To appear in Annual Reviews of Astronomy and Astrophysics (2008)
arXiv:0803.2268

For a PDF version of the article, click here.

NUCLEAR ACTIVITY IN NEARBY GALAXIES

Luis C. Ho


The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara St., Pasadena, CA 91101; e-mail: lho@ociw.edu


Key Words: accretion disks, active galactic nuclei, black holes, LINERs, radio galaxies, Seyfert galaxies


Abstract. A significant fraction of nearby galaxies show evidence of weak nuclear activity unrelated to normal stellar processes. Recent high-resolution, multiwavelength observations indicate that the bulk of this activity derives from black hole accretion with a wide range of accretion rates. The low accretion rates that typify most low-luminosity active galactic nuclei induce significant modifications to their central engine. The broad-line region and obscuring torus disappear in some of the faintest sources, and the optically thick accretion disk transforms into a three-component structure consisting of an inner radiatively inefficient accretion flow, a truncated outer thin disk, and a jet or outflow. The local census of nuclear activity supports the notion that most, perhaps all, bulges host a central supermassive black hole, although the existence of active nuclei in at least some late-type galaxies suggests that a classical bulge is not a prerequisite to seed a nuclear black hole.


Table of Contents

INTRODUCTION

SPECTRAL CLASSIFICATION OF GALACTIC NUCLEI
Physical Motivation
Sample Spectra
Diagnostic Diagrams
Starlight Subtraction
Other Classification Criteria

SURVEYS OF NEARBY GALACTIC NUCLEI
The Palomar Survey
Other Surveys
Detection Rates
Broad Emission Lines
Robustness and Completeness

HOST GALAXY PROPERTIES
Global Parameters
Nuclear Stellar Populations
Influence of Bars and Environment

NUCLEAR PROPERTIES
Ionizing Continuum Radiation
Radio Cores
X-ray Cores
Circumnuclear Thermal Plasma
Broad-line Region
Torus
Narrow-line Region Kinematics
Spectral Energy Distribution
Luminosity Function
Bolometric Luminosities and Eddington Ratios

EXCITATION MECHANISMS
Nonstellar Photoionization
Contribution from Fast Shocks
Contribution from Stellar Photoionization
Energy Budget
The Nature of Transition Objects and a Unified View of LLAGNs

IMPLICATIONS FOR BLACK HOLE DEMOGRAPHICS

IMPLICATIONS FOR ACCRETION PHYSICS
Why Are LLAGNs So Dim?
The Disk-Jet Connection
The Central Engine of LLAGNs

CONCLUDING REMARKS

REFERENCES

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