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Advances in Astronomy
Volume 2012 (2012), Article ID 782030, 17 pages
http://dx.doi.org/10.1155/2012/782030
Review Article

AGN Obscuration and the Unified Model

1Dipartimento di Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
2INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
3Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
4INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, Firenze, Italy
5High Energy Astrophysics Division, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 0218, USA

Received 31 August 2011; Accepted 29 December 2011

Academic Editor: Isabelle Gavignaud

Copyright © 2012 Stefano Bianchi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Linked References

  1. R. Antonucci, “Unified models for active galactic nuclei and quasars,” Annual Review of Astronomy and Astrophysics, vol. 31, no. 1, pp. 473–521, 1993. View at Scopus
  2. R. R. J. Antonucci, “Optical spectropolarimetry of radio galaxies,” The Astrophysical Journal, vol. 278, pp. 499–520, 1984.
  3. C. K. Seyfert, “Nuclear emission in spiral nebulae,” The Astrophysical Journal, vol. 97, p. 28, 1943.
  4. R. R. J. Antonucci and J. S.. Miller, “Diffuse pulmonary hemorrhage: a review and classification,” The Astrophysical Journal, vol. 297, pp. 621–632, 1985. View at Scopus
  5. J. H. Krolik and M. C. Begelman, “An X-ray heated wind in NGC 1068,” The Astrophysical Journal, vol. 308, pp. L55–L58, 1986. View at Publisher · View at Google Scholar
  6. J. H. Krolik and M. C. Begelman, “Molecular tori in seyfert galaxies—feeding the monster and hiding it,” The Astrophysical Journal, vol. 329, pp. 702–711, 1988.
  7. S. Kaspi, D. Maoz, H. Netzer, B. M. Peterson, M. Vestergaard, and B. T. Jannuzi, “The relationship between luminosity and broad-line region size in active galactic nuclei,” The Astrophysical Journal, vol. 629, no. 1, pp. 61–71, 2005. View at Publisher · View at Google Scholar · View at Scopus
  8. B. M. Peterson, “Reverberation mapping of active galactic nuclei,” Publications of the Astronomical Society of the Pacific, vol. 105, pp. 247–268, 1993.
  9. H. D. Tran, “Hidden broad-line Seyfert 2 galaxies in the CfA and 12 μm samples,” Astrophysical Journal Letters, vol. 554, no. 1, pp. L19–L23, 2001. View at Publisher · View at Google Scholar
  10. S. Veilleux, R.-W. Goodrich, and G. J. Hill, “Infrared spectroscopy of seyfert 2 galaxies: a look through the obscuring torus? II,” The Astrophysical Journal, vol. 477, article 631, 1997. View at Publisher · View at Google Scholar
  11. S. Veilleux, R.-W. Goodrich, and G. J. Hill, “Infrared spectroscopy of seyfert 2 galaxies: a look through the obscuring torus? II,” The Astrophysical Journal, vol. 477, no. 2, pp. 631–660, 1997. View at Scopus
  12. N. M. Nagar, E. Oliva, A. Marconi, and R. Maiolino, “NGC 5506 unmasked as a narrow line Seyfert 1: a direct view of the broad line region using near-IR spectroscopy,” Astronomy and Astrophysics, vol. 391, no. 2, pp. L21–L24, 2002. View at Scopus
  13. D. Lutz, R. Maiolino, A. F. M. Moorwood, et al., “Infrared spectroscopy around 4 mu m of Seyfert 2 galaxies: Obscured broad line regions and coronal lines,” Astronomy and Astrophysics, vol. 396, pp. 439–448, 2002.
  14. R. Riffel, A. Rodríguez-Ardila, and M. G. Pastoriza, “A 0.8-2.4 μm spectral atlas of active galactic nuclei,” Astronomy and Astrophysics, vol. 457, no. 1, pp. 61–70, 2006. View at Publisher · View at Google Scholar · View at Scopus
  15. H.-B. Cai, X.-W. Shu, Z.-Y. Zheng, and J. X. Wang, “The study of Seyfert 2 galaxies with and without infrared broad lines,” Research in Astronomy and Astrophysics, vol. 10, no. 5, pp. 427–437, 2010. View at Publisher · View at Google Scholar · View at Scopus
  16. D. A. Smith and C. Done, “Unified theories of active galactic nuclei: a hard X-ray sample of Seyfert 2 galaxies,” Monthly Notices of the Royal Astronomical Society, vol. 280, no. 2, pp. 355–377, 1996. View at Scopus
  17. T. J. Turner, I. M. George, K. Nandra, and R. F. Mushotzky, “ASCA observations of type 2 seyfert galaxies. I. Data analysis results,” The Astrophysical Journal Supplement Series, vol. 113, no. 1, article 23, 1997. View at Publisher · View at Google Scholar
  18. R. Maiolino, M. Salvati, L. Bassani et al., “Heavy obscuration in X-ray weak AGNs,” Astronomy and Astrophysics, vol. 338, no. 3, pp. 781–794, 1998. View at Scopus
  19. L. Bassani, M. Dadina, R. Maiolino et al., “A three-dimensional diagnostic diagram for Seyfert 2 galaxies: probing X-ray absorption and compton thickness,” Astrophysical Journal, Supplement Series, vol. 121, no. 2, pp. 473–482, 1999. View at Scopus
  20. G. Matt, M. Guainazzi, F. Frontera et al., “Hard X-ray detection of NGC 1068 with BeppoSAX,” Astronomy and Astrophysics, vol. 325, no. 2, pp. L13–L16, 1997. View at Scopus
  21. G. Matt, M. Guainazzi, R. Maiolino et al., “One more surprise from the circinus galaxy: beppoSAX discovery of a transmission component in hard X-rays,” Astronomy and Astrophysics, vol. 341, no. 2, pp. L39–L42, 1999. View at Scopus
  22. R. W. Pogge, “Extended ionized gas in the Seyfert 2 galaxy NGC 4388,” The Astrophysical Journal, vol. 332, pp. 702–710, 1988.
  23. I. N. Evans, H. C. Ford, A. L. Kinney, R. R. J. Antonucci, L. Armus, and S. Caganoff, “HST imaging of the inner 3 arcseconds of NGC 1068 in the light of [O III] λ5007,” The Astrophysical Journal, vol. 369, no. 2, pp. L27–L30, 1991. View at Scopus
  24. A. S. Wilson and Z. I. Tsvetanov, “Ionization cones and radio ejecta in active galaxies,” Astronomical Journal, vol. 107, no. 4, pp. 1227–1234, 1994. View at Scopus
  25. F. K. B. Barbosa, T. Storchi-Bergmann, R. C. Fernandes, C. Winge, and H. Schmitt, “Gemini/GMOS IFU gas velocity 'tomography' of the narrow line region of nearby active galaxies,” Monthly Notices of the Royal Astronomical Society, vol. 396, no. 1, pp. 2–18, 2009. View at Publisher · View at Google Scholar · View at Scopus
  26. R. Maiolino and G. H. Rieke, “Low-luminosity and obscured seyfert nuclei in nearby galaxies,” The Astrophysical Journal, vol. 454, no. 1, pp. 95–105, 1995. View at Scopus
  27. N. M. Nagar, A. S. Wilson, J. S. Mulchaey, and J. F. Gallimore, “Radio structures of Seyfert galaxies. VIII. A distance- and magnitude-limited sample of early-type galaxies,” Astrophysical Journal, Supplement Series, vol. 120, no. 2, pp. 209–245, 1999. View at Scopus
  28. E. A. Pier and J. H. Krolik, “Radiation-pressure-supported obscuring tori around active galactic nuclei,” The Astrophysical Journal, vol. 399, no. 1, pp. L23–L26, 1992. View at Scopus
  29. G. L. Granato, L. Danese, and A. Franceschini, “Thick tori around active galactic nuclei: the case for extended tori and consequences for their X-ray and infrared emission,” The Astrophysical Journal, vol. 486, no. 1, pp. 147–159, 1997. View at Scopus
  30. J. H. Krolik, “AGN obscuring tori supported by infrared radiation pressure,” The Astrophysical Journal, vol. 661, no. 1, pp. 52–59, 2007. View at Publisher · View at Google Scholar · View at Scopus
  31. K. Wada and K. Tomisaka, “Molecular gas structure around an active galactic nucleus with nuclear starburst: three-dimensional non-LTE calculations of CO lines,” The Astrophysical Journal, vol. 619, no. 1 I, pp. 93–104, 2005. View at Publisher · View at Google Scholar · View at Scopus
  32. Y. Watabe and M. Umemura, “Obscuration of active galactic nuclei by circumnuclear starbursts,” The Astrophysical Journal, vol. 618, no. 2 I, pp. 649–656, 2005. View at Publisher · View at Google Scholar · View at Scopus
  33. S. Nayakshin, “Warped accretion discs and the unification of active galactic nuclei,” Monthly Notices of the Royal Astronomical Society, vol. 359, no. 2, pp. 545–550, 2005. View at Publisher · View at Google Scholar · View at Scopus
  34. A. Caproni, Z. Abraham, and H. J. Mosquera Cuesta, “Bardeen-petterson effect and the disk structure of the seyfert galaxy NGC 1068,” The Astrophysical Journal, vol. 638, no. 1 I, pp. 120–124, 2006. View at Publisher · View at Google Scholar · View at Scopus
  35. A. Caproni, M. Livio, Z. Abraham, and H. J. M. Cuesta, “Warping and precession in galactic and extragalactic accretion disks,” The Astrophysical Journal, vol. 653, no. 1 I, pp. 112–126, 2006. View at Publisher · View at Google Scholar · View at Scopus
  36. A. Lawrence and M. Elvis, “Misaligned disks as obscurers in active galaxies,” The Astrophysical Journal, vol. 714, no. 1, pp. 561–570, 2010. View at Publisher · View at Google Scholar · View at Scopus
  37. M. Elitzur and I. Shlosman, “The AGN-obscuring torus: the end of the "doughnut" paradigm?” The Astrophysical Journal, vol. 648, no. 2, pp. L101–L104, 2006. View at Publisher · View at Google Scholar · View at Scopus
  38. M. Nenkova, M. M. Sirocky, R. Nikutta, Z. Ivezi, and M. Elitzur, “AGN dusty Tori. II. observational implications of clumpiness,” The Astrophysical Journal, vol. 685, no. 1, pp. 160–180, 2008. View at Publisher · View at Google Scholar · View at Scopus
  39. S. F. Hönig and T. Beckert, “Active galactic nuclei dust tori at low and high luminosities,” Monthly Notices of the Royal Astronomical Society, vol. 380, no. 3, pp. 1172–1176, 2007. View at Publisher · View at Google Scholar · View at Scopus
  40. S. F. Hönig and M. Kishimoto, “The dusty heart of nearby active galaxies: II. from clumpy torus models to physical properties of dust around AGN,” Astronomy and Astrophysics, vol. 523, no. 2, article no. A27, 2010. View at Publisher · View at Google Scholar · View at Scopus
  41. G. Risaliti, “The BeppoSAX view of bright compton-thin Seyfert 2 galaxies,” Astronomy and Astrophysics, vol. 386, no. 2, pp. 379–398, 2002. View at Scopus
  42. G. Risaliti, M. Elvis, G. Fabbiano, A. Baldi, and A. Zezas, “Rapid compton-thick/compton-thin transitions in the seyfert 2 galaxy NGC 1365,” The Astrophysical Journal, vol. 623, no. 2, pp. L93–L96, 2005. View at Publisher · View at Google Scholar · View at Scopus
  43. G. Risaliti, M. Elvis, G. Fabbiano, A. Baldi, A. Zezas, and M. Salvati, “Occultation measurement of the size of the X-ray-emitting region in the active galactic nucleus of NGC 1365,” The Astrophysical Journal, vol. 659, no. 2, pp. L111–L114, 2007. View at Publisher · View at Google Scholar · View at Scopus
  44. G. Risaliti, M. Salvati, M. Elvis et al., “The XMM-Newton long look of NGC 1365: uncovering of the obscured X-ray source,” Monthly Notices of the Royal Astronomical Society, vol. 393, no. 1, pp. L1–L5, 2009. View at Publisher · View at Google Scholar
  45. R. Maiolino, G. Risaliti, M. Salvati et al., “Comets orbiting a black hole,” Astronomy and Astrophysics, vol. 517, no. 6, article A47, 2010. View at Publisher · View at Google Scholar · View at Scopus
  46. M. Elvis, G. Risaliti, F. Nicastro, J. M. Miller, F. Fiore, and S. Puccetti, “An unveiling event in the type 2 active galactic nucleus NGC 4388: a challenge for a parsec-scale absorber,” The Astrophysical Journal, vol. 615, no. 1, pp. L25–L28, 2004. View at Publisher · View at Google Scholar · View at Scopus
  47. S. Puccetti, F. Fiore, G. Risaliti, M. Capalbi, M. Elvis, and F. Nicastro, “Rapid NH changes in NGC 4151,” Monthly Notices of the Royal Astronomical Society, vol. 377, no. 2, pp. 607–616, 2007. View at Publisher · View at Google Scholar · View at Scopus
  48. S. Bianchi, E. Piconcelli, M. Chiaberge, E. J. Bailon, G. Matt, and F. Fiore, “How complex is the obscuration in active galactic nuclei? New clues from the suzaku monitoring of the X-ray absorbers in NGC 7582,” The Astrophysical Journal, vol. 695, no. 1, pp. 781–787, 2009. View at Publisher · View at Google Scholar
  49. G. Risaliti, E. Nardini, M. Salvati et al., “X-ray absorption by broad-line region clouds in Mrk 766,” Monthly Notices of the Royal Astronomical Society, vol. 410, no. 2, pp. 1027–1035, 2011. View at Publisher · View at Google Scholar · View at Scopus
  50. R. Maiolino, A. Marconi, M. Salvati et al., “Dust in active nuclei I. Evidence for "anomalous" properties,” Astronomy and Astrophysics, vol. 365, no. 2, pp. 28–36, 2001. View at Scopus
  51. T. Maccacaro, G. C. Perola, and M. Elvis, “X-ray observations of emission line galaxies with the einstein observatory,” Space Science Reviews, vol. 30, no. 1–4, pp. 61–65, 1981. View at Publisher · View at Google Scholar · View at Scopus
  52. T. Maccacaro, G. C. Perola, and M. Elvis, “X-ray observations with the Einstein Observatory of emission-line galaxies,” The Astrophysical Journal, vol. 257, p. 47, 1982.
  53. S. Bianchi, G. Matt, I. Balestra, and G. C. Perola, “The origin of the iron lines in NGC 7213,” Astronomy and Astrophysics, vol. 407, no. 2, pp. L21–L24, 2003. View at Scopus
  54. S. Bianchi, G. Matt, I. Balestra, M. Guainazzi, and G. C. Perola, “X-ray reprocessing in Seyfert galaxies: simultaneous XMM-Newton/BeppoSAX observations,” Astronomy and Astrophysics, vol. 422, no. 1, pp. 65–76, 2004. View at Publisher · View at Google Scholar · View at Scopus
  55. A. P. Lobban, J. N. Reeves, D. Porquet, et al., “Evidence for a truncated accretion disc in the low-luminosity Seyfert galaxy, NGC 7213?” Monthly Notices of the Royal Astronomical Society, vol. 408, no. 1, pp. 551–564, 2010. View at Publisher · View at Google Scholar
  56. S. Bianchi, F. La Franca, G. Matt et al., “A broad-line region origin for the iron Kα line in NGC 7213,” Monthly Notices of the Royal Astronomical Society, vol. 389, no. 1, pp. L52–L56, 2008. View at Publisher · View at Google Scholar
  57. T. Storchi-Bergmann, J. S. Mulchaey, and A. S. Wilson, “Infrared emission in Seyfert 2 galaxies: reprocessed radiation from a dusty torus?” The Astrophysical Journal, vol. 395, no. 2, pp. L73–L77, 1992. View at Scopus
  58. A. Alonso-Herrero, A. C. Quillen, C. Simpson, A. Efstathiou, and M. J. Ward, “The nonstellar infrared continuum of seyfert galaxies,” Astronomical Journal, vol. 121, no. 3, pp. 1369–1384, 2001. View at Publisher · View at Google Scholar · View at Scopus
  59. E. Oliva, L. Origlia, R. Maiolino, and A. F. M. Moorwood, “Starbursts in active galaxy nuclei: observational constraints from IR stellar absorption lines,” Astronomy and Astrophysics, vol. 350, no. 1, pp. 9–16, 1999. View at Scopus
  60. M. Suganuma, Y. Yoshii, Y. Kobayashi et al., “Reverberation measurements of the inner radius of the dust torus in nearby seyfert 1 galaxies,” The Astrophysical Journal, vol. 639, no. 1, pp. 46–63, 2006. View at Publisher · View at Google Scholar · View at Scopus
  61. R. Maiolino, O. Shemmer, M. Imanishi et al., “Dust covering factor, silicate emission, and star formation in luminous QSOs,” Astronomy and Astrophysics, vol. 468, no. 3, pp. 979–992, 2007. View at Publisher · View at Google Scholar · View at Scopus
  62. E. Treister, J. H. Krolik, and C. Dullemond, “Measuring the fraction of obscured quasars by the infrared luminosity of unobscured quasars,” The Astrophysical Journal, vol. 679, no. 1, pp. 140–148, 2008. View at Publisher · View at Google Scholar · View at Scopus
  63. L. J. Greenhill, C. R. Gwinn, R. Antonucci, and R. Barvainis, “VLBI imaging of water maser emission from the nuclear torus of NGC 1068,” The Astrophysical Journal, vol. 472, no. 1, pp. L21–L24, 1996. View at Scopus
  64. L. J. Greenhill, J. M. Moran, and J. R. Herrnstein, “The distribution of H2O maser emission in the nucleus of NGC 4945,” The Astrophysical Journal, vol. 481, no. 1, pp. L23–L26, 1997. View at Scopus
  65. L. J. Greenhill, P. T. Kondratko, J. E. J. Lovell et al., “The discovery of H2O maser emission in seven active galactic nuclei and at high velocities in the Circinus galaxy,” The Astrophysical Journal, vol. 582, no. 1, pp. L11–L14, 2003. View at Publisher · View at Google Scholar · View at Scopus
  66. P. T. Kondratko, L. J. Greenhill, and J. M. Moran, “The parsec-scale accretion disk in NGC 3393,” The Astrophysical Journal, vol. 678, no. 1, pp. 87–95, 2008. View at Publisher · View at Google Scholar · View at Scopus
  67. P. T. Kondratko, L. J. Greenhill, and J. M. Moran, “Evidence for a geometrically thick self-gravitating accretion disk in NGC 3079,” The Astrophysical Journal, vol. 618, no. 2, pp. 618–634, 2005. View at Publisher · View at Google Scholar · View at Scopus
  68. J. F. Gallimore, S. A. Baum, and C. P. O'Dea, “A direct image of the obscuring disk surrounding an active galactic nucleus,” Nature, vol. 388, no. 6645, pp. 852–854, 1997. View at Publisher · View at Google Scholar · View at Scopus
  69. W. Jaffe, K. Meisenheimer, H. J. A. Röttgering et al., “The central dusty torus in the active nucleus of NGC 1068,” Nature, vol. 429, no. 6987, pp. 47–49, 2004. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  70. D. Raban, W. Jaffe, H. Röttgering, K. Meisenheimer, and K. R. W. Tristram, “Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: revealing the inner funnel of dust,” Monthly Notices of the Royal Astronomical Society, vol. 394, no. 3, pp. 1325–1337, 2009. View at Publisher · View at Google Scholar · View at Scopus
  71. K. R. W. Tristram, K. Meisenheimer, W. Jaffe et al., “Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy,” Astronomy and Astrophysics, vol. 474, no. 3, pp. 837–850, 2007. View at Publisher · View at Google Scholar · View at Scopus
  72. L. Burtscher, W. Jaffe, D. Raban, K. Meisenheimer, K. R. W. Tristram, and H. Röttgering, “Dust emission from a parsec-scale structure in the seyfert 1 nucleus of NGC 4151,” The Astrophysical Journal, vol. 705, no. 1, pp. L53–L57, 2009. View at Publisher · View at Google Scholar · View at Scopus
  73. K. R. W. Tristram, D. Raban, K. Meisenheimer et al., “Parsec-scale dust distributions in Seyfert galaxies Results of the MIDI AGN snapshot survey,” Astronomy and Astrophysics, vol. 502, no. 1, pp. 67–84, 2009. View at Publisher · View at Google Scholar · View at Scopus
  74. K. R. W. Tristram and M. Schartmann, “On the size-luminosity relation of AGN dust tori in the mid-infrared,” Astronomy and Astrophysics, vol. 531, article A99, 2011. View at Publisher · View at Google Scholar
  75. M. Kishimoto, S. F. Honig, R. Antonucci, et al., “The innermost dusty structure in active galactic nuclei as probed by the Keck interferometer,” Astronomy & Astrophysics, vol. 527, article A121, 2011.
  76. G. C. Perola, G. Matt, M. Cappi et al., “Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies,” Astronomy and Astrophysics, vol. 389, no. 3, pp. 802–811, 2002. View at Scopus
  77. S. Bianchi, M. Guainazzi, G. Matt, N. Fonseca Bonilla, and G. Ponti, “CAIXA: a catalogue of AGN in the XMM-Newton archive: I. Spectral analysis,” Astronomy and Astrophysics, vol. 495, no. 2, pp. 421–430, 2009. View at Publisher · View at Google Scholar · View at Scopus
  78. I. De La Calle Pérez, A. L. Longinotti, M. Guainazzi et al., “FERO: finding extreme relativistic objects: I. Statistics of relativistic Fe Kα lines in radio-quiet Type 1 AGN,” Astronomy and Astrophysics, vol. 524, no. 2, article no. A50, 2010. View at Publisher · View at Google Scholar · View at Scopus
  79. K. Nandra, P. M. O'Neill, I. M. George, and J. N. Reeves, “An XMM-Newton survey of broad iron lines in Seyfert galaxies,” Monthly Notices of the Royal Astronomical Society, vol. 382, no. 1, pp. 194–228, 2007. View at Publisher · View at Google Scholar · View at Scopus
  80. K. Nandra, “On the origin of the iron Ko line cores in active galactic nuclei,” Monthly Notices of the Royal Astronomical Society, vol. 368, pp. L62–L66, 2006.
  81. X. W. Shu, T. Yaqoob, and J. X. Wang, “Chandra High-energy Grating Observations of the Fe Kα Line Core in Type II Seyfert Galaxies: A Comparison with Type I Nuclei,” The Astrophysical Journal, vol. 738, p. 147, 2011.
  82. R. Gilli, R. Maiolino, A. Marconi et al., “The variability of the Seyfert galaxy NGC 2992: the case for a revived AGN,” Astronomy and Astrophysics, vol. 355, no. 2, pp. 485–498, 2000. View at Scopus
  83. G. Matt, S. Bianchi, M. Guainazzi et al., “Chandra and XMM-Newton observations of Tololo 0109-383,” Astronomy and Astrophysics, vol. 399, no. 2, pp. 519–523, 2003. View at Scopus
  84. M. Nenkova, Z. Ivezi, and M. Elitzur, “Dust emission from active galactic nuclei,” The Astrophysical Journal, vol. 570, no. 1, pp. L9–L12, 2002. View at Publisher · View at Google Scholar · View at Scopus
  85. M. Elitzur and I. Shlosman, “The AGN-obscuring torus: the end of the "doughnut" paradigm?” The Astrophysical Journal, vol. 648, no. 2, pp. L101–L104, 2006. View at Publisher · View at Google Scholar · View at Scopus
  86. S. F. Hönig, T. Beckert, K. Ohnaka, and G. Weigelt, “Radiative transfer modeling of three-dimensional clumpy AGN tori and its application to NGC 1068,” Astronomy and Astrophysics, vol. 452, no. 2, pp. 459–471, 2006. View at Publisher · View at Google Scholar · View at Scopus
  87. G. L. Granato and L. Danese, “Thick tori around active galactic nuclei—a comparison of model predictions with observations of the infrared continuum and silicate features,” Monthly Notices of the Royal Astronomical Society, vol. 268, no. 1, p. 235, 1994.
  88. R. Maiolino, “Prospects for AGN studies with ALMA,” New Astronomy Reviews, vol. 52, no. 6, pp. 339–357, 2008. View at Publisher · View at Google Scholar · View at Scopus
  89. Y. Shi, G. H. Rieke, D. C. Hines et al., “9.7 μm silicate features in active galactic nuclei: new insights into unification models,” The Astrophysical Journal, vol. 653, no. 1 I, pp. 127–136, 2006. View at Publisher · View at Google Scholar · View at Scopus
  90. R. Nikutta, M. Elitzur, and M. Lacy, “On the 10 μm silicate feature in active galactic nuclei,” The Astrophysical Journal, vol. 707, no. 2, pp. 1550–1559, 2009. View at Publisher · View at Google Scholar · View at Scopus
  91. G. Matt, “Dust lanes, thick absorbers, and the unification model for Seyfert galaxies,” Astronomy and Astrophysics, vol. 355, no. 2, pp. L31–L33, 2000.
  92. C. D. P. Lagos, N. D. Padilla, M. A. Strauss, S. A. Cora, and L. Hao, “Host galaxy-active galactic nucleus alignments in the sloan digital sky survey data release 7,” Monthly Notices of the Royal Astronomical Society, vol. 414, no. 3, pp. 2148–2162, 2011. View at Publisher · View at Google Scholar
  93. M. A. Malkan, V. Gorjian, and R. Tam, “A hubble space telescope 1 imaging survey of nearby active galactic nuclei,” Astrophysical Journal, Supplement Series, vol. 117, no. 1, pp. 25–88, 1998. View at Scopus
  94. M. Guainazzi, G. Matt, and G. C. Perola, “X-ray obscuration and obscured AGN in the local universe,” Astronomy and Astrophysics, vol. 444, no. 1, pp. 119–132, 2005. View at Publisher · View at Google Scholar · View at Scopus
  95. S. Bianchi, M. Chiaberge, E. Piconcelli, and M. Guainazzi, “A multiwavelength map of the nuclear region of NGC 7582,” Monthly Notices of the Royal Astronomical Society, vol. 374, no. 2, pp. 697–702, 2007. View at Publisher · View at Google Scholar · View at Scopus
  96. E. Schinnerer, A. Eckart, L. J. Tacconi, R. Genzel, and D. Downes, “Bars and warps traced by the molecular gas in the Seyfert 2 galaxy NGC 1068,” The Astrophysical Journal, vol. 533, no. 2, pp. 850–868, 2000. View at Scopus
  97. F. Boone, S. García-Burillo, F. Combes et al., “High-resolution mapping of the physical conditions in two nearby active galaxies based on 12CO(1-0), (2-1), and (3-2) lines,” Astronomy and Astrophysics, vol. 525, no. 1, article A18, 2010. View at Publisher · View at Google Scholar · View at Scopus
  98. M. Krips, S. Martin, and A. Eckart, “SMA/PdBI multiple line observations of the nearby Seyfert2 galaxy NGC 1068: shock related gas kinematics and heating in the central 100pc?” The Astrophysical Journal, vol. 736, article 37, 2011.
  99. M. Polletta, D. Weedman, S. Hönig, C. J. Lonsdale, H. E. Smith, and J. Houck, “Obscuration in extremely luminous quasars,” The Astrophysical Journal, vol. 675, no. 2, pp. 960–984, 2008. View at Publisher · View at Google Scholar · View at Scopus
  100. A. Martínez-Sansigre, S. Rawlings, M. Lacy et al., “A population of high-redshift type 2 quasars - I. Selection criteria and optical spectra,” Monthly Notices of the Royal Astronomical Society, vol. 370, no. 3, pp. 1479–1498, 2006. View at Publisher · View at Google Scholar · View at Scopus
  101. G. Risaliti, R. Maiolino, and M. Salvati, “The distribution of absorbing column densities among Seyfert 2 galaxies,” The Astrophysical Journal, vol. 522, no. 1, pp. 157–164, 1999. View at Scopus
  102. A. J. Young, A. S. Wilson, and P. L. Shopbell, “A Chandra X-ray study of NGC 1068. I. Observations of extended emission,” The Astrophysical Journal, vol. 556, no. 1, pp. 6–23, 2001. View at Publisher · View at Google Scholar · View at Scopus
  103. C. A. Heisler, S. L. Lumsden, and J. A. Bailey, “Visibility of scattered broad-line emission in Seyfert 2 galaxies,” Nature, vol. 385, no. 6618, pp. 700–702, 1997. View at Publisher · View at Google Scholar · View at Scopus
  104. D. M. Alexander, “The unified model and the Seyfert 2 infrared dichotomy,” Monthly Notices of the Royal Astronomical Society, vol. 320, no. 1, pp. L15–L19, 2001. View at Publisher · View at Google Scholar
  105. Q. Gu, R. Maiolino, and D. Dultzin-Hacyan, “Nuclear obscuration and scattering in Seyfert 2 galaxies,” Astronomy and Astrophysics, vol. 366, no. 3, pp. 765–770, 2001. View at Scopus
  106. F. Nicastro, “Broad emission line regions in active galactic nuclei: the link with the accretion power,” The Astrophysical Journal Letters, vol. 530, no. 2, article L65, 2000. View at Publisher · View at Google Scholar
  107. M. Elitzur and L. C. Ho, “On the disappearance of the broad-line region in low-luminosity active galactic nuclei,” The Astrophysical Journal Letters, vol. 701, no. 2, article L91, 2009. View at Publisher · View at Google Scholar
  108. J. R. Trump, C. D. Impey, B. C. Kelly, et al., “Accretion rate and the physical nature of unobscured active galaxies,” The Astrophysical Journal, vol. 733, no. 1, article 60, 2011.
  109. S. Bianchi, A. Corral, F. Panessa et al., “NGC 3147: a “true” type 2 Seyfert galaxy without the broad-line region,” Monthly Notices of the Royal Astronomical Society, vol. 385, no. 1, pp. 195–199, 2008. View at Publisher · View at Google Scholar · View at Scopus
  110. F. Panessa, F. J. Carrera, S. Bianchi et al., “Unabsorbed Seyfert 2 galaxies: the case of “naked” AGN,” Monthly Notices of the Royal Astronomical Society, vol. 398, no. 4, pp. 1951–1960, 2009. View at Publisher · View at Google Scholar · View at Scopus
  111. M. Brightman and K. Nandra, “On the nature of unabsorbed Seyfert 2 galaxies,” Monthly Notices of the Royal Astronomical Society, vol. 390, no. 3, pp. 1241–1249, 2008. View at Publisher · View at Google Scholar · View at Scopus
  112. S. Bianchi, in prep., 2012.
  113. A. Marinucci, S. Bianchi, F. Nicastro, G. Matt, and A. D. Goulding, “The link between the hidden broad line region and the accretion rate in seyfert 2 galaxies,” The Astrophysical Journal. In press.
  114. Y. Shi, G. H. Rieke, P. Smith et al., “Unobscured type 2 active galactic nuclei,” The Astrophysical Journal, vol. 714, no. 1, pp. 115–129, 2010. View at Publisher · View at Google Scholar · View at Scopus
  115. F. Nicastro, A. Martocchia, and G. Matt, “The lack of broad-line regions in low accretion rate active galactic nuclei as evidence of their origin in the accretion disk,” The Astrophysical Journal, vol. 589, no. 1, pp. L13–L16, 2003. View at Publisher · View at Google Scholar · View at Scopus
  116. W. Bian and Q. Gu, “The eddington ratios in seyfert 2 galaxies with and without hidden broad-line regions,” The Astrophysical Journal, vol. 657, no. 1, pp. 159–166, 2007. View at Publisher · View at Google Scholar
  117. X. W. Shu, J. X. Wang, P. Jiang, L. L. Fan, and T. G. Wang, “Investigating the nuclear obscuration in two types of Seyfert 2 galaxies,” The Astrophysical Journal, vol. 657, no. 1 I, pp. 167–176, 2007. View at Publisher · View at Google Scholar · View at Scopus
  118. Y. Wu, E. Zhang, Y. Liang, C. Zhang, and Y. Zhao, “The different nature of seyfert 2 galaxies with and without hidden broad-line regions,” The Astrophysical Journal, vol. 730, no. 2, p. 121, 2011. View at Publisher · View at Google Scholar
  119. A. Capetti, D. J. Axon, F. Macchetto, W. B. Sparks, and A. Boksenberg, “Radio outflows and the origin of the narrow-line region in seyfert galaxies,” The Astrophysical Journal, vol. 469, no. 2, pp. 554–563, 1996. View at Scopus
  120. S. Bianchi, M. Chiaberge, D. A. Evans, et al., “High-resolution X-ray spectroscopy and imaging of Mrk 573,” Monthly Notices of the Royal Astronomical Society, vol. 405, no. 1, pp. 553–563, 2010. View at Publisher · View at Google Scholar
  121. R. W. Goosmann and G. Matt, “Spotting the misaligned outflows in NGC 1068 using X-ray polarimetry,” Monthly Notices of the Royal Astronomical Society, vol. 415, no. 4, pp. 3119–3128, 2011. View at Publisher · View at Google Scholar
  122. A. C. Fabian, K. Iwasawa, C. S. Reynolds, and A. J. Young, “Broad iron lines in active galactic nuclei,” Publications of the Astronomical Society of the Pacific, vol. 112, no. 775, pp. 1145–1161, 2000. View at Scopus
  123. M. Guainazzi, S. Bianchi, I. de La Calle Perez, M. Dovciak, and A. L. Longinotti, “On the driver of relativistic effect strength in Seyfert galaxies,” Astronomy & Astrophysics, vol. 531, article A131, 2011. View at Publisher · View at Google Scholar
  124. G. Risaliti, M. Salvati, and A. Marconi, “[O iii] equivalent width and orientation effects in quasars,” Monthly Notices of the Royal Astronomical Society, vol. 411, no. 4, pp. 2223–2229, 2011. View at Publisher · View at Google Scholar
  125. Y. Ueda, M. Akiyama, K. Ohta, and T. Miyaji, “Cosmological evolution of the hard X-ray active galactic nucleus luminosity function and the origin of the hard X-ray background,” The Astrophysical Journal, vol. 598, no. 2 I, pp. 886–908, 2003. View at Publisher · View at Google Scholar · View at Scopus
  126. A. T. Steffen, A. J. Barger, L. L. Cowie, R. F. Mushotzky, and Y. Yang, “The changing active galactic nucleus population,” The Astrophysical Journal, vol. 596, no. 1, pp. L23–L26, 2003. View at Publisher · View at Google Scholar · View at Scopus
  127. F. La Franca, F. Fiore, A. Comastri et al., “The HELLAS2XMM survey. VII. The hard X-ray luminosity function of AGNs up to z=4: more absorbed AGNs at low luminosities and high redshifts,” The Astrophysical Journal, vol. 635, no. 2 I, pp. 864–879, 2005. View at Publisher · View at Google Scholar · View at Scopus
  128. A. Akylas, I. Georgantopoulos, A. Georgakakis, S. Kitsionas, and E. Hatziminaoglou, “XMM-Newton and Chandra measurements of the AGN intrinsic absorption: dependence on luminosity and redshift,” Astronomy and Astrophysics, vol. 459, no. 3, pp. 693–701, 2006. View at Publisher · View at Google Scholar · View at Scopus
  129. A. J. Barger and L. L. Cowie, “The number density of intermediate- and high-luminosity active galactic nuclei at z~2-3,” The Astrophysical Journal, vol. 635, no. 1 I, pp. 115–122, 2005. View at Publisher · View at Google Scholar · View at Scopus
  130. P. Tozzi, R. Gilli, V. Mainieri et al., “X-ray spectral properties of active galactic nuclei in the Chandra Deep Field South,” Astronomy and Astrophysics, vol. 451, no. 2, pp. 457–474, 2006. View at Publisher · View at Google Scholar · View at Scopus
  131. C. Simpson, “The luminosity dependence of the type 1 active galactic nucleus fraction,” Monthly Notices of the Royal Astronomical Society, vol. 360, no. 2, pp. 565–572, 2005. View at Publisher · View at Google Scholar · View at Scopus
  132. T. Dwelly and M. J. Page, “The distribution of absorption in AGN detected in the XMM-Newton observations of the CDFS,” Monthly Notices of the Royal Astronomical Society, vol. 372, no. 4, pp. 1755–1775, 2006. View at Publisher · View at Google Scholar · View at Scopus
  133. E. Treister and C. M. Urry, “Active galactic nuclei unification and the X-ray background,” The Astrophysical Journal, vol. 630, no. 1 I, pp. 115–121, 2005. View at Publisher · View at Google Scholar · View at Scopus
  134. J.-M. Wang and E.-P. Zhang, “The unified model of active galactic nuclei. II. Evolutionary connection,” The Astrophysical Journal, vol. 660, no. 2 I, pp. 1072–1092, 2007. View at Publisher · View at Google Scholar · View at Scopus
  135. G. Hasinger, “Absorption properties and evolution of active galactic nuclei,” Astronomy and Astrophysics, vol. 490, no. 3, pp. 905–922, 2008. View at Publisher · View at Google Scholar · View at Scopus
  136. R. Della Ceca, A. Caccianiga, P. Severgnini et al., “The cosmological properties of AGN in the XMM-Newton Hard Bright Survey,” Astronomy and Astrophysics, vol. 487, no. 1, pp. 119–130, 2008. View at Publisher · View at Google Scholar · View at Scopus
  137. E. Treister, S. Virani, E. Gawiser, et al., “Optical spectroscopy of x-ray sources in the extended chandra deep field south,” The Astrophysical Journal, vol. 693, no. 2, p. 1713, 2009. View at Publisher · View at Google Scholar
  138. M. Brusa, F. Civano, A. Comastri et al., “The XMM-newton wide-field survey in the cosmos field (XMM-COSMOS): demography and multiwavelength properties of obscured and unobscured luminous active galactic nuclei,” The Astrophysical Journal, vol. 716, no. 1, pp. 348–369, 2010. View at Publisher · View at Google Scholar · View at Scopus
  139. R. Maiolino, O. Shemmer, M. Imanishi et al., “Dust covering factor, silicate emission, and star formation in luminous QSOs,” Astronomy and Astrophysics, vol. 468, no. 3, pp. 979–992, 2007. View at Publisher · View at Google Scholar · View at Scopus
  140. R. Gilli, A. Comastri, and G. Hasinger, “The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era,” Astronomy and Astrophysics, vol. 463, no. 1, pp. 79–96, 2007. View at Publisher · View at Google Scholar · View at Scopus
  141. J.-M. Wang, E.-P. Zhang, and B. Luo, “Evolutionary consequences of dusty tori in active galactic nuclei,” The Astrophysical Journal, vol. 627, no. 1, article L5, 2005. View at Publisher · View at Google Scholar · View at Scopus
  142. R. Mor and B. Trakhtenbrot, “Hot-dust clouds with pure-graphite composition around type-i active galactic nuclei,” The Astrophysical Journal Letters, vol. 737, no. 2, p. L36, 2011. View at Publisher · View at Google Scholar
  143. M. Rowan-Robinson, I. Valtchanov, and K. Nandra, “AGN dust tori: the X-ray-infrared connection,” Monthly Notices of the Royal Astronomical Society, vol. 397, no. 3, pp. 1326–1337, 2009. View at Publisher · View at Google Scholar · View at Scopus
  144. K. Iwasawa and Y. Taniguchi, “The X-ray baldwin effect,” The Astrophysical Journal, vol. 413, no. 1, pp. L15–L18, 1993. View at Scopus
  145. K. L. Page, P. T. O'Brien, J. N. Reeves, and M. J. L. Turner, “An X-ray Baldwin effect for the narrow Fe Kα lines observed in active galactic nuclei,” Monthly Notices of the Royal Astronomical Society, vol. 347, no. 1, pp. 316–322, 2004. View at Publisher · View at Google Scholar · View at Scopus
  146. P. Jiang, J. X. Wang, and T. G. Wang, “On the X-ray Baldwin effect for narrow Fe Ka emission lines,” The Astrophysical Journal, vol. 644, no. 2, pp. 725–732, 2006. View at Publisher · View at Google Scholar · View at Scopus
  147. M. Guainazzi, S. Bianchi, and M. Dovčiak, “Statistics of relativistically broadened Fe Kα lines in AGN,” Astronomische Nachrichten, vol. 327, no. 10, pp. 1032–1038, 2006. View at Publisher · View at Google Scholar · View at Scopus
  148. S. Bianchi, M. Guainazzi, G. Matt, and N. F. Bonilla, “On the Iwasawa-Taniguchi effect of radio-quiet AGN,” Astronomy and Astrophysics, vol. 467, no. 1, pp. L19–L22, 2007. View at Publisher · View at Google Scholar · View at Scopus
  149. A. Lawrence, “The relative frequency of broad-lined and narrow-lined active galactic nuclei—implications for unified schemes,” Monthly Notices of the Royal Astronomical Society, vol. 252, pp. 586–592, 1991.
  150. A. Lamastra, G. C. Perola, and G. Matt, “A model for the X-ray absorption in compton-thin AGN,” Astronomy and Astrophysics, vol. 449, no. 2, pp. 551–558, 2006. View at Publisher · View at Google Scholar · View at Scopus
  151. J. Fischer, E. Sturm, E. González-Alfonso et al., “Herschel-PACS spectroscopic diagnostics of local ULIRGs: conditions and kinematics in Markarian 231,” Astronomy and Astrophysics, vol. 518, no. 14676, article L36, 2010. View at Scopus
  152. C. Feruglio, R. Maiolino, E. Piconcelli et al., “Quasar feedback revealed by giant molecular outflows,” Astronomy and Astrophysics, vol. 518, no. 13, article L155, 2010. View at Publisher · View at Google Scholar · View at Scopus
  153. D. S. N. Rupke and S. Veilleux, “Warm molecular hydrogen in the galactic wind of M82,” The Astrophysical Journal, vol. 729, no. 2, article L27, 2011. View at Publisher · View at Google Scholar · View at Scopus
  154. E. Sturm, E. Gonzalez-Alfonso, S. Veilleux, et al., “Massive molecular outflows and negative feedback in ULIRGs observed by Herschel-PACS,” The Astrophysical Journal, vol. 733, no. 1, p. L16, 2011. View at Publisher · View at Google Scholar
  155. M. Cano-Diaz, R. Maiolino, A. Marconi, H. Netzer, O. Shemmer, and G. Cresci, “Observational evidence of quasar feedback quenching star formation at high redshift,” Astronomy & Astrophysics, vol. 537, article L8, 2012. View at Publisher · View at Google Scholar
  156. D. R. Ballantyne, Y. Shi, G. H. Rieke, J. L. Donley, C. Papovich, and J. R. Rigby, “Does the AGN unified model evolve with redshift? Using the X-ray background to predict the mid-infrared emission of AGNs,” The Astrophysical Journal, vol. 653, no. 2, pp. 1070–1088, 2006. View at Publisher · View at Google Scholar · View at Scopus
  157. E. Treister and C. M. Urry, “The evolution of obscuration in active galactic nuclei,” The Astrophysical Journal, vol. 652, pp. L79–L82, 2006. View at Scopus
  158. L. J. Tacconi, R. Genzel, R. Neri et al., “High molecular gas fractions in normal massive star-forming galaxies in the young Universe,” Nature, vol. 463, no. 7282, pp. 781–784, 2010. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  159. E. Daddi, F. Bournaud, F. Walter et al., “Very high gas fractions and extended gas reservoirs in z=1.5 disk galaxies,” The Astrophysical Journal, vol. 713, no. 1, pp. 686–707, 2010. View at Publisher · View at Google Scholar · View at Scopus
  160. L. Shao, D. Lutz, R. Nordon et al., “Star formation in AGN hosts in GOODS-N,” Astronomy and Astrophysics, vol. 518, no. 1, article L26, 2010. View at Publisher · View at Google Scholar · View at Scopus
  161. Y. Shi, G. H. Rieke, P. Ogle, L. Jiang, and A. M. Diamond-Stanic, “Cosmic evolution of star formation in type-1 quasar hosts since z=1,” The Astrophysical Journal, vol. 703, no. 1, pp. 1107–1122, 2009. View at Publisher · View at Google Scholar · View at Scopus
  162. C. Ricci, R. Walter, T. J.-L. Courvoisier, and S. Paltani, “Reflection in Seyfert galaxies and the unified model of AGN,” Astronomy & Astrophysics, vol. 532, article A102, 2011. View at Publisher · View at Google Scholar
  163. A. Marconi, E. Oliva, P. P. van der Werf et al., “The elusive active nucleus of NGC 4945,” Astronomy and Astrophysics, vol. 357, no. 1, pp. 24–36, 2000. View at Scopus
  164. R. Della Ceca, L. Ballo, F. Tavecchio et al., “An enshrouded active galactic nucleus in the merging starburst system Arp 299 revealed by BeppoSAX,” The Astrophysical Journal, vol. 581, no. 1, pp. L9–L13, 2002. View at Publisher · View at Google Scholar · View at Scopus
  165. R. Maiolino, A. Comastri, R. Gilli et al., “Elusive active galactic nuclei,” Monthly Notices of the Royal Astronomical Society, vol. 344, no. 4, pp. L59–L64, 2003. View at Publisher · View at Google Scholar · View at Scopus
  166. L. Ballo, V. Braito, R. Della Ceca, L. Maraschi, F. Tavecchio, and M. Dadina, “ARP 299: a second merging system with two active nuclei?” The Astrophysical Journal, vol. 600, no. 2, pp. 634–639, 2004. View at Scopus
  167. A. Caccianiga, P. Severgnini, R. Della Ceca, T. Maccacaro, F. J. Carrera, and M. J. Page, “Elusive AGN in the XMM-Newton bright serendipitous survey,” Astronomy and Astrophysics, vol. 470, no. 2, pp. 557–570, 2007. View at Publisher · View at Google Scholar · View at Scopus
  168. M. Imanishi, R. Maiolino, and T. Nakagawa, “Spitzer infrared low-resolution spectroscopic study of buried active galactic nuclei in a complete sample of nearby ultraluminous infrared galaxies,” The Astrophysical Journal, vol. 709, no. 2, pp. 801–815, 2010. View at Publisher · View at Google Scholar · View at Scopus
  169. M. Imanishi, C. C. Dudley, R. Maiolino, P. R. Maloney, T. Nakagawa, and G. Risaliti, “A Spitzer IRS low-resolution spectroscopic search for buried AGNs in nearby ultraluminous infrared galaxies: a constraint on geometry between energy sources and dust,” Astrophysical Journal, Supplement Series, vol. 171, no. 1, pp. 77–100, 2007. View at Scopus
  170. M. Imanishi, “Luminous buried active galactic nuclei as a function of galaxy infrared luminosity revealed through spitzer low-resolution infrared spectroscopy,” The Astrophysical Journal, vol. 694, no. 2, pp. 751–764, 2009. View at Publisher · View at Google Scholar · View at Scopus
  171. G. Risaliti, M. Imanishi, and E. Sani, “A quantitative determination of the AGN content in local ULIRGs through L-band spectroscopy,” Monthly Notices of the Royal Astronomical Society, vol. 401, no. 1, pp. 197–203, 2010. View at Publisher · View at Google Scholar · View at Scopus
  172. E. Nardini, G. Risaliti, M. Salvati et al., “Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5-8 μm spectroscopy,” Monthly Notices of the Royal Astronomical Society, vol. 399, no. 3, pp. 1373–1402, 2009. View at Publisher · View at Google Scholar · View at Scopus
  173. E. Nardini, G. Risaliti, M. Salvati, et al., “Spectral decomposition of starbursts and active galactic nuclei in 5—8 μm Spitzer-IRS spectra of local ultraluminous infrared galaxies,” Monthly Notices of the Royal Astronomical Society, vol. 385, no. 1, pp. L130–L134, 2008. View at Publisher · View at Google Scholar
  174. D. A. Evans, W.-F. Fong, M. J. Hardcastle, et al., “A Radio through X-Ray Study of the Jet/Companion-Galaxy Interaction in 3C 321,” The Astrophysical Journal, vol. 675, pp. 1057–1066.
  175. A. D. Goulding and D. M. Alexander, “Towards a complete census of AGN in nearby Galaxies: a large population of optically unidentified AGN,” Monthly Notices of the Royal Astronomical Society, vol. 398, no. 3, pp. 1165–1193, 2009. View at Publisher · View at Google Scholar · View at Scopus
  176. J. P. Perez-Beaupuits, H. W. W. Spoon, M. Spaans, and J. D. Smith, “The deeply obscured AGN of NGC 4945. I. Spitzer-IRS maps of [Ne V], [Ne II], H2 0-0 S(1), S(2), and other tracers,” Astronomy & Astrophysics, vol. 533, article A56, 2011.
  177. L. Armus, V. Charmandaris, J. Bernard-Salas et al., “Observations of ultraluminous infrared galaxies with the infrared spectrograph on the spitzer space telescope. II. The iras bright galaxy sample,” The Astrophysical Journal, vol. 656, no. 1, pp. 148–167, 2007. View at Publisher · View at Google Scholar · View at Scopus
  178. Y. Ueda, S. Eguchi, Y. Terashima, et al., “Suzaku observations of active galactic nuclei detected in the swift BAT survey: discovery of a "New Type" of buried supermassive black holes,” The Astrophysical Journal, vol. 664, pp. L79–L82, 2007.
  179. K. Noguchi, Y. Terashima, and H. Awaki, “A new sample of buried active galactic nuclei selected from the second xmm-newton serendipitous source catalogue,” The Astrophysical Journal, vol. 705, no. 1, pp. 454–467, 2009. View at Publisher · View at Google Scholar · View at Scopus
  180. L. Jiang, X. Fan, W. N. Brandt et al., “Dust-free quasars in the early Universe,” Nature, vol. 464, no. 7287, pp. 380–383, 2010. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  181. H. Hao, M. Elvis, F. Civano et al., “Hot-dust-poor type 1 active galactic nuclei in the COSMOS survey,” The Astrophysical Journal, vol. 724, no. 1, pp. L59–L63, 2010. View at Publisher · View at Google Scholar · View at Scopus
  182. H. Hao, M. Elvis, F. Civano, and A. Lawrence, “Hot-dust-poor quasars in mid-infrared and optically selected samples,” The Astrophysical Journal, vol. 733, no. 2, article 108, 2011. View at Publisher · View at Google Scholar