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.
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.
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
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.
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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“