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Broad-line active galactic nuclei rotate faster than narrow-line ones


The super-massive black holes of 106M to 109M that reside in the nuclei of active galaxies1 (AGN) are surrounded by a region emitting broad lines, probably associated with an accretion disk. The diameters of the broad-line regions range from a few light-days to more than a hundred light-days1, and cannot be resolved spatially. The relative significance of inflow, outflow, rotational or turbulent motions in the broad-line regions as well as their structure (spherical, thin or thick accretion disk) are unknown despite intensive studies over more than thirty years2,3. Here we report a fundamental relation between the observed emission linewidth full-width at half-maximum (FWHM) and the emission line shape FWHM/σline in AGN spectra. From this relation we infer that the predominant motion in the broad-line regions is Keplerian rotation in combination with turbulence. The geometry of the inner region varies systematically with the rotation velocity: it is flattest for the fast-rotating broad-line objects, whereas slow-rotating narrow-line AGN have a more spherical structure. Superimposed is the trend that the line-emitting region becomes geometrically thicker towards the centre within individual galaxies. Knowing the rotational velocities, we can derive the central black-hole masses more accurately; they are two to ten times smaller than has been estimated previously.

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Figure 1: Observed and modelled Hβ linewidth ratios FWHM/ σ line versus linewidth FWHM.
Figure 2: Observed and modelled He  ii λ = 4,686 Å linewidth ratios FWHM/ σ line versus linewidth FWHM.
Figure 3: Observed and modelled C  iv λ = 1,550 Å linewidth ratios FWHM/ σ line versus linewidth FWHM.


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We acknowledge discussions with W. Glatzel and S. Dreizler. This work has been supported by the Niedersachsen-Israel Research Cooperation Program ZN2318.

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W.K. had the basic ideas. M.Z. made the detailed model calculations. Both authors discussed the results at length.

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Correspondence to Wolfram Kollatschny.

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The authors declare no competing financial interests.

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Kollatschny, W., Zetzl, M. Broad-line active galactic nuclei rotate faster than narrow-line ones. Nature 470, 366–368 (2011).

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