Discovering the UniverseDiscovering the UniverseNinth EditionNinth Edition

Neil F. Comins • William J. Kaufmann III

CHAPTER 17CHAPTER 17Quasars and Other Active GalaxiesQuasars and Other Active Galaxies

WHAT DO YOU THINK?

1. What does “quasar” stand for?2. What do quasars look like?3. Where do quasars get their energy?

In this chapter you will discover…In this chapter you will discover… bright and unusual objects, called active galaxies distant, luminous quasars the unusual spectra and small volumes of quasars the extremely powerful BL Lac objects supermassive black holes that serve as central engines

for radio galaxies, quasars, Seyfert galaxies, and BL Lac objects

Cygnus A (3C 405)

This radio image was produced from observations made at the Very Large Array. Most of the radio emissions from Cygnus A come from the radio lobes located on either side of the peculiar galaxy seen in the inset, a Hubble Space Telescope image. Each of the two radio lobes extend about 160,000 ly from the optical galaxy and contain a brilliant, condensed region of radio emission. (Inset) At the heart of this system of gas lies a strange-looking galaxy that has a redshift that corresponds to a recessional speed of 5% of the speed of light. According to the Hubble law, Cygnus A is therefore 635 million ly from Earth. Because Cygnus A is one of the brightest radio sources in the sky, this remote galaxy’s energy output must be enormous.

Quasar 3C 48

For several years, astronomers erroneously believed that this object is simply a peculiar, nearby star that happens to emit radio waves. Actually, the redshift of this starlike object is so great that, according to the Hubble law, it must be roughly 4 billion ly away.

Quasar 3C 273

This combined X-ray and infrared view shows the starlike object associated with the radio source 3C 273 and the luminous jet it has created. The jet is also visible in the radio and visible parts of the spectrum. By 1963, astronomers determined that the redshift of this quasar is so great that, according to the Hubble law, it is nearly 2 billion ly from Earth.

Spectra of 3C 273

The visible and infrared spectra of 3C 273 are dominated by four bright emission lines caused by hydrogen. This radiation is redshifted nearly 16% from its rest wavelengths.

History of Quasar Formation

The greater the redshift of a quasar, the farther it is from Earth and the farther back in time we are seeing it. By observing the number of quasars found at different redshifts, astronomers can calculate how the density of quasars has changed over time.

Brightness of 3C 279

This graph shows variations in the brightness of the quasar 3C 279. Note the especially large outburst observed in 1937. These data were obtained by carefully examining old photographic plates in the files of the Harvard College Observatory.

Seyfert Galaxy NGC 1566

This Sc galaxy is a Seyfert galaxy some 50 Mly (16 Mpc) from Earth in the southern constellation Dorado (the Goldfish). The nucleus of this galaxy is a strong source of radiation whose spectrum shows emission lines of highly ionized atoms.

Peculiar Galaxy NGC 5128 (Centaurus A)Radio galaxy NGC 5128 is 11 million ly from Earth. At visible wavelengths a dust lane crosses the face of the galaxy. Superimposed on this visible image is a false-color radio image (green) showing that vast quantities of radio radiation pour from matter ejected from the galaxy perpendicular to the dust lane, along with radio emission (rose-colored) along the dust lane, and X-ray emission (blue) detected by NASA’s Chandra X-Ray Observatory. The X rays may be from material ejected by the black hole or from the collision of Centaurus A with a smaller galaxy. (Inset) This X-ray image from the Einstein Observatory shows that NGC 5128 has a bright X-ray nucleus. An X-ray jet protrudes from the nucleus along a direction perpendicular to the galaxy’s dust lane.

Head-Tail Source NGC 1265

This active elliptical galaxy is moving at a high speed through the intergalactic medium. Because of this motion, the two tail jets trail the galaxy at its head, giving this radio source a distinctly windswept appearance.

Binary Head-Tail Source

This combined radio and X-ray image of 3C 75 shows the head-tail sources emanating from supermassive black holes in a pair of galaxies that are in the process of merging. The black holes are separated by 25,000 ly and are 300 million ly away from Earth.

BL Lacertae

This photograph shows fuzz around BL Lacertae (arrow). The redshift of this fuzz indicates that BL Lacertae is about 900 Mly (280 Mpc) from Earth. BL Lac objects appear to be giant elliptical galaxies with bright quasarlike nuclei, much as Seyfert galaxies are spiral galaxies with quasarlike nuclei. BL Lac objects contain much less gas and dust than do Seyfert galaxies.

Elliptical Galaxy M32

This small galaxy is a satellite of M31, a portion of which is seen at the left of this wide-angle photograph. Both galaxies are roughly 2.5 million ly from Earth. (Inset) High-resolution image from the Hubble Space Telescope that shows the center of M32. Note the concentration of stars at the nucleus of the galaxy. The nucleus is only 175 ly across.

Giant Elliptical Galaxy M87

M87 is located near the center of the sprawling, rich Virgo cluster, which is about 50 million ly from Earth. Embedded in this radio image of gas is the galaxy M87 from which the gas has been ejected (bottom inset). Images at different radio and visible wavelengths reveal a variety of details about the structure of the jets of gas. M87’s extraordinarily bright nucleus and the gas jets result from a 3-billion-solar-mass black hole, whose gravity causes huge amounts of gas and an enormous number of stars to crowd around it.

Sombrero Galaxy (M104)

This spiral galaxy in Virgo is nearly edge on to our Earth-based view. Spectroscopic observations indicate that a billion-solar-mass black hole is located at the galaxy’s center. You can see the bright region in the galaxy’s center created by stars and gas that orbit the black hole.

Supermassive Black Holes as Engines for Galactic Activity

(a) In the accretion disk around a supermassive black hole, inswirling gas heats and expands. Pulled inward, compressed, and heated further, some of it is eventually expelled perpendicular to the disk in two jets. (b) The giant elliptical galaxy NGC 4261 is a double-radio source located in the Virgo cluster, about 100 million ly from Earth. A visible-light photograph of the galaxy (white) is combined with a radio image (orange and yellow) to show both the visible galaxy, which does not emit much radio energy, and its jets, which do. (Inset) This HST image of the nucleus of NGC 4261 shows a disk of gas and dust about 800 ly in diameter, orbiting a supermassive black hole.

Focusing Jets by Pressure

(a) If a high-speed jet of gas or liquid encounters little pressure (from the surrounding air, in this image), then it will spread out. (b) If the jet encounters high pressure, such as occurs when it enters water, then it will maintain longer its shape as a column.

Focusing Jets by Magnetic Fields

The hot, ionized accretion disk (red-yellow) around the black hole rotates and creates a magnetic field that is twisted into spring-shaped spirals above and below the disk. Some of the accretion disk’s gas falling toward the black hole is overheated and squirted at high speeds into the two tubes created by the magnetic fields. The fields keep the gas traveling directly outward from above and below the disk, thus creating the two jets.

Orientation of the Central Engine and Its Jets

BL Lacertae objects, quasars, and double-radio sources appear to be the same type of object viewed from different directions. If one of the jets is aimed almost directly at Earth, we see a BL Lac object. If the jet is somewhat tilted to our line of sight, we see a quasar. It if is tilted farther, we see an active galaxy. If the jets are nearly perpendicular to our line of sight, we see a double-radio source.

Gravitational Lensing of Quasars

Image from the Hubble Space Telescope that shows the gravitational lensing of a quasar in the constellation of Pegasus. The quasar, about 8 billion ly from Earth, is seen as four separate images that surround a galaxy that is only 400 million ly away. This pattern is called an Einstein cross. The diffuse image at the center of the Einstein cross is the core of the intervening galaxy. The physical effect that creates these multiple images is the same as that seen for galaxies.

Summary of Key IdeasSummary of Key Ideas The development of radio astronomy in the late 1940s

led to the discovery of very powerful and extremely distant energy sources.

Quasars and Other Active Galaxies An active galaxy is an extremely luminous galaxy that has An active galaxy is an extremely luminous galaxy that has

one or more unusual features: an unusually bright, starlike one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects.BL Lacertae objects.

A quasar, or quasi-stellar radio source, is an object that A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from corresponds to a distance of billions of light-years from Earth, according to the Hubble law.Earth, according to the Hubble law.

To be seen from Earth, a quasar must be very luminous, To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the quasars indicate that they cannot be much larger than the diameter of our solar system.diameter of our solar system.

Quasars and Other Active Galaxies An active spiral galaxy with a bright, starlike nucleus and An active spiral galaxy with a bright, starlike nucleus and

strong emission lines in its spectrum is categorized as a strong emission lines in its spectrum is categorized as a Seyfert galaxy.Seyfert galaxy.

An active elliptical galaxy is called radio galaxy. It has a An active elliptical galaxy is called radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream bright nucleus and a pair of radio-bright jets that stream out in opposite directions.out in opposite directions.

BL Lacertae (BL Lac) objects (some of which are called BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively blazars) have bright nuclei whose cores show relatively rapid variations in luminosity.rapid variations in luminosity.

Double-radio sources contain active galactic nuclei Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.particles that emerge from an active galaxy.

Supermassive Central Engines Many galaxies contain huge concentrations of matter at Many galaxies contain huge concentrations of matter at

their centers.their centers. Some matter that spirals in toward a supermassive black Some matter that spirals in toward a supermassive black

hole is squeezed into two oppositely directed beams that hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space.carry particles and energy into intergalactic space.

The energy sources from quasars, Seyfert galaxies, BL The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of surround supermassive black holes at the centers of galaxies.galaxies.

Key TermsKey Termsactive galactic nuclei (AGN)active galaxyblazarBL Lacertae (BL Lac) objectdouble-radio sourceEinstein crossEinstein ringhead-tail sourcepeculiar galaxy (pec)quasar (quasi-stellar radio source)quasi-stellar object (QSO)radio galaxyradio lobeSeyfert galaxy

WHAT DID YOU THINK?What does “quasar” stand for? Quasi-stellar radio source.

WHAT DID YOU THINK?What do quasars look like? They look like stars, but they emit much

more energy than any star.

WHAT DID YOU THINK?Where do quasars get their energy? A quasar is thought to be powered by a

supermassive black hole with millions or billions of solar masses at the center of a galaxy.