How do we know it exists?

• Scale 10kPc (30 000 light years)

• Uses Doppler shift of light from star in spiral galaxy to give velocity (red shift)

• Expect velocity to fall off with distance from centre

  • ...but it doesn’t

• Only part can be explained by gas clouds

NGC 6503

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0

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Radius (kpc)

Velocity (km/s)

gas

halo

disk

observations

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