Formation and Evolution of Galaxies

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Galaxy Formation and Structure in the Universe
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The Formation and Evolution of Galaxies
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The Formation Evolution of Galaxies
past
present
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Begin with galaxy fragments
By obtaining images that can see very faint
objects, we can look for galaxies that are very
far away. Because it takes light a long time to
reach us, we are seeing these galaxies as they
were in the distant past. The deepest images show
galaxy fragments (irregular galaxies) smaller
than the normal galaxies near us today.
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Begin with galaxy fragments
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Making supermassive black holes
past
present
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Making supermassive black holes
As two galaxy fragments collide and merge, the
black holes in them also merge, forming one
larger black holes in the center of the combined
galaxy.
This X-ray image of from the Chandra Observatory
shows two black holes orbiting each other in the
center of a galaxy. They will probably merge
together in about 400 million years.
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Making Quasars
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Quasar Black Hole consuming nearby stars
In a galaxy that has just formed by the merger of
smaller ones, the stars have all kinds of orbits,
some of which come too close to the supermassive
black hole. As the black hole consumes these
stars, a lot of light is emitted from the
accretion disk, and we see this as a Quasar.
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Seeing the Galaxies that contain Quasars
The light from the matter accreting onto these
black holes is very bright and normally makes it
hard to see the surrounding galaxies. But sharp
images with Hubble have been able to detect the
galaxies.
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Normal Galaxy no stars left to eat
After a while, the black hole consumes all of the
stars within its reach, and only the stars in
safe orbits remain. With matter to accrete, we
dont see anything from the black hole, and it is
dormant. This is a normal galaxy (like ours).
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Active Galaxies fresh food
If normal galaxies interact with each other, the
orbits of the stars are disrupted, and providing
fresh victims for the black hole. This is an
active galaxy.
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When Galaxies Collide

• It is not uncommon for galaxies to
  gravitationally interact with each other, and
  even collide.

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When Galaxies Collide

• When galaxies collide, the stars do not.
  (Theyre much too far apart.) However,
• Galaxies can be tidally distorted, or even torn
  apart.
• The Hubble types of the galaxies can change.
• The gas clouds within each galaxy can collide.
  The increased density of gas can cause lots of
  star formation.
• The galaxy can then produce lots of supernovae
  (from all the young O and B stars).
• Exactly what happens depends on the relative
  sizes of the galaxies, their orientation, the
  direction of their rotation, and their original
  Hubble types.

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Two Large Galaxies
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Two Large Galaxies
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Two Large Galaxies
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Two Large Galaxies make an Elliptical Galaxy
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Young Ellipticals

• A few ellipticals even show traces of past
  interactions

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Young Ellipticals

• A few ellipticals even show traces of past
  interactions

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Explaining the Properties of Ellipticals

• When two large galaxies merge, the collision of
  clouds of gas and dust in these galaxies triggers
  the formation of a huge number of stars,
  including a lot of massive stars. When these
  massive stars die, they produce a lot of
  supernova explosions, which eject most of the
  dust and gas from the galaxy. As a result, no new
  stars can be made, and so when we see ellipticals
  today, only old stars are present.

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Mergers of Smaller Galaxies Probably Make Spirals
past
present
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Why is a Spiral Galaxy Flat?
Begin with a large cloud (or multiple gas
clouds) of mostly hydrogen gas

• Small gas clouds coalesce to make a large cloud.
• Self gravity begins to collapse the cloud.
• As the cloud gets smaller, it begins to rotate
  faster.
• Centripetal force prevents collapse of gas in the
  plane of rotation.

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Why is a Spiral Galaxy Flat?
Centripetal force does not strongly effect the
gas that is above and below the rotation plane.
So

• Gas falling in from the top collides with gas
  falling in from the bottom. This gas sticks
  together and forms the galaxys disk.
• All star formation now occurs in a disk (where
  all the gas is). The gas and stars rotate around
  the galactic center.

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The Milky Way and a Dwarf Galaxy
?A small dwarf galaxy
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The Milky Way and a Dwarf Galaxy
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The Milky Way and a Dwarf Galaxy
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The Formation Evolution of Galaxies
past
present
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Summary  Galaxy Formation and Evolution

• Formation of galaxies
• Galaxies begin as small galaxy fragments
• Gas colliding with gas shocks and forms stars
• Small initial angular momentum defines the plane
  of a disk
• Collapse leading to disk galaxies
• Evolution of galaxies
• Galaxy mergers create active galaxies by feeding
  the central black hole(s)
• Central black holes also eventually merge
• Merger of two large galaxies creates burst of
  intense star formation and quasar activity,
  leading to the end of star formation and an
  elliptical galaxy
• Merger of two smaller galaxies leads to star
  formation, a brief period as an active galaxy,
  and ultimately a large spiral galaxy
• Signs of merger activity Active galaxy, intense
  star formation, strong dust bands, tidal tails
• Galaxies around us
• Peak of star formation was 75 of the way back to
  the Big Bang
• Distant galaxies are therefore mostly young and
  star-forming
• Most galaxies today are old and their black holes
  are dormant

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The Structure of the Universe Distances
Motions of Galaxies
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Definition of Cosmology

• Study of the origin, evolution, and fate of the
  universe

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The Distances to Galaxies

• In general, galaxies are too far away to observe
  RR Lyrae or main sequence stars. We need a
  brighter standard candle.

RR Lyrae stars are not the only stars that appear
in the Instability Strip. Another type of star,
called Cepheids, appear in this strip, and thus
pulsate in a regular fashion. Because Cepheids
are brighter than RR Lyrae stars, they can be
detected at larger distances.
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The Cepheids of the Large Magellanic Cloud

• Cepheid variables can be 100 times brighter than
  RR Lyr stars, but they do not all have the same
  brightness. They are difficult to measure in the
  Milky Way due to dust, but many Cepheids exist in
  the Large Magellanic Cloud, our nearest
  (non-dwarf) galaxy.

The LMC is close enough so that we can identify
its RR Lyrae stars. We therefore know its
distance.
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The Cepheid Period-Luminosity Relation

• In 1912, Henrietta Leavitt showed that LMC
  Cepheids had a range of brightness (some
  extremely luminous, some faint). But the
  brighter the Cepheid, the longer it took to
  pulsate. This Period-Luminosity relation makes
  Cepheids a standard candle.

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The Distance Ladder
Using Cepheids as a standard candle, we can
measure distances to galaxies as far away as 25
Mpc. If we want to measure distances beyond this
limit, we need a brighter standard candle.
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The Tully-Fisher Relation

• According to Newton, the rotation speed of a
  galaxy depends on its mass, and the greater the
  mass, the brighter the galaxy.

If we can translate mass into absolute
luminosity, we can have a standard candle that is
as bright as a galaxy. And we can do this by
calibrating the relationship using galaxies whose
distances are known from Cepheids.
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Type Ia Supernovae

• When an accreting 1.4 M? white dwarf goes over
  the Chandrasekhar limit, it becomes a Type Ia
  supernova, which can be seen across the universe.

We can determine exactly how bright SN Ia are by
measuring their brightness in galaxies with known
Cepheid distances.
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The Distance Ladder
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The Velocities of Galaxies
In 1912, Vesto Slipher obtained spectra of
galaxies, which he used to measure their
velocities through the Doppler shift of
absorption lines.
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The Velocities of Galaxies
Moving Toward Us
Moving Away From Us
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The Hubble Law

• Edwin Hubble estimated the distances to Sliphers
  galaxies. He found that the larger the distance,
  the faster the galaxy was moving (away from us).
  In fact, the relationship between velocity and
  distance was simply
• V H D
• where V is velocity (km/s)
• D is distance (in Mpc)
• H is the Hubble Constant

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The Hubble Law

• The Hubble Law is not perfect. In addition to
  its cosmological flow, each galaxy has a peculiar
  (random) velocity of ?300 km/s. But at large
  distances, the Hubble flow dwarfs this component.

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The Distance Ladder
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Mapping Galaxies in the Universe
Galaxies are not randomly distributed. They are
in clusters and filaments separated by huge empty
voids.
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Galaxies on the Surfaces of Bubbles
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The Size of the Universe

• If you were to make a universe, would you give it
  a finite size, or make it infinite?

If you make it finite
In a finite universe, gravity eventually takes
over and causes a big collapse.
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The Size of the Universe

• If you were to make a universe, would you give it
  a finite size, or make it infinite?

If you make it infinite
In an infinite universe, we would see the surface
of a star at every point in the sky, so the night
sky would be bright! This is Olbers paradox.
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The Cosmological Constant

• In 1918, Einstein realized the difficulty with a
  finite universe, and the impossibility of an
  infinite universe. So to keep the universe from
  collapsing, he postulated the existence of a
  Cosmological Constant (i.e., an extra
  anti-gravity term to counteract attraction).
  This is represented by ?.

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The Dynamic Universe

• But the Hubble Law solves both the problem of
  universe collapse and Olbers paradox.
• Since the galaxies are moving away from each
  other, gravity will not necessarily cause the
  universe to collapse. So a finite universe is
  possible.
• The larger the distance, the larger the velocity.
  Galaxies at the other end of the universe have
  their light Doppler shifted out of the optical,
  explaining why the night sky is dark. An
  infinite universe is possible.

Einsteins reaction The Cosmological Constant
was my greatest blunder.
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The Cosmological Principle

• Since we are not at the center of our Solar
  System, our Galaxy, or our Local Group of
  galaxies, it is exceedingly likely that were
  also not at the center of the universe. We
  therefore adopt the cosmological principle, which
  states that the universe (on average) must look
  the same to everyone, no matter where he/she/it
  is. In other words,
• the universe is homogeneous (i.e., smooth)
• the universe is isotropic (no special direction)
• Then why should the galaxies all be moving away
  from us!

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Summary the distance ladder
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Summary structure of the universe

• Galaxies have an ordered distribution, as if on
  the surfaces of bubbles
• A finite, motionless universe would collapse
• An infinite universe would produce a bright
  night-time sky (Olbers paradox)
• Hubble Law solves both dilemmas and allows for
  both to be possible
• In an expanding universe, Hubble Law is seen by
  everyone