In New Astronomy Reviews, Volume 53, Issue 7-10, p. 140-148.
astro-ph/0908.0386

For a PDF version of the article, click here.

WHAT BROAD EMISSION LINES TELL US ABOUT HOW ACTIVE GALACTIC NUCLEI WORK

C. Martin Gaskell


Astronomy Department, University of Texas, Austin, TX 78712, USA


Abstract. I review progress made in understanding the nature of the broad-line region (BLR) of active galactic nuclei (AGNs) and the role BLRs play in the AGN phenomenon. The high equivalent widths of the lines imply a high BLR covering factor, and the absence of clear evidence for absorption by the BLR means that the BLR has a flattened distribution and that we always view it near pole-on. The BLR gas is strongly self-shielding near the equatorial plane. Velocity-resolved reverberation mapping has long strongly excluded significant outflow of the BLR and shows instead that the predominant motions are Keplerian with large turbulence and a significant net inflow. The rotation and turbulence are consistent with the inferred geometry. The blueshifting of high-ionization lines is a consequence of scattering off inflowing material rather than the result of an outflowing wind. The rate of inflow of the BLR is sufficient to provide the accretion rate needed to power the AGN. Because the motions of the BLR are gravitationally dominated, and the BLR structure is very similar in most AGNs, consistent black hole masses can be determined. The good correlation between these estimates and masses predicted from the bulge luminosities of host galaxies provides strong support for the similarity of AGN continuum shapes and the correctness of the BLR picture presented. It is concluded that although many mysteries remain about the details of how AGNs work, a general overall picture of the torus and BLR is becoming clear.


Keywords: Active Galactic Nuclei - Broad-Line Region - Accretion processes - Dust


Table of Contents

INTRODUCTION

THE STRUCTURE OF THE BROAD-LINE REGION AND TORUS

THE KINEMATICS OF THE BLR AND TORUS
Determining the Direction of Motion
The overall velocity field of the BLR

ORIENTATION EFFECTS

THE ACCURACY OF AGN BLACK HOLE MASS DETERMINATIONS

WHAT DRIVES BLR MOTIONS?

CONCLUSIONS AND UNSOLVED PROBLEMS

REFERENCES

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