The first spectroscopic dust reverberation programme on active galactic nuclei: The torus in NGC 5548

H. Landt, M. J. Ward, D. Kynoch, C. Packham, G. J. Ferland, A. Lawrence, J. U. Pott, J. Esser, K. Horne, D. A. Starkey, D. Malhotra, M. M. Fausnaugh, B. M. Peterson, R. J. Wilman, R. A. Riffel, T. Storchi-Bergmann, A. J. Barth, C. Villforth, H. Winkler

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25 Scopus citations


We have recently initiated the first spectroscopic dust reverberation programme on active galactic nuclei in the near-infrared. Spectroscopy enables measurement of dust properties, such as flux, temperature, and covering factor, with higher precision than photometry. In particular, it enables measurement of both luminosity-based dust radii and dust response times. Here we report results from a 1 yr campaign on NGC 5548. The hot dust responds to changes in the irradiating flux with a lag time of ∼70 light-days, similar to what was previously found in photometric reverberation campaigns. The mean and rms spectra are similar, implying that the same dust component dominates both the emission and the variations. The dust lag time is consistent with the luminosity-based dust radius only if we assume a wavelength-independent dust emissivity law, i.e. a blackbody, which is appropriate for grains of large sizes (of a few μm). For such grains the dust temperature is ∼1450 K. Therefore, silicate grains have most likely evaporated and carbon is the main chemical component. But the hot dust is not close to its sublimation temperature, contrary to popular belief. This is further supported by our observation of temperature variations largely consistent with a heating/cooling process. Therefore, the inner dust-free region is enlarged and the dusty torus rather a 'dusty wall', whose inner radius is expected to be luminosity-invariant. The dust-destruction mechanism that enlarges the dust-free region seems to also partly affect the dusty region. We observe a cyclical decrease in dust mass with implied dust reformation times of ∼5-6 months.

Original languageEnglish
Pages (from-to)1572-1589
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Oct 21 2019

Bibliographical note

Funding Information:
HL thanks Nancy Levenson for supporting this project in its initial stages and Chris Done for the many enlightening conversations on the physics of accretion discs. HL, MJW, and DK acknowledge the Science and Technology Facilities Council (STFC) for support through grant ST/P000541/1. CP acknowledges support from the National Science Foundation (NSF) grant no. 1616828. GJF acknowledges support by NSF (1816537), NASA (ATP 17-ATP17-0141), and STScI (HST-AR-15018). KH acknowledges support from STFC grant ST/R000824/1.

Funding Information:
★E-mail: † Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration (NASA).

Publisher Copyright:
© 2019 The Author(s).


  • Galaxies: Seyfert
  • Infrared: Galaxies
  • Quasars: Emission lines
  • Quasars: Individual: NGC 5548

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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