Journal Article

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

Kollatschny, Wolfram, Zetzl, Matthias

2011

Overview

Super-massive black holes cut down to size? Active galactic nuclei (AGN) are the most luminous objects in the Universe. They are powered by super-massive black holes of between 10 million and 10 billion times the mass of the Sun, and are surrounded by a broad emission line region probably associated with an accretion disk. The kinematics and structure of the central broad-line region are poorly understood despite intensive studies over more than 30 years. Wolfram Kollatschny and Matthias Zetzl now show that there is a fundamental relationship between observed emission line width and emission line shape in the spectra of AGN, from which they infer that the geometry of the inner region is flattest for the fast-rotating broad-line objects, while slow-rotating narrow-line AGN have a more spherical structure. Knowing the rotational velocities, it is possible to derive more accurate estimates for the masses of the central black holes — which turn out to be 2–10 times smaller than previously estimated. The super-massive black holes of 10 6 to 10 9 solar masses that reside in the nuclei of active galaxies (AGN) are surrounded by a region emitting broad emission lines, probably associated with an accretion disk, which cannot be resolved spatially. The relative significance of inflow, outflow, rotational, or turbulent motions in the broad-line region as well as their structure (spherical and/or thin/thick accretion disk) are unknown. This study reports a fundamental relation between the observed emission line width and shape in AGN spectra, from which it is inferred that the geometry of the inner region is flattest for the fast-rotating broad-line objects whereas slow-rotating narrow-line AGN have a more spherical structure. Knowing the rotational velocities one can derive more accurately the central black hole masses, which are two to ten times smaller than previously estimated. The super-massive black holes of 10 6 M ⊙ to 10 9 M ⊙ that reside in the nuclei of active galaxies 1 (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-days 1 , 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 years 2 , 3 . Here we report a fundamental relation between the observed emission linewidth full-width at half-maximum (FWHM) and the emission line shape FWHM/ σ line 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33/34/863 / Accretion / Accretion disks / Active and peculiar galaxies (including bl lacertae objects, blazars, seyfert galaxies, markarian galaxies, active galactic nuclei) / Active galaxies / Astronomy / Black holes (astronomy) / Characteristics and properties of external galaxies and extragalactic objects / Chemical properties / Comparative analysis / Earth, ocean, space / Emission / Emissions / Exact sciences and technology / Fluid dynamics / Galaxies / Humanities and Social Sciences / Kinematics, dynamics, and rotation / letter / multidisciplinary / Observations / Quasars. Active or peculiar galaxies, objects, and systems / Ratios / Rotation / Science / Science (multidisciplinary) / Spectra / Spectral linewidth / Stellar systems. Galactic and extragalactic objects and systems. The universe / Turbulence / Turbulent flow