Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-03-15
Ann.Rev.Astron.Astrophys.46:475-539,2008
Astronomy and Astrophysics
Astrophysics
To appear in Annual Reviews of Astronomy and Astrophysics (2008). 76 pages
Scientific paper
10.1146/annurev.astro.45.051806.
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.
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