Download The Milky Way Galaxy

Announcements
Assigned studying: Chapter 16.
Syllabus updates:
1) Details of Final Exam posted.
2) HW #8 will be posted on
Thursday 4; due on Thursday 11.
Starry Night
Saint-Rémy: June, 1889
Van Gogh
Out in the sky the great dark clouds are
massing; I look far out into the pregnant
night …
Paul Laurence Dunbar
Ships That Pass in the Night
Galaxies
Survey Questions
1.
2.
3.
4.
What are the key characteristics of the
four basic types of galaxies?
What is the Hubble Tuning Fork Scheme?
Does the scheme really tell you a
formation sequence of galaxies (NO)?
How do galaxies form?
Galaxies take one of four different morphologies
SPIRALS
Spirals have a disk with dust and gas: star formation
The tightness of a spiral
galaxy’s arms is correlated to
the size of its nuclear bulge
Type Sa
Type Sb
Type Sc
The tighter the arms, the bigger the bulge
Variety of Spiral Arms
Flocculent spirals
(fleecy)
Grand-design spirals
(highly organized)
We easily see these spiral arms
because they contain numerous
bright O and B stars which illuminate
dust in the arms.
However, stars in
total are very
evenly distributed
throughout the
disk.
BARRED SPIRALS
BARRED SPIRALS
The bar is not a spiral arm.
It is made of old stars, like
those in the bulge
Bars of stars run through the
nuclear bulges of barred spiral
galaxies
Type SBa
Type SBb
Type SBc
ELLIPTICALS
Type E0
Type E3
Type E7
ELLIPTICALS
Ellipticals:
have no dust and no cold gas,
hence no star formation.
Made mostly of old, metalenriched stars.
They like to cluster a lot
Discussion Question
Given their color, do you expect elliptical galaxies to
have more or less new star formation than
spirals? Why?
1) ellipticals have more new star formation
2) ellipticals have less new star formation
Elliptical galaxies display a
variety of sizes and masses
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Giant elliptical galaxies
can be 20 times larger
than the Milky Way
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Dwarf elliptical
galaxies can be as small
as a big globular cluster.
They are extremely
common and can contain
as few as a million stars
IRREGULAR
Irregulars mostly
have gas, dust and
star formation.
They contain both old
and young stars
The classification scheme of galaxy morphology is
known as the Hubble Tuning Fork Scheme
Galaxies like to cluster:
eg. M81 group
Optical Image
Radio Image
Hercules
Cluster
The ideas of galaxy formation
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Spiral galaxies form from the collapse of spinning gas cloud
inside Dark Matter halos
Elliptical galaxies form from the mergers of disk galaxies, or
from clouds with low spinning (probably wrong for big galaxies;
might work for early galaxies)
Peculiar galaxies are formed through the interactions of
galaxies
Most popular theory: CDM (Cold Dark Matter)hierarchical
cosmology. Structures form from bottom to top: small ones first,
bigger ones later by merging.
CDM predicts hierarchical assembly of cosmic structures. Many
of its predictions are verified.
Galaxy Merging: evolution of morphology, trigger of star formation
Billions of stars all tug on each other instead of just one
planet tugged by the gravity of the Sun.
How two spirals collide and merge,
making tidal tails and, at the end,
an elliptical galaxy
Survey Questions
1.
2.
3.
What are the key characteristics of the
four basic types of galaxies?
What is the Hubble Tuning Fork Scheme?
Does the scheme really tell you a
formation sequence of galaxies? NO!!!!!
It is just a morphological classification
scheme
How did we find Dark Matter:
The mass of galaxies and rotation curves
v2
A rotation curve is just a
plot of rotational velocity vs
distance for objects in
(roughly) circular orbits.
v1
M
For a set of objects orbiting
a common point, what is
the orbital velocity for
different objects at
different distances from
the common point?
If you think about it, this is just Kepler’s Third Law applied to galaxies
Rotation Curve for a rigid body
no gravity involved, no information
on mass
Straight because objects are
rigidly held.
Rotation Curve for our Solar System
gravity involved, information on mass
Curve decreases because
mass that generates the
gravity is all at the center
(Sun is much, much greater
than all planets together.
Rotation curve of the MW disk
(and of all other galaxies)
It is essentially Kepler’s Third Law
One uses it to measure the mass of galaxies
Rotation Curves of Galaxies
From blue / red shift of spectral
lines across the galaxy
 infer rotational velocity
Observe frequency of
spectral lines across a
galaxy.
Plot of rotational velocity
vs. distance from the
center of the galaxy:
Rotation Curve
Determining the Masses of Galaxies
Based on rotation curves, use Kepler’s 3rd law to
infer
masses of galaxies.
Evidence for Dark Matter

Evidence #1 - The amount of mass that
we can “see” by counting the stars, the
gas and the dust in a galaxy is not enough
compared to the observed gravitational
mass measured with the rotation curves.
Evidence for Dark Matter
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Evidence #2 - The amount of mass that
we can “see” in a cluster of galaxies does
not account for the observed gravitational
pull on the galaxies in the cluster (as
evidenced by the large variation in
velocities of the galaxies).
Hercules
Cluster
Galaxies in a cluster have
more erratic velocities than
we expect from the amount
of matter we can “see” in
the cluster
Evidence for Dark Matter
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Evidence #3 - The intracluster gas is too
hot to be retained by the gravity of the
visible matter in a cluster.
If all the mass there really were only that
of visible matter, its gravity would not be
enough to retain the hot gas, which would
evaporate rapidly. Since the gas is there,
there must be more gravity, hence dark
matter.
Cluster of galaxies: Hydra A
Optical
X-ray
Evidence #4:
gravitational lensing
Just as black holes and other massive
objects curve space around them,
galaxies and galaxy clusters curve space.
When the alignment of a large mass and
background objects is right, a dramatic
effect is observed. This is known as a
gravitational lens.
Lensing by a dense
cluster of galaxies.
Using Einstein’s laws
of general relativity,
we can estimate the
mass of the lens.
About 10 times more
mass is present than
one would estimate by
summing the mass of
the visible galaxies.
Foreground
Cluster of
Galaxies
Background
lensed galaxies
Our picture of the mass around
galaxies now looks something like this
Dark matter outweighs
Normal matter on average
by 10 to 1
So what is this missing mass?
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The only way to explain
the rotation curve of our
galaxy is to say that there
is lots and lots of mass
that is not emitting light.
The halo of our galaxy
must be full of it. The
halo outweighs the disk
by a factor of 10.
As far as we can tell, this
mass doesn’t emit any
light at any frequency.
What is the form of the missing mass?
Dark Matter Possibilities
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Here is the first lists of candidate materials
for the dark matter that dominates the
mass in our galaxy.
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Black Holes
Black Dwarfs
Brown Dwarfs
Unknown particles
Baryonic Matter
(e, p, n … the same stuff
that we’re made of)
Non baryonic matter
Does dark matter matter?
Yes! Dark Matter Rules the
Universe
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Since dark matter outweighs other matter
by a factor of 10, nearly all the large scale
gravitational interactions in the Universe
are dominated by the dark matter:
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Dark matter holds galaxies together – without
it the outer stars would escape
Dark matter holds clusters of galaxies
together – without it, the clusters would
disperse
Dark matter causes these structures to form
in the first place!
Which of the following is NOT a good reason for
learning more about dark matter?
dark matter provides the gravitational force
that holds galaxies together
the total amount of dark matter determines the
fate of our Universe
the Earth would be destroyed if it ever ran into a
clump of cold dark matter
dark matter is responsible for the formation of
structure in our Universe
our understanding of dark matter may reveal a
new, undiscovered form of matter
What is the best evidence for dark matter
in galaxies?
1) the orbital speeds of gas clouds and stars
2) the orbital distances of gas clouds and stars
3) the low density of stars in spiral arms
4) the large orbital eccentricities of disk objects
5) the lack of heavy elements in globular cluster
stars
Dark Matter Summary
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90% of the universe is made of matter that does
not emit detectable radiation at any wavelength.
Evidence to date suggests that only a very small
fraction of the dark matter is made of familiar
matter (baryons).
Dark matter is not stars or stellar remnants,
galaxies, dust clouds, or anything else made of
protons, neutrons, or electrons – as far as we
can tell, it is an as of yet undiscovered form of
matter.
This is one of the biggest mysteries facing astronomers –
What is our Universe made of?
Survey Questions
1.
2.
3.
4.
What is a rotation curve of a galaxy?
How can a rotation curve be used to
measure the mass of the galaxy?
Where is most of the gravitational mass
in a galaxy?
What are the lines of evidence for dark
matter in the universe?