Active galaxies are much more luminous than normal galaxies and have spectra that are nonstellar in nature, indicating that the energy they emit is not simply the accumulated light of many stars. Most of the energy from active galaxies is emitted in the radio and infrared parts of the electromagnetic spectrum. The fraction of observed galaxies that display activity increases with increasing distance from the Milky Way, indicating that galaxies were generally more active in the past than they are today.
A Seyfert galaxy looks like a normal spiral except that it has an extremely bright central galactic nucleus, whose luminosity can in many cases exceed that of the rest of the galaxy. Spectral lines from Seyfert nuclei are very broad, indicating rapid internal motion. In addition, Seyfert luminosities can vary by large amounts in fractions of a year, implying that the region emitting most of the radiation is much less than 1 light year across.
Radio galaxies are active galaxies that emit most of their energy in the radio part of the spectrum. They are generally comparable to the Seyferts in total energy output. Unlike Seyferts, they are usually associated with elliptical galaxies. In a core—halo radio galaxy, the energy is emitted from a small central nucleus, as in a Seyfert. In a lobe radio galaxy, the energy comes from enormous radio lobes that dwarf the visible galaxy and lie far outside it. The lobes are usually symmetrically placed with respect to the center of the visible galaxy.
Many active galaxies have high-speed jets of matter shooting out from their central nuclei. In lobe radio galaxies, astronomers believe that the jets transport energy from the nucleus, where it is generated, to the lobes, where it is radiated into space. The jets often appear to be made up of distinct "blobs" of gas, suggesting that the process generating the energy is intermittent.
The generally accepted explanation for the observed properties of active galaxies is that the energy is generated by accretion of galactic gas onto a supermassive (billion—solar mass) black hole lying at the center of the nucleus. As material spirals down toward the hole it heats up and releases enormous amounts of energy. The small size of the accretion disk explains the compact extent of the emitting region, and the high-speed orbits of gas in the hole's intense gravity account for the rapid motion observed. Typical active-galaxy luminosities require the consumption of about one solar mass of material every few years.
Some of the infalling matter is blasted out into space, producing magnetized jets that create and feed the extended radio lobes. Charged particles spiraling around the magnetic field lines produce synchrotron radiation whose spectrum is consistent with the nonstellar radiation observed in radio galaxies and Seyferts.
Quasars, or quasi-stellar objects, were first discovered as starlike radio sources with unknown broad spectral lines. In the early 1960s, astronomers realized that the unfamiliar lines are actually those of familiar elements, but redshifted to wavelengths much longer than normal. Even the closest quasars lie at great distances from us. They are the most luminous objects known.
Quasars exhibit the same basic features as active galaxies, and astronomers believe that their power source is also basically the same. This source—a black hole—must be scaled-up in a quasar to a consumption rate of many stars per year. If that is the case, the brightest quasars consume so much fuel that their energy-emitting lifetimes must be relatively short. Quasars probably represent a brief phase of violent activity early in the life of a galaxy.
Some quasars have been observed to have double or multiple images. These result from gravitational lensing, in which the gravitational field of a foreground galaxy or galaxy cluster bends and focuses the light from the more distant quasar. Analysis of this bending provides a means of determining the masses of galaxy clusters—including the dark matter—far beyond the optical images of the galaxies themselves.
Quasars, active galaxies, and normal galaxies may represent an evolutionary sequence. When galaxies began to form, conditions might have been suitable for the formation of large black holes at their centers. If a lot of gas was available at those early times, a highly luminous quasar could have been the result. As the fuel supply diminished, the quasar dimmed, and the galaxy in which it was embedded became visible as an active galaxy. At even later times the fuel supply declined to the point at which the nucleus became virtually inactive, and a normal galaxy was all that remained.
1. Active galaxies can emit thousands of times more energy than our own Galaxy. (Hint)
2. The "extra" radiation emitted by active galaxies is due to the tremendous number of stars they contain. (Hint)
3. Active galaxies emit most radiation at optical wavelengths. (Hint)
4. The spectrum of an active galaxy is not well described by a blackbody curve. (Hint)
5. Most core—halo radio galaxies are spirals. (Hint)
6. The size of a billion—solar mass black hole is about 20 A.U. (Hint)
7. Nearby active galaxies are most likely the result of interactions between galaxies. (Hint)
8. A redshift greater than 1 implies a recessional velocity greater than the speed of light. (Hint)
9. All quasars are far away. (Hint)
10. Quasar luminosities are so large that astronomers have no idea what process can account for them. (Hint)
11. Other than a small amount of visible light, quasars emit all their radiation at radio wavelengths. (Hint)
12. Many nearby normal galaxies may once have been quasars. (Hint)
13. Quasars emit much more energy than normal galaxies. (Hint)
14. The quasar stage of a galaxy ends because the central black hole uses up all the matter around it. (Hint)
15. Astronomers still have no evidence that quasars are found in the hearts of young galaxies. (Hint)
1. Active galaxies are more common at _____ distances. (Hint)
2. Active galaxies generally emit more radiation at _____ wavelengths. (Hint)
3. Seyfert galaxies appear like normal spirals, but with a very bright galactic _____. (Hint)
4. In a core—halo radio galaxy, most of the radio radiation is emitted from the _____ . (Hint)
5. Lobe radio galaxies emit radio radiation from regions that are typically much _____ in size than the optical galaxy. (Hint)
6. Radio lobes are always found aligned with the _____ of the optical galaxy. (Hint)
7. For all types of active galaxies the original source of the tremendous energy emitted is the galactic _____ . (Hint)
8. The energy source of an active galaxy is unusual in that there is a large amount of energy emitted from a region less than _____ in diameter. (Give size and unit.) (Hint)
9. The mass of the black hole responsible for energy production in the active galaxy M87 is thought to be approximately _____ solar masses. (Hint)
10. The amount of mass that must be consumed by a supermassive black hole to provide the energy for an active galaxy is about _____ per _____ . (Hint)
11. In visible light, quasars have a _____ appearance. (Hint)
12. The distance to a quasar in light years is not simply equal to the time in years since the quasar emitted the light we see, because of the _____ of the universe. (Hint)
13. Quasar spectra were understood when it was discovered that their radiation is _____ by an unexpectedly large amount. (Hint)
14. The fact that a typical quasar would consume an entire galaxy's worth of mass in 10 billion years suggests that quasar lifetimes are relatively _____ . (Hint)
15. The image of a distant quasar can be split into several images because of gravitational lensing by a foreground _____ along the line of sight. (Hint)
1. Name two basic differences between normal galaxies and active galaxies. (Hint)
2. Describe some of the basic properties of Seyfert galaxies. (Hint)
3. What distinguishes a core—halo radio galaxy from a lobe radio galaxy? (Hint)
4. What is the evidence that the radio lobes of some active galaxies consist of material ejected from the galaxy's center? (Hint)
5. What conditions result in a head—tail radio galaxy? (Hint)
6. Briefly describe the leading model for the central engine of an active galaxy. (Hint)
7. How is the process of synchrotron emission related to observations of active galaxies? (Hint)
8. What was it about the spectra of quasars that was so unexpected and surprising? (Hint)
9. Why do astronomers prefer to speak in terms of redshifts rather than distances to faraway objects? (Hint)
10. How do we know that quasars are extremely luminous? (Hint)
11. How are the spectra of distant quasars used to probe the space between us and them? (Hint)
12. How are BL Lac objects related to other active galaxies? (Hint)
13. What evidence do we have that quasars represent an early stage of galaxy evolution? (Hint)
14. What happened to the energy source at the center of a quasar? (Hint)
15. What are the arguments for and against the idea that quasars may in fact be nearby and not at cosmological distances? (Hint)
1. A Seyfert galaxy is observed to have broadened emission lines indicating an orbital speed of 1000 km/s at a distance of 1 pc from its center. Assuming circular orbits, use Kepler's laws (see Section 23.6) to estimate the mass within this 1-pc radius.
2. Calculate the energy flux—that is, the energy received per unit area per unit time—that would be observed at Earth from a 1037-W Seyfert nucleus located at the Galactic center, neglecting the effects of interstellar extinction. Using the data presented in Appendix 3, Table 4, compare this energy flux with that received from Sirius A, the brightest star in the night sky. From what you know about active galaxy energy emission, is it reasonable to ignore interstellar extinction? (Hint)
3. Centaurus A—from one radio lobe to the other—spans about 1 Mpc. It lies at a distance of 4 Mpc from Earth. What is the angular size of Centaurus A? Compare this value with the angular diameter of the Moon. (Hint)
4. Assuming a jet speed of 0.75 c, calculate the time taken for material in Cygnus A's jet to cover the 500 kpc between the galaxy's nucleus and its radio-emitting lobes.
5. Assuming the same efficiency as indicated in the text, calculate the amount of energy an active galaxy would generate if it consumed one Earth mass of material every day. Compare this value with the luminosity of the Sun. (Hint)
6. Based on the data presented in the text, calculate the orbital speed of material orbiting at a distance of 0.5 pc from the center of M87. (Hint)
7. A certain quasar has a redshift of 0.17 and the same apparent brightness as the Sun would have if it were placed at a distance of 500 pc. Assuming a Hubble constant of 65 km/s/Mpc (and neglecting the complications described in More Precisely 25-1), calculate the quasar's luminosity. (Hint)
8. Assuming an energy-generation efficiency (that is, the ratio of energy released to total mass—energy available) of 10 percent, calculate how long a 1041-W quasar can shine if a total of 1010 solar masses of fuel is available. (Hint)
9. Light from a distant quasar passes 10 Mpc from the center of an intervening galaxy cluster before being deflected to a detector on Earth. If Earth, the cluster, and the quasar are all aligned, the quasar is 750 Mpc away, and the cluster lies midway between Earth and the quasar, calculate the angle through which the galaxy cluster bends the quasar's light. (Hint)
10. Light from a distant star is deflected by 1.75" as it grazes the Sun. (More Precisely 22-2) Given that the deflection angle is proportional to the mass of the gravitating body and is inversely proportional to the minimum distance between the light ray and the body, calculate the mass of the galaxy cluster in the previous question.
Here are three observing projects that are increasingly challenging.
1. In the previous chapter you were given directions for finding the Virgo Cluster of galaxies. M87, in the central part of this cluster, is the nearest core—halo radio galaxy. M87 has coordinates RA = 12h 30.8m, dec = +12 ° 24'. At magnitude 8.6 it should not be difficult to find in an 8-inch telescope. Its distance is roughly 20 Mpc. Describe its nucleus; compare what you see with other nearby ellipticals in the Virgo Cluster.
2. NGC 4151 is the brightest Seyfert galaxy. Its coordinates are RA = 12h 10.5m, dec = +39 ° 24', and it can be found below the Big Dipper in Canes Venatici. At magnitude 10—12 (it is variable), it should be visible in an 8-inch telescope, but it will be challenging to find. Its distance is 13.5 Mpc. As in the case of M87, describe its nucleus and compare with what you have seen for other galaxies.
3. 3C 273 is the nearest and brightest quasar. However, that does not mean it will be easy to find and see! Its coordinates are RA = 12h 29.2m, dec = +2 ° 03'. It is located in the southern part of the Virgo Cluster but is not associated with it. At magnitude 12—13 (again, it is variable), it may require a 10- or 12-inch telescope to see, but try it first with an 8-inch. It should appear as a very faint star. The significance of seeing this object is that it is 640 Mpc distant. The light you are seeing left this object over 2 billion years ago! 3C 273 is the most distant object observable with a small telescope. If you can find the three objects listed here, you have started to become an accomplished observer!
