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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.

• RR Lyrae variables

– periods are all less

than 1 day.

Figure 23.5, Chaisson and McMillan,

6th ed. Astronomy Today, © 2008 Pearson Prentice Hall

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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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• Average luminosity is related to pulsation period.

• If luminosity and apparent brightness are known, distance

can be determined

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LuminosityApparent Brightness

4 d

Cepheid Variables

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.

2

LuminosityApparent Brightness

4 d

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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)

Artist’s view of the Local Group

Figure 16.30, Arny and Schneider, 5th ed. Explorations,

© 2008 The McGraw-Hill Companies, Inc.

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

Which distance measuring method would be most

reliable for a nearby galaxy in our Local Group?

A. Parallax

B. Spectroscopic parallax

C. Cepheid variables

D. Supernovas

E. Tully-Fisher

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Which distance measuring method would be most

reliable for a nearby galaxy in our Local Group?

A. Parallax

B. Spectroscopic parallax

C. Cepheid variables

D. Supernovas

E. 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

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

• Period can be measured from the velocity of gas

• Observe H to

measure mass

outside of stellar

part of the disk.

32 aPMTOT

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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Figure 25.2,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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.

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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

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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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

Formation of Milky Way