The Milky Way

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Galaxies

• Chapter 16

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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.
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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
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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
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Galaxies

• Star systems like our Milky Way

• Contain a few thousand to tens of billions of
  stars.

• Large variety of shapes and sizes

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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
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Galaxy Classification
Sa
E0, , E7
Large nucleus tightly wound arms
E0 Spherical
E1
Sb
Sc
Small nucleus loosely wound arms
E7 Highly elliptical
E6
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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
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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

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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
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Galaxy Types
(SLIDESHOW MODE ONLY)
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Distance Measurements to Other Galaxies (1)

• Cepheid Method Using Period Luminosity
  relation for classical Cepheids
• Measure Cepheids 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.
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Cepheid Distance Measurement
Repeated brightness measurements of a Cepheid
allow the determination of the period and thus
the absolute magnitude.
? Distance
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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
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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 H0d
H0 70 km/s/Mpc is the Hubble constant

• Measure vr through the Doppler effect ? infer
  the distance

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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

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Galaxy Sizes and Luminosities
Vastly different sizes and luminosities
From small, low-luminosity irregular galaxies
(much smaller and less luminous than the Milky
Way) to giant ellipticals and large spirals, a
few times the Milky Ways size and luminosity
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Rotation Curves of Galaxies
From blue / red shift of spectral lines across
the galaxy ? infer rotational velocity
Plot of rotational velocity vs. distance from the
center of the galaxy Rotation Curve
Observe frequency of spectral lines across a
galaxy.
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Determining the Masses of Galaxies
Based on rotation curves, use Keplers 3rd law to
infer
masses of galaxies
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Masses and Other Properties of Galaxies
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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
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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.

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Clusters of Galaxies
Galaxies generally do not exist in isolation, but
form larger clusters of galaxies.
Rich clusters 1,000 or more galaxies, diameter
of 3 Mpc, condensed around a large, central
galaxy
Poor clusters Less than 1,000 galaxies (often
just a few), diameter of a few Mpc, generally not
condensed towards the center
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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
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Our Galaxy Cluster The Local Group
Milky Way
Andromeda galaxy
Small Magellanic Cloud
Large Magellanic Cloud
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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
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Interacting Galaxies
Cartwheel Galaxy
Particularly in rich clusters, galaxies can
collide and interact.
Galaxy collisions can produce ring galaxies and
tidal tails.
NGC 4038/4039
Often triggering active star formation starburst
galaxies
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Tidal Tails
Example for galaxy interaction with tidal
tails The Mice
Computer simulations produce similar structures.
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Simulations of Galaxy Interactions
Numerical simulations of galaxy interactions have
been very successful in reproducing tidal
interactions like bridges, tidal tails, and rings.
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Mergers of Galaxies
Radio image of M 64 Central regions rotating
backward!
NGC 7252 Probably result of merger of two
galaxies, a billion years ago
Small galaxy remnant in the center is rotating
backward!
Multiple nuclei in giant elliptical galaxies
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Galactic Cannibalism

• Collisions of large with small galaxies often
  result in complete disruption of the smaller
  galaxy.
• Small galaxy is swallowed by the larger one.

NGC 5194

• This process is called galactic cannibalism

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Starburst Galaxies
Starburst galaxies are often very rich in gas and
dust bright in infrared ultraluminous infrared
galaxies
M 82
Cocoon Galaxy
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Large Scale Structure
Superclusters clusters of clusters of galaxies
Superclusters appear aligned along walls and
filaments.
Vast regions of space are completely empty
voids
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The Farthest Galaxies
The most distant galaxies visible by HST are seen
at a time when the universe was only 1 billion
years old.
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New Terms
spiral nebula island universe ShapleyCurtis
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 ultralumi
nous infrared galaxy starburst galaxy
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Quiz Questions
9. What must we know about an object to use it as
a distance indicator? a. The objects
luminosity. b. The objects linear size. c. The
objects age. d. Either a or b above. e. Either
a, b, or c above.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Answers
1. c 2. e 3. a 4. c 5. e 6. e 7. c 8. e 9. d 10. a
11. e 12. a 13. a 14. e 15. d 16. b 17. d 18. a 19
. e 20. c
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Hubble
(SLIDESHOW MODE ONLY)