Neil F Comins William J Kaufmann III Discovering

Neil F. Comins • William J. Kaufmann III Discovering the Universe Tenth Edition CHAPTER 17 Quasars and Other Active Galaxies

In this chapter, you will discover… l l l 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 (3 C 405) (a) Radio image 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 light-years 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 light-years from Earth. Because Cygnus A is one of the brightest radio sources in the sky, this remote galaxy’s energy output must be enormous. (b) An X-ray image of Cygnus A taken by the Chandra X-ray Observatory showing that some of the jet particles from the central source have spread throughout the galaxy and, striking other gas and dust, caused this debris to heat and emit X rays.

Active Galaxies and Quasars l l l An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects. A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law. To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system.

Quasar 3 C 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 light-years away.

Quasar 3 C 273 This combined X-ray and infrared view shows the starlike object associated with the radio source 3 C 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 light-years from Earth.

Spectra of 3 C 273 The visible and infrared spectra of 3 C 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 3 C 279 This graph shows variations in the brightness of the quasar 3 C 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.

Other Active Galaxies l l An active spiral galaxy with a bright, starlike nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy. An active elliptical galaxy is called radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions. BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity. Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.

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 light-years 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 3 C 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 light-years and are 300 million light-years 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 M 32 This small galaxy is a satellite of M 31, a portion of which is seen at the left of this wide-angle photograph. Both galaxies are roughly 2. 5 million light-years from Earth. (Inset) High-resolution image from the Hubble Space Telescope that shows the center of M 32. Note the concentration of stars at the nucleus of the galaxy. The nucleus is only 175 light-years across.

Giant Elliptical Galaxy M 87 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 M 87 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. M 87’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 (M 104) 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 doubleradio source located in the Virgo cluster, about 100 million light-years 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 light-years 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.

Supermassive Central Engines l l l Many galaxies contain huge concentrations of matter at their centers. Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space. The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.

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 light-years from Earth, is seen as four separate images that surround a galaxy that is only 400 million lightyears 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 Ideas l The development of radio astronomy in the late 1940 s led to the discovery of very powerful and extremely distant energy sources.

Quasars and Other Active Galaxies l l l An active galaxy is an extremely luminous galaxy that has one or more unusual features: an unusually bright, starlike nucleus; strong emission lines in its spectrum; rapid variations in luminosity; and jets or beams of radiation that emanate from its core. Active galaxies include quasars, Seyfert galaxies, radio galaxies, double-radio sources, and BL Lacertae objects. A quasar, or quasi-stellar radio source, is an object that looks like a star but has a huge redshift. This redshift corresponds to a distance of billions of light-years from Earth, according to the Hubble law. To be seen from Earth, a quasar must be very luminous, typically about 100 times brighter than an ordinary galaxy. Relatively rapid fluctuations in the brightness levels of some quasars indicate that they cannot be much larger than the diameter of our solar system.

Quasars and Other Active Galaxies l l An active spiral galaxy with a bright, starlike nucleus and strong emission lines in its spectrum is categorized as a Seyfert galaxy. An active elliptical galaxy is called radio galaxy. It has a bright nucleus and a pair of radio-bright jets that stream out in opposite directions. BL Lacertae (BL Lac) objects (some of which are called blazars) have bright nuclei whose cores show relatively rapid variations in luminosity. Double-radio sources contain active galactic nuclei located between two characteristic radio lobes. A head-tail radio source shows evidence of jets of high-speed particles that emerge from an active galaxy.

Supermassive Central Engines l l l Many galaxies contain huge concentrations of matter at their centers. Some matter that spirals in toward a supermassive black hole is squeezed into two oppositely directed beams that carry particles and energy into intergalactic space. The energy sources from quasars, Seyfert galaxies, BL Lac objects, radio galaxies, and double-radio sources are probably matter ejected from the accretion disks that surround supermassive black holes at the centers of galaxies.

Key Terms active galactic nuclei (AGN) active galaxy blazar BL Lacertae (BL Lac) object double-radio source Einstein cross Einstein ring head-tail source peculiar galaxy (pec) quasar (quasi-stellar radio source) quasi-stellar object (QSO) radio galaxy radio lobe Seyfert galaxy