Download Galaxies and the Distance Ladder

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Lecture 34
Galaxies and Dark Matter
January 15c, 2014
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M 100 -- Spiral Galaxy
A grand design
spiral galaxy.
Click here for
more info about
this image.
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NGC 5090 and NGC 5091
The spiral galaxy
NGC 5091 may be
colliding with
elliptical galaxy
NGC 5090. Click
here for more info
on the galaxy pair.
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Small Magellanic Cloud
Our nearest neighbor,
an irregular galaxy.
Click here for more
info on this picture.
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Distances to Galaxies
• Galaxies were first thought to be star forming
regions.
• It was proposed that the “spiral nebula” were “island
universes”.
• Spectroscopic parallax is only good for distances up
to 10,000 pc.
Andromeda Galaxy
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Using Cepheid Variable Stars to
Measure Distances
• Variable stars are stars whose brightness varies
in a very smooth, predictable way.
• Cepheid variables
– periods vary from
1-100 days.
Figure 23.5, Chaisson and McMillan,
6th ed. Astronomy Today, © 2008 Pearson Prentice Hall
• RR Lyrae variables
– periods are all less
than 1 day.
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• Cepheids are stars
that have moved off
of the MS.
– Star is expanding and
contracting.
– Luminosity rises and
falls.
Figure 23.6,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Cepheid Variables
• Average luminosity is related to pulsation period.
• If luminosity and apparent brightness are known, distance
can be determined
Luminosity
Apparent Brightness 
4 d 2
Figure 23.7,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Supernovae as Standard Candles
• Type Ia supernovae all reach the same maximum
luminosity, about 3 × 109 solar luminosities
• If supernova is observed in another galaxy and
the peak apparent brightness is measured, the
distance can be calculated.
Luminosity
Apparent Brightness 
4 d 2
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Tully-Fisher Relation for
Determining Distances
• Used to determine distances to galaxies where
individual stars cannot be seen.
• Relates speed of rotation and luminosity of a
galaxy.
– The faster a galaxy rotates, the higher the luminosity.
• If apparent brightness and luminosity are known,
distance can be determined
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• How is rotational speed measured?
– Doppler Shift
• Tully-Fisher calibrated using
nearby galaxies with variable stars
Figure 24.11,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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The Distance Ladder
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Distribution of Galaxies
• Most galaxies are clustered
– Milky Way has 3 nearby companions
– ~40 galaxies in Local Group (Size ~1 Mpc)
Map of three-quarters of the members of the Local Group
Figure 16-18, Comins and Kaufmann,
7th ed. Discovering the Universe,
© 2005 W.H. Freeman and Company
Artist’s view of the Local Group
Figure 16.30, Arny and Schneider, 5th ed. Explorations,
© 2008 The McGraw-Hill Companies, Inc.
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Which distance measuring method would be most
reliable for a nearby galaxy in our Local Group?
A.
B.
C.
D.
E.
Parallax
Spectroscopic parallax
Cepheid variables
Supernovas
Tully-Fisher
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Which distance measuring method would be most
reliable for a nearby galaxy in our Local Group?
A.
B.
C.
D.
E.
Parallax
Spectroscopic parallax
Cepheid variables
Supernovas
Tully-Fisher
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Clusters of Galaxies
• Galaxies often found in clusters
– Rich cluster: many hundreds of galaxies
– Poor cluster (or group): only a few dozen
galaxies
• Held together by gravity
• Milky Way is near the Virgo Cluster of
~2500 galaxies (Size ~3Mpc across)
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Figure 16.36, Arny and Schneider,
5th ed. Explorations, © 2008 The McGraw-Hill Companies
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Virgo Cluster – SDSS Image
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Clusters and Superclusters
• Large clusters
– More Ellipticals found near the center
– More Spirals found in outer regions
• Superclusters
– clusters of clusters of galaxies.
– In between clusters -- no gas gas been detected
• Most must have been swept up during galaxy
formation.
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Figures 25.23 and 24,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Abell 2218
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Role of Interactions
• Small interactions may start formation of
spiral structure.
• Strong interactions (collisions, cannibalism)
may alter structure completely
– Spirals lose structure, become ellipticals.
– Large galaxies “eat” many other galaxies,
become very large
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Galaxy Merger
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Antennae Galaxy
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Which of the following is NOT true concerning
the Local Group?
A. It contains about 40 member galaxies.
B. It is roughly spherical in shape with the most
massive galaxies near the center.
C. It is a poor cluster.
D. It is the galaxy cluster to which the Milky
Way belongs.
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Which of the following is NOT true concerning
the Local Group?
A. It contains about 40 member galaxies.
B. It is roughly spherical in shape with the
most massive galaxies near the center.
C. It is a poor cluster.
D. It is the galaxy cluster to which the Milky
Way belongs.
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Measuring the Mass of Galaxies
– Kepler’s Third Law
• Mass inside of radius of rotation can be measured
using Kepler’s Third Law
M TOT P 2  a 3
• Period can be measured from the velocity of gas
• Observe H to
measure mass
outside of stellar
part of the disk.
Figure 23.21, Chaisson and McMillan,
6th ed. Astronomy Today, © 2008 Pearson Prentice Hall
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Measuring the Observable Mass
• Add up mass of all of the stars, gas, dust, etc.
that can be observed.
• Observable Mass and Mass measured by
rotation ARE NOT THE SAME!!
Expected rotation
curve from
observable mass
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Dark Matter
• There is more mass than we can account for
with known stars and gas.
• The missing mass cannot be observed so it
is called dark matter
• In the Milky Way, about half of the matter
is dark matter.
• Other galaxies: 0-90% is dark matter.
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Andromeda Galaxy Rotation Curve
The M31 major axis mean optical radial velocities and the rotation
curve, r <120 arcmin, superposed on the M31 image from the Palomar
Sky Survey. Velocities from radio observations are indicated by
triangles, 90< r <150 arcmin. Rotation velocities remain flat well beyond
the optical galaxy, implying that the M31 cumulative mass rises linearly
with radius. (Image by Vera Rubin and Janice Dunlap.)
Physics Today, December 2006, p. 9
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Using the
Doppler effect
to measure
velocities, we
find that the
actual mass of
the galaxy
cluster is 10 to
100 times more
than what is
suggested by the
luminous matter.
Figure 25.2,
Chaisson and McMillan,
6th ed. Astronomy Today,
© 2008 Pearson Prentice Hall
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Dark Matter revealed by Galaxy Cluster Collision
This image was made by superimposing a picture made a visible wavelength, an image made at X-ray wavelengths
(revealing hot gas as red blobs), and a map of dark matter (blue blobs) deduced from gravitational lensing.
Figure 16.35, Arny and Schneider, 5th ed. Explorations, © 2008 The McGraw-Hill Companies, Inc.
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Dark Matter in Galaxies and Clusters
• Dark matter has
been detected
– in galaxies
(by rotation curves)
– in clusters
(by galactic motions and
by gravitational lensing)
• Mass of cluster ~
10-100 times
observable mass
• If true, universe is
90% Dark Matter.
Figure 25.1b, Chaisson and McMillan,
6th ed. Astronomy Today, © 2008 Pearson Prentice Hall
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• Where is the dark matter?
– Believed to be distributed in large halo
surrounding galaxy.
– Also in clusters?
• What is dark matter? -- not sure
– Brown dwarfs = protostars that never started
fusion.
– Black dwarfs = cooled white dwarfs.
– Black holes
– Sub-atomic particles
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Which of the following is NOT a reason why
astronomers believe that dark matter exists?
A. They can detect it with radio telescopes.
B. The outer parts of galaxies rotate faster than
expected on the basis of the luminous matter.
C. The galaxies in clusters move faster than expected
on the basis of the luminous matter.
D. It explains some of the gravitational lensing that is
observed at long distances.
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Which of the following is NOT a reason why
astronomers believe that dark matter exists?
A. They can detect it with radio telescopes.
B. The outer parts of galaxies rotate faster than
expected on the basis of the luminous matter.
C. The galaxies in clusters move faster than expected
on the basis of the luminous matter.
D. It explains some of the gravitational lensing that is
observed at long distances.
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Galaxy Formation
• No set theory for galaxy formation yet.
Galaxies form from one
large cloud of gas
Galaxies form from the
merger of a few
medium size gas clouds
Galaxies form from
many small gas clouds
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Questions on Galaxy Formation
• Why are ellipticals and spirals so different?
– Ellipticals have mainly older stars = stars
formed early on and little new star formation
has occurred.
– Spirals have new and old stars = more
continuous star formation
• What role do interactions play in creating
galaxies?
• Spiral galaxies are more common at large
distances (in the past) -- where are they
now?
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Formation of Milky Way
One model for the formation of the Milky Way. There still remains debate among
scientists about the role that collisions and mergers play in the formation of large
galaxies like the Milky way.
Figure 15.27, Arny and Schneider,
5th ed. Explorations, © 2008 The McGraw-Hill Companies