Transcript Chapter 27

Chapter 27
The Evolution and Distribution of
Galaxies
What happens to galaxies over billions of years?
How did galaxies form?
Do they evolve over time?
How are the galaxies distributed, and why?
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Distant Galaxies
• Taking advantage of the constant speed of light, we
can look back in time to see how galaxies have
evolved.
• The most distant galaxies we can see are the oldest.
• Using the Hubble Telescope, astronomers can see
back more than 10 billion years, to see galaxies soon
after they’d formed.
• The earliest galaxies tend to be small and irregularly
shaped.
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HST Deep Field
• This picture was taken
by pointing the
Hubble telescope at a
tiny region of space,
empty of nearby stars
or galaxies.
• It required combining
many hours of
observations to make
this picture.
• More than 10,000
galaxies are found in
this picture!
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Small Galaxies in Deep Field
• These are some of the most distant galaxies yet seen.
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Looking Back in Time
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Mergers and Collisions
• If the earliest galaxies were small and irregular, how
did the large, structured galaxies we have today form?
• It is believed that the first galaxies combined -- either
through mergers or collisions.
– The difference between a merger and a collision is all in the
“attitude”.
– Violent mergers are called collisions.
– The merger of a larger galaxy with a smaller one is called
cannibalism.
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Galaxies Can Collide
• The collision of two
galaxies is
evidenced in some
distinctive patterns.
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Colliding Galaxies
The Galaxy that Collided
with the Cartwheel Galaxy
A Ring Galaxy
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Star Birth in Distant Galaxy
Blue color is evidence of O
type stars. O stars have short
lives.
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Collisions Initiate New Stars
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Cartwheel Galaxy
Note the
blue stars
(O type).
Known as
a
starburst.
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The Mice
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One Galaxy From Two
• NGC 6240
• Note the two nuclei visible in
the center.
• Probably the collision of two
spiral galaxies.
• Large infrared output
indicative of heating of dust
clouds at the center of the
galaxy.
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Star Formation Peaked Early
• Measurements and
predictions suggest
that the rate of star
formation peaked
around 3 or 4 billion
years after the big
bang.
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Formation of Galaxies
• Bottom up picture of
galaxy formation.
• Small, irregular
clusters of stars pull
together under
gravity to form more
structured galaxies.
• Galaxies are pulled
together by gravity
to form clusters and
superclusters.
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The Local Group
• Our Milky Way is part of a cluster called the Local
Group (an imaginative name, yes?).
• The Local Group spreads over a region about 3 million
LY in radius.
• The Local Group has two large spirals, one small
spiral, two ellipticals, 13 irregulars, and 14 dwarf
ellipticals.
• There may be more irregular and dwarf ellipticals.
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The Local Group
• Three
dimensional
view of the Local
Group.
• The Milky Way is
at the center
due to our
choice.
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Virgo Cluster
• Closest large cluster to us
• Moderately-rich
• Called Virgo Cluster because it is in the direction of the Virgo
constellation.
• It has many hundreds of galaxies (mostly spirals and irregulars)
distributed into an irregular shape about 10 million light years
across.
• It is about 49 to 59 million light years from us.
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Virgo Cluster (cont’d)
• Some ellipticals in the
central part of the cluster
• Includes giant elliptical at
the center (M87) that
has become so large by
gobbling up nearby
galaxies.
• Total mass of the Virgo
cluster is large enough
that its gravity pulls
nearby groups of
galaxies (including the
Local Group) toward it.
The center of the Virgo cluster has three giant ellipticals, M 87
(bottom left corner) and M 84, M 86 (center right). M 86 may
be a lenticular S0 type instead.
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Local Supercluster
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Mass Near Milky Way
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Superclusters
• The clustering phenomenon does not stop with galaxies.
• Galaxy clusters “attract” each other in superclusters of 10s
to 100s of clusters.
• Mutual gravity binds them together into long filaments (thin,
string-like structures) 300 to 900 million LY long, 150 to 300
million LY wide, and 15 to 30 million LY thick on average.
• Discovery of these huge structures made recently from years
of taking Doppler shifts of thousands of galaxies.
• Doppler shifts of the galaxies converted to distances using
Hubble Law.
• Between the filamentary superclusters are HUGE voids with
very few (if any) galaxies.
• Voids are typically 150 million light years across.
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The Coma Cluster
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• Hickson
Compact
Group 87
• group of
galaxies in
orbit about
each other
Mapping the Universe
• Two pioneers in the
mapping of the
structure of the
universe are Margaret
Geller and John
Huchra.
• Have taken thousands
of spectra of galaxies
along thin pie-shaped
slices of the sky.
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Universe Map
• Would take much too long to
take spectra of galaxies in
every direction in space,
• Map the universe in slices.
• This slice map took about 15
years of data-collection to
generate!
• Geller and Huchra's have
shown surprising results
about the large-scale
structure of the universe.
• Other groups of astronomers
have joined their efforts to
map the universe.
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Superclusters and Voids
• Arrangements of superclusters and voids look
like a bunch of soap bubbles or swiss cheese
with the galaxies on the borders of the huge
holes.
• What produces the long thin strands of clusters
around the huge bubbles of empty space?
• Obviously, gravity is the force at work, but how
has it worked to produce these structures?
• Dark matter must play a significant role but
how it does that is not known.
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The Cosmological Principle
• The Sloan Digital Sky
Survey is making a more
complete map of several
slices of the sky.
• One thing we learn is
that the universe looks
similar in all directions.
• This is embodied in the
“Cosmological Principle”
which says that universe
is “isotropic” and
“homogeneous”.
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Summary
• The earliest galaxies were small, irregular clusters of
stars that have merged over time to produce the
large, structured galaxies we see today.
• Evidence for the merger and collision of galaxies is
visible.
• Galaxies tend to clump into clusters and supercluster,
leaving large empty regions of space called voids.
• From the large scale distribution of galaxies we
deduce the Cosmological Principle, that the universe is
isotropic and homgeneous.
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