Virtually all galaxies show a flat rotation curve.
• Let’s look back at the rotational velocity equation:
•
v = (MinteriorG/r)0.5
• The rotational velocity is constant at big r values.
• What must we conclude?
• 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 don’t know what it is but it has mass. It
surrounds galaxies in a huge dark matter halo.
• It doesn’t interact with light or we could
observe it. That is true even for gas.
Other ways to detect Dark Matter
• Gravitational lensing. The amount that the
light is bent is directly proportional to the
mass bending the light.
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.
• 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 can’t even see it. We can only
measure its presence using velocities.
So we see this….
But it is only the tip of the iceberg
• 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.
The Antennae Galaxies
• 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.
M 87 in the Virgo Cluster
Centaurus A
A radio
emitting
elliptical
galaxy
Bi-Polar
outflows
Composite
image of
Centaurus
A
• 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.
• 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.
• But when we look to very great distances,
virtually all galaxies are in the process of
merging.
30
Why are there so many galaxy mergers when we
look very far away.
30
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
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
• 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.
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)
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.
• 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.
• Let’s think about why this would be.
• 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?
30
What happens when moving atoms in a gas run
into each other?
30
1. They can radiate
light
2. They bounce off
each other into new
directions
3. They explode
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
What happens when they radiate light?
30
30
1. The atoms get hot and
expand
2. The atoms use some of
their kinetic energy and
slow down
3. The atoms explode
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
• 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?
30
What happens when a halo star falls in
toward the center of the Galaxy?
30
1. It’s 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.
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
• 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 doesn’t work.
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.
Today’s 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.
• 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.
Sagittarius Dwarf galaxy and stream
Sagittarius tidal stream of stars.
Tidal streams
from a dwarf
galaxy around
a galaxy.
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.