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Quasars and
Active Galaxies
ORIGINS We explore the significance of quasars as an early phase in the origin
and evolution of large galaxies, such as our Milky Way Galaxy. We find that quasars
allow us to study the distribution of matter, both visible and dark, in the vast space
between us and them.
uasars, 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 inconsequential 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. 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.
Q
AIMS
To describe the discovery
of quasars, the source
of their stupendous power,
and their relationship to
galaxies with unusually
energetic centers.
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.
A Hubble Space Telescope image of quasar PKS 2349-014, whose spectral lines are redshifted
by 17.3 per cent—that is, z 0.173. The loops of gas around this quasar suggest that it is being fueled by gas from the merging of two galaxies. The galaxies above the quasar are probably interacting with it.
17-1
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17
Quasars and Active Galaxies
Figure 17–1 Hubble Space
Telescope images of the central regions of the normal galaxy NGC
7626 (left) and the active galaxy
NGC 5548 (right). Both galaxies
have the same general appearance
(Hubble type), but note the bright,
star-like nucleus in the active galaxy.
Rays of light are a telescope artifact, and indicate that the active nucleus is very compact. The normal
galaxy does not show such rays because its nucleus is much less concentrated.
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 (Fig. 17–1 ). 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.
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 (Fig. 17–2), emits about a million times more energy in
the radio region of the spectrum than does the Milky Way Galaxy. Close scrutiny
Figure 17–2 A radio map of
Cygnus A, with shading indicating
the intensity of the radio emission
in the two giant lobes. An electronic
image of the faint central object
seen at optical wavelengths is superposed at the proper scale.
(lower inset) This optical groundbased view of the faint central object, taken under very good
conditions, reveals some structure.
(upper inset) A Hubble Space Telescope optical image shows yet
more structure. Particles feeding
the large radio lobes come from the
very central part.
Active Galactic Nuclei
30 arc min
NGC 6251
radio
structure
1 arc min
1 arc min
0.1 arc sec
of such galaxies sometimes reveals two long, narrow, oppositely directed “jets”
joining the nuclei and the lobes of radio galaxies (Fig. 17–3). The jets are thought
to consist of charged particles moving 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
(Fig. 17–4).
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 (Fig.
17–5). 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.”
A
B
Figure 17–4 (A) Centaurus A (NGC 5128) looks like an elliptical or S0 galaxy seen through
an extensive dust lane. (B) A Chandra Observatory x-ray view of the jet in Centaurus A.
17-3
Figure 17–3 Radio maps of
the active galaxy NGC 6251 shown
on four scales. (At the distance of
NGC 6251, 30 arc minutes corresponds to about 3 million light
years, 1 arc minute is about
100,000 light years, and 0.1 arc second is about 170 light years.) One
of the radio jets is pointing roughly
in our direction, and thus appears
much brighter than the other jet,
which points away from us.
CHAPTER
17
Figure 17–5 Optical spectrum
of the nucleus of the Seyfert (active) galaxy NGC 5548, an image of
which is shown in Figure 17–1.
Note the broad emission lines,
along with narrower ones. The
broad lines are formed by gas that
is moving very rapidly close to the
center.
Quasars and Active Galaxies
6
O++
5
Apparent brightness
17-4
Hα
4
3
Hγ
2
Hβ
1
0
4000
5000
6000
7000
Observed wavelength (Å)
8000
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.
QUASARS
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.
The Discovery of Quasars
In the late 1950s, as radio astronomy developed, astronomers found that some
celestial objects emit strongly at radio wavelengths. Catalogs 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
catalog 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.
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
Quasars
3C 48
3C 147
3C 273
17-5
3C 196
Figure 17–6 Optical photographs of four of the first known quasars. They appear starlike, but measurements with radio telescopes show that they are much brighter than normal
stars at radio wavelengths.
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 of 13th magnitude—600 times fainter than
the naked-eye limit.
When the positions of other radio sources were determined accurately
enough, it was found that they, too, often coincided with faint, bluish-looking
stars (Fig. 17–6). 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. 3C 273 seemed to
be especially interesting: a jet-like feature stuck out from it, visible at optical
wavelengths (Fig. 17–7) and radio wavelengths (Fig. 17–8).
http://ww.harcourtcollege.com/astro/cosmos/rsce
3C 273 and Its Jet
Hubble Space Telescope
ESO New Technology Telescope
Palomar Sky Survey
Figure 17–7 An optical photograph of 3C 273, the first quasar to
be unambiguously identified. The
object, near the center, looks like a
normal star, especially in the wideangle view shown at the lower
right. However, there is also a faint
“jet” (boxed) pointing away from
the quasar. A Hubble Space Telescope close-up image of the jet is
shown at the upper right.
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17
Figure 17–8 The radio jet of
the quasar 3C 273 stems from the
nucleus.
Quasars and Active Galaxies
Several astronomers, including Maarten Schmidt of Caltech, photographed
the optical spectra of some quasars with the 5-m 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.
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 (Fig. 17–9). 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).
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-
line of hydrogen (i.e., H), 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 immediately realized that the spectrum of quasar
3C 48 looked like that of hydrogen redshifted by an even more astounding
amount: 37%.
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.
H
H
H [0 III]
3C 273
Comparison
(no redshift)
4000 Å H
H
H 5000 Å
6000 Å
Figure 17–9 The spectrum of the quasar 3C 273. The lower spectrum is of a
hot lamp in the telescope dome; it consists of hydrogen, helium, neon, and other
elements that emit lines at known wavelengths. This “comparison spectrum” establishes the scale of wavelength. A color bar shows the colors of the different wavelengths of the comparison spectrum. The upper spectrum is the spectrum of the
quasar. The hydrogen Balmer lines H, H, and H in the quasar spectrum are at longer
wavelengths (labels in red) than in the comparison spectrum (labels in green). The
redshift of 16% corresponds, according to Hubble’s law (with H0 65 km/sec/Mpc),
to a distance of 2.4 billion light-years.
Quasars
17-7
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 (Fig. 17–10); 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.
Similarly, there were solid arguments against a “gravitational redshift” interpretation, 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. 3C 273, for example, has z 0.16, so
v 0.16c 48,000 km/sec. According to Hubble’s law, v H0 d, so if H0 65 km/sec/Mpc, then d v/H0 (48,000 km/sec)/(65 km/sec/Mpc) 740 Mpc
2.4 billion light-years, a sixth of the way back to the origin of the Universe!
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.
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 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 must instead use the relativistic Doppler
formula to calculate the nominal speed.
Even calling it a Doppler effect is
a bit misleading and not entirely correct: 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.
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 H0 d 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. The highest redshift known for a quasar as
of early 2000 is z 5.0 (Fig. 17–11), which means that a feature whose laboratory (rest) wavelength is 1000 Å is observed to be at a wavelength 500 per cent
Figure It Out
The Relativistic Doppler Effect
The relativistic Doppler formula is z [(1 v/c)/(1 v/c)]1/2 1, where z is the
redshift and v is the speed of recession. Note that for v 0, z [(1 0)/
(1 0)]1/2 1 0, the same answer as for the nonrelativitistic approximation. But
for v 0.9c, z [(1 0.9)/(1 0.9)]1/2 1 [1.9/0.1]1/2 1 19
1 3.4,
far greater than the nonrelativistic approximation would have given. So even this z 1
corresponds to a speed less than the speed of light.
Figure 17–10 The idea that
quasars were local, and were
ejected from our galaxy, could explain why they all have high redshifts. (The blue dot shows the
position of the Sun.) But this hypothesis has a number of problems
and was quickly rejected by most
astronomers.
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Quasars and Active Galaxies
Apparent brightness
6
z = 5.0
Ly α
(1216)
4
SilV
(1400)
ClV
(1550)
2
0
6000
A
7000
8000
Wavelength (Å)
9000
B
Figure 17–11 (A) An image of one of the farthest known quasars, SDSS J03380021.
(B) Its spectrum shows that all the emission lines are shifted by 5.0 times their original wavelengths. The high redshift puts the ultraviolet lines like Lyman-, normally at 1216 Å, into the
red and near-infrared regions of the spectrum. (Original wavelengths are shown in parentheses.) At these redshifts, we are looking back to about 1.5 billion years after the birth of the
Universe.
larger, or 1000 Å 5000 Å 6000 Å. (Recall that z /0.) The corresponding
nominal speed of recession is 0.95c, and the quasar’s lookback time is about 13 billion years (in a model where the Universe is 14.5 billion years old). We see the
quasar as it was when the Universe was about 10 per cent of its current age!
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 that of normal stars. Low-redshift quasars tend to look bluish,
because they emit more blue light than typical stars. But the light from highredshift 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. Thousands of quasars are now known, and more are being discovered very rapidly.
The Energy Problem
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. Yet their diameters appeared relatively small (even through Earth’s blurry
atmosphere) compared with galaxies, so their energy must be efficiently produced
from within a small volume. Already this made them unusual and intriguing.
However, these astronomers were in for a big surprise when they figured out
just how small quasars really are. They noticed that some quasars vary in brightness over short time scales—days, weeks, months, or years (Fig. 17–12). 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.
Quasars
13.0
Apparent magnitude (R)
13.5
3C 279
14.0
14.5
15.0
15.5
16.0
16.5
17.0
1995.0
1996.0
1997.0
1998.0
Date (year)
1999.0
Figure 17–12 The apparent brightness of optical radiation from
3C 279 varies on a timescale of months.
The argument goes as follows: suppose we have a glowing, spherical, opaque
object that is 1 light-month in radius (Fig. 17–13). 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 time scale 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 time scale of an observed variation sets an upper limit
(i.e., a maximum value) to the size of the emitting region: the actual size must be
smaller than this upper limit.
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 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
Takes 1 month for variation to be complete
1 Light
month
Takes 1 year for variation to be complete
1 Light year
Figure 17–13 Why a large object can’t fluctuate in brightness as rapidly as a smaller object. Say that each object abruptly brightens at one instant. The wave emitted from the edge
of the object takes longer to reach the observer than light from the near side of the object,
because it has to travel farther. We don’t see the full variation until waves from all parts of
the object reach us. The same is true for both opaque and transparent objects. For the latter,
light from the far side takes longer to reach us than light from the near side.
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Quasars and Active Galaxies
can be 100 times more powerful than an entire galaxy of stars 100,000 light-years
in diameter!
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.
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 even
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.
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.
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, taking up (“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
black hole, but it is typically a few solar masses per year.
The matter generally swirls around the black hole, forming a rotating disk
called an accretion disk (Fig. 17–14). 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. Friction between the gas particles in
the accretion disk causes them to heat up, and they emit electromagnetic radiation, thereby converting part of their kinetic energy into light. 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 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” that emerge from some quasars. No material actually comes from within
the black hole; instead, its origin is the accretion disk. The charged particles in
Quasars
Jet
Accretion
disk
17-11
Figure 17–14 Cross-sectional
view of an accretion disk (doughnut)
surrounding a black hole, with highspeed jets of particles emerging
from the disk’s nozzle. The nozzle
points along the black hole’s rotation axis.
Black
hole
the jets are believed to shoot out perpendicularly to the accretion disk, along the
black hole’s axis of rotation (Figure 17–14). They emit radiation as they are accelerated. In addition to the radio radiation, high-energy photons such as x-rays
can also be produced (Fig. 17–15). The impressive focusing might be provided
by a magnetic field, as in the case of pulsars, or by the central cavity in the disk.
Recall that jets are also seen in radio galaxies, which appear to be closely related
to quasars (Figure 17–3). As discussed in more detail in the following section, 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.
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 almost like a quasar
(Fig. 17–16). 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.
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
Figure 17–16 An active galaxy whose nucleus is much brighter than the surrounding
galactic disk, making the latter hard to detect. This image from a ground-based telescope is
shown in false color to emphasize the faint outer parts. Two stars are shown for comparison
(upper left ).
Figure 17–15 An image of the
quasar PKS 0637-752 (z 0.65) obtained with the Chandra X-ray Observatory. It is so distant that we
see it as it was about 6 billion years
ago. Note the “jet” that extends
about 200,000 light-years out from
the quasar; its power at x-ray energies exceeds that at radio energies,
an important constraint that a successful theory of jet formation will
have to explain.
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CHAPTER
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Quasars and Active Galaxies
Figure 17–17 (A) The quasar
PG 0052251, at redshift z 0.155,
is in the center of a normal spiral
galaxy. (B) The quasar PHL 909, at
redshift z 0.171, is at the core of
a normal elliptical galaxy. Both of
these images were obtained with
the Hubble Space Telescope.
B
A
5
Hα
4
Apparent brightness
Figure 17–18 Hiding the
bright quasar 3C 273 with a dark
disk, as was done here in pre-Hubble
days, has revealed extended light
(“fuzz”) that is almost certainly a
galaxy. Quasars are now thought to
be very powerful events in the centers of galaxies.
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.
More recently, images obtained with the Hubble Space Telescope demonstrate conclusively that quasars live in galaxies, almost always at their centers. www
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 (Fig. 17–17), but in others it is barely visible, and special techniques are
used to reveal it (Fig. 17–18). Further solidifying the association of quasars with
galaxies, Keck telescope 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 (Fig. 17–19).
Hβ
3
Quasar
Hγ
2
1
Fuzz
Ca+
0
5000
Mg
6000
7000
Observed wavelength (Å)
8000
Figure 17–19 Optical spectra of the quasar 4C 31.63
(z 0.296) and its “fuzz,” the latter revealing absorption lines produced by the relatively cool outer parts of normal stars. This
shows that the “fuzz” seen around nearby quasars is really galaxy
light. The quasar is actually much brighter than the “fuzz,” but was
scaled here by an arbitrary amount for clarity.
17-13
Quasars exist almost exclusively at high redshifts and hence large distances.
The peak of the distribution is at z 2 (Fig. 17–20), though new studies at
x-ray wavelengths suggest that it might have been at an even higher redshift.
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.
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?
Hubble Space Telescope images show that in many cases, the galaxy containing
the quasar is interacting or merging with another galaxy (Fig. 17–21). 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.
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 (Fig.
17–22). However, most astronomers blame the association on chance superposition. We now have little reason to doubt the conventional interpretation of quasar
redshifts. 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 (see below) shows that quasars are indeed very distant.
A
B
C
D
Figure 17–21 Quasars in interacting or merging galaxies, in each case imaged with the
Hubble Space Telescope. (A) Debris from a collision between two galaxies fueling a quasar. A
ring galaxy left by the collision is at bottom; a foreground star in our galaxy is at top. (B) A tidal
tail above a quasar, perhaps drawn out by a galaxy that is no longer there. (C) A quasar merging with the bright galaxy that appears just below it. The swirling wisps of dust and gas surrounding them indicate that an interaction is taking place. (D) A pair of merged galaxies have
left loops of gas around this quasar.
Number density of quasars (Gpc3)
Quasars
300
200
100
8 10 12 14
Now
Cosmic time (109 years)
0
2
4
6
Figure 17–20 The number
density of quasars (number per billion cubic parsecs) is plotted versus
cosmic time, for an assumed Universe age of 14 billion years. There
was a bright, spectacular era of
quasars billions of years ago, and
essentially none now remain.
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CHAPTER
17
Quasars and Active Galaxies
Figure 17–22 Two galaxies
with relatively low redshifts are
seen close in the sky to highredshift quasars. Most astronomers
think these are just chance projections along nearly the same lines of
sight; the quasars and nearby galaxies are not physically associated
with each other.
Anomalous Quasar-Galaxy Associations
z = 0.0288
z = 0.018
z = 1.17
z = 1.05
ESO 1327-206/PKS1327-206
Klemola 31AB/PKS 2020-370
Detection of 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. 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.
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 our Solar System. The galaxy NGC 4258 (Fig. 17–23) 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. This implies a mass of about 3.6 107 Suns in the nucleus.
The corresponding density is over 100 million solar masses per cubic lightyear, a truly astonishing number. If the mass consisted of stars, there would be
2 kpc
Figure 17–23 An optical image of the spiral galaxy NGC 4258
that emphasizes star-forming regions. Radio observations of the nucleus have revealed gas orbiting a
central, dark, very massive object,
almost certainly a black hole.
Figure It Out
The Central Mass in a Galaxy
We saw in Chapter 16 that the mass enclosed within a circular orbit of radius R is
M v 2R/G, where v is the orbital speed and G is Newton’s constant of gravitation.
For NGC 4258, we measure v 1120 km/sec at a distance of 0.4 light-year from
the center. Properly converting units, we find that M 7.1 10 40 g 3.6 107
solar masses.
Quasars
A
Figure 17–24 (A) The nucleus of the active elliptical galaxy M87, showing a jet of highspeed charged particles and, enlarged, an unusual spiral disk in the center. (B) The nucleus
(bright point at top) and jet of M87, in a computer-processed view from the Faint Object Camera aboard the Hubble Space Telescope. In the jet, which extends 8000 light-years, the image
reveals detail as small as 10 light-years across.
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!
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 (Fig. 17–24).
Spectra of the gas disk surrounding the nucleus were obtained with the Hubble
Space Telescope (Fig. 17–25), and the derived mass in the nucleus is about 3 billion Suns.
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
supermassive black 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.
Probably the most impressive and compelling case is our own Milky Way
Galaxy. At infrared wavelengths, stars in the highly obscured nucleus were seen
from Earth, and their motions were measured over the course of a few years (Fig.
17–26). The implied mass of the central dark object is 2.6 million Suns, and it
is confined to a volume 0.03 light-year in diameter! 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.
B
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17
Figure 17–25 Spectra of the
regions shown on the image of the
center of M87, taken with the Faint
Object Spectrograph aboard the
Hubble Space Telescope, reveal
Doppler shifts of the gas. The
speed of revolution of 550 km/sec
at this distance of 60 light-years
from the nucleus allows astronomers to calculate how much
mass must be inside those locations to keep the gas in orbit. The
result is about 3 billion solar
masses, after various effects like
the inclination of the disk are taken
into account.
Quasars and Active Galaxies
Approaching
Apparent brightness
Receding
Wavelength
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 (Fig.
17–27), 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
600
∆DEC from Sgr A* (arc sec)
0.2
0
Sgr A*
1995
1996
1997
1998
1999
–0.2
–0.4
Speed (km/sec)
17-16
400
200
–0.6
0.6
0.4
0.2
0
∆RA from Sgr A* (arc sec)
–0.2
0
0.01
0.1
Radius (pc)
Figure 17–26 (left) Stars in the central region of the Milky Way Galaxy, photographed
several times with the Keck-I telescope using a special technique that enhances angular resolution. Some of the stars clearly change position relative to each other over the course of a
few years. (right) The observed speeds of stars versus their distance from the center of our
galaxy. The curve shows the predicted speeds if the center contains a black hole 2.6 million
times as massive as the Sun. Note that 1 pc 3.26 light-years.
Quasars
Relative Dec. (milliarcsec)
0 1991.48
2 1992.45
1992.86
4
1993.13
2.5c
1993.85
2.5c
6 1994.45
2
0
2
Relative R.A. (milliarcsec)
A
B
Figure 17–27 (A) A series of views of the quasar 3C 279 with radio interferometry at
a wavelength of 1.3 cm. The apparent speed translates (for H0 65 km/sec/Mpc) to a
speed of 9.5c, though such “superluminal speeds” can be explained in conventional
terms. (B) Superluminal speeds also appear in the active galaxy M87. The inset shows
features that seem to move at 2.5c. Red shows the brightest and blue shows the faintest
emission.
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.
Astronomers can explain how the components only appear to be separating
at greater than the speed of light even though they are actually 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. If the jet moves a certain distance in our direction in 1 year, 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 less than 1 year, we see the jet’s motion over
1 full year. 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. 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 (Figure 17–14).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.
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
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17-18
CHAPTER
17
Figure 17–28 The appearance
from Earth of an active galaxy can
depend on the orientation relative to
our line of sight of the torus (doughnut) that surrounds it. If we see the
nucleus from a direction within the
indicated cones, the nucleus appears bright, and we can see the
broad emission lines produced by
gas in rapid motion near the supermassive black hole. If instead we
view from outside the cones, then
the active nucleus and central,
rapidly moving clouds can be hidden
at optical wavelengths by gas and
dust in the torus. Narrow emission
lines from clouds farther from the
nucleus (and within the cones),
however, should still be visible.
Quasars and Active Galaxies
Region lit up
by active nucleus
Gas and
dust "torus"
Broad
lines not
directly
visible
Broad lines
directly visible
by an obscuring torus of material (Fig. 17–28). 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.
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 (Fig. 17–29).
Quasars as Probes of the Universe
Quasars are powerful beacons, allowing us to probe the amount and nature of
intervening material. For example, numerous narrow absorption lines are seen
in the spectra of high-redshift quasars (Fig. 17–30). These spectral lines are
Continuum
NGC 5252
N
E
Figure 17–29 Radiation from
the core of this active galaxy, NGC
5252, illuminates matter only within
two oppositely directed cones. Presumably the cones are aligned
along the axis of a rotating black
hole surrounded by a torus of gas
and dust that blocks our direct view
of the active nucleus, as shown in
Figure 17–28.
O+ + emission
Quasars
Figure 17–30 Spectrum of a
high-redshift quasar (bottom), showing a large number of absorption
lines produced by clouds of gas and
galaxies along our line of sight to
the quasar. Most of these lines, especially the ones in the blue (left)
part of the spectrum, correspond to
the hydrogen Lyman- transition
produced by intergalactic clouds at
many different (and generally large)
distances from us. We see the lines
at many different wavelengths because of the different redshifts of
the clouds. The spectrum of 3C 273
(top), a low-redshift quasar, has far
fewer absorption lines, indicating
that intergalactic clouds of gas are
scarce at the present time in the
universe.
100
80
3C 273 z = 0.158
60
Apparent brightness
40
20
0
1000
1050
1100
1150
1200
Emitted wavelength (Å)
1250
1300
1350
1250
1300
1350
100
80
Q1422+2309 z = 3.62
60
40
20
0
1000
1050
1100
1150
1200
Emitted wavelength (Å)
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.
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.
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 (Fig. 17–31A). 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 (Figure 17–31B,C)
A
B
Figure 17–31 (A) Hubble Space Telescope wide-angle view of two quasars, 0957561 A
and B, that are only about 6 seconds of arc apart in the sky. They have the same redshifts
(z 1.4136) and essentially identical spectra. (B) Close-up view of the two quasars, obtained
with a ground-based telescope; they appear slightly oblong because of imperfect tracking by
the telescope during the exposure. An intervening galaxy can be seen only 1 second of arc
from the quasar on the right, in the direction of the quasar on the left. (C) The image of the
quasar on the left has been subtracted from that on the right to reveal just the intervening
galaxy.
17-19
C
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CHAPTER
17
Figure 17–32 The geometry
of gravitational lensing. Light from a
distant quasar can take two different paths on its way to Earth; each
is bent because of the gravitational
field of the intervening galaxy or
cluster of galaxies. The two paths
produce two distinct images of the
same quasar close together in the
sky. Note that the lengths of the
two paths generally differ.
Quasars and Active Galaxies
Path of light ray
Image B
Quasar
Galaxy
Observer
on Earth
Image A
was subsequently found along the same line of sight, but at a smaller redshift.
The most probable explanation is that light from the quasar is bent by the gravity of the cluster (warped spacetime), leading to the formation of two distinct images (Fig. 17–32). The cluster is acting like a gravitational lens.
Since then, over two dozen 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 known as an “Einstein ring,” and
a few are known (Fig. 17–33A). Some gravitationally lensed quasars even have
quadruple quasar images that resemble a cross (Fig. 17–33B,C ). Only gravitational lensing seems to be a reasonable explanation of these objects, the redshifts
of whose components are identical. 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 (Figure 17–32). The variability pattern is not expected to be identical in two entirely different quasars that happen to be bound
in a physical pair.
Multiply imaged (pronounced “mul´te-plee”) quasars are 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. In some cases, the lens itself seems to consist almost entirely of dark matter!
A
B
C
Figure 17–33 (A) This particular quasar is so exactly aligned with the intervening gravitational lens that its radio radiation is spread out into an “Einstein ring.” A second object is less
well aligned; its image appears slightly inside the ring on one side and slightly outside the ring
on the other side. (B) The “clover leaf” (H 1413117), a quadruply imaged quasar at redshift
2.55. The four images of comparable brightness are only 1 second of arc apart, and barely resolved in this ground-based photograph. (C) Hubble Space Telescope image of the “Einstein
cross” (Q22030305), a quasar (z 1.70) that is gravitationally lensed into four images by a
relatively nearby galaxy (z 0.039). The nucleus of the galaxy is visible in the center of the
image. The four images of comparable brightness are separated by less than 2 seconds of arc
and have essentially identical spectra. The rare configuration and identical spectra show that
we are indeed seeing gravitational lensing rather than a cluster of quasars.
Topics for Discussion
17-21
Concept Review
The central regions of some galaxies are unusually luminous—
they are very powerful emitters of all kinds of electromagnetic
radiation, and they sometimes spew out enormous jets of particles. It is unlikely that normal stellar processes can account
for these objects, called active galactic nuclei. The mysterious quasars are apparently related: they are relatively bright,
star-like objects that have bizarre optical spectra and in some
cases shine very brightly at radio wavelengths. The discovery
that quasars have high redshifts implied that they are at vast
distances and have astonishing luminosities. Rapid variations
in their brightness were used to deduce that quasars are small,
in some cases less than one light-week across—yet they are
more powerful than entire galaxies 100,000 light-years in diameter. The energy of a quasar probably comes from matter
falling into a supermassive black hole, 10 million to a billion
times the mass of the Sun, in the central region of a galaxy.
The matter forms a rotating accretion disk around the black
hole.
We now know that quasars are indeed phenomena in the
centers of a minority of galaxies. Hubble Space Telescope images of low-redshift quasars reveal nebulous structures that
closely resemble galaxies, and spectra confirm that the extended light comes from stars. Although a few quasars appear
close in the sky to galaxies with much lower redshifts, these
are thought to be chance projections. Quasars lived in the distant past; as the material surrounding the supermassive black
holes was used up, the quasars faded, becoming less active
galaxies and finally normal-looking galaxies. There is strong
evidence for the existence of gigantic black holes in the centers of some galaxies, and even our own Milky Way Galaxy
harbors such a beast. Some quasars appear to be revived
through gravitational interactions and collisions with galaxies,
which direct gas toward the central black hole.
Apparent superluminal speeds of components seen in
some quasars are produced by material moving near (but below) the speed of light, close to our line of sight; they are not
violations of Einstein’s special theory of relativity. The spectra
of quasars often exhibit absorption lines due to material at intermediate distances, allowing astronomers to study galaxies
and clouds of gas that are otherwise too dim to see. Some
quasars look multiple because of gravitational lensing by intervening galaxies or clusters.
Questions
1. Summarize the main observed characteristics of active
galactic nuclei.
2. Describe the historical development of the study of
quasars, and list their peculiar properties.
3. Why is it useful to find the optical objects that correspond
in position with radio sources?
4. What was the key breakthrough in the interpretation of
quasar spectra? What was its significance?
5. Why do you think it was initially difficult for astronomers
to entertain the possibility that the spectra of quasars are
highly redshifted?
6. We observe a quasar with a spectral line at 5000 Å that we
know is normally emitted at 4000 Å. (a) By what percentage is the line redshifted? (b) By approximately what
percentage of the speed of light is the quasar receding? (c)
At what speed is the quasar receding in km/sec? (d) Using Hubble’s law, to what distance does this speed correspond?
7. Explain what is meant by the “lookback time” of a quasar.
8. Outline the argument used to infer that the physical size
of quasars is small.
9. What is the most probable physical mechanism that produces a quasar’s energy? Does it seem paradoxical that
black holes, from which nothing can escape, have something to do with it?
10. What are three differences between quasars and pulsars?
11. What is the evidence suggesting that quasars live in the
centers of galaxies?
12. Summarize what generally happens to a quasar as it ages.
13. Suppose no nearby galaxies exhibited evidence for the presence of supermassive black holes in their centers. Would
this be a problem for the hypothesis of what powers quasars?
14. Why do we think that some nearby quasars are rejuvenated?
15. Explain how we deduce the presence and measure the mass
of a supermassive black hole in the center of a galaxy.
16. Describe how quasars can be used as probes of matter between them and us.
17. What do we mean by a “gravitationally lensed quasar”?
Topics for Discussion
If you wanted to determine whether high-redshift quasars are
sometimes associated with low-redshift galaxies (thereby casting doubt on the interpretation of quasars being very distant
objects), what kinds of observations might you conduct? Would
you use statistical arguments?