Universe Tenth Edition Chapter 24 Quasars and Active Galaxies Roger Freedman Robert Geller William Kaufmann III

24-1 The distinctive features of quasars 24-2 The role of supermassive black holes in powering active galactic nuclei (AGN) 24-3 How quasars can form accretion disks and jets 24-4 That active galactic nuclei can look very different depending on their orientation 24-5 The fate of active galactic nuclei and their potential to occasionally flare up By reading this chapter, you will learn

Cygnus A

A Quasar and Its Host Galaxy 24-1: Quasars are the ultraluminous centers of distant galaxies

A Quasar and Its Host Galaxy

The Spectrum of a Quasar

Quasars Are Extinct

Brightness Variations of an AGN 24-2: Supermassive black holes are the “central engines” that power active galactic nuclei

A Limit in the Speed of Variations in Brightness

Accretion Disks

Jets from a Supermassive Black Hole 24-3: Quasar accretion disks and jets

Giant Elliptical Galaxy M87

Peculiar Galaxy NGC 5128 (Centaurus A)

Superluminal Motion in 3C 273

An Explanation of Superluminal Motion

A Dusty Torus Around a Supermassive Black Hole 24-4:The unified model explains much of the diversity of AGN

A Unified Model of Active Galaxies

Providing Fresh “Fuel” for a Supermassive Black Hole 24-5: The evolution of AGN

The Radio Galaxy Centaurus A

Dead Quasar Flare

Radio Galaxy 3C 75

Key Ideas Quasars or active galactic nuclei (AGN): A quasar looks like a star but has a huge redshift. According to the Hubble law, these redshifts show that quasars are typically billions of light-years away from Earth To be seen at such large distances, quasars must be very luminous, typically about 1000 times brighter than an ordinary galaxy. The peak of quasar activity took place when the universe was just over 2 billion years old; the era of quasar activity has long since passed. Supermassive black holes power quasars: At the center of a quasar is a black hole with millions or even billions of solar masses.

Key Ideas Rapid fluctuations in the brightness of active galaxies indicate that the region that emits radiation is quite small. The Eddington limit relates the mass of a black hole to the maximum luminosity it can emit from its surroundings; if more luminous, radiation pressure would push away accreting gas and shut off the black hole’s fuel source.

Key Ideas Accretion and jets around black holes: Hot gas accreting around the black hole is thought to give a quasar its intense luminosity. About 10% of AGN also have strong radio emission coming from jets, as well as lobes of radio emission as the jets collide with sur- rounding gas on scales much larger than the host galaxy. Models for a thin accretion disk describe heat and emission pro- duced as portions of gas on different orbits rub past each other, although no specific model of accretion has been verified. As gases spiral in toward the supermassive black hole, some of the gas may be redirected to form two jets of high-speed particles that are aligned perpendicularly to the accretion disk. When a jet is directed toward us, we can observe apparent superluminal motion.

Key Ideas Unified model of AGN: A thick dusty doughnut-shaped torus sur- rounds a quasar that, when viewed edge-on, blocks a direct view of the luminous accreting gas. A quasar is only visible if our viewing angle allows us to see the region in the center of the torus. When viewing the torus somewhat edge-on, the most luminous part of a quasar is blocked. Some emission is still observed, and the object appears as an active galaxy with a different emission spectrum. While the viewing angle changes what we observe, the central engine—an accreting black hole—is fundamentally the same. This insight unifies a diverse group of AGN.