Type 1 active galactic nuclei display broad emission lines, which are regarded as arising from photoionized gas moving in the gravitational potential of a supermassive black hole1,2. However, the origin of this broad-line region gas is unresolved so far1,2,3. Another component is the dusty torus4 beyond the broad-line region—probably an assembly of discrete clumps5,6,7—which can hide the region from some viewing angles and make them observationally appear as type 2 objects. Here, we report that these clumps moving within the dust sublimation radius, such as the molecular cloud G2 discovered in the Galactic Centre8, will be tidally disrupted by the black hole, resulting in some gas becoming bound at smaller radii while other gas is ejected and returns to the torus. The clumps fulfill necessary conditions to be photoionized9. Specific dynamical components of tidally disrupted clumps include spiral-in gas as inflow, circularized gas and ejecta as outflow. We calculated various profiles of emission lines from these clouds, and found that they generally agree with Hβ profiles of Palomar–Green quasars10. We found that the asymmetry, shape and shift of the profiles strongly depend on [O iii] luminosity, which we interpret as a proxy of dusty torus angles. Tidally disrupted clumps from the torus may represent the source of the broad-line region gas.
This is a preview of subscription content, access via your institution
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 digital issues and online access to articles
$119.00 per year
only $9.92 per issue
Rent or buy this article
Prices vary by article type
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
Ho, L. C. Nuclear activity in nearby galaxies. Annu. Rev. Astron. Astrophys. 46, 475–539 (2008).
Gaskell, C. M. What broad emission lines tell us about how active galactic nuclei work. New Astron. Rev. 53, 140–148 (2009).
Korista, K. What’s emitting the broad emission lines? ASP Conf. Series 162, 165–176 (1999).
Antonucci, R. R. J. Unified models for active galactic nuclei and quasars. Annu. Rev. Astron. Astrophys. 31, 473–521 (1993).
Jaffe, W. et al. The central dusty torus in the active nucleus of NGC 1068. Nature 429, 47–49 (2004).
Elitzur, M. The obscuring torus in AGN. New Astron. Rev. 50, 728–731 (2006).
Nenkova, M., Sirocky, M. M., Ivezić, Ž. & Elitzur, M. AGN dusty tori. I. Handling of clumpy media. Astrophys. J. 685, 147–159 (2008).
Gillessen, S. et al. A gas cloud on its way towards the supermassive black hole at the Galactic Centre. Nature 481, 51–54 (2012).
Osterbrock, D. E. & Ferland, G. J. Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (University Science Books, Sausalito, CA, 2006).
Boroson, T. A. & Green, R. F. The emission-line properties of low-redshift quasi-stellar objects. Astrophys. J. Suppl. Ser. 80, 109–135 (1992).
Barvainis, R. Hot dust and the near-infrared bump in the continuum spectra of quasars and active galactic nuclei. Astrophys. J. 320, 537–544 (1987).
Laor, A. & Draine, B. T. Spectroscopic constraints on the properties of dust in active galactic nuclei. Astrophys. J. 402, 441–468 (1993).
Elvis, M. et al. Atlas of quasar energy distributions. Astrophys. J. Suppl. Ser. 95, 1–68 (1994).
Krolik, J. H. & Begelman, M. C. Molecular tori in Seyfert galaxies—feeding the monster and hiding it. Astrophys. J. 329, 702–711 (1988).
Arav, N., Barlow, T. A., Laor, A., Sargent, W. L. W. & Blandford, R. D. Are AGN broad emission lines formed by discrete clouds? Analysis of Keck high-resolution spectroscopy of NGC 4151. Mon. Not. R. Astron. Soc. 297, 990–998 (1998).
Laor, A. & Netzer, H. Massive thin accretion discs—I. Calculated spectra. Mon. Not. R. Astron. Soc. 238, 897–916 (1989).
Begelman, M. C., Frank, J., & Shlosman, I. Theory of Accretion Disks Vol. 290 (eds Meyer, F., Duschi, W. J., Frank, J. & Meyer-Hofmeister, E.) 373–386 (Springer, Berlin, 1989).
Collin, S. & Zahn, J.-P. Star formation and evolution in accretion discs around massive black holes. Astron. Astrophys. 344, 433–449 (1999).
Draine, B. T. Interstellar dust grains. Annu. Rev. Astron. Astrophys. 41, 241–289 (2003).
Bentz, M. et al. The low-luminosity end of the radius–luminosity relationship for active galactic nuclei. Astrophys. J. 767, 149 (2013).
Rees, M. J. Tidal disruption of stars by black holes of 106–108 solar masses in nearby galaxies. Nature 333, 523–528 (1988).
Burkert, A. et al. Physics of the Galactic Center cloud G2, on its way toward the supermassive black hole. Astrophys. J. 750, 58 (2012).
Pfuhl, O. et al. The Galactic Center cloud G2 and its gas streamer. Astrophys. J. 798, 111 (2015).
McCourt, M., O’Leary, R. M., Madigan, A.-M. & Quataert, E. Magnetized gas clouds can survive acceleration by a hot wind. Mon. Not. R. Astron. Soc. 449, 2–7 (2015).
Simpson, C. The luminosity dependence of the type 1 active galactic nucleus fraction. Mon. Not. R. Astron. Soc. 360, 565–572 (2005).
Maiolino, R. et al. Dust covering factor, silicate emission, and star formation in luminous QSOs. Astron. Astrophys. 468, 979–992 (2007).
Reyes, R. et al. Space density of optically selected type 2 quasars. Astron. J. 136, 2373–2390 (2008).
Risaliti, G. et al. Occultation measurement of the size of the X-ray-emitting region in the active galactic nucleus of NGC 1365. Astrophys. J. 659, L111–L114 (2007).
Krolik, J. H., McKee, C. F. & Tarter, C. B. Two-phase models of quasar emission line regions. Astrophys. J. 249, 422–442 (1981).
Mathews, W. G. Structure and stability of quasar clouds. Astrophys. J. 305, 187–203 (1986).
Shen, Y., Greene, J. E., Strauss, M. A., Richards, G. T. & Schneider, D. P. Biases in virial black hole masses: an SDSS perspective. Astrophys. J. 680, 169–190 (2008).
Wang, J.-M. et al. Star formation in self-gravitating discs in active galactic nuclei. II. Episodic formation of broad-line regions. Astrophys. J. 746, 137 (2012).
Hamann, F., Korista, K. T., Ferland, G. J., Warner, C. & Baldwin, J. Metallicities and abundance ratios from quasar broad emission lines. Astrophys. J. 564, 592–603 (2002).
Warner, C., Hamann, F. & Dietrich, M. A relation between supermassive black hole mass and quasar metallicity? Astrophys. J. 596, 72–84 (2003).
Matsuoka, K., Nagao, T., Marconi, A., Maiolino, R. & Taniguchi, Y. The mass–metallicity relation of SDSS quasars. Astron. Astrophys. 527, A100 (2011).
Nenkova, M., Ivezic, Z. & Elitzur, M. Dust emission from active galactic nuclei. Astrophys. J. 570, L9–L12 (2002).
Mor, R., Netzer, H. & Elitzur, M. Dusty structure around type-I active galactic nuclei: clumpy torus narrow-line region and near-nucleus hot dust. Astrophys. J. 705, 298–313 (2009).
Mor, R. & Netzer, H. Hot graphite dust and the infrared spectral energy distribution of active galactic nuclei. Mon. Not. R. Astron. Soc. 420, 526–541 (2012).
Lira, P. et al. Modeling the nuclear infrared spectral energy distribution of type II active galactic nuclei. Astrophys. J. 764, 159 (2013).
Netzer, H. Revisiting the unified model of active galactic nuclei. Annu. Rev. Astron. Astrophys. 53, 365–408 (2015).
Ichikawa, K. et al. The differences in the torus geometry between hidden and non-hidden broad line active galactic nuclei. Astrophys. J. 803, 57 (2015).
Fuller, L. et al. Investigating the dusty torus of Seyfert galaxies using SOFIA/FORCAST photometry. Mon. Not. R. Astron. Soc. 462, 2618–2630 (2016).
Hernácuten-Caballero, A., Hatziminaoglou, E., Alonso-Herrero, A. & Mateos, S. The near-to-mid infrared spectrum of quasars. Mon. Not. R. Astron. Soc. 463, 2064–2078 (2016).
Audibert, A., Riffel, R., Sales, D. A., Pastoriza, M. G. & Ruschel-Dutra, D. Probing the active galactic nucleus unified model torus properties in Seyfert galaxies. Mon. Not. R. Astron. Soc. 464, 2139–2173 (2017).
Gaskell, C. M. A redshift difference between high and low ionization emission-line regions in QSOs—evidence for radial motions. Astrophys. J. 263, 79–86 (1982).
Hu, C. et al. Hβ profiles in quasars: evidence for an intermediate-line region. Astrophys. J. 683, L115–L118 (2008).
Hu, C. et al. A systematic analysis of Fe ii emission in quasars: evidence for inflow to the central black hole. Astrophys. J. 687, 78–96 (2008).
Müller-Sánchez, F. et al. The central molecular gas structure in LINERs with low-luminosity AGNs: evidence for gradual disappearance of the torus. Astrophys. J. 763, L1 (2013).
Vollmer, B. & Beckert, T. Turbulent viscosity in clumpy accretion discs: application to the Galaxy. Astron. Astrophys. 382, 872–887 (2002).
Hopkins, P. F., Hayward, C. C., Narayanan, D. & Hernquist, L. The origins of active galactic nuclei obscuration: the ‘torus’ as a dynamical, unstable driver of accretion. Mon. Not. R. Astron. Soc. 420, 320–339 (2012).
Xu, Y.-D., Narayan, R., Quataert, E., Yuan, F. & Baganoff, F. K. Thermal X-ray iron line emission from the Galactic Center black hole Sagittarius A*. Astrophys. J. 640, 319–326 (2006).
Schartmann, M. et al. Simulations of the origin and fate of the Galactic Center cloud G2. Astrophys. J. 755, 155 (2012).
Cowie, L. L. & McKee, C. F. The evaporation of spherical clouds in a hot gas. I. Classical and saturated mass loss rates. Astrophys. J. 211, 135–146 (1977).
Mathews, W. G. & Ferland, G. What heats the hot phase in active nuclei? Astrophys. J. 323, 456–467 (1987).
Sutherland, R. & Dopita, M. A. Cooling functions for low-density astrophysical plasmas. Astrophys. J. Suppl. Ser. 88, 253–327 (1993).
Mathews, W. G. & Doane, J. S. Can quasar clouds form in thermal instabilities? Astrophys. J. 352, 423–432 (1990).
Krause, M., Burkert, A. & Schartmann, M. Stability of cloud orbits in the broad-line region of active galactic nuclei. Mon. Not. R. Astron. Soc. 411, 550–556 (2011).
Plewa, P. M., Schartmann, M. & Burkert, A. Dynamics of gas and dust clouds in active galactic nuclei. Mon. Not. R. Astron. Soc. 431, L127–L130 (2013).
Shadmehri, M. On the orbital motion of cold clouds in broad-line regions. Mon. Not. R. Astron. Soc. 451, 3671–3678 (2015).
Evans, C. R. & Kochanek, C. S. The tidal disruption of a star by a massive black hole. Astrophys. J. 346, L13–L16 (1989).
Leighly, K. M. & Moore, J. R. HST STIS ultraviolet spectral evidence of outflow in extreme narrow-line Seyfert 1 galaxies. I. Data and analysis. Astrophys. J. 611, 107–124 (2004).
Koshida, S. et al. Reverberation measurements of the inner radius of the dust torus in 17 Seyfert galaxies. Astrophys. J. 788, 159 (2014).
Osterbrock, D. E. & Shaw, R. A. The relative number of Seyfert 2 galaxies. I. Spectra of emission-line galaxies in the Wasilewski field. Astrophys. J. 327, 89–98 (1088).
Tovmassian, H. M. On the relative number of Seyfert 1 and Seyfert 2 galaxies and the opening angle of dust torus. Astron. Nachr. 322, 87–91 (2001).
Cao, X. On the dust tori in Palomar–Green quasars. Astrophys. J. 619, 86–92 (2005).
Wang, J.-M., Zhang, E.-P. & Luo, B. Evolutionary consequences of dusty tori in active galactic nuclei. Astrophys. J. 627, L5–L8 (2005).
Marin, F. Are there reliable methods to estimate the nuclear orientation of Seyfert galaxies? Mon. Not. R. Astron. Soc. 460, 3679–3705 (2016).
Ghisellini, G., Padovani, P., Celotti, A. & Maraschi, L. Relativistic bulk motion in active galactic nuclei. Astrophys. J. 407, 65–82 (1993).
Baskin, A., Laor, A. & Hamann, F. The average absorption properties of broad absorption line quasars at 800 < λrest < 3000 Å and the underlying physical parameters. Mon. Not. R. Astron. Soc. 432, 1525–1543 (2013).
Baskin, A. & Laor, A. What controls the C IV line profile in active galactic nuclei? Mon. Not. R. Astron. Soc. 356, 1029–1044 (2005).
Krawczyk, C. M. et al. Mining for dust in type 1 quasars. Astron. J. 149, 203 (2015).
Shlosman, I., Vitello, P. A. & Shaviv, G. Active galactic nuclei—internal dynamics and formation of emission clouds. Astrophys. J. 294, 96–105 (1985).
Murray, N., Chiang, J., Grossman, S. A. & Voit, G. M. Accretion disc winds from active galactic nuclei. Astrophys. J. 451, 498–509 (1995).
Proga, D. & Kallman, T. R. Dynamics of line-driven disc winds in active galactic nuclei. II. Effects of disc radiation. Astrophys. J. 616, 688–695 (2004).
Czerny, B. & Hryniewicz, K. The origin of the broad line region in active galactic nuclei. Astron. Astrophys. 525, L8 (2011).
Baskin, A., Laor, A. & Stern, J. Radiation pressure confinement II. Application to the broad-line region in active galactic nuclei. Mon. Not. R. Astron. Soc. 438, 604–619 (2014).
Emmering, R. T., Blandford, R. D. & Shlosman, I. Magnetic acceleration of broad emission-line clouds in active galactic nuclei. Astrophys. J. 385, 460–477 (1992).
Konigl, A. & Kartje, J. F. Disc-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes. Astrophys. J. 434, 466–467 (1994).
Gaskell, C. M. & Harrington, P. Z. Partial obscuration of innermost regions of active galactic nuclei by outflowing dusty clouds as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities. Preprint at https://arxiv.org/abs/1704.06455 (2017).
Collin-Souffrin, S., Dyson, J. E., McDowell, J. C. & Perry, J. J. The environment of active galactic nuclei. I. A two-component broad emission line model. Mon. Not. R. Astron. Soc. 232, 539–550 (1988).
Xue, S.-J., Cheng, F.-Z. & Kwan, J. Kinematic models of BLR gas and line profiles of He I λ5876 and Hβ in AGNs. Sci. China Ser. A 37, 487–496 (1994).
Eracleous, M., Livio, M., Halpern, J. P. & Storchi-Bergmann, T. Elliptical accretion discs in active galactic nuclei. Astrophys. J. 438, 610–622 (1995).
Goad, M. R., Korista, K. T. & Ruff, A. J. The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus. Mon. Not. R. Astron. Soc. 426, 3086–3111 (2012).
Elvis, M. Quasar rain: the broad emission line region as condensations in the warm accretion disc wind. Preprint at https://arxiv.org/abs/1703.02956 (2017).
Gaskell, C. M. & Goosmann, R. W. The case for inflow of the broad-line region of active galactic nuclei. Astrophys. Space Sci. 361, 67 (2016).
Peterson, B. M. In: Proc. Sci. Narrow-Line Seyfert 1 Galaxies and Their Place in the Universe (eds Peterson, L. et al.) 032 (NLS1, Trieste, Italy, 2011).
Murray, N. & Chiang, J. Photoionization of disc winds. Astrophys. J. 494, 125–138 (1998).
Kaspi, S. et al. Reverberation mapping of high-luminosity quasars: first results. Astrophys. J. 659, 997–1007 (2007).
Crenshaw, M., Kraemer, S. B. & George, I. M. Mass loss from the nuclei of active galaxies. Annu. Rev. Astron. Astrophys. 41, 117–167 (2003).
Proga, D., Stone, J. M. & Kallman, T. R. Dynamics of line-driven disc winds in active galactic nuclei. Astrophys. J. 543, 686–696 (2000).
Tombesi, F. & Cappi, M. On the presence of ultrafast outflows in the WAX sample of Seyfert galaxies. Mon. Not. R. Astron. Soc. 443, L104–L108 (2014).
Denney, K. D. et al. Diverse kinematic signatures from reverberation mapping of the broad-line region in AGNs. Astrophys. J. 704, L80–L84 (2009).
Grier, C. J. et al. The structure of the broad-line region in active galactic nuclei. I. Reconstructed velocity-delay maps. Astrophys. J. 764, 47 (2013).
Du, P. et al. Supermassive black holes with high accretion rates in active galactic nuclei. VI. Velocity-resolved reverberation mapping of the Hβ line. Astrophys. J. 820, 27 (2016).
Beltrametti, M. Thermal instabilities in radiatively driven winds—application to emission line clouds of quasars and active galactic nuclei. Astrophys. J. 250, 18–30 (1981).
Moriya, T. J., Tanaka, M., Morokuma, T. & Ohsuga, K. Superluminous transients at AGN centers from interaction between black-hole disc winds and broad-line region clouds. Astrophys. J. Lett. 843, L19 (2017).
Baldwin, J., Ferland, G., Korista, K. & Verner, D. Locally optimally emitting clouds and the origin of quasar emission lines. Astrophys. J. 455, L119–L122 (1995).
Du, P. et al. Supermassive black holes with high accretion rates in AGNs. I. First results from a new reverberation mapping campaign. Astrophys. J. 782, 45 (2014).
Du, P. et al. Supermassive black holes with high accretion rates in AGNs. IV. Hβ time lags and implications for super-Eddington accretion. Astrophys. J. 806, 22 (2015).
Du, P. et al. Supermassive black holes with high accretion rates in AGNs. V. A new size-luminosity scaling relation for the BLR. Astrophys. J. 825, 126 (2016).
Pei, L. et al. Space telescope and optical reverberation mapping project. V. Optical spectroscopic campaign and emission-line analysis for NGC 5548. Astrophys. J. 837, 131 (2017).
Li, Y.-R., Wang, J.-M. & Bai, J.-M. A non-parametric approach to constrain the transfer function in reverberation mapping. Astrophys. J. 831, 206 (2016).
Nicastro, F. Broad emission line regions in active galactic nuclei: the link with the accretion power. Astrophys. J. 530, L16–L68 (1999).
Ho, L. C., Kim, M. & Terashima, Y. The low-mass, highly accreting black hole associated with the active galactic nucleus 2XMM J123103.2 + 110648. Astrophys. J. 759, L16 (2012).
Miniutti, G. et al. A high Eddington-ratio, true Seyfert 2 galaxy candidate: implications for broad-line region models. Mon. Not. R. Astron. Soc. 433, 1764–1777 (2013).
Pancoast, A., Brewer, B. J. & Treu, T. Geometric and dynamical models of reverberation mapping data. Astrophys. J. 730, 139 (2011).
Li, Y.-R.,Wang, J.-M., Ho, L. C., Du, P. & Bai, J.-M. A Bayesian approach to estimate the size and structure of the BLR in AGNs using RM data. Astrophys. J. 779, 110 (2013).
Zamfir, S., Sulentic, J. W., Marziani, P. & Dultzin, D. Detailed characterization of Hβ emission line profile in low-z SDSS quasars. Mon. Not. R. Astron. Soc. 403, 1759–1786 (2010).
Runnoe, J. C. et al. A large systematic search for close supermassive binary and rapidly recoiling black holes. II. Continued spectroscopic monitoring and optical flux variability. Astrophys. J. Suppl. Ser. 221, 7 (2015).
Wills, B. J. & Browne, I. W. A. Relativistic beaming and quasar emission lines. Astrophys. J. 302, 56–63 (1986).
The authors thank T. Boroson for sending the PG quasar spectra data. This research is supported by the National Key Program for Science and Technology Research and Development (grant 2016YFA0400701), grants NSFC-11173023, -11133006, -11373024, -11233003 and -11473002, and the Key Research Program of Frontier Sciences at the Chinese Academy of Sciences (grant QYZDJ-SSW-SLH007).
The authors declare no competing financial interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Electronic supplementary material
About this article
Cite this article
Wang, JM., Du, P., Brotherton, M.S. et al. Tidally disrupted dusty clumps as the origin of broad emission lines in active galactic nuclei. Nat Astron 1, 775–783 (2017). https://doi.org/10.1038/s41550-017-0264-4