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        Chapter 17
Galaxies With Active Nuclei
This chapter is important for two reasons. First, it draws
together ideas from many previous chapters to show how
nature uses the same basic rules on widely different
scales. Matter flowing into a protostar, into a white dwarf,
into a neutron star, or into the heart of a galaxy must obey
the same laws of physics, so we see the same geometry
and the same phenomena. The only difference is the level
of violence.
Second, this chapter is important because the most distant
objects we can see in the universe are the most luminous
galaxies, and many of those are erupting in outbursts and
are thus peculiar. By studying these galaxies, our attention
is drawn out in space to the edge of the visible universe
and back in time to the earliest stages of galaxy formation.
In other words, we are led to think of the origin and
evolution of the universe, the subject of the next chapter.
I. Active Galaxies
    A. Seyfert Galaxies
    B. Double-Lobed Radio Sources
    C. Testing The Black Hole Hypothesis
    D. The Search for a Unified Model
    E. Black Holes and Galaxy Formation

II. Quasars
     A. The Discovery of Quasars
     B. Quasar Distances
     C. Evidence of Quasars in Distant Galaxies
     D. Superluminal Expansion
     E. A Model Quasar
     F. Quasars Through Time
Active Galaxies
    Galaxies with extremely violent energy
    release in their nuclei (pl. of nucleus).

      “Active Galactic Nuclei” (= AGN)

       Up to many thousand times more
     luminous than the entire Milky Way;
        energy released within a region
     approx. the size of our solar system!
The Spectra of Galaxies
                          Taking a spectrum of
                          the light from a normal

 The light from the galaxy should be mostly star
 light, and should thus contain many absorption
 lines from the individual stellar spectra.
Seyfert Galaxies
               Unusual spiral galaxies:
• Very bright cores
• Emission line spectra.
• Variability: ~ 50 % in
    a few months

                           NGC 1566

                                      Most likely power
                                   Accretion onto a
                                supermassive black hole
   Circinus Galaxy                 (~107 – 108 Msun)
Interacting Galaxies
                                      Seyfert galaxy NGC 4151
Seyfert galaxy NGC 7674

Active galaxies are often
associated with interacting
galaxies, possibly result of
recent galaxy mergers.

Often: gas outflowing at high velocities, in opposite directions
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Radio image
of Cygnus A                Hot spots: Energy in
                               the jets is released in

Material in the jets moves
with almost the speed of
light (“Relativistic jets”).
 Radio Galaxies
 Centaurus A (“Cen A” = NGC 5128): the closest AGN to us.
                                               Jet visible in
                                               radio and X-
                                               rays; show
                                               bright spots
                                               in similar

                                               Infrared image
Radio image superposed                         reveals warm
   on optical image                            gas near the
Radio Galaxies (2)
              Radio image   3C129: Evidence
                            for the galaxy
                            moving through

Radio image
of 3C 75

               3C 75: Evidence for two nuclei
                recent galaxy merger
Jet Deflection

Formation of Radio Jets
                Jets are powered by accretion of
                matter onto a supermassive black hole

                                Black Hole
        Accretion Disk

                 Twisted magnetic fields help to
                 confine the material in the jet and to
                 produce synchrotron radiation.
The Jets of M 87
 M 87 = Central, giant elliptical galaxy in the Virgo
 cluster of galaxies

Optical and radio observations detect
a jet with velocities up to ~ 1/2 c.
M31 at Many Wavelengths

Evidence for Black Holes in AGNs
Elliptical galaxy M 84:
Spectral line shift indicates high-velocity rotation of
gas near the center.
    Visual image
                             NGC 7052:
                             Stellar velocities indicate the
                             presence of a central black hole.
Model for Seyfert Galaxies
                                          Seyfert I:
                                          Strong, broad
                                          emission lines from
Gas clouds
                                          rapidly moving gas
                                          clouds near the BH

Emission lines

          UV, X-rays

                                                       Seyfert II:
                           Supermassive      Weaker, narrow
          Accretion disk   black hole     emission lines from
                                          more slowly moving
   Dense dust torus                       gas clouds far from
                                                       the BH
Other Types of AGN and AGN Unification

                         Cyg A (radio emission)

                     Radio Galaxy:
                     Powerful “radio lobes”
                     at the end points of the
                     jets, where power in the
                     jets is dissipated.
Other Types of AGN and AGN Unification (2)
                    Quasar or BL Lac object
                    (properties very similar to
                    quasars, but no emission

                    Emission from the jet pointing
                    towards us is enhanced
                    (“Doppler boosting”) compared
                    to the jet moving in the other
                    direction (“counter jet”).
The Dust Torus in NGC 4261

Dust Torus is directly visible with Hubble Space
Black Holes in Normal Galaxies

                                 sources are
                                 mass black

             The Andromeda galaxy M 31:

             No efficient accretion onto the
                   central black hole
Black Holes and Galaxy Formation

               Interactions of galaxies not
               only produce tidal tails etc.,

                   but also drive matter
                    towards the center

                triggering AGN activity.

                 Such interactions may
                 also play a role in the
                   formation of spiral
Active nuclei in elliptical galaxies with even
more powerful central sources than Seyfert

Also show strong variability over time
scales of a few months.

Also show very strong, broad emission lines in
their spectra.
The Spectra of Quasars

                    Spectral lines show
                    a large red shift of

                    z = Dl / l0 = 0.158

The Quasar 3C 273
Quasar Red Shifts
                     Quasars have been
            z=0     detected at the highest
                       red shifts, up to
z = 0.178

 z = 0.240                z = Dl/l0

   z = 0.302          Our old formula
                         Dl/l0 = vr/c
       z = 0.389
                    is only valid in the
                    limit of low speed,
                           vr << c
Quasar Red Shifts (2)

  The full, relativistic expression always gives
  speeds less than c, but extremely large distance:

                   Several Gpc.
Studying Quasars
The study of high-redshift quasars allows
astronomers to investigate questions of:
 1) Large scale structure of the universe
 2) Early history of the universe
 3) Galaxy evolution
 4) Dark matter
Observing quasars at high redshifts:
• distances of several Gpc
• Look-back times of many billions of years
• The universe was only a few billion years old!
Probing Dark Matter with High-z Quasars:
Gravitational Lensing

                    Light from a distant quasar is bent
                        around a foreground galaxy
                    → two images of the same quasar!

                   Light from a quasar behind a galaxy
                   cluster is bent by the mass in the
                             Use to probe the distribution of
                                       matter in the cluster.
Evidence for Quasars in Distant

                  Quasar 0351+026 at the
                 same red shift as a galaxy

                   evidence for quasar
                  activity due to galaxy
Galaxies Associated with Quasars

                Two images of the same
                  quasar, 1059+730

                 New source probably a
                 supernova in the host
                  galaxy of the quasar
Host Galaxies of Quasars
Host galaxies
of most
quasars can
not be seen
because they
are outshined
by the bright
emission from
the AGN.

 Blocking out the light from the center of the quasar 3C
 273, HST can detect the star light from its host galaxy.
Gallery of Quasar Host Galaxies

Elliptical galaxies; often merging / interacting galaxies
Superluminal Motion
           Individual radio knots in quasar jets:

                 Sometimes apparently moving
                    faster than speed of light!

          time effect:

          Material in
            the jet is
         catching up
        with the light
              it emits
New Terms
radio galaxy
active galaxy
active galactic nucleus
Seyfert galaxy
double-lobed radio
double-exhaust model
hot spot
unified model
BL Lac object
relativistic Doppler
gravitational lens
superluminal expansion
relativistic jet model
Discussion Questions
1. Do you think that our galaxy has ever been an active
galaxy? Could it have hosted a quasar when it was

2. If a quasar is triggered in a galaxy’s core, what would
it look like to people living in the outer disk of the
galaxy? Could life continue in that galaxy? (Begin by
deciding how bright a quasar would look seen from the
outer disk, considering both distance and dust.)
Quiz Questions
1. Which characterizes the visible part of the spectrum for most

a. They have absorption lines of singly ionized calcium (Ca II).
b. They have absorption lines of neutral atomic hydrogen (H I).
c. They have emission lines of carbon monoxide (CO)
d. Both a and b above.
e. All of the above.
Quiz Questions
2. How are the spectra of Seyfert galaxies different from most

a. They have broad absorption lines of highly ionized elements.
b. They have broad emission lines of highly ionized elements.
c. They have narrow absorption lines of highly ionized
d. They have narrow emission lines of highly ionized elements.
e. They have a continuous spectrum.
Quiz Questions
3. What conditions can create broad emission lines of highly
ionized elements?

a. High-temperature gas must be present.
b. Low-density gas must be present.
c. The gas must be rotating at high speeds.
d. Both a and b above.
e. All of the above
Quiz Questions
4. Seyfert galaxies have a spectrum with broad emission lines
of ionized elements. What other unusual features do Seyfert
galaxies have?

a. They have small, highly luminous nuclei that fluctuate
b. They have small, dark nuclei that fluctuate rapidly.
c. They have large, highly luminous nuclei that fluctuate rapidly.
d. They have large, dark nuclei that fluctuate rapidly.
e. They all have highly red shifted spectral lines.
Quiz Questions
5. What is the difference between type 1 and type 2 Seyfert

a. Type 1 Seyferts are very luminous at ultraviolet wavelengths.
b. Type 1 Seyferts are very luminous at X-ray wavelengths.
c. Type 2 Seyferts have broader emission lines.
d. Both a and b above.
e. All of the above.
Quiz Questions
6. Galaxies in close pairs are three times more likely to be
Seyfert galaxies than are isolated galaxies. What general
conclusion can be drawn from this statistical fact?

a. Most Seyfert galaxies found in galaxy pairs are type 2.
b. Seyfert galaxies in pairs are smaller than isolated Seyfert
c. The majority of Seyfert galaxies in pairs are spiral galaxies.
d. Isolated Seyfert galaxies are most likely to be type 1.
e. Seyfert galaxies are very likely the result of galaxy
Quiz Questions
7. What is at the center of Seyfert galaxies?

a. Globular star clusters.
b. Clusters of about one million neutron stars.
c. Supermassive black holes.
d. Dwarf elliptical galaxies.
e. None of the above.
Quiz Questions
8. In the double-exhaust model, how does a double-lobed radio
source form?

a. The high-energy source at the center of the central galaxy is
transforming energy into matter and antimatter that flow out in
opposite directions to form the lobes.
b. The magnetic field of the central galaxy pulls the hot ionized
intragalactic matter toward the two galactic poles and forms
two feeding lobes.
c. The central galaxy experiences gravitational harassment due
to a near collision with another galaxy.
d. Tidal interaction with a nearby galaxy drags matter out into
the two opposing radio lobes.
e. The lobes are inflated by bipolar jets of excited gas emerging
from the central galaxy.
Quiz Questions
9. What observational evidence leads us to believe that AGNs
contain supermassive black holes?

a. Broad emission lines of ionized gases indicate that gas near
the center of an AGN is orbiting at high speeds.
b. Short-duration fluctuations in brightness limit the size of the
object at the center of an AGN.
c. High-resolution imaging reveals dark regions the size of an
event horizon at the center of some AGNs.
d. Both a and b above.
e. All of the above.
Quiz Questions
10. How do blazars (BL Lac objects) differ from type 1 Seyfert

a. Blazars are much more luminous than type 1 Seyfert
b. The luminosity of blazars fluctuates more rapidly than type 1
Seyfert galaxies.
c. The luminosity of blazars fluctuates more slowly than type 1
Seyfert galaxies.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
11. Blazars are more luminous and fluctuate much more rapidly
than both type 1 and type 2 Seyfert galaxies. How does the
unified model of an AGN’s supermassive black holes and
accretion disk explain these differences?

a. Blazars and Seyfert galaxies are different views of the
accretion disk of an AGN.
b. Blazars are the face-on view of the accretion disks of AGNs.
c. Seyfert type 1 galaxies are AGNs with the accretion disk
tipped slightly from face-on.
d. Seyfert type 2 galaxies are AGN accretion disks with an
edge-on view.
e. All of the above.
Quiz Questions
12. Why are galaxies with active nuclei more often found in
close galaxy pairs and in rich clusters of galaxies?

a. Galactic harassment is more likely under these
b. Galactic merger is more likely under these circumstances.
c. Galactic cannibalism is more likely under these
d. Galactic interactions transfer material onto the central
supermassive black holes.
e. All of the above.
Quiz Questions
13. Which of the following are characteristics of quasars?

a. Quasars have bright emission line spectra.
b. The spectra of quasars have large blue shifts.
c. Quasars are larger than the largest spiral galaxies.
d. Quasars are more abundant now than at anytime in the
history of the universe.
e. All of the above.
Quiz Questions
14. What evidence do we have that quasars are small?

a. Some quasars emit strongly at radio wavelengths.
b. They have rapid fluctuations in brightness.
c. They have large red shifts in their spectra.
d. They are very distant.
e. All of the above.
Quiz Questions
15. What evidence do we have that quasars are very far away?

a. Their spectral lines have large red shifts.
b. Some are gravitationally lensed by distant galaxies.
c. Some light from quasars contains less red shifted absorption
lines of distant galaxies.
d. Some quasars have nearby galaxies with similar red shifts in
their spectra.
e. All of the above.
Quiz Questions
16. How do quasars resemble the AGN in Seyfert galaxies?

a. They have jets and pairs of opposing radio lobes.
b. They are small and very luminous.
c. They have new chemical elements never found on Earth.
d. Both a and b above.
e. All of the above.
Quiz Questions
17. How can a quasar jet eject material at apparent
superluminal speed?

a. The accretion disk surrounding the supermassive black hole
at its center can add more than one solar mass per year.
b. The intense magnetic field of a quasar can accelerate ions to
a speed greater than the speed of light.
c. The jet ejects material at nearly the speed of light almost
directly toward Earth.
d. Both a and b above.
e. All of the above.
Quiz Questions
18. What does it mean that quasars are most common at a red
shift of about 2?

a. The average quasar has a recessional speed that is twice
the speed of light.
b. Most quasars cannot be imaged at visible wavelengths.
c. Quasars were more plentiful in the past.
d. Both a and b above.
e. All of the above.
Quiz Questions
19. Where are all the dead quasars?

a. They have dissipated into the ether.
b. They lurk quietly at the hearts of galaxies.
c. They have decayed to become gas and dust.
d. Fortunately, they are many 1000s of Mpc from Earth.
e. The residents of cosmic wormholes have consumed them.
Quiz Questions
20. Why are most quasars so far away?

a. There is a greater volume of the universe that is far away
than is nearby.
b. Quasars were more abundant in the past, when galaxies
were close together.
c. The jets have moved most of them to great distances from
d. Both a and b above.
e. All of the above

1.    d   11.   e
2.    b   12.   e
3.    e   13.   a
4.    a   14.   b
5.    d   15.   e
6.    e   16.   d
7.    c   17.   c
8.    e   18.   c
9.    d   19.   b
10.   d   20.   d

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