Announcements

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Announcements
Assigned reading Chapter 16. Course evaluation
10 minutes before end of class, with Hansung.
Starry Night Saint-Rémy June, 1889 Van Gogh
Out in the sky the great dark clouds are massing
I look far out into the pregnant night Paul
Laurence Dunbar Ships That Pass in the Night
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Galaxies
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Survey Questions

• What are the key characteristics of the four
  basic types of galaxies?
• What is the Hubble Tuning Fork Scheme?
• Does the scheme really tell you a formation
  sequence of galaxies (NO)?
• How do galaxies form?

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Galaxies take one of four different morphologies
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SPIRALS
Spirals have a disk with dust and gas star
formation
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The tightness of a spiral galaxys arms is
correlated to the size of its nuclear bulge
Type Sa
Type Sb
Type Sc
The tighter the arms, the bigger the bulge
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Variety of Spiral Arms
Flocculent spirals (fleecy)
Grand-design spirals (highly organized)
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We easily see these spiral arms because they
contain numerous bright O and B stars which
illuminate dust in the arms.
However, stars in total are very evenly
distributed throughout the disk.
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BARRED SPIRALS
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BARRED SPIRALS
The bar is not a spiral arm. It is made of old
stars, like those in the bulge
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Bars of stars run through the nuclear bulges of
barred spiral galaxies
Type SBa
Type SBb
Type SBc
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ELLIPTICALS
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ELLIPTICALS
Ellipticals have no dust and no cold gas, hence
no star formation. Made mostly of old,
metal-enriched stars. They like to cluster a lot
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Discussion Question
Given their color, do you expect elliptical
galaxies to have more or less new star formation
than spirals? Why? 1) ellipticals have more
new star formation 2) ellipticals have less new
star formation
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Elliptical galaxies display a variety of sizes
and masses

• Giant elliptical galaxies can be 20 times larger
  than the Milky Way
• Dwarf elliptical galaxies can be as small as a
  big globular cluster. They are extremely common
  and can contain as few as a million stars

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IRREGULAR
Irregulars mostly have gas, dust and star
formation. They contain both old and young stars
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The classification scheme of galaxy morphology is
known as the Hubble Tuning Fork Scheme
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Galaxies like to clustereg. M81 group
Optical Image
Radio Image
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Hercules Cluster
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The ideas of galaxy formation

• Spiral galaxies form from the collapse of
  spinning gas cloud
• Elliptical galaxies form from the mergers of disk
  galaxies, or from clouds with low spinning
  (probably wrong for big galaxies might work for
  early galaxies)
• Peculiar galaxies are formed through the
  interactions of galaxies
• Most popular theory CDM (Cold Dark
  Matter)hierarchical cosmology. Structures form
  from bottom to top small ones first, bigger ones
  later by merging.
• CDM predicts hierarchical assembly of cosmic
  structures. Many of its predictions are verified.

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Galaxy Merging evolution of morphology, trigger
of star formation
Billions of stars all tug on each other instead
of just one planet tugged by the gravity of the
Sun.
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How two spirals collide and merge, making tidal
tails and, at the end, an elliptical galaxy
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Survey Questions

• What are the key characteristics of the four
  basic types of galaxies?
• What is the Hubble Tuning Fork Scheme?
• Does the scheme really tell you a formation
  sequence of galaxies? NO!!!!! It is just a
  morphological classification scheme

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How did we find Dark Matter The mass of
galaxies and rotation curves
A rotation curve is just a plot of rotational
velocity vs distance for objects in (roughly)
circular orbits. For a set of objects
orbitinga common point, what isthe orbital
velocity for different objects at different
distances from the common point?
M
If you think about it, this is just Keplers
Third Law applied to galaxies
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Rotation Curve for a rigid bodyno gravity
involved, no information on mass
Straight because objects arerigidly held.
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Rotation Curve for our Solar Systemgravity
involved, information on mass
Curve decreases because mass that generates the
gravity is all at the center (Sun is much, much
greater than all planets together.
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Rotation curve of the MW disk(and of all other
galaxies)
It is essentially Keplers Third Law One uses it
to measure the mass of galaxies
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Evidence for Dark Matter

• Evidence 1 - The amount of mass that we can
  see by counting the stars, the gas and the dust
  in a galaxy is not enough compared to the
  observed gravitational mass measured with the
  rotation curves.

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Evidence for Dark Matter

• Evidence 2 - The amount of mass that we can
  see in a cluster of galaxies does not account
  for the observed gravitational pull on the
  galaxies in the cluster (as evidenced by the
  large variation in velocities of the galaxies).

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Hercules Cluster
Galaxies in a cluster have more erratic
velocities than we expect from the amountof
matter we can see in the cluster
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Evidence for Dark Matter

• Evidence 3 - The intracluster gas is too hot to
  be retained by the gravity of the visible matter
  in a cluster.
• If all the mass there really were only that of
  visible matter, its gravity would not be enough
  to retain the hot gas, which would evaporate
  rapidly. Since the gas is there, there must be
  more gravity, hence dark matter.

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Cluster of galaxies Hydra A
Optical
X-ray
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Evidence 4 gravitational lensing
Just as black holes and other massive objects
curve space around them, galaxies and galaxy
clusters curve space. When the alignment of a
large mass and background objects is right, a
dramatic effect is observed. This is known as a
gravitational lens.
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Lensing by a densecluster of galaxies. Using
Einsteins laws of general relativity,we can
estimate themass of the lens. About 10 times
moremass is present thanone would estimate
bysumming the mass ofthe visible galaxies.
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ForegroundCluster ofGalaxies
Background lensed galaxies
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Our picture of the mass around galaxies now looks
something like this
Dark matter outweighs Normal matter on average
by 10 to 1
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So what is this missing mass?

• The only way to explain the rotation curve of our
  galaxy is to say that there is lots and lots of
  mass that is not emitting light.
• The halo of our galaxy must be full of it. The
  halo outweighs the disk by a factor of 10.
• As far as we can tell, this mass doesnt emit any
  light at any frequency.

What is the form of the missing mass?
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Dark Matter Possibilities

• Here is the first lists of candidate materials
  for the dark matter that dominates the mass in
  our galaxy.
• Black Holes
• Black Dwarfs
• Brown Dwarfs
• Unknown particles

Baryonic Matter (e, p, n the same stuffthat
were made of) Non baryonic matter
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Does dark matter matter?
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Yes! Dark Matter Rules the Universe

• Since dark matter outweighs other matter by a
  factor of 10, nearly all the large scale
  gravitational interactions in the Universe are
  dominated by the dark matter
• Dark matter holds galaxies together without it
  the outer stars would escape
• Dark matter holds clusters of galaxies together
  without it, the clusters would disperse
• Dark matter causes these structures to form in
  the first place!

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Which of the following is NOT a good reason for
learning more about dark matter? dark matter
provides the gravitational force that holds
galaxies together the total amount of dark
matter determines the fate of our Universe the
Earth would be destroyed if it ever ran into a
clump of cold dark matter dark matter is
responsible for the formation of structure in
our Universe our understanding of dark matter
may reveal a new, undiscovered form of matter
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What is the best evidence for dark matter in
galaxies? 1) the orbital speeds of gas clouds
and stars 2) the orbital distances of gas
clouds and stars 3) the low density of stars in
spiral arms 4) the large orbital eccentricities
of disk objects 5) the lack of heavy elements
in globular cluster stars
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Dark Matter Summary

• 90 of the universe is made of matter that does
  not emit detectable radiation at any wavelength.
• Evidence to date suggests that only a very small
  fraction of the dark matter is made of familiar
  matter (baryons).
• Dark matter is not stars or stellar remnants,
  galaxies, dust clouds, or anything else made of
  protons, neutrons, or electrons as far as we
  can tell, it is an as of yet undiscovered form of
  matter.

This is one of the biggest mysteries facing
astronomers What is our Universe made of?
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Survey Questions

• What is a rotation curve of a galaxy?
• How can a rotation curve be used to measure the
  mass of the galaxy?
• Where is most of the gravitational mass in a
  galaxy?
• What are the lines of evidence for dark matter in
  the universe?