Transcript Chapter 20

Chapter 17
Quasars and Active
Galaxies
Introduction
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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.
Introduction
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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.
17.1 Active Galactic Nuclei
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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.
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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.
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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.
17.1 Active Galactic Nuclei
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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.
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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.
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Clusters of ordinary stars rarely, if ever, produce so
much x-ray and radio radiation.
17.1 Active Galactic Nuclei
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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.
17.1 Active Galactic Nuclei
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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).
17.1 Active Galactic Nuclei
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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).
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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.”
17.1 Active Galactic Nuclei
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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.
17.2 Quasars: Denizens
of the Distant Past
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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.
17.2a The Discovery of Quasars
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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.
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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.
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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.
17.2a The Discovery of Quasars
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Special techniques were developed to pinpoint the source of the
radio waves in a few instances.
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When the radio source winked out, we knew that the Moon had
just covered it while moving slowly across the background of
stars.
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Specifically, the occultation (hiding) of 3C 273 by the Moon
provided an unambiguous identification with an optical star-like
object.
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.
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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.
17.2a The Discovery of Quasars
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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.
17.2a The Discovery of Quasars
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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).
17.2b Puzzling Spectra
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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.
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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.
17.2b Puzzling Spectra
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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).
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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).
17.2b Puzzling Spectra
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This possibility had not been recognized because nobody
expected stars to have such large redshifts.
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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.
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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%.
17.2b Puzzling Spectra
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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 radioquiet variety, unless we explicitly mention the radio
properties.
17.2c The Nature of the Redshift
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How were the high redshifts produced?
The Doppler effect is the most obvious
possibility.
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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.
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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.
17.2c The Nature of the Redshift
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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.
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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.
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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!
17.2c The Nature of the Redshift
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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.
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Other quasars found during the 1960s had redshifts of 0.2 to 1,
and hence are billions of light-years away.
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Quasar 3C 273 turns out to be one of the closest quasars.
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.
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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.
17.2c The Nature of the Redshift
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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.
17.2c The Nature of the Redshift
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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 + 6400 Å
= 7400 Å. (Recall that z = /0 .)
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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!
17.2c The Nature of the Redshift
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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.
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Quasars have also been found in maps of the sky made with xray satellites, and of course with ground-based radio surveys.
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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.
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.
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Tens of thousands of quasars are now known, and more are being
discovered very rapidly, especially by the Sloan Digital Sky Survey.
17.3 How Are Quasars Powered?
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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.
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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.
17.3a A Big Punch from a Tiny Volume
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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).
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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.
17.3a A Big Punch from a Tiny Volume
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The argument goes as follows: Suppose we have a glowing, spherical,
opaque object that is 1 light-month in radius (see figure).
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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.
17.3a A Big Punch from a Tiny Volume
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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.
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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!
17.3b What Is the Energy Source?
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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?
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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.
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After all, we don’t expect huge explosions from tiny
firecrackers.
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.
17.3b What Is the Energy Source?
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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.
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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.
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To produce that much nuclear energy, a larger volume of material
would be needed.
17.3b What Is the Energy Source?
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The annihilation of matter and antimatter is energetically
feasible, since it is 100% efficient.
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turned into photons (radiation), and in principle a very small
volume can therefore be tremendously powerful.
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However, the observed properties of quasars do not
support this hypothesis.
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Specifically, matter–antimatter collisions tend to emit excess
amounts of radiation at certain wavelengths, and this is not
the case for quasars.
17.3b What Is the Energy Source?
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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.
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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.
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But to produce the prodigious power of quasars, a very strong gravitational
field is needed.
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.
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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).
17.3c Accretion Disks and Jets
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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.
17.3c Accretion Disks and Jets
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Note that energy is radiated before the matter is
swallowed by the black hole—nothing escapes from
within the black hole itself.
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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.
17.3c Accretion Disks and Jets
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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).
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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).
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The impressive focusing might be provided by a
magnetic field, as in the case of pulsars, or by
the central cavity in the disk.
17.3c Accretion Disks and Jets
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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.
17.3c Accretion Disks and Jets
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Recently, indirect evidence for accretion disks surrounding
a central, supermassive black hole has been found in
several active galaxies from observations with various xray 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.
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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).
17.3c Accretion Disks and Jets
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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.
17.4 What Are Quasars?
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The idea that quasars are energetic
phenomena at the centers of galaxies is now
strongly supported by observational evidence.
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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).
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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.
17.4 What Are Quasars?
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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).
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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.
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In any case, the data strongly suggested that quasars could indeed
be extreme examples of galaxies with bright nuclei.
17.4 What Are Quasars?
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More recently, images obtained with the Hubble Space
Telescope demonstrate conclusively that quasars live in
galaxies, almost always at their centers.
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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.
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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).
17.4 What Are Quasars?
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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.
17.4 What Are Quasars?
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With lookback times of about 10 billion years, quasars must be
denizens of the young Universe.
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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.
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What happened to them?
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.
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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.
17.4 What Are Quasars?
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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?
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High-resolution images (see figures) show that in many cases, the galaxy
containing the quasar is interacting or merging with another galaxy.
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Why hasn’t all of the gas in the central region been used up?
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.
17.4 What Are Quasars?



Adaptive optics is now allowing highresolution 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).
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).
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.
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.
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.
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!
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).
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.
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.
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.
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).


Thus, there is evidence that the formation of the supermassive
black hole is related to the earliest stages of formation of
galaxies.


But recall from Chapter 16 that the bulges of spiral galaxies are
old, as are elliptical galaxies (which resemble the bulges of spiral
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.
17.6 Finding Supermassive Black Holes
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.
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).
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.


In the interval between our observations, the jet had several
times longer to move than we would naively think it had.


So in fewer than 5 years, we see the jet’s motion over 5 full years.
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.
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.
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.
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).
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.
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.
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.
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.
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).
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.
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.


The variability pattern is not expected to be identical in two entirely
different quasars that happen to be bound in a physical pair.


This delay occurs because the light travels along two different paths of
unequal length to form the two quasar images; see figure.
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.