Download Rich and Poor Galaxy Clusters

Which of the following is least easily explainable as a result of
interaction between galaxies?
a) Some galaxies have long "tails" of stars.
b) Rich, regular clusters are dominated by central giant ellipticals.
c) Both spiral and elliptical galaxies are seen at very high redshift.
d) Some galaxies seems to be undergoing bursts of star formation.
• Rich clusters:
Rich and Poor Galaxy Clusters
– Contain hundreds to
thousands of member galaxies
– Are roughly spherical, with
the largest galaxies near the
center
– Contain mostly elliptical and
type S0 galaxies
– Lots of hot gas and dust
• Poor clusters
– Contain only tens of galaxies
– Have a ragged, irregular
appearance
– More spiral and irregular
galaxies
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Superclusters
• Clusters of clusters are called
superclusters
– Contain a few to many dozen
clusters of galaxies
– Can be Mpc across!
– The Local Group is part of the
Local Supercluster, shown at left.
• The Local Supercluster is heading
toward a region of space known
as the Great Attractor, where there
are a large number of massive
superclusters
• There may be super-superclusters!
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The most likely reason that clusters of galaxies have
more elliptical than spiral galaxies is that in the high
density cluster environment
a) spirals merge to form ellipticals.
b) intracluster gas strips galaxies of the gas needed for
star formation.
c) near-misses between galaxies makes them rounder.
d) galaxies are older and their brighter disk stars have
burned out.
Large Scale Structure in the Universe
• Using modern technology,
astronomers have mapped the
location of galaxies and
clusters of galaxies in three
dimensions
• Redshift is used to determine
distance to these galaxies
• Galaxies tend to form long
chains or shells in space,
surrounded by voids
containing small or dim
galaxies
• This is as far as we can see!
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Seyfert Galaxies
• Seyfert galaxies are
spiral galaxies with
extremely luminous
central bulges
• Light output of the
bulge is equal to the
light output of the
whole Milky Way!
• Radiation from Seyfert
galaxies fluctuates
rapidly in intensity
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Radio Galaxies
• Radio Galaxies emit
large amounts of
energy in the radio part
of the spectrum
• Energy is generated in
two regions
– Galactic nucleus
– Radio lobes on either
side of the galaxy
• Energy generated by
energetic electrons
– Synchrotron radiation
– Electrons are part of the
gas shooting out of the
core in narrow jets
Quasars
• Quasars are small, extremely
luminous, extremely distant
galactic nuclei
– Bright radio sources
– Name comes from Quasi-Stellar
Radio Source, as they appeared to
be stars!
– Can have clouds of gas near them,
or jets racing from their cores
– Spectra are heavily redshifted,
meaning they are very far away
– Energy output is equivalent to one
supernova going off every hour!
• The HST was able to image a
quasar, showing it to be the active
core of a distant galaxy
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Energy Source for Active
Galactic Nuclei
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•
•
•
•
Active galactic nuclei emit a
tremendous amount of
radiation over a broad range
of wavelengths
A black hole can be both
very small, and have an
accretion disk that can emit
enough radiation
Likely that at the centers of
these galactic nuclei, there
are supermassive black holes
Intense magnetic fields in
the accretion disk pump
superheated gas out into jets
that leave the nucleus
There are still many
questions to be answered…
Figure 77.08b
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