Chapter 24 - Quasars and Other Active Galaxies
CHAPTER 24
QUASARS AND OTHER ACTIVE GALAXIES
CHAPTER OUTLINE AND LECTURE NOTES
1. Quasars
Some of the more alert students may notice that the greatest distances in Figure 24.3 are
about 25 billion light years. Yet for the same assumptions (a Hubble constant of 70
km/s/Mpc and a flat universe dominated by dark energy) Chapter 26 gives the age of the
universe as about 13.5 billion years. How could we see objects 25 billion light years away
if the universe is only 13.5 billion years old? There is really no paradox, however, because
the distances in Figure 24.3 are the present distances of the galaxies, not their distances at
the time the light we are now receiving was emitted.
2. The Active Galaxy Zoo
I have somewhat arbitrarily discussed IRAS galaxies in the section on active galaxies. The
most luminous IRAS galaxies are probably appropriate in this section because they may
turn out to be dust-shrouded quasars. Starburst galaxies may turn out to have nothing to do
with the quasar phenomenon except that their activity is powered by interactions with other
galaxies. I didn’t want to divide the IRAS galaxies into the most luminous IRAS galaxies
in this chapter and starburst galaxies in Chapter 23.
3. Massive Black Holes and Active Galaxies
Its hard to see what would constitute “proof” that AGN contain black holes. As I state in
the textbook, the best evidence is that nothing else can explain the phenomena found in
AGN. This still leaves room for skeptics to declare that the case hasn’t yet been proved.
4. Evolution of Quasars
The section “When Did Quasars Form?” discusses how the space density of quasars
increases with distance out to a redshift of about 2.5 and then decreases for greater
distances. I sometimes compare this situation to that of paleontologists digging through
layers of strata to find dinosaur bones. In the case of the dinosaur bones, digging deeper is
actually exploring further back in time. No dinosaur bones are found until the depth
corresponding to 65 million years is reached. Starting at that depth, dinosaur bones are
numerous until the digging reaches a depth when dinosaurs had recently evolved and were
becoming more numerous. Eventually the digging reaches the depth corresponding to the
time when dinosaurs originated. At greater depths there are no dinosaur bones. The
analogy to distance as time and probing the evolution of quasars is quite apparent to most
students.
5. Quasars As Probes of the Universe
Figure 24.29 shows the deflection of light from a quasar by a cluster of galaxies. The
banana shaped thing in the upper left is the distorted and displaced image of the quasar.
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Chapter 24 - Quasars and Other Active Galaxies
KEY TERMS
active galactic nucleus — The nucleus of an active galaxy.
active galaxy — A galaxy whose nucleus is unusually bright and small. Seyfert galaxies, BL
Lacertae objects, and quasars are examples of active galaxies.
blazar — A type of active galaxy named for BL Lacertae, the first of the type discovered.
Blazars show rapid, unpredictable variations in brightness.
broad line region — The high density region in a quasar where broad emission
lines are formed.
Eddington luminosity — The maximum luminosity that a body could emit without driving
away surrounding material.
Einstein ring — The ring or near ring into which the image of a distant quasar is distorted if the
quasar lies directly behind a galaxy or cluster of galaxies producing a gravitational lens.
gravitational lens — A massive body that bends light passing near it. A gravitational lens can
distort or focus the light of background sources of electromagnetic radiation.
jet — A narrow beam of gas ejected from a star or the nucleus of an active galaxy.
lookback time — The length of time that has elapsed since the light we are now receiving from
a distant object was emitted.
luminosity function — The distribution of stars or galaxies according to their luminosities. A
luminosity function is often expressed as the number of objects per unit volume of space
that are brighter than a given absolute magnitude or luminosity.
Lyman  forest — The large number of absorption lines seen at wavelengths just longer than
the wavelength of the Lyman  line of hydrogen in the spectrum of a quasar. The Lyman 
forest is caused by absorption by gas clouds lying between the quasar and the Earth.
narrow line region — The low density region in a quasar where narrow emission lines are
formed.
quasar — A distant galaxy, seen as it was in the remote past, with a very small, luminous
nucleus.
radio galaxy — A galaxy that is a strong source of radio radiation.
Seyfert galaxy — A barred or normal spiral galaxy with a small, very bright nucleus.
starburst galaxy — A galaxy in which a very large number of stars have recently formed.
superluminal motion — The apparent separation of components of a quasar at speeds faster
than the speed of light.
V/Vmax test — A statistical method used to determine whether quasars have changed over time.
ANSWERS TO QUESTIONS AND PROBLEMS
Conceptual Questions
1. Quasars are extremely blue in color.
2. The broad line region has a higher density and less matter than the narrow line region.
3. Quasars vary in brightness in a few years or less.
4. 4 light years across
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Chapter 24 - Quasars and Other Active Galaxies
5. Superluminal motions will occur if components of quasars move almost directly at us at
speeds just less than the speed of light.
6. Seyfert 1 galaxies have both broad and narrow emission lines whereas Seyfert 2 galaxies
have only narrow emission lines.
7. The broad line region of Seyfert galaxy is within a torus of dust that blocks the broad line
region when viewed from near the plane of the dust, resulting in the classification of the
galaxy as a Seyfert 2. Viewed from above the torus, the broad line region can be seen and
the galaxy is classified as Seyfert 1.
8. They are carried into the lobes by jets from the center of the galaxy.
9. The lobes are regions in which electrons in the jets are slowed and emit synchrotron
radiation.
10. A jet pointed toward us has its brightness enhanced whereas a jet pointed away from us has
its brightness diminished. In cases where only one jet is seen, the other jet (pointing away
from us) is too dim to be seen.
11. Blazars are small, bright, and show brightness variations. Unlike quasars, they have weak
emission lines or no lines at all.
12. If a jet is pointed directly at us, the continuous radiation it produces appears so bright that it
overwhelms the emission line spectrum and we see a blazar. If the jet isn’t pointed directly
at us, we see quasar-like emission lines.
13. The great luminosities of quasars would blow surrounding matter away unless the black
holes were massive enough to attract surrounding gas.
14. If objects are uniformly distributed through space, some of them will be barely detectable
and have values of V/V(max) greater than 0.5. Other objects will be so near that they could
be detected if they were much farther away. These will have values of V/V(max) less than
0.5.
15. This shows that most quasars are very distant and are seen as they were in the remote past.
16. Quasars have dimmed by more than 100 times since they formed.
17. Despite many searches, few quasars with redshifts greater than 3 have been found. This
shows that there were few quasars in the early universe.
18. Many galaxies have regions in their cores that resemble very weak quasars. Also, massive
but not very luminous objects have been found in the centers of several nearby galaxies.
19. An Einstein Ring is produced if the distant quasar lies directly behind the galaxy or
galaxies producing the gravitational lens.
Problems
1. 3 × 1013 solar luminosities
2. 3 × 107 solar masses
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Chapter 24 - Quasars and Other Active Galaxies
Figure-based Questions
1. 0.9
2. 3200 Mpc
3. 0.6
4. 1.1
5. 0.4 milliarcseconds/y, 250 years
6. 2c
7. 7° or 23°
8. Seyfert 2 galaxy
9. Blazar
10. 0.8
11. The space density of old quasars was about 5 times as large
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