Virtually all galaxies show a flat rotation curve.

Let’s 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?

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

Composite image of Centaurus A Bi-Polar outflows

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.

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

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?

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

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

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?

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

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.