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Chapter 16
Galaxies
Guidepost
The preceding chapter was about our Milky Way Galaxy, an
important object to us but only one of the many billions of
galaxies visible in the sky. We can no more understand
galaxies by understanding a single example, the Milky Way,
than we could understand humanity by understanding a
single person. This chapter expands our horizon to discuss
the different kinds of galaxies and their complex histories.
We take two lessons from this chapter. First, galaxies are not
solitary beasts; they collide and interact with each other.
Second, most of the matter in the universe is invisible. The
galaxies we see are only the tip of a cosmic iceberg.
We will carry the lessons of this chapter into the next, where
we will discuss violently active galaxies, and on into Chapter
18, where we discuss the universe as a whole.
Outline
I. The Family of Galaxies
A. The Discovery of Galaxies
B. The Shapes of Galaxies
C. How Many Galaxies?
II. Measuring the Properties of Galaxies
A. Distance
B. The Hubble Law
C. Diameter and Luminosity
D. Mass
E. Supermassive Black Holes in Galaxies
F. Dark Matter in Galaxies
Outline (continued)
III. The Evolution of Galaxies
A. Clusters of Galaxies
B. Colliding Galaxies
C. The Origin and Evolution of Galaxies
D. The Farthest Galaxies
Galaxies
• Star systems like our Milky Way
• Contain a few thousand to tens of billions of stars.
• Large variety of shapes and sizes
Galaxy Diversity
Even seemingly
empty regions
of the sky
contain
thousands of
very faint, very
distant galaxies
Large variety of
galaxy
morphologies:
Spirals
Ellipticals
Irregular
(some interacting)
The Hubble Deep Field:
10-day exposure on an apparently empty field in the sky
Galaxy Classification
E0, …, E7
Sa
E0 =
Spherical
Large
nucleus;
tightly
wound arms
E1
Sb
Sc
E7 = Highly
elliptical
E6
Small
nucleus;
loosely
wound arms
Gas and Dust in Galaxies
Spirals are rich in
gas and dust
Ellipticals are almost
devoid of gas and dust
Galaxies with disk and bulge,
but no dust are termed S0
Barred Spirals
• Some spirals show a
pronounced bar structure
in the center
• They are termed
barred spiral galaxies
• Sequence:
SBa, …, SBc,
analogous to regular
spirals
Irregular Galaxies
Often: result of galaxy
collisions / mergers
Often: Very active star formation
(“Starburst galaxies”)
The Cocoon
Galaxy
NGC 4038/4039
Some: Small (“dwarf galaxies”)
satellites of larger galaxies
(e.g., Magellanic Clouds)
Large
Magellanic
Cloud
Galaxy Types
(SLIDESHOW MODE ONLY)
Distance Measurements to Other
Galaxies (1)
a) Cepheid Method: Using Period – Luminosity relation
for classical Cepheids:
Measure Cepheid’s Period  Find its luminosity 
Compare to apparent magnitude  Find its distance
b) Type Ia Supernovae (collapse of an accreting white
dwarf in a binary system):
Type Ia Supernovae have well known standard
luminosities  Compare to apparent magnitudes  Find
its distances
Both are “Standard-candle” methods:
Know absolute magnitude (luminosity)  compare to
apparent magnitude  find distance.
Cepheid Distance Measurement
Repeated
brightness
measurements
of a Cepheid
allow the
determination
of the period
and thus the
absolute
magnitude.
 Distance
The Most Distant Galaxies
At very large
distances, only
the general
characteristics
of galaxies can
be used to
estimate their
luminosities 
distances.
Cluster of galaxies at ~ 4 to 6 billion light years
Distance Measurements to Other
Galaxies (2): The Hubble Law
E. Hubble (1913):
Distant galaxies are moving away from our Milky Way, with
a recession velocity, vr, proportional to their distance d:
vr = H0*d
H0 ≈ 70 km/s/Mpc is the
Hubble constant
• Measure vr
through the
Doppler effect 
infer the distance
The Extragalactic Distance Scale
• Many galaxies are typically millions or
billions of parsecs from our galaxy.
• Typical distance units:
Mpc = Megaparsec = 1 million parsec
Gpc = Gigaparsec = 1 billion parsec
• Distances of Mpc or even Gpc  The
light we see left the galaxy millions or
billions of years ago!!
• “Look-back times” of millions or billions of years
Galaxy Sizes and Luminosities
Vastly different sizes
and luminosities:
From small, lowluminosity irregular
galaxies (much
smaller and less
luminous than the
Milky Way) to giant
ellipticals and large
spirals, a few times
the Milky Way’s
size and luminosity
Rotation Curves of Galaxies
From blue / red shift of spectral
lines across the galaxy
 infer rotational velocity
Observe frequency of
spectral lines across a
galaxy.
Plot of rotational velocity vs.
distance from the center of
the galaxy: Rotation Curve
Determining the Masses of Galaxies
Based on rotation curves,
use Kepler’s 3rd law to infer
masses of galaxies
Masses and Other Properties of
Galaxies
Supermassive Black Holes
From the
measurement of
stellar velocities
near the center of
a galaxy:
Infer mass in the
very center 
central black
holes!
Several million, up
to more than a
billion solar masses!
 Supermassive
black holes
Dark Matter
Adding “visible” mass in:
• stars,
• interstellar gas,
• dust,
…etc., we find that most of the mass is “invisible”!
• The nature of this “dark matter” is not
understood at this time.
• Some ideas: brown dwarfs, small black holes,
exotic elementary particles.
Clusters of Galaxies
Galaxies generally do not exist in isolation,
but form larger clusters of galaxies.
Rich clusters:
Poor clusters:
1,000 or more galaxies,
diameter of ~ 3 Mpc,
condensed around a large,
central galaxy
Less than 1,000 galaxies
(often just a few),
diameter of a few Mpc,
generally not condensed
towards the center
Hot Gas in Clusters of Galaxies
Space between galaxies is not empty, but filled with
hot gas (observable in X-rays)
That this gas remains gravitationally bound provides
further evidence for dark matter.
Visible light
X-rays
Coma Cluster of Galaxies
Our Galaxy Cluster: The Local Group
Milky Way
Small Magellanic
Cloud
Large Magellanic
Cloud
Andromeda
galaxy
Neighboring Galaxies
Some galaxies of our local group are difficult to observe
because they are located behind the center of our Milky
Way, from our view point.
Spiral Galaxy Dwingeloo 1
Interacting Galaxies
Cartwheel Galaxy
Particularly in rich
clusters, galaxies can
collide and interact.
Galaxy collisions
can produce
ring galaxies and
NGC 4038/4039
tidal tails.
Often triggering active
star formation:
starburst galaxies
Tidal Tails
Example for galaxy
interaction with tidal tails:
The Mice
Computer simulations
produce similar
structures.
Simulations of Galaxy Interactions
Numerical
simulations
of galaxy
interactions
have been
very
successful
in
reproducing
tidal
interactions
like bridges,
tidal tails,
and rings.
Mergers of Galaxies
NGC 7252:
Probably
result of
merger of two
galaxies, ~ a
billion years
ago:
Small galaxy
remnant in the
center is
rotating
backward!
Radio image of M 64: Central
regions rotating backward!
Multiple
nuclei in giant
elliptical
galaxies
Galactic Cannibalism
NGC 5194
• Collisions of large
with small galaxies
often result in
complete disruption
of the smaller
galaxy.
• Small galaxy is
“swallowed” by the
larger one.
• This process is called
“galactic cannibalism”
Starburst Galaxies
M 82
Starburst galaxies
are often very rich in
gas and dust; bright
in infrared:
ultraluminous
infrared galaxies
Cocoon Galaxy
Large Scale Structure
Superclusters
= clusters of
clusters of
galaxies
Superclusters
appear aligned
along walls and
filaments.
Vast regions of
space are
completely empty:
“voids”
The Farthest Galaxies
The most distant galaxies visible by HST are seen at a
time when the universe was only ~ 1 billion years old.
New Terms
spiral nebula
island universe
Shapley–Curtis Debate
elliptical galaxy
spiral galaxy
barred spiral galaxy
irregular galaxy
megaparsec (Mpc)
distance indicator
standard candle
distance scale
look-back time
Hubble law
Hubble constant (H)
rotation curve
rotation curve method
cluster method
velocity dispersion method
rich cluster
poor cluster
ring galaxy
galactic cannibalism
ultraluminous infrared
galaxy
starburst galaxy
Discussion Questions
1. From what you know about star formation and the
evolution of galaxies, do you think the Infrared
Astronomy Satellite should have found irregular
galaxies to be bright or faint in the infrared? Why or why
not? What about starburst galaxies? What about
elliptical galaxies?
2. Imagine that we could observe a gas cloud at such a
high look-back time that it is just beginning to form one
of the first galaxies. Further, suppose we discovered
that the gas was metal rich. Would that support or
contradict our understanding of galaxy formation?
Quiz Questions
1. How was William Parsons (Lord Rosse) in 1845 able to see
spiral structure in some nebulae, whereas others had not
noticed this spiral structure before?
a. He had incredibly large pupils and keen eyesight.
b. His observatory was located on a high, dry mountain peak.
c. His telescope, with a diameter of 72 inches, was the largest
in the world.
d. No nebulae had been observed with a telescope before his
time.
e. His long time exposure photographs that revealed the spiral
structure.
Quiz Questions
2. What did William Parsons (Lord Rosse) think the spiral
nebulae were?
a. Spiral clusters of low luminosity stars located nearby.
b. New planetary systems in the process of formation.
c. Spiral star clusters located in the Milky Way.
d. Dying high-mass stars.
e. Island universes.
Quiz Questions
3. What was the topic of the Shapley-Curtis Debate of 1920?
a. The location of the spiral nebulae.
b. The size of the Milky Way Galaxy.
c. The period-luminosity relationship of Cepheid variable stars.
d. The period-luminosity relationship of RR Lyrae variable
stars.
e. The time-sharing schedule of the new 100 inch diameter
telescope.
Quiz Questions
4. Edwin Hubble resolved the Shapley-Curtis debate in 1924 by
measuring the distance to large, bright spiral nebulae. What
distance method did Hubble employ?
a. The parallax method.
b. The Hubble Law method.
c. The Cepheid variable star method.
d. The spectroscopic parallax method.
e. The RR Lyrae variable star method.
Quiz Questions
5. Galaxies with active star formation also have which of the
following?
a. Plenty of gas and dust.
b. O and B associations.
c. Emission nebulae
d. A bluish tint.
e. All of the above.
Quiz Questions
6. The Hubble deep field image reveals more than 1500 distant
galaxies in a region about 1 arc minute in diameter. If this
density of galaxies is typical over the whole sky, how many
distant galaxies are hidden from view at one instant by the
Moon, with an angular diameter of about 30 arc minutes?
a. Approximately 150 distant galaxies are hidden behind the
Moon at any given time.
b. Approximately 1500 distant galaxies are hidden behind the
Moon at any given time.
c. Approximately 15,000 distant galaxies are hidden behind the
Moon at any given time.
d. Approximately 150,000 distant galaxies are hidden behind
the Moon at any given time.
e. More than 1,000,000 distant galaxies are hidden behind the
Moon at any given time.
Quiz Questions
7. How does a Sa galaxy differ from a Sc galaxy?
a. The Sa galaxy has a smaller nuclear bulge.
b. The Sa galaxy has more loosely wound spiral arms.
c. The Sa galaxy has less gas and dust.
d. Both a and c above.
e. Both b and c above.
Quiz Questions
8. What gives elliptical galaxies a redder color than spiral
galaxies?
a. Elliptical galaxies are more massive, and thus have a larger
gravitational red shift.
b. Elliptical galaxies are more distant, and thus have more
interstellar reddening.
c. Elliptical galaxies are more distant, and thus have larger red
shifts.
d. Elliptical galaxies have a higher percentage of iron.
e. Elliptical galaxies have less gas and dust.
Quiz Questions
9. What must we know about an object to use it as a distance
indicator?
a. The object’s luminosity.
b. The object’s linear size.
c. The object’s age.
d. Either a or b above.
e. Either a, b, or c above.
Quiz Questions
10. Which of the following is NOT a distance indicator used in
galactic astronomy?
a. White dwarfs.
b. Cepheid variable stars.
c. Planetary nebulae.
d. Type Ia supernovae.
e. Brightest globular cluster.
Quiz Questions
11. What observable property of a standard candle must be
measured to determine its distance?
a. Age.
b. Mass.
c. Luminosity.
d. Angular size.
e. Apparent magnitude.
Quiz Questions
12. Why is a supernova type Ia standard candle better to use in
measuring very long distances than either the brightest
globular cluster or Cepheid variable star standard candles?
a. Type Ia supernovae are more luminous.
b. Globular star clusters and Cepheid variables exist only in the
Milky Way Galaxy.
c. Type Ia supernovae are very common.
d. The calibration of Type Ia supernovae is more precise.
e. Both c and d above.
Quiz Questions
13. When viewing a distant galaxy, the amount of look-back
time in years is equal to the
a. distance to the galaxy in light years.
b. round-trip distance to the galaxy in light years.
c. time that has passed since the galaxy was first discovered.
d. exposure time of a photograph that is taken of that galaxy.
e. time since you last looked at the galaxy.
Quiz Questions
14. What is the distance to a galaxy that has a recessional
speed of 3500 km/s if the Hubble constant is 70 km/s/Mpc?
a. 10 Mpc.
b. 20 Mpc.
c. 30 Mpc.
d. 40 Mpc.
e. 50 Mpc.
Quiz Questions
15. What parameter must we know to determine the linear
diameter and luminosity of a particular galaxy?
a. The type of galaxy.
b. The age of the galaxy.
c. The mass of the galaxy.
d. The distance to the galaxy.
e. The amount of gas and dust in the galaxy.
Quiz Questions
16. Which type of galaxy has the largest range in both size and
mass?
a. Spiral.
b. Elliptical
c. Irregular.
d. Both a and b have the same range.
e. All of the above have the same range.
Quiz Questions
17. What assumption is made in determining mass by the
rotation curve method, the cluster method, and the velocity
dispersion method?
a. That Newton's law of gravity is valid.
b. That the system being measured is holding itself together.
c. That spiral galaxies rotate faster than elliptical galaxies.
d. Both a and b above.
e. All of the above.
Quiz Questions
18. Which feature of a galaxy seems to be related to the mass
of the galaxy's central supermassive black hole?
a. The size of the galaxy's central bulge.
b. The rotation curve of the galaxy.
c. The overall color of the galaxy.
d. The mass of the galaxy.
e. The age of the galaxy.
Quiz Questions
19. Which of the following is the least important factor in
galactic evolution?
a. Collision of galaxies.
b. The merger of larger galaxies.
c. The harassment by galaxies that pass nearby.
d. Small galaxies are cannibalized by larger galaxies.
e. The elemental composition of the material from which
galaxies form.
Quiz Questions
20. In a few billion years our own Milky Way may merge with
the Andromeda galaxy. Which type of galaxy is most likely to
result from such a merger?
a. An irregular galaxy.
b. A larger spiral galaxy.
c. An elliptical galaxy.
d. Either a or c above.
e. Either b or c above.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
c
e
a
c
e
e
c
e
d
a
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
e
a
a
e
d
b
d
a
e
c
Hubble
(SLIDESHOW MODE ONLY)