Hidden Broad-line Region Research Articles

Infrared Photometric Study for theIRAS1 Jy Sample of Ultraluminous Infrared Galaxies

The Two Micron All Sky Survey observation of the IRAS 1 Jy sample of ultraluminous infrared galaxies (ULIRGs) is presented in this paper. Together with the IRAS photometric data, the properties and energy sources of the IRAS 1 Jy sample of ULIRGs in the near- and far-infrared are discussed. It is shown that most sources, not only Seyfert galaxies but also H II and LINER galaxies, have a radiation mechanism with a power-law continuum in the near- infrared. All Seyfert 1 sources and some Seyfert 2 sources with hidden broad-line regions (BLRs) have much redder colors and steeper spectral indices in the near- infrared, indicative of an extremely strong energy source from the central active galactic nucleus. Almost all sources have thermal radiation indicative of starburst domination in the IRAS region; in particular, all Seyfert 1 sources and many Seyfert 2 sources with hidden BLRs have lower [25]-[60] colors, indicative of a flatter spectral continuum and a rather warm color temperature in the 25-60 mu m region. However, all kinds of sources have similar [60]-[100] color distributions, indicative of similar energy sources from the enhanced heating in active star-forming regions and very cold material and also indicative of the quite small influence from the central activity in the 60-100 mu m region. In addition, all Seyfert 1 sources and some Seyfert 2 sources with hidden BLRs have much larger absolute magnitudes in the K band compared with other sources. The results presented in this paper may further help us to understand the physical properties of ULIRGs.

Dust Morphology of Hidden Broad‐Line Region and Non–Hidden Broad‐Line Region Seyfert 2 Galaxies

We investigate the nuclear dust properties of hidden broad-line region (HBLR) and non-HBLR Seyfert 2 galaxies. Optical images obtained from the Hubble Space Telescope for a selected sample of HBLR and non-HBLR Seyfert 2 galaxies are fitted with the Galfit package to probe the inner structures of these galaxies within the central 1 kpc regions. Most of the galaxies show complicated dust features in these regions. However, the dust morphology shows no significant difference between the HBLR and non-HBLR Seyfert 2 galaxies. Dust masses inside the 1 kpc nuclear regions (M1 kpc) are estimated from the obscuration levels in the central regions of these galaxies. We compare our results with other observed properties, including [O III], far-infrared, and radio emission. We find that the HBLR and non-HBLR Seyfert 2 galaxies show different near-infrared colors and M1 kpc-FIR correlations, indicating that these two classes of Seyfert 2 galaxies are dominated by different emission mechanisms. We suggest that they are intrinsically different and cannot be explained by the standard unification model.

ISOobservations and models of galaxies with hidden broad line regions

We present ISO mid-infrared spectrophotometry and far-infrared photometry of galaxies with Hidden Broad Line Regions (HBLR). We also present radiative transfer models of their spectral energy distributions which enable us to separate the contributions from the dusty disc of the AGN and the dusty starbursts. We find that the combination of tapered discs (discs whose thickness increases with distance from the central source in the inner part but stays constant in the outer part) and starbursts provide good fits to the data. The tapered discs dominate in the mid-infrared part of the spectrum and the starbursts in the far-infrared. After correcting the AGN luminosity for anisotropic emission we find that the ratio of the AGN luminosity to the starburst luminosity, L(AGN)/L(SB), ranges from about unity for IRAS14454-4343 to about 13 for IRAS01475-0740. Our results suggest that the warm IRAS colours of HBLR are due to the relatively high L(AGN)/L(SB). Our fits are consistent with the unified model and the idea that the infrared emission of AGN is dominated by a dusty disc in the mid-infrared and starbursts in the far-infrared.

Detection of the Polarized Broad Emission Line in the Seyfert 2 Galaxy Markarian 573

We report the discovery of the scattered emission from a hidden broad-line region (BLR) in a Seyfert 2 galaxy, Mrk 573, based on our recent spectropolarimetric observation performed at the Subaru Telescope. This object has been regarded as a type 2 active galactic nucleus (AGN) without a hidden BLR by previous observations. However, our high-quality spectrum of the polarized flux of Mrk 573 shows prominent broad (∼3000 km s-1) Hα emission, broad weak Hβ emission, and subtle Fe II multiplet emission. Our new detection of these indications for the presence of the hidden BLR in the nucleus of Mrk 573 is thought to be because of the high signal-to-noise ratio of our data, but the possibility of a time variation of the scattered BLR emission is also mentioned. Some diagnostic quantities such as the IRAS color, the radio power, and the line ratio of the emission from the narrow-line region of Mrk 573 are consistent with the distributions of such quantities of type 2 AGNs with a hidden BLR. Mrk 573 is thought to be an object whose level of the AGN activity is the weakest among the type 2 AGNs with a hidden BLR. In terms of the systematic differences between the type 2 AGNs with and without a hidden BLR, we briefly comment on an interesting Seyfert 2 galaxy, Mrk 266 SW, which may possess a hidden BLR but has been treated as a type 2 AGNs without a hidden BLR.

On the Nature of Low‐Luminosity Narrow‐Line Active Galactic Nuclei

There is clear observational evidence that some narrow-line (type 2) active galactic nuclei (AGNs) have a hidden broad-line region (BLR) and are thus intrinsically broad-line (type 1) AGNs. Does this AGN unification apply for all type 2 AGNs? Indirect arguments suggest that some type 2 AGNs, i.e., AGNs having no obscured BLR, do exist, but it is not clear why the BLR is missing in these AGNs. Here we point out a possible natural explanation. The observed radius-luminosity relation for the BLR implies an increasing line width with decreasing luminosity for a given black hole mass (MBH). In addition, there appears to be an upper limit to the observed width of broad emission lines in AGNs of Δvmax ~ 25,000 km s-1, which may reflect a physical limit above which the BLR may not be able to survive. Thus, at a low enough luminosity the BLR radius shrinks below the Δvmax radius, leaving no region where the BLR can exist, although the AGN may remain otherwise normal. The implied minimum bolometric luminosity required to sustain a BLR with Δv < 25,000 km s-1 is Lmin ~ 1041.8(MBH/108 M☉)2. All AGNs with L < Lmin are expected to be type 2 AGNs, i.e., narrow-line AGNs without a hidden BLR. Predictions for the true nature of low-luminosity AGNs in two samples of nearby galaxies are provided. These can be used to test the above Lmin conjecture and the predictions of other models for the size and origin of the BLR.

The Lack of Broad-Line Regions in Low Accretion Rate Active Galactic Nuclei as Evidence of Their Origin in the Accretion Disk

In this Letter, we present evidence suggesting that the absence or presence of hidden broad-line regions (HBLRs) in Seyfert 2 galaxies is regulated by the rate at which matter accretes onto a central supermassive black hole, in units of the Eddington rate. Evidence is based on data from a subsample of type 2 active galactic nuclei extracted from the Tran spectropolarimetric sample and made up of all those sources that also have good-quality X-ray spectra available and for which a bulge luminosity can be estimated. We use the intrinsic (i.e., unabsorbed) X-ray luminosities of these sources and their black hole masses (estimated by using the well-known relationship between nuclear mass and bulge luminosity in galaxies) to derive the nuclear accretion rate in Eddington units. We find that virtually all HBLR sources have accretion rates larger than a threshold value of thres 10-3 (in Eddington units), while non-HBLR sources lie at thres. These data nicely fit predictions from a model proposed by Nicastro in which the broad-line regions (BLRs) are formed by accretion disk instabilities occurring in proximity of the critical radius at which the disk changes from gas pressure dominated to radiation pressure dominated. This radius diminishes with decreasing ; for low enough accretion rates (and therefore luminosities), the critical radius becomes smaller than the innermost stable orbit and BLRs cannot form.

The Unified Model and Evolution of Active Galaxies: Implications from a Spectropolarimetric Study

We extend the analysis presented in Paper I of a spectropolarimetric survey of the CfA and 12 lm samples of Seyfert 2 galaxies (S2s). We confirm that polarized (hidden) broad-line region (HBLR) S2s tend to have hotter circumnuclear dust temperatures, show mid-IR spectra more characteristic of Seyfert 1 galaxies (S1s), and are intrinsically more luminous than non-HBLR S2s. The levels of obscuration and circumnuclear star formation, however, appear to be similar between HBLR and non-HBLR S2 galaxies, based on an examination of various observational indicators. HBLR S2s, on average, share many similar large-scale, presumably isotropic, characteristics with S1s, as would be expected if the unified model is correct, while non-HBLR S2s generally do not. The active nuclear engines of non-HBLR S2s, then, appear to be truly weaker than HBLR S2s, which in turn are fully consistent with being S1s viewed from another direction. There is also evidence that the fraction of detected HBLRs increases with the radio power of the active galactic nucleus. Thus, all S2 galaxies may not be intrinsically similar in nature, and we speculate that evolutionary processes may be at work. Subject headings: galaxies: active — galaxies: Seyfert — polarization On-line material: color figures

Comparisons of Infrared Colors and Emission-line Intensities between Two types of Seyfert 2 Galaxies

We study the relation between the infrared colors, [OIII] emission lines, gaseous absorbing column density (NH),and the detectability of the polarized (hidden) broad-line region (HBLR) in a large sample of 75 Seyfert 2 galaxies (Sy2s). From the indicators of star-formation activity, f60/f100 and LFIR/LB, we find some evidence that the Sy2s without HBLR show higher star-formation activities than those with HBLR, in agreement with previous prediction. Also, we confirm that the HBLR Sy2s tend to have a larger luminosity ratio of the core to the host galaxy, suggesting that the HBLR Sy2s display more powerful AGN activity. However, the level of obscuration found in previous papers is nearly indistinguishable between the two types of Sy2s. The results support the statement that the non-HBLR Sy2s, with a weaker core component and a stronger star-formation activity component, are intrinsically different from the HBLR Sy2s, which are Sy1 systems with a hidden powerful AGN core and a low star-formation activity. The indications are that the non-HBLR Sy2s might be at an earlier evolutionary phase than the HBLR Sy2s.

Hidden Broad-Line Seyfert 2 Galaxies in the C[CLC]f[/CLC]A and 12 μ[CLC]m[/CLC] Samples

We report the results of a spectropolarimetric survey of the CfA and 12 μm samples of Seyfert 2 (S2) galaxies. Polarized (hidden) broad-line regions (HBLRs) are confirmed in a number of galaxies, and several new cases (F02581-1136, MCG -3-58-7, NGC 5995, NGC 6552, NGC 7682) are reported. The 12 μm S2 galaxy sample shows a significantly higher incidence of HBLRs (50%) than its CfA counterpart (30%), suggesting that the latter may be incomplete in hidden active galactic nuclei. Compared to the non-HBLR S2 galaxies, the HBLR S2 galaxies display distinctly higher radio power relative to their far-infrared output and hotter dust temperature as indicated by the f25/f60 color. However, the level of obscuration is indistinguishable between the two types of S2 galaxies. These results strongly support the existence of two intrinsically different populations of S2 galaxies: one harboring an energetic, hidden S1 nucleus with a broad-line region and the other a pure S2 galaxy, with a weak or absent S1 nucleus and a strong, perhaps dominating starburst component. Thus, the simple purely orientation-based unification model is not applicable to all Seyfert galaxies.

High-Ionization Nuclear Emission-Line Region of Seyfert Galaxies

Recently, Murayama & Taniguchi proposed that a significant part of the high-ionization nuclear emission-line region (HINER) in Seyfert nuclei arises from the inner wall of dusty tori because type 1 Seyfert nuclei (S1s) show excess HINER emission with respect to type 2 Seyfert nuclei (S2s). This means that the radiation from the HINER is anisotropic, and thus statistical properties of the HINER emission can be used to investigate the viewing angle toward dusty tori for various types of Seyfert nuclei. In order to investigate viewing angles toward narrow-line Seyfert 1 galaxies (NLS1s) and intermediate types of Seyfert galaxies (i.e., type 1.5, 1.8, and 1.9 Seyfert galaxies, hereafter S1.5, S1.8, and S1.9, respectively), we apply this HINER test to them. We also apply the same test for S2s with and without the hidden broad-line region. The sample of Seyfert nuclei analyzed here consists of 124 Seyfert nuclei compiled from the literature. Our main results and suggestions are as follows: (1) The NLS1s are viewed more face-on toward dusty tori than the S2s. However, the HINER properties of the NLS1s are indistinguishable from those of the S1s. (2) The S1.5s appear to compose heterogeneous populations; e.g., (a) some of them may be seen from an intermediate viewing angle between S1s and S2s, (b) some S1.5s are basically S1s but a significant part of the broad-line region (BLR) emission is accidentally obscured by dense, clumpy gas clouds, or (c) some S1.5s are basically S2s, but a part of the BLR emission can be seen from some optically thin regions of the dusty torus. (3) The S1.8s, the S1.9s, and the objects showing either a broad Paβ line or polarized broad Balmer lines are seen from a large inclination angle, and the emission from the BLRs of such objects reaches us through optically thin parts of dusty tori. These three results support strongly the current unified model of Seyfert nuclei. Finally, (4) the line ratios of [Fe X] λ6374 to the low-ionization emission lines are rather more isotropic than those of [Fe VII] λ6087. Therefore, it is suggested that the [Fe X] λ6374 emission is not useful for investigating the viewing angle toward the dusty torus in Seyfert nuclei. The most plausible reason seems to be that the [Fe X] λ6374 emission is spatially extended, and thus its strength tends to show less viewing-angle dependence.

The Electron-scattering Region in Seyfert Nuclei

The electron-scattering region (ESR) is one of the important ingredients of Seyfert nuclei because it makes it possible to observe the hidden broad-line region (HBLR) in some type 2 Seyfert nuclei (S2's). However, little is known about its physical and geometrical properties. Using the number ratio of S2's with and without HBLR, we statistically investigate where the ESR is in Seyfert nuclei. Our analysis suggests that the ESR is located at a radius between ~0.01 and ~0.1 pc from the central engine. We also briefly discuss a possible origin of the ESR.

Mid-Infrared and Optical Spectroscopy of Ultraluminous Infrared Galaxies: A Comparison

New tools from Infrared Space Observatory (ISO) mid-infrared spectroscopy have recently become available to determine the power sources of dust-obscured ultraluminous infrared galaxies (ULIRGs). We compare ISO classifications—starburst or active galactic nucleus (AGN)—with classifications from optical spectroscopy and with optical/near-infrared searches for hidden broad-line regions. The agreement between mid-infrared and optical classification is excellent if optical LINER spectra are assigned to the starburst group. The starburst nature of ULIRG LINERs strongly supports the suggestion that LINER spectra in infrared-selected galaxies, rather than being an expression of the AGN phenomenon, are due to shocks that are probably related to galactic superwinds. Differences between ISO and optical classification provide clues on the evolution of ULIRGs and on the configuration of obscuring dust. We find few ISO AGNs with optical H II or LINER identification, suggesting that highly obscured AGNs exist but are not typical for the ULIRG phenomenon in general. Rather, our results indicate that strong AGN activity, once triggered, quickly breaks the obscuring screen at least in certain directions, thus becoming detectable over a wide wavelength range.

Hubble Space Telescopeand VLA Observations of Seyfert 2 Galaxies: The Relationship between Radio Ejecta and the Narrow‐Line Region

We present HST/WFPC2 images, in narrowband filters containing the [O III] λ5007 and Hα + [N II] emission lines and their adjacent continua, of a sample of seven Seyfert 2 galaxies selected because they possess either extended emission-line regions in ground-based observations or a hidden broad-line region in polarized light. Six of the galaxies have also been observed with the VLA in order to obtain radio maps of better quality and angular resolution than those in the literature. We find detailed correspondences between features in the radio and emission-line images, clearly indicating strong interactions between the radio jets and the interstellar medium. Such interactions play a major role in determining the morphology of the NLR, as the radio jets sweep up and compress ambient gas, producing ordered structures with enhanced surface brightness in line emission. In at least three galaxies, namely, Mrk 573, ESO 428-G14, and Mrk 34 (and perhaps also NGC 7212), off-nuclear radio lobes coincide with regions of low gaseous excitation (as measured by the [O III]/(Hα + [N II]) ratio). In Mrk 573 and NGC 4388, there is a clear trend for low-brightness ionized gas to be of higher excitation. These results may be understood if radio lobes and regions of high emission-line surface brightness are associated with high gas densities, reducing the ionization parameter. [O III]/(Hα + [N II]) excitation maps reveal bipolar structures that can be interpreted as either the ionization cones expected in the unified scheme or widening, self-excited gaseous outflows. Only NGC 4388 and Mrk 573 show a clearly defined, straight-edged ionization cone.

Status of Agn Unification Models

Unification of Active Galactic Nuclei is obviously multidimensional. Dimensions include the mass and angular momentum of the postulated central black hole, the mass accretion rate through the accretion disk surrounding the black hole, and the orientation of this central engine plus associated dust with respect to the line of sight (e.g., Blandford 1990). As for orientation related unification, considerable progress has been made over the past decade. This review will deal with this geometric unification.Mechanisms in AGN include:1.relativistic beaming in radio jets (as manifested by rapid variability from radio to ϒ-rays, superluminal motion, synchrotron self-Compton2.emission by an optically thick, non-spherical body (e.g., hot accretion disk);3.external collimation and shadowing by an anisotropic dust distribution (as manifested by the existence of hidden continua, observed through polarization or photon deficit in extended emission line regions, and hidden broad line regions, observed through polarization or in the less opaque near infrared part of the spectrum).These anisotropic mechanisms introduce selection biases, they lead to incorrect inferred luminosities and make us overestimate the AGN diversity. They furthermore imply unification at some level.

Hidden broad line regions and anisotropy in AGN

Hidden broad line regions and anisotropy in AGN

Multidirectional views of the active nucleus of NGC 1068

view Abstract Citations (250) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multidirectional Views of the Active Nucleus of NGC 1068 Miller, J. S. ; Goodrich, R. W. ; Mathews, William G. Abstract Previous spectropolarimetric work has shown that the type 2 Seyfert galaxy NGC 1068 contains a broadline region hidden from our direct view and visible only in scattered, polarized light. New observations, including imaging polarimetry as well as spectropolarimetry, allow us to confirm and extend the earlier work. Off-nucleus regions of high polarization are identified as places in the body of the galaxy where dust reflects light from the hidden broad-line region. This allows for the first time the ability to obtain spectra of an active nucleus as it would appear from several different viewing directions. From these other directions the spectrum of NGC 1068 would appear as a normal Seyfert 1 galaxy. We derive a value of 1.44 x 10^42^ ergs s^-1^ for the Hβ luminosity that would be seen in the absence of obscuration, also typical of Seyfert 1 galaxies. The broad Hβ line as seen reflected in the external dusty regions is significantly narrower than that observed in the nuclear polarized flux, adding additional weight to the view that electron scattering is responsible for the nuclear polarization. The observational data are used to constrain a theoretical model for the electron-scattering region. There is an inherent difficulty in having the same electrons responsible for scattering X-rays and the optical light, since the low electron temperature implied by the scattered broad Hβ line width must be reconciled with the high level of ionization implied by the lack of soft X-ray absorption and the observed energy of the Fe Kα line. However, models satisfying constraints from both optical and X-ray data are possible. We assume a conical electron- scattering region and find that a cone opening half-angle of ~30^deg^ viewed from an angle of 32^deg^ is consistent with the observations, with the inner radius of the scattering region ~10^20^ cm from the nucleus, the electron density ~400-600 cm^-3^, and the mean electron temperature ~(2-3)x10^5^ K. The luminosity necessary to maintain the required ionization level is larger than the observed bolometric luminosity, suggesting the surprising result that the continuum source must be inherently anisotropic. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170406 Bibcode: 1991ApJ...378...47M Keywords: Active Galactic Nuclei; Astronomical Polarimetry; Seyfert Galaxies; Astronomical Spectroscopy; Charge Coupled Devices; Cosmic Dust; Electron Scattering; Luminosity; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1068; GALAXIES: NUCLEI; GALAXIES: SEYFERT; POLARIZATION; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (1) NED (1)

Extended broad-line emission in the obscured Seyfert 1 Nucleus of NGC 4388

view Abstract Citations (52) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extended Broad-Line Emission in the Obscured Seyfert 1 Nucleus of NGC 4388 Shields, Joseph C. ; Filippenko, Alexei V. Abstract We present spectroscopic observations of extended, off-nuclear broad Hα emission (FWZI~ 4000 km s^-1^) in NGC 4388, a galaxy often classified as a type 2 Seyfert. These features are interpreted as scattered radiation from a Seyfert 1 nucleus that is obscured along our line of sight, possibly by the normal interstellar medium in this galaxy. NGC 4388 strongly resembles NGC 1068, another object with a hidden broad-line region, and may be useful for understanding the relationship between Seyfert 1 and Seyfert 2 galaxies. A scattering origin for the off-nuclear broad emission may be tested by spectropolarimetric observations. Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/185265 Bibcode: 1988ApJ...332L..55S Keywords: Emission Spectra; Galactic Nuclei; H Alpha Line; Seyfert Galaxies; Spectral Line Width; Astronomical Spectroscopy; Radio Astronomy; X Ray Astronomy; Astronomy; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4388; GALAXIES: NUCLEI; GALAXIES: SEYFERT full text sources ADS | data products SIMBAD (2) NED (2)