Lecture 5: Galaxies

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Lecture 5Galaxies

• Dr Michael Burton

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The Structure of our Galaxy

• How did we find out?
• Herschels 18th Century map
• Analogy lighthouses above the fog
• Globular Clusters
• RR Lyrae stars (pulsating red giants)
• Flattened disk in shape of Spiral
• 100,000 light years across, 300,000 million stars
• Dust lanes in disk (young stars)
• Central Bulge (old stars)
• Halo (globular clusters)

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Galactic Rotation

• Rotation curve is constant with distance
• V 250 km/s
• There must be Dark Matter (unseen matter
  exerting a gravitational pull)
• Winding of spiral arms dilemma
• Density Wave (cf. traffic jam on highway)
• Star formation induced by compression of gas
  clouds as they pass through the spiral arm

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The Realm of Galaxies

• Island universes or nebulae in our Galaxy?
• The Great Debate on the Scale of the Universe
• Spirals galaxies
• active star formation (young and old stars).
• Elliptical galaxies
• no gas, star formation has finished (all old
  stars).
• Irregular galaxies
• ill-defined shape

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The Expansion of the Universe

• Measure distance of galaxy
• e.g. apparent size, brightness of supernova
• Measure velocity of galaxy
• Doppler shift
• Hubbles Law
• Speed H0 x Distance
• H0 is Hubbles Constant
• 71 ? 4 kilometres per second per megaparsec
• (megaparsec million parsecs 3 million light
  years)
• 1 / H0 age of Universe (13.7?0.2 billion years)

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Galaxy Clusters

• Milky Way is in the Local Group
• a small cluster of galaxies
• Galaxies are clustered.
• Clusters combine to form superclusters.
• Large Scale Structure (e.g. the Great Wall)
• 2dF instrument on the AAT has measured 250,000
  galaxies.

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Galaxy Mergers and Cannibalism

• Galaxy Mergers
• Stars pass right by one another (do not collide!)
• Gas collides, induces star formation
• Galactic Cannibalism
• Forms the dominant cluster member
• Galaxies today built up by mergers over first 12
  billion years of the Universe.

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Galaxy Formation

• Hubble Deep Field
• a pencil beam to the early universe
• We see the building blocks of galaxies, merging
  together and undergoing star formation
• Theories for Galaxy Formation
• Collapse of a single huge gas cloud?
• Coalescence of many smaller clouds?
• Perhaps a bit of both??

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Quasars andActive Galactic Nuclei

• Central engine a supermassive black hole
• Surrounded by accretion disk
• Emitting jets of plasma at relativistic speeds

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The Story of the Universe

• MAP satellite measures the Cosmic Microwave
  Background Radiation
• Pins down tightly several parameters that
  describe the nature of our Universe
• The Universe is 13.7?0.2 billion years old
• The Universe will expand for ever
• Ordinary matter only makes up 4 of it
• Dark Matter is 23, Dark Energy is 73