Tidally disrupted dusty clumps as the origin of broad emission lines in active galactic nuclei

Abstract

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.

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Fig. 1: Scheme of the present model.
Fig. 2: Various profiles of the models with a wide range of parameters.
Fig. 3: Best-fit models characterizing the four different types of Hβ profile.
Fig. 4: Correlations of asymmetries, shapes and shifts with [O iii] luminosity as a proxy of torus angles.

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Acknowledgements

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).

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J.-M.W. conceived the project, presented the idea and built up the current model. J.-M.W. and P.D. made the calculations. J.-M.W. and M.S.B. wrote the manuscript. Y.-Y.S. and J.-M.W. fitted the Hβ profiles of the PG quasars. C.H., Z.-X.Z. and Y.S. measured the PG quasar spectra. J.-M.W. and Y.-R.L. discussed clump physics. All authors discussed the contents of the paper.

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Correspondence to Jian-Min Wang.

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Wang, J., 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

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