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

For a PDF version of the article, click here.

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