This is the estimated fraction of elusive AGN among galaxies not classified as Seyfert. Veilleux et al.
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We again illustrate this result in Fig. However, for completeness, we also include in Fig. We note that the fractions of elusive AGN inferred above are probably lower limits because we have assumed that galaxies not matching the selection criteria of Kewley et al. The obvious counterexample of NGC demonstrates that this assumption is a conservative one. Moreover, there are some examples of classical Seyfert nuclei without a detected radio core, and therefore elusive AGN without a radio core may also exist.
Finally, a large fraction of the elusive AGN in Table 2 are relatively well-studied objects with better X-ray data longer integrations and more detailed analysis. Elusive AGN may have been missed in other objects which have received less attention. It is not clear why elusive AGNs do not produce the typical Seyfert-type emission lines in their optical spectra.
Active galactic nucleus
However, most of the objects in our sample are very nearby and have been studied in great detail, and generally no optical Seyfert-like signatures were found but see notes to Table 2. The X-ray spectra of elusive AGN provide some hints of their nature. This strongly suggests that the elusive nature of this class of nuclei is associated with heavy obscuration. The dark-grey histogram shows the contribution by Sy2, while the light-grey histogram shows the contribution by elusive AGN.
Histograms marked with arrows show the fraction of objects with a lower limit on N H.
One possibility is that the narrow-line region NLR is also heavily obscured along our line of sight. However, spectroscopic observations in the near-IR and mid-IR, where dust extinction is much reduced, did not detect the expected narrow emission lines in several of these objects Marconi et al. Moreover, the NLR is generally too extended to be completely obscured.
Another possibility is that the nuclear pc-scale absorber is not distributed in the torus-like geometry typically assumed for Seyfert 2 nuclei, but covers the nuclear UV source in all directions. Detailed studies of NGC favour this scenario Marconi et al. Indeed, the clouds located in the cavity produced by the nuclear starburst superwind are very faint and only weakly ionized, implying that they are not exposed to the strong photoionizing UV continuum which is expected to be associated with the X-ray emission.
As suggested by Done et al.
Notably, our sample includes some of the objects with the deepest silicate absorption, e. NGC Maiolino et al. By combining new and past hard X-ray observations we have defined a small sample of optically elusive AGN, i. After accounting for selection effects, we estimate that the fraction of galaxies with elusive AGN may be relatively high, i.
Our results also imply that the ratio between obscured and unobscured AGN is higher than estimated previously, i.
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Nearly all elusive AGN are heavily absorbed in the X-rays Compton-thick , suggesting that their elusive nature is associated with heavy obscuration. The geometry of the obscuring medium may be different than in Sy2 galaxies. In particular, the extension of the Compton-thick medium may completely embed the nuclear source, preventing UV photons from escaping and producing the narrow-line region.
The X-ray spectral properties of elusive AGN may have important implications for the X-ray background. However, an assessment of the contribution of elusive AGN to the keV background bump requires a better knowledge of their luminosity distribution. We are grateful to D. Alexander, M. Veron and P.
Veron for useful comments. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents.
Elusive active galactic nuclei R. Oxford Academic. Journal RSS feed. Sign up for new issue notifications. The broad emission lines in the spectra of active galactic nuclei respond to variations in the luminosity of the central continuum source with a delay due to light-travel time effetrs within the emission-line region.
It is therefore possible through the process of "reverberation mapping" to determine the geometry and kinematics of the emission-line region by careful monitoring of the continuum variations and the resulting emission-line response.
Types of AGN:
Image: An artist's impression of the accretion disc around the supermassive black hole that powers an active galaxy. At the heart of virtually every large galaxy lurks a supermassive black hole with a mass of a million to more than a billion times that of the Sun.
Most of these black holes are dormant, but a few per cent are 'active' meaning that they are drawing inwards material from their host galaxy, this forms an accretion disc that feeds the black hole. As the material spirals through the disc it releases vast amounts of energy as heat and light. These active galaxies are the most powerful, continuously emitting sources in the Universe, often more than a hundred billion times more powerful than the Sun. This prodigous luminosity means active galaxies can be seen over huge distance and are among the most distant objects that have ever been detected.
Active galaxies emit radiation over the entire spectrum from radio waves through optical visible light up to X-rays and gamma-rays, but are so distant that it is not possible to image the accreion disc or black hole. In order to study active galaxies, and gain a better undertanding of the physics of accretion flows and black holes and how they interact with their host galaxies, we must understand the spectrum of the light and how it changes with time.
At Leicester, the programme of observational research into active galaxies makes extensive use of multi-wavelength ground- and space-based observational facilities, several of which include instrumentation built at Leicester. These include using X-ray observations with ESA's XMM-Newton and NASA's RXTE and Chandra missions of the X-ray emission to probe the space closest around the black hole, detecting the echoes of light that bounce off the accretion disc using ground-based telescopes, and measuring the 'winds' of gas that is blown off the surface of the disc by the radiation the 'exhaust' of the accretion process using ultraviolet and X-ray observations.
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For further details of current research click this link.