Seyfert Nuclei Research Articles

Near‐Infrared and X‐Ray Obscuration to the Nucleus of the Seyfert 2 Galaxy NGC 3281

We present the results of a near-infrared and X-ray study of the Seyfert 2 galaxy NGC 3281. Emission from the Seyfert nucleus is detected in both regions of the electromagnetic spectrum, allowing us to infer both the equivalent line of sight hydrogen column density, N=71.0+ 11.3−12.3×1026 m-2, and the extinction due to dust, AV = 22 ± 11 mag (90% confidence intervals). We infer a ratio of NH/AV that is an order of magnitude larger than that determined along lines of sight in the Milky Way and discuss possible interpretations. We consider the most plausible explanation to be a dense cloud in the foreground of both the X-ray and infrared-emitting regions that obscures the entire X-ray source but only a fraction of the much larger infrared source.

Radio continuum evidence for nuclear outflow in the Circinus galaxy

Australia Telescope Compact Array radio continuum maps of the edge-on galaxy in Circinus are presented for four frequencies. The unresolved core of the galaxy measures less than 20 pc in diameter and has a flat spectral index of a = -0.06 ± 0.15 (S ∝ v). An arcsecond-scale jet close to the core is directed similarly to the previously detected larger scale plumes. The core is surrounded by what we believe to be a diffuse radio counterpart to the recently discovered r = 200 pc starburst ring. The large radio lobes are shown to consist of central plume features surrounded by edge-brightened emission. The polarized emission from these lobes is mainly contained within these distinct plume and edge-brightened features, the latter showing very high degrees of polarization (> 50 per cent at λ = 6 cm). A study of the depolarization and rotation measures indicates that with respect to the galactic disc, the northern radio lobe is in the foreground and directed towards us whilst the opposite is true for the southern lobe. Analysis of the spectral indices along the plume structures shows slight steepening of values away from the core (α ≃ -0.8), indicative of ageing of electrons along the flow. Brighter hotspots of emission present in both north and south plumes show significantly flatter spectral indices (α ≃ -0.4). This is similar to the spectral index behaviour observed in higher energy astrophysical jets. High field strengths in the edge-brightened regions (up to 18 μG) of the radio lobes are probably owing to compression of the frozen halo field, indicative of interaction between the ejecta driven bow shock with the ambient halo medium. Comparison with other galaxies displaying similar features suggests that although the nuclear starburst is no doubt contributing significantly to the concentration of cosmic rays in the radio lobes, a Seyfert nucleus is needed to explain the collimated jet observed in these objects.

Dusty Tori of Seyfert Nuclei Probed by the Water Vapor Maser Emission: How Large Are the Dusty Tori?

Geometrical and physical properties of dusty tori of Seyfert nuclei probed by the water vapor maser emission at 22 GHz are discussed. We assume that the dusty torus has a simple cylindrical form and the maser emission can be detected only when we observe the torus from almost edge-on views. The observed low frequency of occurrence of the water vapor maser emission (less than 10 percent) suggests that the torus is a vertically thin cylinder whose outer radius between a few pc and ~ 10 pc. However, the observed masing regions are concentrated in the inner 1 pc regions of the torus. This property can be explained by that only the inner a few pc regions have physical conditions enough to cause the maser emission; the temperature is as high as several hundred K and the density is as high as ~ 10^(10) cm^(-3).

Dynamical Mass of Type 2 Seyfert Nuclei

We have derived the masses of central objects (MBH) of nine type 2 Seyfert nuclei using the observational properties of the hidden broad Hβ emission. We obtain the average dynamical mass log (MBH/M☉) 8.00 ± 0.51 - 0.475 log (τes/1), where τes is the optical depth for electron scattering. If τes ~ 1, this average mass is almost comparable with those of type 1 Seyfert nuclei. However, if τes 1, as is usually considered, the average mass of type 2 Seyfert nuclei may be more massive than that of type 1 Seyfert nuclei. We discuss implications for issues concerning both the current unified model of Seyfert nuclei and physical conditions of the electron scattering regions.

X-ray spectroscopy of the broad-line radio galaxy 3C 111

We present an ASCA observation of the broad-line radio galaxy 3C 111. The X-ray spectrum is well described by a model consisting of a photoelectrically absorbed power-law form. The inferred absorbing column density is significantly greater than expected on the basis of 21-cm measurements of Galactic H I. Whilst this may be the result of intrinsic absorption from a circumnuclear torus or highly warped accretion disc, inhomogeneities and molecular gas within the foreground giant molecular cloud may also be responsible for some of this excess absorption. We also claim a marginal detection of a broad iron Kα line which is well explained as being a fluorescent line originating from the central regions of a radiatively efficient accretion disc. This line appears weak in comparison to those found in (radio-quiet) Seyfert nuclei. We briefly discuss the implications of this fact.

Star formation in southern Seyfert galaxies

We have produced radio maps, using the ATCA, of the central regions of six southern Seyfert 2 galaxies (NGC 1365, 4945, 6221, 6810, 7582, and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self--absorption. In each case the radio to far--infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [FeII] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR, and [FeII] line emission are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and HII regions) rather than with the AGN.

Where Is the Coronal Line Region in Active Galactic Nuclei?

We report the new finding that type 1 Seyfert nuclei (S1s) have excess [FeVII]6087 emission with respect to type 2s (S2s). The S1s exhibit broad emission lines which are attributed to ionized gas within 1 pc of the black hole, whereas the S2s do not show such broad lines. The current unified model of active galactic nuclei explains this difference as that the central 1 pc region in the S2s is hidden from the line of sight by a dusty torus if we observe it from a nearly edge-on view toward the torus. Therefore, our finding implies that the coronal line region (CLR) traced by the [FeVII]6087 emission resides in the inner wall of such dusty tori. On the other hand, the frequency of occurrence of the CLR in the optical spectra is nearly the same between the S1s and the S2s. Moreover, some Seyfert nuclei exhibit a very extended (~ 1 kpc) CLR. All these observational results can be unified if we introduce a three-component model for the CLR; 1) the inner wall of the dusty torus, 2) the clumpy ionized region associated with the narrow line region at distance from ~ 10 to ~ 100 pc, and 3) the extended ionized region at distance ~ 1 kpc.

Radiative acceleration of coronal gas in Seyfert nuclei

The line ratios from coronal gas in Seyferts can be successfully fitted with photoionized clouds of high densities and low volume filling factor. The ionization parameter implied is sufficiently high that models must consider the effect of radiation pressure from the active nucleus. In spite of the nucleus gravitational force, radiation pressure is sufficiently strong to compress and radially accelerate the internally stratified gas clouds provided these contain small amounts of dust (~10% of solar neighborhood value). This radial acceleration could explain the blueshift of the coronal lines relative to systemic velocity without the need of invoking an ambient `pushing' wind. Embedded dust has the interesting effect of making the photoionized clouds marginally ionization bounded instead of matter bounded.

Radiative acceleration of coronal gas in Seyfert nuclei

The line ratios from coronal gas in Seyferts can be successfully fitted with photoionized clouds of high densities and low volume filling factor. The ionization parameter implied is sufficiently high that models must consider the effect of radiation pressure from the active nucleus. In spite of the nucleus gravitational force, radiation pressure is sufficiently strong to compress and radially accelerate the internally stratified gas clouds provided these contain small amounts of dust (~10% of solar neighborhood value). This radial acceleration could explain the blueshift of the coronal lines relative to systemic velocity without the need of invoking an ambient `pushing' wind. Embedded dust has the interesting effect of making the photoionized clouds marginally ionization bounded instead of matter bounded.

Extended Broad Balmer Emission in the Seyfert 1 Galaxy Markarian 509

We present two-dimensional spectroscopic observations of the circumnuclear region of Mrk 509. The spectra show extended (off-nuclear) broad Balmer emission (FWZI>15,000 km s-1) in a region of about 8'' in diameter (8 kpc if H -->0=50 km s-1 Mpc-1). These features are interpreted as reflected radiation coming from the unresolved active nucleus. The nearly isotropic appearance of the scattering region can be explained within the framework of the unified picture for active galactic nuclei, if the scatterers lie outside the galaxy's disk; otherwise, the present observations would not support simple anisotropic models for the Seyfert nucleus in Mrk 509.

High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy Tololo 0109−383

We present a detailed optical spectroscopic analysis of the high-ionization Seyfert galaxy Tololo 0109-383, which was classified originally as a type 2 Seyfert nucleus. The high-ionization nuclear emission-line region (HINER) (traced by e.g., [Fe X] λ6374) of this galaxy is found to be spatially extended. Although the total radial extent of the HINER amounts to ~1.1 kpc, ~70% of the HINER emission is concentrated in the inner 220 pc in radius. The inner HINER in Tololo 0109-383 can be interpreted by the photoionization model of Ferguson, Korista, & Ferland, while the extended HINER suggests that low-density interstellar medium is also photoionized by high-energy continuum emission as described by Korista & Ferland. We find an extended low-ionization emission line region traced by Hα, [N II], and [S II] emission lines with very narrow widths (<200 km s-1) at ~2.2 kpc east of the nucleus. This region is characterized by (1) the observed [N II]/Hα ratio, which is ~0.8; (2) no measurable [O III] emission; and (3) a low ionization parameter, U ~ 6 × 10-5. These properties can also be interpreted in terms of a photoionization model. As well as confirmation of the presence of broad emission in both Hα and Hβ lines, we also find optical Fe II emission features in our spectra. Since the optical Fe II emission is one of the important characteristics of type 1 active galactic nuclei, it is shown that Tololo 0109-383 harbors a type 1 central engine in its nucleus. Since the soft X-ray–to–infrared (60 μm) luminosity ratio of Tololo 0109-383 is higher than those of typical type 2 Seyfert nuclei, we consider that blocking by a dusty torus may be incomplete and thus we are able to observe the type 1 nature of Tololo 0109-383.

A Geometrical Relationship between Broad-Line Clouds and an Accretion Disk around Active Galactic Nuclei

Recent hard X-ray spectroscopy of active galactic nuclei has strongly suggested that double-peaked, very broad Fe K emission arises from an accretion disk around the central engine. Model fitting of the observed Fe K emission line profile makes it possible to estimate a probable inclination angle of the accretion disk. In order to study the geometrical relationship between the accretion disk and broad emission-line regions (BLRs), we investigate the correlation between the inclination angle of the accretion disk and the velocity width of BLRs for 18 type-1 Seyfert galaxies. We found that there may be a negative correlation between them, i.e., Seyfert nuclei with a more face-on accretion disk tend to have larger BLR velocity widths, suggesting that the BLRs are not coplanar with respect to the accretion disk. The most probable interpretation may be that the BLRs arise from outer parts (r ∼ 0.01 pc) of a warped accretion disk illuminated by the central engine.

Seyfert Activity and Nuclear Star Formation in the Circinus Galaxy

We present high angular resolution (0.15-0.5) near infrared images and spectroscopy of the Circinus galaxy, the closest Seyfert 2 galaxy known. The data reveal a non-stellar nuclear source at 2.2 microns. The coronal line region and the hot molecular gas emission extend for 20-50 pc in the ionization cone. The data do not show evidence for a point-like concentration of dark mass; we set an upper limit of 4*10^6 Mo to the mass of a putative black hole. We find evidence for a young nuclear stellar population, with typical ages between 4*10^7 and 1.5*10^8 yrs. The luminosity of the starburst inside a few hundred pc is comparable to the intrinsic luminosity of the Seyfert nucleus, and the two of them together account for most of the observed bolometric luminosity of the galaxy. Within the central 12 pc the starburst has an age of about 7*10^7 yrs and radiates about 2% of the luminosity of the active nucleus. We discuss the implications of these results for models that have been proposed for the starburst-AGN connection.

5.14. NMA survey of HCN and CO emission from nearby active galaxies

In order to study the spatial variation of molecular gas properties and their relation to the nuclear activities of galaxies, we have conducted an imaging survey of HCN(1-0) and CO(1-0) emission from nearby active galaxies with the Nobeyama Millimeter Array. Here we present the results of three galaxies, NGC 3504 (Starburst nucleus), NGC 4736 (Post-Starburst nucleus), and NGC 6951 (Seyfert nucleus with a circumnuclear star-forming ring).In the observations of these three galaxies, we find that (1) the HCN distribution is significantly different from that of CO, and that (2) the HCN emission spatially correlates better with the massive star-forming region than CO emission (Figure 1, 2(a), and 2(b)). For instance, in the Post-Starburst nucleus of NGC 4736, we find a remarkable decrease of the HCN emission, despite of strong CO nuclear concentration. The upper limit (2σ) of the integrated intensity ratio I(HCN)/I(CO) within the central r &lt; 150 pc region is 0.04, which is surprisingly low compared with the value in other galactic nucleus (cf. ratio = 0.08 within r &lt; 300 pc in Milky Way, Jackson et al. 1996, ApJ, 456, 91). Presumably the dense molecular gas must have been consumed by a past starburst event in the nucleus. These results suggest a tight correlation between the gas density measured by the I(HCN)/I(CO) ratio and the star-forming activities in the central region of galaxies.It seems that there exist a spatial anti-correlation between the I(HCN)/ I(CO) ratio and the gas velocity dispersion in NGC 6951 (Figure 2(c)). The ratio is not enhanced in the CO twin peaks which are interpreted as shock regions due to gas orbit crowding. These imply the importance of gravitational instability on the formation of dense molecular gas and successive massive starformation in this region.

6.8. A multi wavelength study of the circumnuclear region of NGC 1365

The nuclear region of the supergiant barred spiral galaxy NGC 1365 contains bright “hot spots”, as seen at optical wavelengths, as well as a number of non-thermal radio continuum sources, some of which remain unresolved at 0.25” × 0.10” resolution (Sandqvist et al. 1995, A&amp;A 295, 585).The distribution of [OIII] λ5007 emission from the nuclear region supports the scenario of an [OIII] cone emanating from the Seyfert nucleus. The velocity field of the high excitation gas in the cone has been modeled by Hjelm &amp; Lindblad (1996, A&amp;A 305, 727) in terms of an accelerated bipolar conical outflow. Such conical or biconical high-excitation emission-line structures extending from the position of the nucleus are found in several active galactic nuclei.

A Near‐Infrared Spectroscopic Survey of LINER Galaxies

This paper reports the results of a near-infrared spectroscopic survey of LINER galaxies undertaken with a new infrared spectrograph at the 5 m Hale telescope. The galaxy sample includes 11 LINERs with spectra covering the [Fe II] (1.2567 μm), Paβ (1.2818 μm), H2 [1-0 S(1), 2.1218 μm], and Brγ (2.1655 μm) near-infrared emission lines and one additional galaxy with only [Fe II] and Paβ line coverage. All of the LINERs with infrared line detections have [Fe II] and/or H2 emission, with about half (four of nine) having extremely high ratios (>2) of [Fe II] to Paβ. The strength of the H2 and [Fe II] lines is well correlated with the optical [O I] line, with many LINERs having higher ratios of [Fe II]/Paβ, H2/Brγ, and [O I]/Hα than other galaxy types. The LINERs with the highest [Fe II]/Paβ ratios (termed strong [Fe II] LINERs) show evidence for recent star formation. Shocks from compact supernova remnants may enhance the [Fe II] emission in these strong [Fe II] LINERs. The LINERs with lower [Fe II]/Paβ ratios (termed weak [Fe II] LINERs) are more consistent with Seyfert-like activity, including higher ionization states, some X-ray sources, and some broad Hα detections. The [Fe II] luminosity and the [Fe II]/Paβ ratio in these objects are more easily explained by hard X-ray excitation than in the strong [Fe II] LINERs. These weak [Fe II] LINERs are considered prime candidates for being low-luminosity Seyfert nuclei.

11.17. Radio emission from low-luminosity active galactic nuclei

“Low-luminosity” active galactic nuclei (LLAGNs) typically have ≳ mag, as opposed to −18 to −30 mag for luminous Seyfert 1 nuclei and QSOs. The physical origin of the class of emission-line nuclei known as low-ionization nuclear emission-line regions (LINERs; Heckman 1980) remains controversial. Based on the close resemblance of some LINERs to Seyfert nuclei, LINERs can arguably be considered another manifestation of the AGN phenomenon. An alternative view is that stellar processes account for LINER energy. The energetics alone do not provide a useful discrimator between the two possible mechanisms.

5.6. Mapping the nucleus of NGC1068 in CO(2-1)

Molecular gas in active galactic nuclei (AGN) may play multiple roles: it can fuel a massive black hole, collimate jets, and/or obscure a broad-line region from most viewing angles. In order to understand the physical state and kinematics of molecular gas in AGN, we are using the Owens Valley Millimeter Array to map several nearby active galxies at high resolution in the CO(2–1) line. Our first target is NGC 1068, which hosts an obscured broad-line Seyfert nucleus as well as an extended starburst. We acquired data from 35 baselines out to 242 m, with 10.4 km s−1 resolution. After calibration with MMA (Scoville et al. 1992), we subtracted a 23 mJy nuclear continuum source from the visibility data and made CLEAN maps with AIPS. The resulting sensitivity in each line channel is 32 mJy per 1.8″ × 1.5″ robustly-weighted beam (1″ = 70 pc at 14.4 Mpc).

3.33. A close look at NGC1068 by adaptive optics: dust torus and micro-spiral structure

As the closest Seyfert nucleus, NGC 1068 is a key object to establish on firm grounds the model of active galactic nuclei (AGN). Two important questions relative to AGNs are : i) can we actually see the parsec-scale torus of dust and molecular gas around the central engine predicted by the popular Unified Model model (e.g. Antonucci, 1993)?; ii) how the gas is brought down to the parsec scale to feed the monster, and, in particular, is there a micro spiral/bar system that could produce the required torque ? Adaptive Optics in the near-IR has potential advantages to make a significant progress in those two questions : the low dust opacity and the excellent angular resolution.

Radio Emission from Low-Luminosity Active Galactic Nuclei

AbstractMany nearby galaxies show optical evidence for low-luminosity AGNs that are far less luminous than classical Seyfert nuclei and QSOs. LINERs, the most common variety of such emission-line objects, comprise ~ 1/3 of nearby galaxies, and may serve as an important “missing link“ between normal and Seyfert galaxies. To shed light on the physical origin of LINERs, which remains controversial, we are conducting a high-resolution continuum survey of a representative sample of galaxies using the VLA to search for compact radio cores. We additionally discuss the dramatic radio variability in the LINER nucleus of M81.