Seyfert 2型星系中的星暴活动

星暴和活动星系核之间的联系是活动星系研究领域最重要、最活跃的研究课题之一。Seyfert星系由于距离较近、数目较多和相对低的核活动,已成为研究星暴和活动星系核之间联系的理想天体。综述了活动星系核中存在星暴的观测证据和Seyflert2型星系核区星暴活动的最新研究结果,着重讨论了存在两类Seyflert2型星系(一类是被遮挡的Seyfert1型星系,另一类是“真正”的Seyfert2型星系即不存在宽线区的Seyfert2型星系)的可能性．通过比较具有和不具有偏振宽线的Seyflert2型星系在红外、射电、光学和X射线光谱性质的差别,发现具有偏振宽线的Seyfert2型星系在本质上和Seyflert1型星系是同一类天体,差别只在于观测者视线方向的不同;而不具有偏振宽线的Seyfert2型星系是一些星系核活动较弱而星暴活动占主导的天体,这些星系从射电、红外、光学到硬X射线,都具有和星暴星系相似的性质。由于这些星系中核的吸积率将比Seyflert1型星系低近两个量级,因而它们很有可能是一些没有宽发射线区的Seyfert2型星系,即所谓的“真正”Seyflert2型星系。     

The infrared luminosity of the torus and the visibility of scattered broad line emission in Seyfert 2 galaxies

A number of studies have shown that the visibility of scattered broad emission lines in Seyfert 2 galaxies is strongly dependent on the IRAS     flux ratio, where those Seyfert 2 galaxies with 'warm' IRAS colours show polarized broad line emission. It is now clear that this effect is owing to the increasing dominance of the galactic rather than the active galactic nucleus (AGN) emission at 60 μm in less-luminous 'cool' Seyfert 2 galaxies. However, we present evidence that the 25-μm emission is a good measure of the AGN luminosity for most Seyfert 2 galaxies. Using this result, we show that the visibility of scattered broad line emission has a dependence on the AGN luminosity. The observations can be interpreted self-consistently if the scaleheight of the scattering zone varies with central source luminosity whilst the scaleheight of the obscuring torus is approximately constant.     

Seyfert galaxies and the radio–far-infrared correlation

We have observed a sample of 149 Seyfert galaxies and radio-quiet quasars at 13 cm with both a 275-km radio interferometer and the 6-km compact array of the Australia Telescope. The high-resolution observations searched for the presence of compact, high-brightness-temperature radio emission from the active nucleus. The low-resolution observations measured the total radio emission from the galaxy disc and Seyfert core and lobes. From these we draw the following conclusions. (i) Seyfert galaxies that lack compact radio cores display a correlation between radio and far-infrared (FIR) emission similar to the correlation displayed by normal spirals, albeit with greater scatter. The correlation is found to be intrinsic and is not an artefact of the richness effect. (ii) A very different radio–FIR correlation is displayed by those Seyferts that harbour compact radio cores. These tend to be more radio-loud than either normal spirals or the Seyferts that lack compact cores. The compact core emission thus seems to be responsible for the generally poor radio–FIR correlation displayed by Seyfert galaxies. (iii) The radio–FIR correlation is not significantly improved by subtracting off the 0.1-arcsec (20- to 200-pc) compact radio emission from the total radio emission. This suggests that the emission from the active galactic nucleus has significant structure on scales larger than 0.1 arcsec. Perhaps these structures are the 'linear' radio features that have been seen previously in Seyfert nuclei.     

Relationship between Infrared and Radio Emission of Seyfert Galaxies

The relationships between the monochromatic luminosity of Seyfert galaxies at frequencies of 0.408, 1.49, and 4.85 GHz and the integrated luminosity in the far infrared (IR) range are investigated. At all radio frequencies they are linear and equally close. Some Seyfert galaxies, of morphological types S0/a, E, and S0, have a far higher radio luminosity than Seyfert spiral galaxies with the same IR luminosity. Most of them are found to have compact central radio components. Seyfert spiral galaxies follow the same relationship between radio and IR emission as non-Seyfert spiral galaxies. The relationships between radio and IR luminosity for the individual groups of galaxies of spectral types Sy 1-Sy 1.5 and Sy 1.8-Sy 2 are also linear.     

Active Galactic Nuclei

The term Active Galactic Nuclei includes QSOs (Quasi-Stellar Objects), Seyfert galaxies, BL Lac objects and nuclei showing unusual amounts of star formation. South African work has centred on the photometric properties of these objects, both in the optical(UBVRI) and the infrared(JHKL). Photometry has been used to investigate the nature of the relevant emission mechanisms by studying both the colours of the nuclear components and their variations. In particular, it has been demonstrated convincingly that circumnuclear dust shells are present in several Seyfert 1 galaxies at temperatures of around 1500 K.It has also been shown that the variable component in many Seyfert 1 galaxies retains a constant spectral shape independent of its actual flux at a given time. Further, there is evidence that this shape is largely the same in all Seyfert 1 galaxies. Other studies have concentrated on surveys of morphological, spectroscopic and photometric characteristics of certain classes of galaxies such as those found by the IRAS satellite and those listed in the Arp-Madore catalogue of interacting galaxies.     

High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample – II. The properties of compact radio components

We discuss the properties of compact nuclear radio components in Seyfert galaxies from the extended 12-μm AGN sample of Rush et al. Our main results can be summarized as follows.
Type 1 and type 2 Seyferts produce compact radio components which are indistinguishable in strength and aspect, indicating that their central engines are alike, as proposed by the unification model. Infrared IRAS fluxes are more closely correlated with low-resolution radio fluxes than high-resolution radio fluxes, suggesting that they are dominated by kiloparsec-scale, extranuclear emission regions; extranuclear emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend to have stronger nuclear radio emission than late-type Seyfert galaxies. V-shaped extended emission-line regions, indicative of 'ionization cones', are usually found in sources with large, collimated radio outflows. Hidden broad lines are most likely to be found in sources with powerful nuclear radio sources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm flux densities have well-matched properties.     

The ROSAT Bright Survey: I. Identification of an AGN sample with hard ROSAT X-ray spectra

The ROSAT Bright Survey (RBS) aims to completely optically identify the more than 2000 brightest sources detected in the ROSAT all-sky survey at galactic latitudes |b| > 30° (excluding LMC, SMC, Virgo cluster). This paper presents a subsample of 66 bright point-like ROSAT survey sources with almost hard PSPC spectra, the hardness ratio HR1 is > 0.5 for most of the sources. Teh subsample could be nearly completely identified by low-resolution optical spectroscopy with the following breakdown into object classes: 31 Seyfert galaxies, 22 BL Lac candidates, 5 clusters of galaxies, 1 cataclysmic variable, and 5 bright stars. Only one object remained unidentified and one X-ray source was a spurious detection. The redshift distrbution peaks around 0.06 for the Seyferts and around 0.13 for the BL Lac candidates. Observations with medium spectral resolution were obtained for most of the new Seyfert galaxies. A large fraction (20 objects) are type 1 Seyfert galaxies, the other fraction includes Seyfert galaxies of type 1.5 – 1.8 (5 objects), two LINERs, and 4 possible narrow-line Seyfert 1 galaxies (NLS1). About one third of the new Seyfert's have nearby companion galaxies displaying either emission or absorption lines at the same redshift. Among them are a couple of systems showing direct morphological evidence for interaction. The large fraction of interacting galaxies among our sample suggests a scenario where interaction is the main trigger of AGN activity.     

MERLIN radio observations of CfA Seyferts

We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is         , and approximately 25 per cent of the components have a spectral index flatter than     . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions.     

Stellar indices and kinematics in Seyfert 1 nuclei

We present spectra of six type 1 and two type 2 Seyfert galaxies, a starburst galaxy and a compact narrow-line radio galaxy, taken in two spectral ranges centred around the near-infrared Ca  ii triplet (∼8600 Å), and the Mgb stellar feature at 5180 Å. We measured the equivalent widths (EWs) of these features and the Fe₅₂ and Fe₅₃ spectral indices.
We found that the strength of the infrared Ca  ii triplet (CaT) in type 1 Seyfert galaxies with prominent central point sources is larger than what would be expected from the observed strength of the blue indices. This could be explained by the presence of red supergiants in the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these galaxies could also be diluted by the strong Fe  ii multiplets that can be seen in their spectra.
We have also measured the stellar‐ and gas-velocity dispersions of the galaxies in the sample. The stellar velocity dispersions were measured using both the Mgb and CaT stellar features. The velocity dispersion of the gas in the narrow-line region (NLR) was measured using the strong emission lines [O  iii ] λλ 5007, 4959 and [S  iii ] λ 9069. We compare the gas- and star-velocity dispersions and find that the magnitudes of both are correlated in Seyfert galaxies.
Most of the Seyfert 1 galaxies that we observe have stellar‐velocity dispersions somewhat greater than that of the gas in the NLR.     

How do we see the nuclear region (r<0.1 pc) of narrow-line Seyfert 1 galaxies?

We propose two statistical tests to investigate how we see the nuclear region (r<0.1 pc) of narrow-line Seyfert 1 galaxies (NLS1s). 1) The high-ionization nuclear emission-line region (HINER) test: Seyfert 1 galaxies (S1s) have systematically higher flux ratios of [Fe vii] λ6087 to [O iii] λ5007 than Seyfert 2 galaxies (S2s). This is interpreted in that a significant part of the [Fe vii] λ6087 emission arises from the inner walls of dusty tori that cannot be seen in S2s [Murayama, T., Taniguchi, Y., 1998a. ApJ 497, L9; 1998b. ApJ 503, L115]. 2) The mid-infrared test: S1s have systematically higher flux ratios of the L band (3.5 μm) to the IRAS 25 μm band than S2s. This is also interpreted in that a significant part of the L band emission arises from the inner walls of dusty tori, because the tori are optically thick enough to absorb the L band emission if the tori are viewed nearly edge on [Murayama, T. et al., 2000. ApJ 528, 179]. Applying these tests to a sample of NLS1s, we have found that the NLS1s possibly have nearly the same properties as S1s.     

The X-ray spectra of optically selected Seyfert 2 galaxies: are there any Seyfert 2 galaxies with no absorption?

We present an X-ray spectral analysis of a sample of eight bona fide Seyfert 2 galaxies, selected on the basis of their high [O  iii ] λ 5007 flux, from the Ho et al. spectroscopic sample of nearby galaxies. We find that, in general, the X-ray spectra of our Seyfert 2 galaxies are complex, with some of our objects having spectra different from the 'typical' spectrum of X-ray selected Seyfert 2 galaxies. Two (NGC 3147 and 4698) show no evidence for intrinsic absorption. We suggest that this is a result of the fact that when the torus suppresses the intrinsic medium and hard energy flux, underlying emission from the host galaxy, originating in circumnuclear starbursts, and scattering from warm absorbers contributes in these energy bands more significantly. Our ASCA data alone cannot discriminate whether low-absorption objects are Compton-thick active galactic nuclei (AGNs) with a strong scattered component or lack an obscuring torus. The most striking example of our low absorption Seyfert 2 is NGC 4698. Its spectrum could be explained by either a dusty warm absorber or a lack of broad-line clouds so that its appearance as a Seyfert 2 is intrinsic and not a result of absorption.     

ROSAT results on narrow-line Seyfert 1 galaxies

The excellent soft X-ray sensitivity of the PSPC detector onboard the ROSAT satellite provided the first chance to study precisely the spectral and timing properties of Narrow-Line Seyfert 1 galaxies. ROSAT observations of Narrow-Line Seyfert 1 galaxies have revealed (1) the existence of a giant soft X-ray excess, (2) a striking, clear correlation between the strength of the soft X-ray excess emission and the FWHM of the Hβ line, (3) the general absence of significant soft X-ray absorption by neutral hydrogen above the Galactic column, (4) short doubling time scales down to about 1000 s, (5) the existence of persistent giant (above a factor of 10), and rapid (less than 1 day) X-ray variability in extragalactic sources. The soft X-ray results on Narrow-Line Seyfert 1 galaxies indicate that their black hole regions are directly visible, further supporting the Seyfert 1 nature of these objects. The extreme X-ray properties of Narrow-Line Seyfert 1 galaxies make them ideal objects for understanding many of the problems raised generally by the Seyfert phenomenon.     

Spectrophotometric observations of the nuclei of the seyfert galaxies NGC 4051, NGC 7469, and NGC 1275

A digital television complex, equipped with an original slitless spectrograph with transpaarent diffraction gratings, and operating on the 0.5-meter telescope of the Crimean Astrophysical Observatory, was used for spectrophotometric observations of the nuclei of Seyfert galaxies. The absolute energy distributions in the spectra of the nuclei of the Seyfert galaxies NGC 4501, NGC 7469, and NGC 1275 in the wavelength range 4000–7000 Å were obtained. Synthetic stellar magnitudes in the V band were calculated. The apparatus can be used to investigate the spectral variability of emission from the nuclei of Seyfert galaxies on a time scale of tens of minutes or more.Translated from Astrofizika, Vol. 39, No. 1, pp. 101–110, January–March, 1996.     

Seyfert Galaxies: A Perspective with the Infrared Space Observatory (ISO)

We present data from the Infrared Space Observatory (ISO) of a complete set of Seyfert galaxies (Seyfert 1 and Seyfert 2) in a wavelength range seldom explored so far. These data allow a detailed study of the far IR Spectral Energy Distribution of these sources. A Bayesian inversion method has been used to invert the Spectral energy Distribution (SED) of these sources yielding two fundamental results, namely that the far IR SED of Seyfert Galaxies can be explained solely through thermal reradiation of high energy photons by dust. And, that this thermal emission is made up of two or three independent components. Comparisons between the parameters obtained from the analysis of the IR SEDs and the emission in other wavelengths ranges (radio and X-rays) show that the host galaxies are key elements in the understanding of the IR emission of Seyferts. Besides, the nuclear properties seem to be adequately explained with the Unified Models of AGN, that state that the differences seen between the broad (Seyfert 1) and narrow (Seyfert 2) line objects are due to geometrical orientation effects. This revised version was published online in July 2006 with corrections to the Cover Date.     

The radio emission of galaxies with low-luminosity active nuclei

We have compared the radio continuum emission of three samples of bright (B _t ^o <13) galaxies with LINER,HII region-like, and Seyfert nuclei. Basically, we have found that the objects of the first class are the weakest total and central radio emitters (for total and central unit light, respectively), whereas the Seyfert galaxies contain the most powerful central radio sources.     

Active Galactic Nuclei

D01 Direct evidence of the receding ‘torus’ around active galactic nuclei of FRII radio galaxies and quasars D02 Infrared emission from a clumpy and dusty torus around AGN D03 Size and properties of AGN narrow–line regions from emission–line diagnostics D04 Structural Variability of Intraday Variable Sources D05 Stability of self‐gravitating accretion disks in galactic centers D06 Supermassive Binary Black Holes in AGN D07 The extreme flare in III Zw 2: Evolution of a radio jet in a Seyfert galaxy D08 Radio Linear and Circular Polarization from M81* D09 A fundamental relation between Supermassive Black Holes and Dark Matter Haloes D10 Hunting for radio‐quiet BL Lacs – the 2dF BL Lac survey D11 The Eddington limit in accretion discs D12 Molecular Tori in AGN: A search using excited states of OH D13 The X‐Ray Properties of Radio‐Loud Core‐Dominated AGN: The 2 cm‐X‐Sample D14 The X‐Ray Properties of Radio‐Loud Core‐Dominated AGN: Extension to the High Redshift Regime D15 Line Profile Variability in AGN D16 Jet Superwind Interaction D17 Radio Interferometric Observations of AGN – Probing the Nucleus of M87 with 20 Schwarzschild radii resolution D18 The ISO–2MASS AGN survey D19 Supermassive binary black holes driving the activity of galactic nuclei D20 Proton acceleration at quasi‐perpendicular shocks: A case study for Active Galactic Nuclei D21 Super‐luminal shocks in Active Galactic Nuclei D22 Unconventional quasars from the variability and proper motion survey D23 Radio observations of starburst and AGN activity in Ultraluminous Infrared Galaxies D24 Kinematics in Active Galactic Nuclei at Parsec Scales: the VLBA 2cm Survey D25 Three‐dimensional models of clumpy tori in Seyfert galaxies D26 Silicate emission in active galaxies ‐ From LINERs to QSOs D27 Discovery of 10 µm silicate emission in quasars. – Evidence of the AGN unification scheme. D28 Near‐IR adaptive optics imaging of luminous infrared galaxies D29 Interferometric observations of the Circinus galaxy with MIDI D30 Infrared Interferometry of the Seyfert Galaxy NGC 1068     

Positron annihilation radiation from Seyfert nuclei and the cosmic diffuse γ-ray background

Observational features in the low energy γ-ray spectra of Seyfert galaxies are interpreted in terms of broad and blueshifted annihilation radiation. The possibility that this ‘line’ emission is the underlying source of the bump in the cosmic diffuse γ-ray background is investigated. The observational data available in the literature are then used to estimate the maximum temperature of the electron positron plasma and the average positron annihilation rate for Seyfert galaxies. The assumption that other active nuclei, namely radio galaxies and quasars, are powered by the same annihilation mechanism is used to set limits on their contribution to the cosmic diffuse γ-ray background.     

The fraction of galaxies that contain active nuclei and their accretion rates

We investigate the relationship between the present-day optical luminosity function of galaxies and the X-ray luminosity function of Seyfert 1s to determine the fraction of galaxies that host Seyfert 1 nuclei and their Eddington ratios. The local type 1 active galactic nuclei (AGN) X-ray luminosity function is well reproduced if ∼1 per cent of all galaxies are type 1 Seyferts which have Eddington ratios of ∼10⁻³. However, in such a model the X-ray luminosity function is completely dominated by AGN in E and S0 galaxies, contrary to the observed mix of Seyfert host galaxies. To obtain a plausible mix of AGN host galaxy morphologies requires that the most massive black holes in E and S0 galaxies accrete with lower Eddington ratios, or have a lower incidence of Seyfert activity, than the central black holes of later-type galaxies.     

The Steep Balmer Decrement of Quasars and Seyfert I Galaxies and its Origin

Observations of QSOs and Seyfert I galaxies show an anomalously steep Balmer decrement, which is in contradiction to the prediction of the traditional recombination theory. This is a long standing puzzle in past three decades in the study of AGNs. In this paper, we provide an alternative approach to solve this puzzle by using a newly recognized line emission mechanism, namely, the “Cerenkov line-like radiation”. For this purpose, we collected about a hundred of QSOs and Seyfert I galaxies, whose Balmer decrements have been measured and published in the past 30 years, and make the theoretical calculations on the observed Balmer decrements using an improved formula for the Cerenkov line intensity. The agreement between the calculations and observations is excellent. Therefore, we argue that the broad hydrogen lines of the QSOs and Seyfert I galaxies mainly originate from the Cerenkov line-like radiation of relativistic electrons. If this suggestion is further confirmed, our knowledge about the physics of AGNs will be greatly changed.     

Relationship between Infrared and Radio Emission of Galaxies of the Liner Type

The relationships among the monochromatic luminosities at 0.408, 1.4, and 4.85 GHz and the integrated luminosity in the far-infrared (IR) range for galaxies of the liner type (galaxies with low-ionization, narrow-emission regions) are investigated. At all the frequencies there is a close correlation between radio luminosity and IR luminosity for liners. The character of the relationship between the radio and IR luminosities of spiral liners differs from that for spiral normal and spiral Seyfert galaxies. For the latter this relationship is linear in a wide radio range. For spiral liners it may have a nonlinear character. The scatter (standard deviation) around the regression line for the relationship between radio and IR luminosities is considerably larger for spiral liners. The ratio of the emission fluxes in the far-IR range and in the radio range is larger, on the average, for spiral liners than for normal and Seyfert spiral galaxies.