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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, supermassive 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?
If quasars appear fairly bright to us here on
earth, but they are billions of light years away,
What must we conclude?
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1. Quasars have a radius
that is much bigger
than a galaxy
2. Quasars extremely old
3. Quasars have an
enormous luminosity
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• B = L/4πd2
• 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
Width is 2 hours
Time
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
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• 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 lighthours 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 intergalactic 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 supermassive 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
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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?
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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
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• 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.