Narrow-line Active Galactic Nuclei Research Articles

Optical Identification of theASCALarge Sky Survey

We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2-7 keV band above 3.5 ?. The flux limit corresponds to ~1 ? 10-13 ergs cm-2 s-1 in the 2-10 keV band. The sources are identified with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentified. All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of log NH(cm-2) = 22-23 at the object's redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3 ? 0.3. The hardness may be explained by the reflection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with log NH(cm-2) = 22-23 at the object's redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet. Based on the log N- log S relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH(cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3 ? 10-11 ergs cm-2 s-1 to 2 ? 10-13 ergs cm-2 s-1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%. In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z = 0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z = 0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of log NH(cm-2) = 22-23 in the redshift range 0.5-2, or in the X-ray luminosity range larger than 1044 ergs s-1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.

ASCA observations of deep ROSAT fields -- IV. Infrared and hard X-ray observations of an obscured high-redshift QSO

We use UKIRT and ASCA observations to determine the nature of a high-redshift (z=2.35) narrow-line AGN, previously discovered by Almaini et al. The UKIRT observations show a broad Hα line while no Hβ line is detected. This together with the red colour (B-K=5.4) suggests that our object is a moderately obscured QSO (AV >3), at optical wavelengths. The ASCA data suggest a hard spectrum, probably the result of a large obscuring column, with Γ =1.93+0.62-0.46, NH∼ 1023 cm-2. The combined ASCA and ROSAT data again suggest a heavily obscured spectrum (NH∼ 1023 cm-2 or AV∼ 100). In this picture, the ROSAT soft X-ray emission may arise from electron scattering, in a similar fashion to local Seyfert 1.9 galaxies. Then, there is a large discrepancy between the moderate reddening witnessed in the infrared and the large X-ray absorbing column. This could be possibly explained on the basis of, e.g., high gas metallicities, or by assuming that the X-ray absorbing column is inside the dust sublimation radius. An alternative explanation can be obtained when we allow for variability between the ROSAT and ASCA observations. Then the best-fitting spectrum is still flat, Γ =1.35+0.16-0.14, but with low intrinsic absorption in better agreement with the IR data, while the ROSAT normalization is a factor of 2 below the ASCA normalization. This object may be one of the bright examples of a type II QSO population at high redshift, previously undetected in optical surveys. The hard X-ray spectrum of this object suggests that such a population could make a substantial contribution to the X-ray background.

On the polarization of Hα scattered by neutral hydrogen in active galactic nuclei

Raman scattering by atomic hydrogen converts the UV continuum around Ly$\beta$ into optical continuum around H$\alpha$, and the basic atomic physics has been discussed in several works on symbiotic stars. We propose that the same process may operate in active galactic nuclei (AGN) and calculate the linear polarization of the broad emission lines Raman-scattered by a high column neutral hydrogen compnent. The conversion efficiency of the Raman scattering process is discussed and the expected scattered flux is computed using the spectral energy distribution of an AGN given by a typical power law. The high column H {\sc i} component in AGN is suggested by many observations encompassing radio through UV and X-ray ranges. When the neutral hydrogen component with a column density $\sim 10^{22} cm^{-2}$ is present around the active nucleus, it is found that the scattered H$\alpha$ is characterized by a very broad width $\sim 20,000 km/s$ and that the strength of the polarized flux is comparable to that of the electron-scattered flux expected from a conventional unified model of narrow line AGN. The width of the scattered flux is mainly determined by the column density of the neutral scatterers where the total scattering optical depth becomes of order unity. The asymmetry in the Raman scattering cross section around Ly$\beta$ introduces red asymmetric polarized profiles around H$\alpha$. The effects of the blended Ly$\beta$ and O {\sc vi} 1034 doublet are also investigated. We briefly discuss the spectropolarimetric observations performed on the Seyfert galaxy IRAS 110548-1131 and the narrow line radio galaxy Cyg A.

Thick Tori around Active Galactic Nuclei: The Case for Extended Tori and Consequences for Their X‐Ray and Infrared Emission

Two families of models of dusty tori in active galactic nuclei (AGNs; moderately thick and extended versus very thick and compact) are tested against available observations. The confrontation suggests that the former class better explains the infrared (IR) broadband spectra of both broad- and narrow-line AGNs, the anisotropy of the emission deduced by comparing IR properties of Seyfert 1 and 2 nuclei, and the results of IR spectroscopy and those of high spatial resolution observations. There is, however, clear evidence for a broad distribution of optical depths. We also examine the relationship between IR and X-ray emission. The data support a view in which the matter responsible for the X-ray absorption is mostly dust free, lying inside the dust sublimation radius. The consequences of these results for the hard X-ray background as well as IR counts and background are discussed.

Near-Infrared Spectrophotometry of the Radio Galaxy 4C 40.36 at [ITAL]z[/ITAL] = 2.269

We present new near-infrared spectrophotometric observations of the high-z radio galaxy 4C 40.36 (z = 2.269) including J- and H-band images and a low-resolution (λ/Δλ ~ 100) spectrum between 1.14 and 1.70 μm. We have detected distinctively strong [O III] λ4959/5007 doublet and other relatively weak emission lines superposed on the faint flat continuum. The measured emission-line ratios are roughly consistent with the typical narrow-line AGN with the solar metal abundance, except for the large [Ne III] λ3869/[O II] λ3727 ratio that is probably caused by the overabundance of neon in this object.

Thick tori around AGNs: Model predictions and observations

We have worked out detailed calculations of the 0.1–1000 μm SEDs for AGN surrounded by dust tori with different opening angles. The radiative transfer equation for a cloud having azimuthal symmetry and containing a mixture of dust grains has been solved by means of a numerical code that takes absorption, emission and scattering into account. The expected continuum and silicate features are compared to the data available for broad and narrow line AGN. We find that models of thick tori extending up to a few hundreds of parsecs, in which physical processes such as shocks significantly reduce the silicate grain abundance within the first few tens of parsecs, are fully consistent with available broad-band data and high resolution IR spectra of Seyfert 1 and 2 nuclei.

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.

Near-infrared observations of FSC 15307+3252

Near-infrared observations of the infrared luminous galaxy FSC 15307+3252 are reported. Imaging shows that there is at least one, and possibly more, companion sources within 1.5 sec. Near-infrared spectra show the lines of a low-redshift Seyfert 2 galaxy in the visible, and the line ratios are characteristic of narrow-line AGNs. We conclude that FSC 15307+3252 represents an extension of the infrared bright systems to higher luminosities.

The relative frequency of broad-lined and narrow-lined active galactic nuclei: implications for unified schemes

I discuss evidence concerning the relative occurrence of narrow-lined and broad-lined active galactic nuclei (AGN), in optical, IR, X-ray, and radio-selected samples. Both narrow-lined AGN and reddened broad-lined AGN occur more frequently at lower source powers. There is marginal evidence that narrow-lined AGN were more common in the past. Narrow-lined objects have weaker [O III] at a given radio power. These data are inconsistent with the simplest unified scheme where a similar thick molecular torus surrounds all AGN, and with the simplest modification, that the torus geometrical thickness is a function of source power.

X-ray and infrared selected AGN. II - Optical spectroscopy

view Abstract Citations (50) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS X-Ray and Infrared Selected AGN. II. Optical Spectroscopy Kirhakos, S. D. ; Steiner, J. E. Abstract In a search for obscured active galactic nuclei we selected 144 x-ray/infrared emitting galaxies. Optical spectroscopy of this sample is presented. A classification according to the nuclear activity shows that 28 are AGN, 39 are transition-type objects and 44 are H II region-like galaxies. Three of the 28 AGN are Seyfert 1 galaxies and the others are of Type 2. We suggest that the objects identified as narrow line AGN are obscured Seyfert 1. Most of the observed galaxies are seen edge-on, indicating that dust may have a flattened distribution coplanar to the disk of the parent galaxy. With the inclusion of the newly identified AGN, the sample of the x-ray emitting Seyfert 2 galaxies is fairly complete above a flux limit of log F_HX_ ~ -10.8. Publication: The Astronomical Journal Pub Date: June 1990 DOI: 10.1086/115453 Bibcode: 1990AJ.....99.1722K Keywords: Active Galactic Nuclei; Astronomical Spectroscopy; Optical Emission Spectroscopy; Seyfert Galaxies; X Ray Sources; Emission Spectra; H Ii Regions; Infrared Radiation; Red Shift; Tables (Data); Astrophysics; GALAXIES: NUCLEI full text sources ADS | data products NED (139) SIMBAD (103) Related Materials (1) Part 1: 1990AJ.....99.1435K