Galaxies

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Galaxies

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

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Introduction

• Galaxies are the large-scale components of the
  Universe.
• Our galaxy is called the Milky Way
• It lies in a cluster of similar galaxies called
  the Local Group
• Galaxies can be spirals, barred spirals,
  elliptical or irregular in shape

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The Milky Way Galaxy

• Basic shape
• disk 80,000 ly across, 1011 stars
• spherical halo surrounding disk
• central bulge
• Our sun is
• 28,000 ly from the center
• orbital speed of 220 km/s
• completes one revolution every 200 My

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Populations of Stars

• Population I
• YOUNG
• in spiral arms
• rich in heavy elements
• Population II
• OLD
• in galactic disk AND halo, no connection with
  spiral arms
• poor in heavy elements

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Spiral Arms

• Stars are denser here, and there are many bright
  nebulas (HII regions)
• Regions of active star formation
• Large amounts of obscuring dust
• Arms do not revolve as collective objects they
  would tangle up
• Instead, they are density waves

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Density Waves

• Proposed by Hayashi
• There are more stars in spiral arms because
• gravitational interactions slow stars down here
• O and B stars dont survive long enough to leave
  the arms
• Different stars are in the arms at different
  times
• A good terrestrial analogue is the traffic jam

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The Galactic Nucleus

• There is a radio source here Sagittarius A
• Visual light cannot penetrate due to dust
  absorption
• Infrared and radio observations indicate very
  rapid (1000 km/s) turbulent motion in the central
  1000 ly region
• Motions can be explained by the gravity of a 106
  M? black hole.

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Spiral Galaxies

• Disk central bulge
• Active star formation
• Example M51 Whirlpool Galaxy (type Sc)

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Barred Spiral Galaxies

• Disk central bulge with bar
• Active star formation
• Example M95 (type SBb)

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Elliptical Galaxies

• All bulge, elliptical shape, no disk, Population
  II stars
• Almost no new star formation
• Example M87 in the Virgo Cluster

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Irregular Galaxies

• Irregular shape
• Star formation can occur, but is uncommon
• Example NGC4449

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The Magellanic Clouds

• Two irregular galaxies orbiting the Milky Way,
  100,000 ly away
• SMC probably collided with Milky Way 200 My ago
• Example LMC with Tarantula Nebula

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The Local Group

• Our galaxy is in the Local Group, which mostly
  contains small galaxies
• M31, the Andromeda Galaxy, is 2 million ly away.
  It is similar in size to our galaxy.

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The Andromeda Galaxy
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Galactic Clusters

• We are close to the edge of the Virgo Cluster
• 50 million ly away
• 7 million ly across
• 1,000 galaxies
• Further away is the Coma Cluster
• 350 million ly away
• 10,000 galaxies

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Galactic Collisions
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Seyfert Galaxies

• Galactic nuclei 10 Milky Way luminosity
• Much energy emitted in radio and infrared
• Probably all powered by supermassive black holes
  in their nuclei

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Quasars

• Typical luminosity of 1013 L? - 100 Milky Way
  luminosity
• We are now able to see that they are the cores of
  galaxies

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The Great Wall

• In 1989, Margaret Geller and John Huchra
  discovered the Great Wall
• 500 Mly across
• 200 Mly wide