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Visibility of scattered broad-line emission in Seyfert 2 galaxies

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Abstract

Active galaxies are thought to be powered by the accretion of gas onto a central massive black hole. Seyfert galaxies—the most common examples of nearby active galaxies—are separated into two classes based on their emission line widths1. Seyfert 1 galaxies exhibit broad emission lines that are attributed to ionized gas within 1 pc of the black hole, whereas the spectra of Seyfert 2 galaxies show only narrower emission lines, believed to originate from a much larger region around the core. The 'unified model' for Seyfert galaxies attributes these differences to the presence of a dusty torus of dense molecular gas surrounding the black hole2: the orientation of Seyfert 2 galaxies is such that the broad-line region is obscured. The detection3 in the polarization spectrum of broad emission lines scattered into our line of sight by free electrons in NGC1068 (the prototypical Seyfert 2 galaxy) and other Seyfert 2 galaxies4–8 has strengthened this view, but all of these galaxies were subject to selection biases. Here we report the results of a systematic search for polarized broad emission lines in a well defined sample of Seyfert 2 galaxies. We show that the ability to detect scattered broad emission lines is related to the far-infrared colours, in the manner predicted by the unified model.

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Authors and Affiliations

  1. Mount Stromlo and Siding Springs Observatory, Institute of Advanced Studies, The Australian National University, Weston Creek Post Office, Canberra, ACT, 2611, Australia

    Charlene A. Heisler

  2. Anglo-Australian Observatory, PO Box 296, Epping, NSW, 2121, Australia

    Stuart L. Lumsden & Jeremy A. Bailey

Authors
  1. Charlene A. Heisler
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  2. Stuart L. Lumsden
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  3. Jeremy A. Bailey
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Heisler, C., Lumsden, S. & Bailey, J. Visibility of scattered broad-line emission in Seyfert 2 galaxies. Nature 385, 700–702 (1997). https://doi.org/10.1038/385700a0

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  • DOI: https://doi.org/10.1038/385700a0

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