously believed. Some 25% of LINERs show
broad Hα emission. If the ratio of LINERs with and without BLRs
is assumed to be the same as the ratio of Seyfert 1s to Seyfert 2s (1:1.6),
and if the low detection rate of broad Hα emission in transition objects
can be attributed to observational selection effects, then at least 60% of
all LINERs may be genuine AGNs. Here are some more reviews on AGN spectra: http://nedwww.ipac.caltech.edu/level5/,
Osterbrock (Astrophysics of Gaseous Nebulae and Active Galactic
Nuclei (Mill Valley: University Science Books), 1989), Shields,
G. A., 1999, PASP, 111, 661 OUR RESEARCHES Nuclei
of emmission-line galaxies, high surface brightness galaxies, active galaxies
have been spectrophotometrically investigated. Ion
abundance, chemical composition, physical conditions, masses and rotational momenta have been estimated. PUBLISHED PAPERS 1. PETROV G. T.,
Pis'ma AJ, v. 5, 267-270, 1979 (in Russian) “Physical conditions in the
nuclei of galaxies with emission lines” 2. PETROV G. T. , Astrofizika, v. 15, 383-392, 1979 (in Russian) “Physical conditions in the nuclei of Seyfert
galaxies of type 1” 3. PETROV G. T.
, C. r. A. S. Armenia SSR, v. 69, 52-56, 1979 (in Russian) “Contents of the ions and chemical abundances in
the nuclei of type 1 Seyfert galaxies and broad lines radio galaxies” 4. PETROV G. T.,
Youth Astrophysicists Conference, 2-5 oct.,1979, Bjurakan “Abundances in the Radio- and Seyfert galaxies” 5. GOLEV V. K., YANKULOVA I. M., PETROV G. T. , Pis'ma
AJ, v. 6, 554-558, 1980 (in Russian) “Preliminary spectrophotometric investigation of
the nucleus of the galaxy NGC 5929” 6. PETROV G. T.,
C. r. A. S. Armenia SSR, v. 70, 46-49, 1980 (in Russian) “Ion abundance and chemical composition in the
nuclei of type 2 Seyfert galaxies and narrow lines radio galaxies” 7. YANKULOVA I.
M., GOLEV V. K., PETROV G. T. ,
Pis'ma AJ, v. 6, 691-695, 1980 (in Russian) “Phisical conditions in the nucleus of the galaxy Mrk 534” 8. YANKULOVA I.
M., PETROV G. T., GOLEV V. K., C. r.
Acad. Sci. Bulg. , v. 33, 1297-1300, 1980 “Preliminary spectrophotometric investigation of
the nucleus of the galaxy NGC 5929” 9. GOLEV V. K., PETROV G. T., YANKULOVA I. M., C. r.
Acad. Sci. Bulg. , v. 33, 1033-1036, 1980 (in Russian) “Spectrophotometric investigation and phisical
conditions in the nucleus of the galaxy Mrk 534” 10. PETROV G.
T.,GOLEV V. K., YANKULOVA I. M.,
Astr. Tsirc. No. 1143, 1-3, 1980 (in Russian) "Spectrophotometry of the nuclei of the
emission line galaxies NGC 7463, Mrk 313, 531 and III Zw 103” 11. PETROV G.
T.,GOLEV V. K., YANKULOVA I. M., C.
r. Acad. Sci. Bulg. , v. 34, 461-464, 1981 “Physical conditions in the double galaxies with emission lines. Mrk 171 a, b” 12. PETROV G. T.,
YANKULOVA I. M., GOLEV V. K.,
Astrofizika, v. 17, 43-51, 1981 (in Russian) “Physical conditions in the nuclei of the emission line
galaxies” 13. YANKULOVA I.
M., PETROV G. T., GOLEV V. K., Astr.
Tsirc. No. 1169, 1-3, 1981 (in Russian) “Some spectrophotometric data about the double
galaxy NGC 3690 + IC 694" 14. MINEVA V. A., PETROV G. T., KOVACHEV B. J., C. r. Acad. Sci. Bulg. , v. 34, 1629-1632, 1981 “Spectrophotometric study of high surface brightness galaxies. I. Arakelian 144” 15. MINEVA V. A.,
PETROV G. T., GOLEV V. K., TSVETANOV
Z. I., Pis'ma AJ, v. 8, 210-213, 1982 (in Russian) Spectrophotometry of the nucleus of the galaxy Arakelian
144. 16. PETROV G. T.,
MINEVA V. A., GOLEV V. K., TSVETANOV Z.
I., Astr. Tsirc. No.1202, 4-5,1982 (in Russian) “Spectrophotometry of the nucleus of the galaxy Arakelian 583” 17. PETROV G. T.,
MINEVA V. A., GOLEV V. K., C. r.
Acad. Sci. Bulg. , v. 35, 137-140, 1982 Spectrophotometric study of galaxies with high surface
brightness. II. Arakelian 583 18. KYAZUMOV
G.A., PETROV G. T., GOLEV V. K.,
TSVETANOV Z., C. r. Acad. Sci. Bulg. , v. 35, 137-140, 1982 “NGC 6503 - rotation, mass and physical conditions in the galaxy nucleus” 19. PETROV G. T.,
KOVACHEV B. J., MINEVA V. A., C. r. Acad. Sci. Bulg. , v. 35, 725-728, 1982 “Physical conditions in the galaxy nuclei with emission lines. Mark 558” 20. MINEVA V. A.,
PETROV G. T., KOVACHEV B. J., C. r. Acad. Sci. Bulg. , v. 36, 713-716, 1983 “Physical conditions in the nucleus of the Seyfert
galaxy NGC 7469. II. Spectrophotometric investigation" 21. PETROV G. T.,
MINEVA V. A., KOVACHEV B. J., KYAZUMOV G., C. r. Acad. Sci. Bulg. , v. 36,
717-719, 1983 “Rotation, mass and physical conditions in the
nucleus of the spiral galaxy NGC 7537” 22. GOLEV V. K., YANKULOVA I. M., PETROV G. T. , Adv.
Space Res., v. 3, 235-237, 1984 “On the physical state in the narrow-line region
of Classical Seyfert galaxy NGC 7469” 23. GOLEV V. K., YANKULOVA I. M., PETROV G. T. , C. r.
Acad. Sci. Bulg. , v. 37, 549-551, 198 “On the physical state in the narrow-line region
of classical Seyfert galaxy NGC 7469” 24. PETROV G. T.,
MINEVA V. A., KYAZUMOV G.A., C. r. Acad. Sci. Bulg. , v. 37, 1287-1289, 1984 “Gas component parameters in the nucleus of the galaxy NGC 5879” 25. PETROV G. T.,
KOVACHEV B. J., MINEVA V. A., Ap & Spa.Sci., v. 116, 333-340, 1985 “Some spectrophotometric data for 31 galaxies from Karachentsev list” 26. PETROV G. T.,
MINEVA V. A., KYAZUMOV G.A., C. r. Acad. Sci. Bulg. , v. 38, 291-294, 1985 “Rotation, mass and physical conditions in the
nucleus of the spiral galaxy NGC 7339 (Karachentsev 570b )” 27. PETROV G. T.,
MINEVA V. A., KYAZUMOV G. A., C. r. Acad. Sci. Bulg. , v. 38, 699-702, 1985 “Physical conditions in the galaxy nuclei with
emission lines. Rotation, mass and parameters of the nucleus of the galaxy
NGC 1084” 28. MARKOV KH.
S., ZHEKOV S. A., PETROV G. T., TSVETANOV Z., Astr. Tsirc. No.1378, 1-3, 1985
(in Russian) “Spectra of galaxies obtained with
"ROZHEN" 2-m telescope of the Bulgarian Academy of Sciences” 29. GOLEV V. K., TSVETANOV Z. I., PETROV G. T., Astr.
Invest.(Bulg. AS), v. 4, 95-105,1985 (in Russian) “Results of a spectroscopic investigation of some
Arakelian galaxies“ 30. PETROV G. T.
, Astr. Tsirc. No.1480, 3-4,1988 (in Russian) “Spectroscopy of the Seyfert galaxy Markarian 609“ 31. PETROV G. T.,
KYAZUMOV G., KOVACHEV B. J., MINEVA V. A., Astr.Invest.(Bulg.AS), v. 5, 3-12,
1989 “Dinamic, mass and physical characteristics of the
spiral Galaxies NGC 1084, 6503, 7339 and 7537“ 32. PETROV G. T.,
MINEVA V. A., C. r.Acad. Sci. Bulg. , v. 41, No.11, 1988 “Masses and rotational momenta of Arakelian galaxies“ 33. PETROV G.,
Astr.Invest. (Bulg. AS), v. 6, p. 3-11, 1991 “CCD-spectra of the galaxy Arakelian 144“ 34. SLAVCHEVA L.,
PETROV G., MIHOV B., C. r., 1998, v. 51, No. 1 “Spectral analysis of Sefert 1 Galaxies“ 35. SLAVCHEVA L.,
MIHOV B., PETROV G., BACHEV R., IAU Symp. 194, p.87, 1999 “Spectrophotometry of selected AGN. Seyfert galaxy Arakelian 564“
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