# 16 Hubble s Law and Dark Matter

```Chapter 16
Galaxies and
Dark Matter
Chapter 16
Galaxies and Dark Matter
UGC 09618 500 Mly (150 Mpc)
Units of Chapter 16
Dark Matter in the Universe
Galaxy Collisions
Galaxy Formation and Evolution
Black Holes and Active Galaxies
The Universe on Very Large Scales
Question 1
A galaxy seen
1 billion
light-years
away means
we see it
a) as it was when the universe was 1 billion
years old.
b) as it will be 1 billion years from now.
c) as it was 1 billion years ago.
d) as it is today, but redshifted 10 percent of
the speed of light.
e) as it was just after the Big Bang.
Question 1
A galaxy seen
1 billion
light-years
away means
we see it
a) as it was when the universe was 1 billion
years old.
b) as it will be 1 billion years from now.
c) as it was 1 billion years ago.
d) as it is today, but redshifted 10 percent of
the speed of light.
e) as it was just after the Big Bang.
Looking farther away in
space means looking
back further in time, to
when the object (and
universe) was younger.
Sloan Digital Sky Survey
SDSS
Dark Matter in the Universe
Other galaxies have rotation curves similar to
ours, allowing measurement of their mass.
Dark Matter in the Universe
Another way to
measure the average
mass of galaxies in a
cluster is to calculate
how much mass is required to keep the cluster
gravitationally bound.
Dark Matter in the Universe
Galaxy mass measurements show that galaxies
need between 3 and 10 times more mass than
can be observed to explain their rotation curves.
The discrepancy is even larger in galaxy
clusters, which need 10 to 100 times more mass.
The total needed is more than the sum of the
dark matter associated with each galaxy.
Dark Matter in the Universe
There is evidence
for intracluster
superhot gas
throughout
clusters, densest
in the center.
Dark Matter in the Universe
swept back, probably because of collisions with
intracluster gas.
Dark Matter in the Universe
It is believed this gas is primordial – dating
from the very early days of the universe.
There is not nearly enough of it to be the
needed dark matter in galaxy clusters.
Question 2
Based on galactic
rotation curves and
motions in clusters
of galaxies, dark
matter
a) makes up about 90 percent of the
matter in the universe.
b) is best detected by the largest optical
telescopes.
c) makes up about 10 percent of the
matter in clusters of galaxies.
d) exists but has no observable effects
on galaxies.
e) is the result of gas and dust.
Question 2
Based on galactic
rotation curves and
motions in clusters
of galaxies, dark
matter
a) makes up about 90 percent of the
matter in the universe.
b) is best detected by the largest optical
telescopes.
c) makes up about 10 percent of the
matter in clusters of galaxies.
d) exists but has no observable effects
on galaxies.
e) is the result of gas and dust.
Galaxy Collisions
The separation between galaxies is usually not
large compared to the size of the galaxies
themselves, and galactic collisions are frequent.
The “cartwheel”
galaxy on the left
appears to be the
collision with another
galaxy, perhaps one
of those on the right.
Question 3
When spiral galaxies
collide, the greatest
impact occurs on
their
a) globular cluster stars.
b) giant molecular clouds.
c) central bulge stars.
d) open clusters.
e) disk stars.
Question 3
When spiral galaxies
collide, the greatest
impact occurs on
their
a) globular cluster stars.
b) giant molecular clouds.
c) central bulge stars.
d) open clusters.
e) disk stars.
Galaxy Collisions
This galaxy collision has led to bursts of star
formation in both galaxies; ultimately they will
probably merge.
Question 4
Collisions
between galaxies
a) are much rarer than collisions between
stars.
b) can transform elliptical galaxies into
spirals.
c) trigger Type II supernova explosions in
the halo.
d) cause gas and dust clouds to collide,
Question 4
Collisions
between galaxies
a) are much rarer than collisions between
stars.
b) can transform elliptical galaxies into
spirals.
c) trigger Type II supernova explosions in
the halo.
d) cause gas and dust clouds to collide,
Galaxies are relatively
close compared with
their size. In clusters of
galaxies, collisions
clearly occur.
Galaxy Collisions
The Antennae galaxies collided fairly recently,
sparking stellar formation. The plot on the right
is the result of a computer simulation of this
kind of collision. Galaxy Collision
Galaxy Collision
Question 5
Due to the density and
collisions among galaxies,
___________ are rare in the
centers of clusters.
a) giant ellipticals
b) irregulars
c) spirals
d) active galaxies
Question 5
Due to the density and
collisions among galaxies,
___________ are rare in the
centers of clusters.
a) giant ellipticals
b) irregulars
c) spirals
d) active galaxies
The gas, dust, and disks of spiral
galaxies are tidally disrupted, and
even destroyed, in the centers of
dense clusters, which are often
dominated by giant elliptical galaxies.
Galaxy Formation and Evolution
Galaxies are believed to
have formed from mergers
of smaller galaxies and
star clusters. Image (c)
shows large star clusters
found some 5000 Mpc
away. They may be
precursors to a galaxy.
Galaxy Formation and Evolution
This Hubble Deep Field view
shows some extremely distant
galaxies. The most
distant appear
irregular, supporting
the theory of
galaxy formation
by merger.
Hubble 1
Hubble 2
Galaxy Formation and Evolution
Each of these starburst galaxies exhibits
massive star formation in the wake of a galactic
collision. In images (a) and (b), the two colliding
galaxies can be clearly seen.
Infrared
Galaxy Formation and Evolution
This appears to be an instance of galactic
cannibalism – the large galaxy has three cores.
Computer enhanced false color
Galaxy Formation and Evolution
This simulation shows how interaction with a
smaller galaxy could turn a larger one into a
spiral.
Merging Clusters of Galaxies
Black Holes and Active Galaxies
These visible and X-ray images show two
supermassive black holes orbiting each other
at a distance of about 1 kpc. They are expected
million years.
Black Holes and Active Galaxies
This galaxy is viewed
mostly from 21-cm
shifts of emissions
from the core show
enormous speeds
very close to a
massive object – a
black hole.
Black Holes and Active Galaxies
Careful measurements show that the mass of
the central black hole is correlated with the size
of the galactic core.
100 Mly Virgo
Black Holes and Active Galaxies
The quasars we see are very distant, meaning
they existed a long time ago. Therefore, they may
represent an early stage in galaxy development.
The quasars in this image are shown with their
host galaxies.
Question 6
The rapid variation
of brightness of
quasars indicates
a) the source of energy is very small.
b) energy is coming from matter and
antimatter.
c) the energy source is rotating rapidly.
d) a chain reaction of supernovas occurs.
e) there are many separate sources of
energy in the core.
Question 6
The rapid variation
of brightness of
quasars indicates
a) the source of energy is very small.
b) energy is coming from matter and
antimatter.
c) the energy source is rotating rapidly.
d) a chain reaction of supernovas occurs.
e) there are many separate sources of
energy in the core.
The size of an object
cannot be larger than the
distance light can travel
in the time it takes to
change its brightness.
Black Holes and Active Galaxies
The end of the quasar epoch seems to have
been about 10 billion years ago; all the
quasars we have seen are older than that.
Why might that be?
The black holes powering the quasars do not
go away; it is believed that many, if not most,
galaxies have a supermassive black hole at
their centers.
Black Holes and Active Galaxies
This figure shows how galaxies may have
evolved, from early irregulars through active
galaxies, to the normal ellipticals and spirals we
see today.
The Universe on Very Large Scales
Galaxy clusters join
in larger groupings,
called superclusters.
This is a 3-D map of
the superclusters
nearest us; we are
part of the Virgo
Supercluster.
The Universe on Very Large Scales
This plot shows
the locations
of individual
galaxies within
the Virgo
Supercluster.
The Universe on Very Large Scales
This slice of a larger galactic survey shows that,
on the scale of 100–200 Mpc, there is structure in
the universe – walls and voids.
Stephen
Gregory
Question 7
The large-scale
distribution of
galaxies in the
universe reveals
a) a smooth, continuous, and homogeneous
arrangement of clusters.
b) large voids, with most of the galaxies
lying in filaments and sheets.
c) a large supercluster at the center of the
universe.
d) a central void with walls of galaxies at the
edge of the universe.
Question 7
The large-scale
distribution of
galaxies in the
universe reveals
a) a smooth, continuous, and homogeneous
arrangement of clusters.
b) large voids, with most of the galaxies
lying in filaments and sheets.
c) a large supercluster at the center of the
universe.
d) a central void with walls of galaxies at the
edge of the universe.
The Universe on Very Large Scales
This survey, extending
out even farther,
shows structure on the
scale of 100–200 Mpc,
but no sign of
structure on a larger
scale than that.
The decreasing
density of galaxies at
the farthest distances
is due to the difficulty
of observing them.
The Universe on Very Large Scales
Quasars are all very distant, and the light
coming to us from them has probably gone
through many interesting regions. We can
learn about the intervening space by careful
study of quasar spectra.
The Universe on Very Large Scales
This “absorption-line forest” is the result of
quasar light passing through hundreds of gas
clouds, each with a different redshift, on its
way to us.
The Universe on Very Large Scales
This appeared at first to be a double quasar, but
on closer inspection the two quasars turned out
to be not just
similar, but
identical – down
to their luminosity
variations. This is
not two quasars
at all – it is two
images of the
same quasar.
Question 8
The lensing of a
distant quasar
is produced in a
foreground
galaxy by its
a) total mass of stars, gas, and dark
matter.
b) central supermassive black hole.
c) globular clusters.
d) magnetic fields.
e) intergalactic gas.
Question 8
The lensing of a
distant quasar
is produced in a
foreground
galaxy by its
a) total mass of stars, gas, and dark
matter.
b) central supermassive black hole.
c) globular clusters.
d) magnetic fields.
e) intergalactic gas.
The twin quasar AC114
has two images of the
same object.
The Universe on Very Large Scales
This could happen via gravitational
lensing. From this we can learn about
the quasar itself, as there is
usually a time difference
between the two paths.
the lensing galaxy
by analyzing the
bending of the
light.
The Universe on Very Large Scales
Here, the intervening galaxy has made four
images of the distant quasar.
The Universe on Very Large Scales
These are two spectacular images of gravitational
lensing.
On the left are distant galaxies being imaged by a
whole cluster.
On the right is a cluster with images of what is
probably a single galaxy.
The Universe on Very Large Scales
On the left is a visible image of a cluster of
galaxies.
On the right, to the same scale, is the dark matter
distribution inferred from galaxy motion.
More Things to Study
1. Identify parts of a waveform such as
amplitude and wavelength
2. What is a photon
3. What is a quanta of energy
4. Telescopes collect light, lots of light
5. What is the advantage of the HST
6. Why do stars twinkle
Summary of Chapter 16
• Galaxy masses can be determined by rotation
curves and galaxy clusters.
• All measures show that a large amount of dark
matter must exist.
• Large galaxies probably formed from the
merger of smaller ones.
• Collisions are also important.
• Merger of spiral galaxies probably results in an
elliptical.
Summary of Chapter 16, cont.
• Quasars, active galaxies, and normal galaxies
may represent an evolutionary sequence.
• Galaxy clusters are gravitationally bound into
superclusters.
• The universe has structure up to 100–200 Mpc;
beyond that, there is no sign of it.
• Quasars can be used as probes of intervening
space, especially if there is galactic lensing.
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