Virtually all galaxies show a flat rotation curve.
Slide 2
Lets look back at the rotational velocity equation: v = (M
interior G/r) 0.5 The rotational velocity is constant at big r
values. What must we conclude?
Slide 3
The mass interior to the orbit is still increasing. Even after
the radius r has gone beyond the last of the stars. It must be
increasing in order to hold v constant. This is dark matter. We
dont know what it is but it has mass. It surrounds galaxies in a
huge dark matter halo. It doesnt interact with light or we could
observe it. That is true even for gas.
Slide 4
Other ways to detect Dark Matter Gravitational lensing. The
amount that the light is bent is directly proportional to the mass
bending the light.
Slide 5
Clusters of Galaxies. Galaxies that are in clusters have
velocities that are many orders of magnitude to large to be bound
by the cluster. This means that either, the galaxy clusters
everywhere in the universe are flying apart, or else there is a
huge amount of dark matter present.
Slide 6
Slide 7
The results from all these different observations is that
around 90 to 95% of the mass in the universe is Dark Matter. When
we look at a galaxy, we are seeing the luminous matter. But that
matter is embedded in a much larger Dark Matter halo which contains
around 90% of the mass of the galaxy. And we cant even see it. We
can only measure its presence using velocities.
Slide 8
So we see this.
Slide 9
But it is only the tip of the iceberg
Slide 10
In order to model galaxy interactions, it is necessary to use
all the mass, not just the luminous mass. In the simulations we
have seen so far, dark matter is explicitly put into the
simulation. Let see what will happen some day when the Milky Way
and Andromeda collide.
Slide 11
The Antennae Galaxies
Slide 12
Slide 13
When this merger is finished, stars will be thrown out of the
new galaxy onto very randomly inclined orbits. They will no longer
be confined to a disk. Also the furious star formation will leave
the resulting galaxy with very little gas. This means very little
new star formation is possible. The result is an elliptical
galaxy.
Slide 14
Slide 15
M 87 in the Virgo Cluster
Slide 16
Centaurus A A radio emitting elliptical galaxy
Slide 17
Composite image of Centaurus A Bi-Polar outflows
Slide 18
Slide 19
Giant elliptical galaxies are usual found at the very center of
galaxy clusters. This is where the density of galaxies is the
highest. The central dominant (CD) elliptical galaxy, continues to
absorb more and more smaller spirals until they grow enormous. Some
CD galaxies have more than 1 trillion stars.
Slide 20
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Slide 26
In dense galaxy clusters, there is clearly an enormous amount
of interactions and mergers occurring. Most galaxies are not in
such dense environments. And the number of mergers and interactions
are much less. A good example is our Local Group of galaxies. The
Local group has three major galaxies, M31, M33 and the Milky Way,
and a few dozen little satellite galaxies.
Slide 27
But when we look to very great distances, virtually all
galaxies are in the process of merging.
Slide 28
Slide 29
Why are there so many galaxy mergers when we look very far
away. 1.We are in a part of the universe where galaxies are very
far apart. 2.We are looking into the distant past, when the
universe was smaller and therefore more dense. 3.Both 1 and 2 30 0
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Slide 30
When we look back 12 billion light years we are looking to a
time when galaxies were just beginning to form. About 1 billion
years after the Big Bang. There are virtually no normal galaxies.
They are all fragments that are in the process of merging.
Slide 31
Galactic dynamics of stars and gas. For many years, starting in
1962, it was expected that the Milky Way formed from a huge
proto-galactic gas cloud. Very similar to the way proto-stars are
believed to form. (Text book Figure 16.14)
Slide 32
Proto-galaxy gas begins as a huge cloud with a small amount of
spin. As gravity pulls the gas closer to the center, star formation
begins. Also, the conservation of angular momentum makes the
flattening disk spin rapidly.
Slide 33
In this model, the first stars to form are out in the halo of
the Galaxy. Since they nearly the first stars to form, they have
very little heavy elements in them. The elements heavier than
hydrogen and helium are made in stars. Since there were no stars to
make these elements, the first stars had virtually none. The gas
continues to spiral in an form a disk. But the stars that are
formed in the halo, do not spiral in. They remain in the halo where
they formed. Lets think about why this would be.
Slide 34
Gas Atoms in gas clouds that are falling into the central
regions of the Galaxy can collide with one another. Since they are
falling in they clearly have kinetic energy. What happens when the
atoms in a gas cloud run into each other?
Slide 35
What happens when moving atoms in a gas run into each other?
1.They can radiate light 2.They bounce off each other into new
directions 3.They explode 30 0 1234567891011121314151617181920
21222324252627282930
Slide 36
What happens when they radiate light? 1.The atoms get hot and
expand 2.The atoms use some of their kinetic energy and slow down
3.The atoms explode 30 0 1234567891011121314151617181920
21222324252627282930
Slide 37
Since atoms can radiate energy, their kinetic energy is used to
produce radiant energy. This slows down the gas and allows it to
fall into a disk around the Milky Way. What about the stars that
formed in the halo? The distances between stars is enormous and the
chance of a collision between stars is a virtual impossibility.
What happens when a halo star falls in toward the center of the
Galaxy?
Slide 38
1.Its eaten by the supermassive black hole 2.At the center it
stops and becomes a bulge star 3.It flies back out into the halo.
30 0 1234567891011121314151617181920 21222324252627282930
Slide 39
So the stars, being collision-less, retain their orbits. They
have no way to get rid of their kinetic energy and slow up. They
just fly back out in to the halo after passing close to the inner
parts of the Galaxy. The gas collides and radiates. It can loose
kinetic energy and slow down. This allows it to form a disk. Stars
that later form in the disk have the same orbits as the gas in the
disk. This is a very good model for Galaxy formation, but
unfortunately it doesnt work.
Slide 40
Predictions from the single collapsing proto-galaxy model. We
would expect stars in the halo to be very old and very heavy
element poor. And they are. We would expect that the disk stars are
younger and more heavy element rich. And they are. We would also
expect that the stars that formed as the gas collapsed into a disk,
should have ever increasing heavy element content, and should have
orbits that begin randomly oriented (outer halo) becoming more
disk-like orbits as the gas flattened in to a disk. This we do NOT
see.
Slide 41
Todays model for the formation of the Milky Way and other
galaxies The Galaxy formed out of the merger of smaller galaxy
fragments. These small star forming galaxies were the first objects
to form in the universe. Each one has its own Dark Matter halo. As
pieces begin to merge, so do the dark matter halos, and the region
becomes one big dark matter halo.
Slide 42
The galactic fragments had already begun to form stars has they
merged together to form the Galaxy. These stars retained their
orbits and made the halo of the Galaxy. The gas collided and sunk
to the center. The Milky Way was built up piece-meal in this
fashion. Today, galaxy interactions between the primary spiral
galaxy and its satellites are much less frequent, because there are
few satellites remaining. The Milky Way is in the process of eating
a satellite galaxy today. This is the Sagittarius Dwarf
galaxy.
Slide 43
Sagittarius Dwarf galaxy and stream
Slide 44
Sagittarius tidal stream of stars.
Slide 45
Slide 46
Tidal streams from a dwarf galaxy around a galaxy.
Slide 47
The Magellanic Clouds There are ~30 satellite galaxies that
orbit the Milky Way. The largest are the Large and Small Magellanic
Clouds. These are irregular galaxies. They are not spiral, disk
galaxies and they are not elliptical.