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Quasars and Active Galaxies

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1 Quasars and Active Galaxies
Chapter 17 Quasars and Active Galaxies

2 Introduction Quasars, and the way in which they became understood, have been one of the most exciting stories of the last forty years of astronomy. First noticed as seemingly peculiar stars, quasars turned out to be some of the most powerful objects in the Universe, and represent violent forces at work. We think that giant black holes, millions or even billions of times the Sun’s mass, lurk at their centers. A quasar shines so brightly because its black hole is pulling in the surrounding gas, causing the gas to glow vividly before being swallowed.

3 Introduction Our interest in quasars is further piqued because many of them are among the most distant objects we have ever detected in the Universe. Since, as we look out, we are seeing light that was emitted farther and farther back in time, observing quasars is like using a time machine that enables us to see the Universe when it was very young. We find that quasars were an early stage in the evolution of large galaxies. As time passed, gas in the central regions was used up, and the quasars faded, becoming less active. Indeed, we see examples of active galaxies relatively near us, and in some of these the presence of a massive black hole has been all but proven.

4 17.1 Active Galactic Nuclei
The central regions of normal galaxies tend to have large concentrations of stars. For example, at infrared wavelengths we can see through our Milky Way Galaxy’s dust and penetrate to the center. When we do so, we see that the bulge of our Galaxy becomes more densely packed with stars as we look closer to the nucleus. With so many stars confined there in a small volume, the nucleus itself is relatively bright. This concentrated brightness appears to be a natural consequence of galaxy formation; gas settles in the central region due to gravity, and subsequently forms stars.

5 17.1 Active Galactic Nuclei
In a minority of galaxies, however, the nucleus is far brighter than usual at optical and infrared wavelengths, when compared with other galaxies at the same distance (see figure). Indeed, when we compute the optical luminosity (power) of the nucleus from its apparent brightness and distance, we have trouble explaining the result in terms of normal stars: It is difficult to cram so many stars into so small a volume. Such nuclei are also often very powerful at other wavelengths, such as x-rays, ultraviolet, and radio. These galaxies are called “active” to distinguish them from normal galaxies, and their luminous centers are known as active galactic nuclei. Clusters of ordinary stars rarely, if ever, produce so much x-ray and radio radiation.

6 17.1 Active Galactic Nuclei
Active galaxies that are extraordinarily bright at radio wavelengths often exhibit two enormous regions (known as “lobes”) of radio emission far from the nucleus, up to a million light-years away. The first “radio galaxy” of this type to be detected, Cygnus A (see figure), emits about a million times more energy in the radio region of the spectrum than does the Milky Way Galaxy.

7 17.1 Active Galactic Nuclei
Close scrutiny of such radio galaxies sometimes reveals two long, narrow, oppositely directed “jets” joining their nuclei and lobes (see figure, left). The jets are thought to consist of charged particles moving at close to the speed of light and emitting radio waves. Sometimes radio galaxies appear rather peculiar when we look at visible wavelengths, and the jet is visible in x-rays, as in the case of Centaurus A (see figure, middle and right).

8 17.1 Active Galactic Nuclei
Optical spectra of the active nuclei often show the presence of gas moving with speeds in excess of 10,000 km /sec, far higher than in normal galactic nuclei. We measure these speeds from the spectra, which have broad emission lines (see figure). Atoms that are moving toward us emit photons that are then blueshifted, while those that are moving away from us emit photons that are then redshifted, thereby broadening the line by the Doppler effect. Early in the 20th century, Carl Seyfert was the first to systematically study galaxies with unusually bright optical nuclei and peculiar spectra, and in his honor they are often called “Seyfert galaxies.”

9 17.1 Active Galactic Nuclei
Although spectra show that gas has very high speeds in supernovae as well, the overall observed properties of active galactic nuclei generally differ a lot from those of supernovae, making it unlikely that stellar explosions are responsible for such nuclei. Indeed, it is difficult to see how stars of any kind could produce the unusual activity. However, for many years active galaxies were largely ignored, and the nature of their central powerhouse was unknown.

10 17.2 Quasars: Denizens of the Distant Past
Interest in active galactic nuclei was renewed with the discovery of quasars (shortened form of “quasi-stellar radio sources”), the recognition that quasars are similar to active galactic nuclei, and the realization that both kinds of objects must be powered by a strange process that is unrelated to stars.

11 17.2a The Discovery of Quasars
In the late 1950s, as radio astronomy developed, astronomers found that some celestial objects emit strongly at radio wavelengths. Catalogues of them were compiled, largely at Cambridge University in England, where the method of pinpointing radio sources was developed. For example, the third such Cambridge catalogue is known as “3C,” and objects in it are given numerical designations like 3C 48. Although the precise locations of these objects were difficult to determine with single-dish radio telescopes (since they had poor angular resolution), sometimes within the fuzzy radio image there was an obvious probable optical counterpart such as a supernova remnant or a very peculiar galaxy. More often, there seemed to be only a bunch of stars in the field—yet which of them might be special could not be identified, and in any case there was no known mechanism by which stars could produce so much radio radiation.

12 17.2a The Discovery of Quasars
Special techniques were developed to pinpoint the source of the radio waves in a few instances. Specifically, the occultation (hiding) of 3C 273 by the Moon provided an unambiguous identification with an optical star-like object. When the radio source winked out, we knew that the Moon had just covered it while moving slowly across the background of stars. Thus, we knew that 3C 273 was somewhere on a curved line marking the front edge of the Moon. When the radio source reappeared, we knew that the Moon had just uncovered it, so it was somewhere on a curved line marking the Moon’s trailing edge at that time. These two curves intersected at two points, and hence 3C 273 must be at one of those points. Though one point seemed to show nothing at all, the other point was coincident with a bluish, star-like object about 600 times fainter than the naked-eye limit.

13 17.2a The Discovery of Quasars
When the positions of other radio sources were determined accurately enough, it was found that they, too, often coincided with faint, bluish-looking stars (see figures). These objects were dubbed “quasi-stellar radio sources,” or “quasars” for short. Optically they looked like stars, but stars were known to be faint at radio wavelengths, so they had to be something else.

14 17.2a The Discovery of Quasars
Object 3C 273 seemed to be especially interesting: A jet-like feature stuck out from it, visible at optical wavelengths (see figures, left and middle) and radio wavelengths (see figure, right).

15 17.2b Puzzling Spectra Several astronomers, including Maarten Schmidt of Caltech, photographed the optical spectra of some quasars with the 5-m (200-inch) Hale telescope at the Palomar Observatory. These spectra turned out to be bizarre, unlike the spectra of normal stars. They showed bright, broad emission lines, at wavelengths that did not correspond to lines emitted by laboratory gases at rest. Moreover, different quasars had emission lines at different wavelengths.

16 17.2b Puzzling Spectra Schmidt made a breakthrough in 1963, when he noticed that several of the emission lines visible in the spectrum of 3C 273 had the pattern of hydrogen—a series of lines with spacing getting closer together toward shorter wavelengths—though not at the normal hydrogen wavelengths (see figure). He realized that he could simply be observing hot hydrogen gas (with some contaminants to produce the other lines) that was Doppler shifted. The required redshift would be huge, about 16% (that is, z = /0 = 0.16), corresponding to 16% of the speed of light (since z  v/c, or v  cz, valid for z less than about 0.2).

17 17.2b Puzzling Spectra This possibility had not been recognized because nobody expected stars to have such large redshifts. Also, the spectral range then available to astronomers, who took spectra on photographic film, did not include the bright Balmer-a line of hydrogen (that is, Ha), which is normally found at 6563 Å but was shifted over to 7600 Å in 3C 273. As soon as Schmidt announced his insight, the spectra of other quasars were interpreted in the same manner. Indeed, one of Schmidt’s Caltech colleagues, Jesse Greenstein, immediately realized that the spectrum of quasar 3C 48 looked like that of hydrogen redshifted by an even more astounding amount: 37%.

18 17.2b Puzzling Spectra Subsequent searches for blue stars revealed a class of “radio-quiet” quasars—their optical spectra are similar to those of quasars, yet their radio emission is weak or absent. These are often called QSOs (“quasi-stellar objects”), and they are about ten times more numerous than “radio-loud” quasars. Consistent with the common practice of using the terms interchangeably, here we will simply use “quasar” to mean either the radio-loud or radio-quiet variety, unless we explicitly mention the radio properties.

19 17.2c The Nature of the Redshift
How were the high redshifts produced? The Doppler effect is the most obvious possibility. But it seemed implausible that quasars were discrete objects ejected like cannonballs from the center of the Milky Way Galaxy (see figure); their speeds were very high, and no good ejection mechanism was known. Also, we would then expect some quasars to move slightly across the sky relative to the stars, since the Sun is not at the center of the Galaxy, but such motions were not seen. Even if these problems could be overcome, we would then have to conclude that only the Milky Way Galaxy (and not other galaxies) ejects quasars—otherwise, we would have seen “quasars” with blueshifted spectra, corresponding to those objects emitted toward us from other galaxies.

20 17.2c The Nature of the Redshift
Similarly, there were solid arguments against a “gravitational redshift” interpretation (recall our discussion of this effect in Chapter 14), one in which a very strong gravitational field causes the emitted light to lose energy on its way out. This possibility was completely ruled out later, as we shall see. If, instead, the redshifts of quasars are due to the expansion of the Universe (as is the case for normal galaxies), then quasars are receding with enormous speeds and hence must be very distant. Quasar 3C 273, for example, has z = 0.16, so v  0.16c  48,000 km /sec. According to Hubble’s law, v = H0d, so if H0 = 71 km /sec/Mpc, then d = v/H0 (48,000 km /sec)/(71 km /sec/Mpc)  680 Mpc  2.2 billion light-years, a sixth of the way back to the origin of the Universe!

21 17.2c The Nature of the Redshift
A few galaxies with comparably high redshifts (and therefore distances) had previously been found, but they were fainter than 3C 273 by a factor of 10 to 1000, and they looked fuzzy (extended) rather than star-like. Quasar 3C 273 turns out to be one of the closest quasars. Other quasars found during the 1960s had redshifts of 0.2 to 1, and hence are billions of light-years away. Note that redshifts greater than 1 do not necessarily imply speeds larger than the speed of light, because the approximation z  v/c is reasonably accurate only when v/c is less than about 0.2. For higher speeds we may instead use the relativistic Doppler formula to calculate the nominal speed. However, even calling it a Doppler effect is misleading and, strictly speaking, incorrect: The redshift is produced by the expansion of space, not by motion through space, and the concept of “speed” then takes on a somewhat different meaning.

22 17.2c The Nature of the Redshift
Similarly, as discussed in Chapter 16 for galaxies, it makes more sense to refer to the “lookback time” of a given quasar (the time it has taken for light to reach us) than to its distance: v =H0d is inaccurate at large redshifts for a number of reasons. The lookback time formula is complicated, but some representative values are given in Table 16 –1.

23 17.2c The Nature of the Redshift
A few dozen quasars with redshifts exceeding 6 have been discovered (see figures). The highest redshift known for a quasar as of late-2005 is z = 6.4, which means that a feature whose laboratory (rest) wavelength is 1000 Å is observed to be at a wavelength 640 per cent larger, or 1000 Å Å = 7400 Å. (Recall that z = /0 .) The corresponding nominal speed of recession is about 0.96c, and the quasar’s lookback time is roughly 12.8 billion years (in a model where the Universe is 13.7 billion years old). We see the quasar as it was when the Universe was about 6.6 per cent of its current age!

24 17.2c The Nature of the Redshift
How do we detect quasars? Many of them are found by looking for faint objects with unusual colors—that is, the relative amounts of blue, green, and red light differ from those of normal stars. Low-redshift quasars tend to look bluish, because they emit more blue light than typical stars. But the light from high-redshift quasars is shifted so much toward longer wavelengths that these objects appear very red, especially since intergalactic clouds of gas absorb much of the blue light. Quasars have also been found in maps of the sky made with x-ray satellites, and of course with ground-based radio surveys. After finding a quasar candidate with any technique, however, it is necessary to take a spectrum in order to verify that it is really a quasar and to measure its redshift. As we have seen, the spectra of quasars are quite distinctive, and are rarely confused with other types of objects. Tens of thousands of quasars are now known, and more are being discovered very rapidly, especially by the Sloan Digital Sky Survey.

25 17.3 How Are Quasars Powered?
Astronomers who conducted early studies of quasars (mid-1960s) recognized that quasars are very powerful, 10 to 1000 times brighter than a galaxy at the same redshift. But while galaxies looked extended in photographs, quasars with redshifts comparable to those of galaxies appeared to be mere points of light, like stars. Their diameters were therefore smaller than those of galaxies, so their energy-production efficiency must have been higher, already making them unusual and intriguing.

26 17.3a A Big Punch from a Tiny Volume
However, these astronomers were in for a big surprise when they figured out just how compact quasars really are. They noticed that some quasars vary in apparent brightness over short timescales—days, weeks, months, or years (see figure). This implies that the emitting region is probably smaller than a few light-days, light-weeks, light-months, or light-years in diameter, in all cases a far cry from the tens of thousands of light-years for a typical galaxy.

27 17.3a A Big Punch from a Tiny Volume
The argument goes as follows: Suppose we have a glowing, spherical, opaque object that is 1 light-month in radius (see figure). Even if all parts of the object brightened instantaneously by an intrinsic factor of two, an outside observer would see the object brighten gradually over a timescale of 1 month, because light from the near side of the object would reach the observer 1 month earlier than light from the edge. Thus, the timescale of an observed variation sets an upper limit (that is, a maximum value) to the size of the emitting region: The actual size must be smaller than this upper limit.

28 17.3a A Big Punch from a Tiny Volume
Although this conclusion can be violated under certain conditions (such as when different regions of the object brighten in response to light reaching them from other regions, creating a “domino effect”), such models generally seem unnatural. Proper use of Einstein’s special theory of relativity (in case the light-emitting material is moving very fast) can also change the derived upper limit to some extent, but the basic conclusion still holds: Quasars are very small, yet they release tremendous amounts of energy. For example, a quasar only 1 light-month across can be 100 times more powerful than an entire galaxy of stars 100,000 light-years in diameter!

29 17.3b What Is the Energy Source?
The nature of the prodigious (yet physically small) power source of quasars was initially a mystery. How does such a small region give off so much energy? After all, we don’t expect huge explosions from tiny firecrackers. There was some indication that these objects might be related to active galactic nuclei: They have similar optical spectra and are bright at radio wavelengths. So, perhaps the same mechanism might be used to explain the unusual properties of both kinds of objects. In fact, maybe active galactic nuclei are just low-power versions of quasars! If so, quasars should be located in the centers of galaxies. Later we will see that this is indeed the case.

30 17.3b What Is the Energy Source?
The fact that the incredible power source of quasars is very small immediately rules out some possibilities. Such a process of elimination is often useful in astronomy; recall, for instance, how we deduced that pulsars are rapidly spinning neutron stars. It turns out that for quasars, chemical energy is woefully inadequate: They cannot be wood on fire, or even chemical explosives, because the most powerful of these is insufficient to produce so much energy within such a small volume. Even nuclear energy, which works well for stars, is not possible for the most powerful quasars. They cannot be radiation from otherwise-unknown supermassive stars or chains of supernovae going off almost all the time, or other more exotic stellar processes, because once again the efficiency of nuclear energy production is not high enough. To produce that much nuclear energy, a larger volume of material would be needed.

31 17.3b What Is the Energy Source?
The annihilation of matter and antimatter is energetically feasible, since it is 100% efficient. That is, all of the mass in a matter–antimatter collision gets turned into photons (radiation), and in principle a very small volume can therefore be tremendously powerful. However, the observed properties of quasars do not support this hypothesis. Specifically, matter–antimatter collisions tend to emit excess amounts of radiation at certain wavelengths, and this is not the case for quasars.

32 17.3b What Is the Energy Source?
The release of gravitational energy, on the other hand, can in some cases be very efficient, and seemed most promising to several theorists studying quasars in the mid-1960s. We have already discussed how the gravitational contraction of a ball of gas (a protostar), for example, both heats the gas and radiates energy. But to produce the prodigious power of quasars, a very strong gravitational field is needed. The conclusion was that a quasar is a supermassive black hole, perhaps 10 million to a billion times the mass of the Sun, in the process of swallowing (“accreting”) gas. The black hole is in the center of a galaxy. The rate at which matter can be swallowed, and hence the power of the quasar, is proportional to the mass of the blackhole, but it is typically a few solar masses per year. Although the Schwarzschild radius of, say, a 50 million solar-mass black hole is 150 million km, this is just 1 A.U. (i.e., 8.3 light-minutes, the distance between the Earth and the Sun), and hence is minuscule compared with the radius of a galaxy (many thousands of light-years).

33 17.3c Accretion Disks and Jets
The matter generally swirls around the black hole, forming a rotating disk called an accretion disk (see figure), a few hundred to a thousand times larger than the Schwarzschild radius of the black hole (and hence up to a few light-days to a lightweek in size). As the matter falls toward the black hole, it gains speed (kinetic energy) at the expense of its gravitational energy, just as a ball falling toward the ground accelerates. Compression of the gas particles in the accretion disk to a small volume, and the resulting friction between the particles, causes them to heat up; thus, they emit electromagnetic radiation, thereby converting part of their kinetic energy into light.

34 17.3c Accretion Disks and Jets
Note that energy is radiated before the matter is swallowed by the black hole—nothing escapes from within the black hole itself. This process can convert the equivalent of about 10% of the rest-mass energy of matter into radiation, more than 10 times more efficiently than nuclear energy. (Recall from Chapter 11 that the fusion of hydrogen to helium converts only 0.7% of the mass into energy.) A spinning, very massive black hole is also consistent with the well-focused “jets of matter and radiation that emerge from some quasars, typically reaching distances of a few hundred thousand light-years.

35 17.3c Accretion Disks and Jets
Again, no material actually comes from within the black hole; instead, its origin is the accretion disk. The charged particles in the jets are believed to shoot out in a direction perpendicular to the accretion disk, along the black hole’s axis of rotation (see figure, top). They emit radiation as they are accelerated. In addition to the radio radiation, high-energy photons such as x-rays can also be produced (see figure, bottom). The impressive focusing might be provided by a magnetic field, as in the case of pulsars, or by the central cavity in the disk.

36 17.3c Accretion Disks and Jets
Recall that jets are also seen in some types of active galaxies, which appear to be closely related to quasars (see figure). As discussed in more detail later in this chapter, we know that the particles move with very high speeds because a jet can sometimes appear to travel faster than the speed of light—an effect that occurs only when an object travels nearly along our line of sight, nearly at the speed of light.

37 17.3c Accretion Disks and Jets
Recently, indirect evidence for accretion disks surrounding a central, supermassive black hole has been found in several active galaxies from observations with various x-ray telescopes (Japan’s ASCA, the European Space Agency’s XMM-Newton Mission, and NASA’s Chandra X-ray Observatory). The specific shape of emission lines from highly ionized iron atoms that must reside very close to the galaxy center resembles that expected if the light is coming from a rotating accretion disk. Moreover, these lines exhibit a “gravitational redshift”—they appear at a somewhat longer wavelength than expected from the recession speed of the galaxy, because the photons lose some energy (and hence get shifted to longer wavelengths) as they climb out of the strong gravitational field near the black hole (see Chapter 14).

38 17.3c Accretion Disks and Jets
Similar emission lines have been seen in x-ray binary systems in which the compact object is likely to be a black hole (see the discussion in Section 14.7). Such lines, in both active galaxies and x-ray binaries, are now being analyzed in detail to detect and study predicted relativistic effects such as the strong bending of light and the “dragging” of space–time around a rotating black hole.

39 17.4 What Are Quasars? The idea that quasars are energetic phenomena at the centers of galaxies is now strongly supported by observational evidence. First of all, the observed properties of quasars and active galactic nuclei are strikingly similar. In some cases, the active nucleus of a galaxy is so bright that the rest of the galaxy is difficult to detect because of contrast problems, making the object look like a quasar (see figures). This is especially true if the galaxy is very distant: We see the bright nucleus as a point-like object, while the spatially extended outer parts (known as “fuzz” in this context) are hard to detect because of their faintness and because of blending with the nucleus.

40 17.4 What Are Quasars? In the 1970s, a statistical test was carried out with quasars. A selection of quasars, sorted by redshift, was carefully examined. Faint fuzz (presumably a galaxy) was discovered around most of the quasars with the smallest redshifts (the nearest ones), a few of the quasars with intermediate redshifts, and none of the quasars with the largest redshifts (the most distant ones). Astronomers concluded that the extended light was too faint and too close to the nucleus in the distant quasars, as expected. In the 1980s, optical spectra of the fuzz in a few nearby quasars revealed absorption lines due to stars, but the vast majority of objects were too faint for such observations. In any case, the data strongly suggested that quasars could indeed be extreme examples of galaxies with bright nuclei.

41 17.4 What Are Quasars? More recently, images obtained with the Hubble Space Telescope demonstrate conclusively that quasars live in galaxies, almost always at their centers. With a clear view of the skies above the Earth’s atmosphere, and equipped with CCDs, the Hubble Space Telescope easily separates the extended galaxy light from the point-like quasar itself at low redshifts. In some cases the galaxy is obvious (see figures, top and middle), but in others it is barely visible, and special techniques are used to reveal it; recall, for example, 3C 273 in the figures. Further solidifying the association of quasars with galaxies, recent ground-based optical spectra of some relatively nearby quasars (z 0.2–0.3) show unambiguous stellar absorption lines at the same redshift as that given by the quasar emission lines (see figure, bottom).

42 17.4 What Are Quasars? Quasars exist almost exclusively at high redshifts and hence large distances. The peak of the distribution is at z  2 (see figures), though new studies at x-ray wavelengths suggest that it might be at an even higher redshift.

43 17.4 What Are Quasars? With lookback times of about 10 billion years, quasars must be denizens of the young Universe. What happened to them? Quasars probably faded with time, as the central black hole gobbled up most of the surrounding gas; the quasar shines only while it is pulling in material. Thus, some of the nearby active and normal galaxies may have been luminous quasars in the distant past, but now exhibit much less activity because of a slower accretion rate. Perhaps even the nucleus of the Milky Way Galaxy, which is only slightly active, was more powerful in the past, when the putative black hole had plenty of material to accrete. Of course, many of the weakly active galaxies we see nearby were probably never luminous enough to be genuine quasars. Either their central black hole wasn’t sufficiently massive to pull in much material, or there was little gas available to be swallowed.

44 17.4 What Are Quasars? Though most quasars are very far away, some have relatively low redshifts (like 0.1). If quasars were formed early in the Universe, how can these quasars still be shining? Why hasn’t all of the gas in the central region been used up? High-resolution images (see figures) show that in many cases, the galaxy containing the quasar is interacting or merging with another galaxy. This result suggests that gravitational tugs end up directing a fresh supply of gas from the outer part of the galaxy (or from the intruder galaxy) toward its central black hole, thereby fueling the quasar and allowing it to continue radiating so strongly. Some quasars may have even faded for a while, and then the interaction with another galaxy rejuvenated the activity in the nucleus.

45 17.4 What Are Quasars? Adaptive optics is now allowing high-resolution imaging from mountaintop observatories in addition to the Hubble Space Telescope. An image with adaptive optics on the Gemini North telescope has enabled the central quasar peak of brightness to be subtracted from the overall image. A flat edge-on disk, interpreted to be the host galaxy, was revealed (see figures).

46 17.5 Are We Being Fooled? A few astronomers have disputed the conclusion that the redshifts of quasars indicate large distances, partly because of the implied enormously high luminosity produced in a small volume. If Hubble’s law doesn’t apply to quasars, maybe they are actually quite nearby. Specifically, Halton Arp has found some cases where a quasar seems associated with an object of a different, lower redshift (see figure).

47 17.5 Are We Being Fooled? However, most astronomers blame the association on chance superposition. There could also be some amplification of the brightnesses of distant quasars, along the line of sight, by the gravitational field of the low-redshift object; this would produce an apparent excess of quasars around such objects. We now have little reason to doubt the conventional interpretation of quasar redshifts (though of course as scientists we should keep an open mind). Quasars clearly reside in the centers of galaxies having the same redshift. They are simply the more luminous cousins of active galactic nuclei, and a plausible energy source has been found. In addition, gravitational lensing shows that quasars are indeed very distant.

48 17.6 Finding Supermassive Black Holes
We argued above, essentially by the process of elimination, that the central engine of a quasar or active galaxy consists of a supermassive black hole swallowing material from its surroundings, generally from an accretion disk. Is there any more direct evidence for this? Well, the high speed of gas in quasars and active galactic nuclei, as measured from the widths of emission lines, suggests the presence of a supermassive black hole. A strong gravitational field causes the gas particles to move very quickly, and the different emitted photons are Doppler shifted by different amounts, resulting in a broad line. On the other hand, alternative explanations such as supernovae might conceivably be possible; they, too, produce high-speed gas, but without having to use a supermassive black hole.

49 17.6 Finding Supermassive Black Holes
Recently, however, very rapidly rotating disks of gas have been found in the centers of several mildly active galaxies. Their motion is almost certainly produced by the gravitational attraction of a compact central object, because we see the expected decrease of orbital speed with increasing distance from the center, as in Kepler’s laws for the Solar System. The galaxy NGC 4258 (see figure) presents the most convincing case, one in which radio observations were used to obtain very accurate measurements. The typical speed is v = 1120 km /sec at a distance of only 0.4 light-year from the center. The data imply a mass of about 3.6  107 solar masses in the nucleus.

50 17.6 Finding Supermassive Black Holes
The corresponding density is over 100 million solar masses per cubic light-year, a truly astonishing number. If the mass consisted of stars, there would be no way to pack them into such a small volume, at least not for a reasonable amount of time: They would rapidly collide and destroy themselves, or undergo catastrophic collapse. The natural conclusion is that a supermassive black hole lurks in the center. Indeed, this is now regarded as the most conservative explanation for the data: If it’s not a black hole, it’s something even stranger!

51 17.6 Finding Supermassive Black Holes
One of the most massive black holes ever found is that of M87, an active galaxy in the Virgo Cluster that sports a bright radio and optical jet (see figures).

52 17.6 Finding Supermassive Black Holes
Spectra of the gas disk surrounding the nucleus were obtained with the Hubble Space Telescope (see figure), and the derived mass in the nucleus is about 3 billion solar masses.

53 17.6 Finding Supermassive Black Holes
If some nearby, relatively normal-looking galaxies were luminous quasars in the past, and a significant fraction even show some activity now, we suspect that supermassiveblack holes are likely to exist in the centers of many large galaxies today. Sure enough, when detailed spectra of the nuclear regions of a few galaxies were obtained (especially with the Hubble Space Telescope), strong evidence was found for rapidly moving stars. The masses derived from Kepler’s third law were once again in the range of a million to a billion Suns. By late-2005, the central regions of several dozen galaxies had been observed in this manner, revealing the presence of supermassive black holes.

54 17.6 Finding Supermassive Black Holes
Probably the most impressive and compelling case is our own Milky Way Galaxy. As we discussed in Chapter 15, stars in the highly obscured nucleus were seen from Earth at infrared wavelengths, and their motions were measured over the course of a few years; see the top figure. The data are consistent with stars orbiting a single, massive, central dark object (see figure, bottom). The implied mass of this object is 3.7 million solar masses, and it is confined to a volume only 0.03 light-year in diameter! The only known explanation is a black hole. Thus, our Galaxy could certainly have been more active in the past, though never as powerful as the most luminous quasars, which require a black hole of 108 to 109 solar masses.

55 17.6 Finding Supermassive Black Holes
In the past few years, it has been found that the mass of the central black hole is proportional to the mass of the bulge in a spiral galaxy, or to the total mass of an elliptical galaxy (see figure on the next slide). But recall from Chapter 16 that the bulges of spiral galaxies are old, as are elliptical galaxies (which resemble the bulges of spiral galaxies). Thus, there is evidence that the formation of the supermassive black hole is related to the earliest stages of formation of galaxies. We don’t yet understand this relation, but clearly it offers a clue to physical processes long ago, when most galaxies were being born. Very recent studies show that for a given bulge mass, the more compact the bulge, the more massive the black hole, suggesting an even closer link between bulge formation and black-hole formation.

56 17.6 Finding Supermassive Black Holes

57 17.7 The Effects of Beaming Radio observations with extremely high angular resolution, generally obtained with the technique of very-long-baseline interferometry, have shown that some quasars consist of a few small components. In many cases, observations over a few years reveal that the components are apparently separating very fast (see figures), given the conversion from the angular change in position we measure across the sky to the actual physical speed in km /sec at the distance of the quasar. Indeed, some of the components appear to be separating at superluminal speeds—that is, at speeds greater than that of light! But Einstein’s special theory of relativity says that no objects can travel through space faster than light, an apparent contradiction.

58 17.7 The Effects of Beaming Astronomers can explain how the components only appear to be separating at greater than the speed of light even though they are actually physically moving at allowable speeds (less than that of light). If one of the components is a jet approaching us almost along our line of sight, and nearly at the speed of light, then according to our perspective the jet is nearly keeping up with the radiation it emits (see figure).

59 17.7 The Effects of Beaming If the jet moves a certain distance in our direction in (say) 5 years, the radiation it emits at the end of that period gets to us sooner than it would have if the jet were not moving toward us. So in fewer than 5 years, we see the jet’s motion over 5 full years. In the interval between our observations, the jet had several times longer to move than we would naively think it had. So it could, without exceeding the speed of light, appear to move several times as far. Whether a given object looks like a quasar or a less-active galaxy with broad emission lines probably depends on the orientation of the jet relative to our line of sight: Jets pointing at us appear far brighter than those that are misaligned. Thus, quasars are probably often beamed roughly toward us, a conclusion supported by the fact that many radio-loud quasars show superluminal motion.

60 17.7 The Effects of Beaming However, if the jet is pointing straight at us, it can greatly outshine the emission lines, and the object’s optical spectrum looks rather featureless, unlike that of a normal quasar. It is then called a “BL Lac object,” after the prototype in the constellation Lacerta, the Lizard. At the other extreme, if the jet is close to the plane of the sky, dust and gas in a torus (doughnut) surrounding the central region may hide the active nucleus from us (see figure). The galaxy nucleus itself may then appear relatively normal, although the active nature of the galaxy could still be deduced from the presence of extended radio emission from the jet.

61 17.7 The Effects of Beaming This general idea of beamed, or directed, radiation probably accounts for many of the differences seen among active galactic nuclei. For example, in one type of Seyfert galaxy, the very broad emission lines are not easily visible, despite other evidence that indicates considerable activity in the nucleus. (For example, bright narrow emission lines can be seen.) We think that in some cases, the broad emission lines are present, but simply can’t be directly seen because they are being blocked by an obscuring torus of material (see figure). But light from the broad lines can still escape along the axis of this torus and reflect off of clouds of gas elsewhere in the galaxy. Observations of these clouds then reveal the broad lines, but faintly.

62 17.7 The Effects of Beaming Similarly, some galaxies hardly show any sort of active nucleus directly—it is too heavily blocked from view by gas and dust along our line of sight, in the central torus. However, radiation escaping along the axis of this torus can still light up exposed parts of the galaxy, indirectly revealing the active nucleus (see figure).

63 17.8 Probes of the Universe Quasars are powerful beacons, allowing us to probe the amount and nature of intervening material at high redshifts. For example, numerous narrow absorption lines are seen in the spectra of high-redshift quasars (see figures). These spectral lines are produced by clouds of gas at different redshifts between the quasar and us. The lines can be identified with hydrogen, carbon, magnesium, and other elements.

64 17.8 Probes of the Universe Analysis of the line strengths and redshifts allows us to explore the chemical evolution of galaxies, the distribution and physical properties of intergalactic clouds of gas, and other interesting problems. The lines are produced by objects that are generally too faint to be detected in other ways. One surprising conclusion is that all of the clouds have at least a small quantity of elements heavier than helium. Since stars and supernovae produced these heavy elements, the implication is that an early episode of star formation preceded the formation of galaxies.

65 17.8 Probes of the Universe Another way in which quasars are probes of the Universe is the phenomenon of gravitational lensing of light (Chapter 16). In fact, such lensing was first confirmed through studies of quasars. In 1979, two quasars were discovered close together in the sky, only a few seconds of arc apart (see figure, left). They had the same redshift, yet their spectra were essentially identical, arguing against a possible binary quasar. A cluster of galaxies with one main galaxy (see figure, middle and right) was subsequently found along the same line of sight, but at a smaller redshift.

66 17.8 Probes of the Universe The most probable explanation is that light from the quasar is bent by the gravity of the cluster (warped space–time), leading to the formation of two distinct images (see figure). The cluster is acting like a gravitational lens.

67 17.8 Probes of the Universe Since then, dozens of gravitationally lensed quasars have been found. For a point lens and an exactly aligned object, we can get an image that is a ring centered on the lensing object. Such a case is called an “Einstein ring,” and a few are known (see figure).

68 17.8 Probes of the Universe Some gravitationally lensed quasars have quadruple quasar images that resemble a cross (see figures, left), or even more complicated configurations (see figures, right). Only gravitational lensing seems to be a reasonable explanation of these objects, the redshifts of whose components are identical.

69 17.8 Probes of the Universe Moreover, in some cases continual monitoring of the brightness of each quasar image has revealed the same pattern of light variability, but with a time delay between the different quasar images. This delay occurs because the light travels along two different paths of unequal length to form the two quasar images; see figure. The variability pattern is not expected to be identical in two entirely different quasars that happen to be bound in a physical pair. The multiple imaging of quasars is an exciting verification of a prediction of Einstein’s general theory of relativity. The lensing details are sensitive to the total amount and distribution of matter (both visible and dark) in the intervening cluster. Thus, gravitationally lensed quasars provide a powerful way to study dark matter.

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