Quasars and Active Galaxies

1
Chapter 17

• Quasars and Active Galaxies

2
Introduction

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

3
Introduction

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

4
17.1 Active Galactic Nuclei

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

5
17.1 Active Galactic Nuclei

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

6
17.1 Active Galactic Nuclei

• Active galaxies that are extraordinarily bright
  at radio wavelengths often exhibit two enormous
  regions (known as lobes) of radio emission far
  from the nucleus, up to a million light-years
  away.

• The first radio galaxy of this type to be
  detected, Cygnus A (see figure), emits about a
  million times more energy in the radio region of
  the spectrum than does the Milky Way Galaxy.

7
17.1 Active Galactic Nuclei

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

8
17.1 Active Galactic Nuclei

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

9
17.1 Active Galactic Nuclei

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

10
17.2 Quasars Denizensof the Distant Past

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

11
17.2a The Discovery of Quasars

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

12
17.2a The Discovery of Quasars

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

13
17.2a The Discovery of Quasars

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

14
17.2a The Discovery of Quasars

• Object 3C 273 seemed to be especially
  interesting A jet-like feature stuck out from
  it, visible at optical wavelengths (see figures,
  left and middle) and radio wavelengths (see
  figure, right).

15
17.2b Puzzling Spectra

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

16
17.2b Puzzling Spectra

• Schmidt made a breakthrough in 1963, when he
  noticed that several of the emission lines
  visible in the spectrum of 3C 273 had the pattern
  of hydrogena series of lines with spacing
  getting closer together toward shorter
  wavelengthsthough not at the normal hydrogen
  wavelengths (see figure).
• He realized that he could simply be observing hot
  hydrogen gas (with some contaminants to produce
  the other lines) that was Doppler shifted.

• The required redshift would be huge, about 16
  (that is, z ??/?0 0.16), corresponding to 16
  of the speed of light (since z ? v/c, or v ? cz,
  valid for z less than about 0.2).

17
17.2b Puzzling Spectra

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

18
17.2b Puzzling Spectra

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

19
17.2c The Nature of the Redshift

• How were the high redshifts produced?
• The Doppler effect is the most obvious
  possibility.
• But it seemed implausible that quasars were
  discrete objects ejected like cannonballs from
  the center of the Milky Way Galaxy (see figure)
  their speeds were very high, and no good ejection
  mechanism was known.

• Also, we would then expect some quasars to move
  slightly across the sky relative to the stars,
  since the Sun is not at the center of the Galaxy,
  but such motions were not seen.
• Even if these problems could be overcome, we
  would then have to conclude that only the Milky
  Way Galaxy (and not other galaxies) ejects
  quasarsotherwise, we would have seen quasars
  with blueshifted spectra, corresponding to those
  objects emitted toward us from other galaxies.

20
17.2c The Nature of the Redshift

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

21
17.2c The Nature of the Redshift

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

22
17.2c The Nature of the Redshift

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

23
17.2c The Nature of the Redshift

• A few dozen quasars with redshifts exceeding 6
  have been discovered (see figures).
• The highest redshift known for a quasar as of
  late-2005 is z 6.4, which means that a feature
  whose laboratory (rest) wavelength is 1000 Å is
  observed to be at a wavelength 640 per cent
  larger, or 1000 Å 6400 Å 7400 Å. (Recall that
  z ??/?0 .)
• The corresponding nominal speed of recession is
  about 0.96c, and the quasars lookback time is
  roughly 12.8 billion years (in a model where the
  Universe is 13.7 billion years old).
• We see the quasar as it was when the Universe was
  about 6.6 per cent of its current age!

24
17.2c The Nature of the Redshift

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

25
17.3 How Are Quasars Powered?

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

26
17.3a A Big Punch from a Tiny Volume

• However, these astronomers were in for a big
  surprise when they figured out just how compact
  quasars really are.
• They noticed that some quasars vary in apparent
  brightness over short timescalesdays, weeks,
  months, or years (see figure).

• This implies that the emitting region is probably
  smaller than a few light-days, light-weeks,
  light-months, or light-years in diameter, in all
  cases a far cry from the tens of thousands of
  light-years for a typical galaxy.

27
17.3a A Big Punch from a Tiny Volume

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

28
17.3a A Big Punch from a Tiny Volume

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

29
17.3b What Is the Energy Source?

• The nature of the prodigious (yet physically
  small) power source of quasars was initially a
  mystery.
• How does such a small region give off so much
  energy?
• After all, we dont expect huge explosions from
  tiny firecrackers.
• There was some indication that these objects
  might be related to active galactic nuclei They
  have similar optical spectra and are bright at
  radio wavelengths.
• So, perhaps the same mechanism might be used to
  explain the unusual properties of both kinds of
  objects.
• In fact, maybe active galactic nuclei are just
  low-power versions of quasars!
• If so, quasars should be located in the centers
  of galaxies.
• Later we will see that this is indeed the case.

30
17.3b What Is the Energy Source?

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

31
17.3b What Is the Energy Source?

• The annihilation of matter and antimatter is
  energetically feasible, since it is 100
  efficient.
• That is, all of the mass in a matterantimatter
  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, matterantimatter collisions tend
  to emit excess amounts of radiation at certain
  wavelengths, and this is not the case for
  quasars.

32
17.3b What Is the Energy Source?

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

33
17.3c Accretion Disks and Jets

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

34
17.3c Accretion Disks and Jets

• Note that energy is radiated before the matter is
  swallowed by the black holenothing escapes from
  within the black hole itself.
• This process can convert the equivalent of about
  10 of the rest-mass energy of matter into
  radiation, more than 10 times more efficiently
  than nuclear energy. (Recall from Chapter 11 that
  the fusion of hydrogen to helium converts only
  0.7 of the mass into energy.)
• A spinning, very massive black hole is also
  consistent with the well-focused jets of matter
  and radiation that emerge from some quasars,
  typically reaching distances of a few hundred
  thousand light-years.

35
17.3c Accretion Disks and Jets

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

36
17.3c Accretion Disks and Jets

• Recall that jets are also seen in some types of
  active galaxies, which appear to be closely
  related to quasars (see figure).

• As discussed in more detail later in this
  chapter, we know that the particles move with
  very high speeds because a jet can sometimes
  appear to travel faster than the speed of
  lightan effect that occurs only when an object
  travels nearly along our line of sight, nearly at
  the speed of light.

37
17.3c Accretion Disks and Jets

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

38
17.3c Accretion Disks and Jets

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

39
17.4 What Are Quasars?

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

40
17.4 What Are Quasars?

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

41
17.4 What Are Quasars?

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

42
17.4 What Are Quasars?

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

43
17.4 What Are Quasars?

• With lookback times of about 10 billion years,
  quasars must be denizens of the young Universe.
• What happened to them?
• Quasars probably faded with time, as the central
  black hole gobbled up most of the surrounding
  gas the quasar shines only while it is pulling
  in material.
• Thus, some of the nearby active and normal
  galaxies may have been luminous quasars in the
  distant past, but now exhibit much less activity
  because of a slower accretion rate.
• Perhaps even the nucleus of the Milky Way Galaxy,
  which is only slightly active, was more powerful
  in the past, when the putative black hole had
  plenty of material to accrete.
• Of course, many of the weakly active galaxies we
  see nearby were probably never luminous enough to
  be genuine quasars.
• Either their central black hole wasnt
  sufficiently massive to pull in much material, or
  there was little gas available to be swallowed.

44
17.4 What Are Quasars?

• Though most quasars are very far away, some have
  relatively low redshifts (like 0.1).
• If quasars were formed early in the Universe, how
  can these quasars still be shining?
• Why hasnt all of the gas in the central region
  been used up?
• High-resolution images (see figures) show that in
  many cases, the galaxy containing the quasar is
  interacting or merging with another galaxy.
• This result suggests that gravitational tugs end
  up directing a fresh supply of gas from the outer
  part of the galaxy (or from the intruder galaxy)
  toward its central black hole, thereby fueling
  the quasar and allowing it to continue radiating
  so strongly.
• Some quasars may have even faded for a while, and
  then the interaction with another galaxy
  rejuvenated the activity in the nucleus.

45
17.4 What Are Quasars?

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

46
17.5 Are We Being Fooled?

• A few astronomers have disputed the conclusion
  that the redshifts of quasars indicate large
  distances, partly because of the implied
  enormously high luminosity produced in a small
  volume.
• If Hubbles law doesnt apply to quasars, maybe
  they are actually quite nearby.

• Specifically, Halton Arp has found some cases
  where a quasar seems associated with an object of
  a different, lower redshift (see figure).

47
17.5 Are We Being Fooled?

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

48
17.6 Finding Supermassive Black Holes

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

49
17.6 Finding Supermassive Black Holes

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

50
17.6 Finding Supermassive Black Holes

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

51
17.6 Finding Supermassive Black Holes

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

52
17.6 Finding Supermassive Black Holes

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

53
17.6 Finding Supermassive Black Holes

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

54
17.6 Finding Supermassive Black Holes

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

55
17.6 Finding Supermassive Black Holes

• In the past few years, it has been found that the
  mass of the central black hole is proportional to
  the mass of the bulge in a spiral galaxy, or to
  the total mass of an elliptical galaxy (see
  figure on the next slide).
• But recall from Chapter 16 that the bulges of
  spiral galaxies are old, as are elliptical
  galaxies (which resemble the bulges of spiral
  galaxies).
• Thus, there is evidence that the formation of the
  supermassive black hole is related to the
  earliest stages of formation of galaxies.
• We dont yet understand this relation, but
  clearly it offers a clue to physical processes
  long ago, when most galaxies were being born.
• Very recent studies show that for a given bulge
  mass, the more compact the bulge, the more
  massive the black hole, suggesting an even closer
  link between bulge formation and black-hole
  formation.

56
17.6 Finding Supermassive Black Holes
57
17.7 The Effects of Beaming

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

58
17.7 The Effects of Beaming

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

59
17.7 The Effects of Beaming

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

60
17.7 The Effects of Beaming

• However, if the jet is pointing straight at us,
  it can greatly outshine the emission lines, and
  the objects optical spectrum looks rather
  featureless, unlike that of a normal quasar.
• It is then called a BL Lac object, after the
  prototype in the constellation Lacerta, the
  Lizard.

• At the other extreme, if the jet is close to the
  plane of the sky, dust and gas in a torus
  (doughnut) surrounding the central region may
  hide the active nucleus from us (see figure).
• The galaxy nucleus itself may then appear
  relatively normal, although the active nature of
  the galaxy could still be deduced from the
  presence of extended radio emission from the jet.

61
17.7 The Effects of Beaming

• This general idea of beamed, or directed,
  radiation probably accounts for many of the
  differences seen among active galactic nuclei.
• For example, in one type of Seyfert galaxy, the
  very broad emission lines are not easily visible,
  despite other evidence that indicates
  considerable activity in the nucleus. (For
  example, bright narrow emission lines can be
  seen.)
• We think that in some cases, the broad emission
  lines are present, but simply cant be directly
  seen because they are being blocked by an
  obscuring torus of material (see figure).

• But light from the broad lines can still escape
  along the axis of this torus and reflect off of
  clouds of gas elsewhere in the galaxy.
• Observations of these clouds then reveal the
  broad lines, but faintly.

62
17.7 The Effects of Beaming

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

63
17.8 Probes of the Universe

• Quasars are powerful beacons, allowing us to
  probe the amount and nature of intervening
  material at high redshifts.
• For example, numerous narrow absorption lines are
  seen in the spectra of high-redshift quasars (see
  figures).

• These spectral lines are produced by clouds of
  gas at different redshifts between the quasar and
  us.
• The lines can be identified with hydrogen,
  carbon, magnesium, and other elements.

64
17.8 Probes of the Universe

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

65
17.8 Probes of the Universe

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

66
17.8 Probes of the Universe

• The most probable explanation is that light from
  the quasar is bent by the gravity of the cluster
  (warped spacetime), leading to the formation of
  two distinct images (see figure).
• The cluster is acting like a gravitational lens.

67
17.8 Probes of the Universe

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

68
17.8 Probes of the Universe

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

69
17.8 Probes of the Universe

• Moreover, in some cases continual monitoring of
  the brightness of each quasar image has revealed
  the same pattern of light variability, but with a
  time delay between the different quasar images.
• This delay occurs because the light travels along
  two different paths of unequal length to form the
  two quasar images see figure.
• The variability pattern is not expected to be
  identical in two entirely different quasars that
  happen to be bound in a physical pair.
• The multiple imaging of quasars is an exciting
  verification of a prediction of Einsteins
  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.