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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.

Small and Large Magellanic Clouds. These are small irregular galaxies that will add stars and gas to the Milky Way.

Sagittarius tidal stream of stars.

Tidal streams from a dwarf galaxy around a galaxy.

Summary of galaxy formation (#1) Galaxy fragments form within a large dark matter halo. Most of these fragments merge to form the large galaxies we see today, in our local portion of the universe. Gas from these early mergers build up the disk of the galaxies and the stars that had originally formed in the fragments are thrown all around the halo of the forming galaxy. In stars form in the disk which are younger and more heavy element enhanced compared to the halo stars, which are old and have very few heavy elements.

Large elliptical galaxies form later, after the large disk galaxies form. They form from the merger of large galaxies. Some small satellite galaxies have survived to this day and are still merging with the large primary galaxies. Most of these galaxies are irregular in shape because they are undergoing tidal stresses from the primary galaxy.

What is at the very centers of galaxies? In the galactic nucleus. All large galaxies contain central, super- massive black holes in their nucleus. The idea that super-massive black holes exist at the center of galaxies first arose in the 1960s with the discovery of quasi-stellar radio sources. (quasars) The quasars emitted enormous amounts of radio waves but looked like stars.

Bright Quasar

Bright Quasars Quasar Star in our Galaxy

When the spectra of quasars were examined it was found that they fantastically large cosmological redshifts. This meant they were extremely far away. Some were billions of light years distant. If quasars appear fairly bright to us here on earth, but they are billions of light years away, What must we conclude?

1.Quasars have a radius that is much bigger than a galaxy 2.Quasars extremely old 3.Quasars have an enormous luminosity

B = L/4πd 2 So quasars have enormous luminosities. They can often have more than 100 times the luminosity of an entire galaxy. Although not know at the time of discovery, the quasars are located at the centers of galaxies. We can see this today in Hubble Space Telescope images.

Not only are quasars found at the center of galaxies, they are found in galaxies that are in the process of merging with other galaxies. Although in the images, quasars look like they are large in diameter this is not the case. Quasars often change their brightness. They tend to flare up brightly and then dim. A given flare-up might last for a couple hours. Here is an example.

Example of a quasar flare. Brightness Time Width is 2 hours

The entire quasar flares up and then returns to its normal brightness Light coming from the flare up of the quasar

From the two previous slides, estimate how big the quasar is. 1.About the size of the Earth 2.About 10 light years in diameter 3.About 2 light hours in diameter

If the entire quasar flares up, then the amount of time from when the flare began to when it finally ended, sets strong constraints on how big the quasar can be. This is because the light has to travel to us from different locations in the quasar. If the quasar brightens and dims in a span of two hours, then it can not be larger than two light- hours across. This is similar to the size of our solar system.

This creates a huge problem. How can something that is about the size of our solar system produce 100 times the energy of the entire galaxy. The only object that come close is a supernova explosion. But the light coming from a quasar doesn’t fade away light a supernova, and the spectrum of the quasar is fairly constant over large spans of wavelength. So it doesn’t look like a supernova explosion.

Other evidence Bi-polar outflows

Bi-polar outflows occur when an object has an accretion disk and some of the accreting gas is redirected to the magnetic poles and accelerated outward. Examples – proto-stars that are accreting material Neutron stars that are in the pulsar phase

So the energy that is being radiated from a quasar is produced in the accretion disk and the bi-polar outflows. In the accretion disk, charged particles spiral around and rub against each other. This releases light. In the bi-polar outflows, atoms shot out along the poles, run into material in the inter- galactic medium and radiate light. Usually radio waves.

The only think in the universe that is small in size and can accelerate accreting gas to nearly the speed of light. And that can then produce the energies that are observed is a super- massive black hole. The Milky Way is much closer than a typical quasar, so we can directly examine our nucleus. Here is what we find.

These stars are orbiting at 15,000 km/s

These measurements are equivalent to measuring the size of a quarter (coin) that is about 7000 miles away.

How can stars orbit something at 15,000 km/s 1.The stars are being held in the center of the galaxy by the dark matter halo around the galaxy 2.They must be stars that came from a merged galaxy 3.Something inside their orbit has a hell of a lot of mass

Mass of central object is 2.2 million solar masses. That would be equivalent to have 2.2 million suns crammed into a volume that is a little bigger than the solar system. Here is a globular star cluster that has about 1 million times the mass of the Sun.

But it has a diameter of about 150 light years. Not, 1 light day.

So where is this massive object that is at the center of this orbit?

It is a super-massive black hole. But if this is the case, why don’t we have a quasar at the center of the Milky Way?

The black hole is currently accreting only a very small amount of material. There is a small amount of gamma-radiation coming from the black hole, but it is very feeble compared a quasar.

Quasars are very distant and emit a tremendous amount of energy. They appear to be in galaxies that are under going mergers. In the more local universe, there are galaxies with active nuclei. They are called Active Galactic Nuclei (AGN). They seem to be very similar to quasars but generally emit less energy than quasars. Then there are normal galaxies with super massive black holes that emit virtually no energy.

M 87, the giant elliptical galaxy in Virgo is an AGN.

So is Centaurus A

What must happen in order to produce a quasar or AGN? 1.Galaxies must be merging in order to fed the beast 2.A galaxy must have more gas it than the Milky Way does 3.Super nova must be continually exploding

To be a quasar or an AGN the black hole must be fed. It has to be accreting material. This requires that gas or stars be sent into the immediate vicinity of the central massive black hole. The way to accomplish this it to merge galaxies. This introduces new material to the central nucleus. In the distant past, during galaxy formation, there were many mergers, and many powerful quasars. Today the mergers are less frequent and usually not as much material. But when they happen they can produce AGN. When the material is all accreted by the black hole, the nucleus becomes normal once again.

Evolution of the Universe Ancient stars (age > 12 billion years) have virtually no processed heavy elements. While stars like the Sun (Age = 4.5 billion years) have a thousand times more, and new stars have 20 times what the Sun has. Galaxy very long ago were only merging galaxy fragments. They weren’t the large, well defined galaxies we see today. Quasars we prominent in the early universe, during galaxy formation. They are much less so in the local universe.