Nuclear Activity in Nearby Galaxies

TL;DR

This paper reviews the nature of weak nuclear activity in nearby galaxies, highlighting the role of black hole accretion at low rates and the structural changes in their central engines, with implications for galaxy evolution.

Contribution

It provides a comprehensive overview of low-luminosity active galactic nuclei, emphasizing the structural modifications of accretion flows at low accretion rates and their prevalence in galaxy bulges.

Findings

Most bulges host a supermassive black hole.

Low accretion rates lead to disappearance of broad-line regions.

Faint sources show transformed accretion disk structures.

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.