the size of our solar system.

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Question 1 The length of the jets we see in
radio images of radio galaxies is comparable to
Review Quiz No. 27

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• the size of our solar system.
• a few pc.
• the diameter of the host galaxy.
• many times the diameter of the host galaxy.
• the diameter of superclusters.

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Cosmic Jets and Radio Lobes
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Many active galaxies show powerful radio jets
Hot spots Energy in the jets is released in
interaction with surrounding material
Radio image of Cygnus A
Material in the jets moves with almost the speed
of light (Relativistic jets).
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Question 2Why do we only see one jet in this
radio image of a quasar?

• There is only one jet (on only one side of the
  core).
• The counter-jet is emitting mostly invisible
  gamma-rays in our direction.
• The counter-jet is emitting mostly invisible
  X-rays in our direction.
• The counter-jet is much less energetic.
• The emission from the counter-jet is
  Doppler-boosted away from us.

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Question 3What kind of object do you observe if
you happen to look at a radio galaxy from the
direction in which the jet is pointing?

• A Seyfert galaxy.
• A blazar (i.e., a quasar or a BL Lac object)
• A gamma-ray burst.
• An X-ray binary.
• A Herbig-Haro Object.

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Other Types of AGN and AGN Unification
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Quasar or BL Lac object (properties very similar
to quasars, but no emission lines)
Emission from the jet pointing towards us is
enhanced (Doppler boosting) compared to the jet
moving in the other direction (counter jet).
Observing direction
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Question 4What is the cause of the observed
superluminal motions in blazar jets?

• The material in the jet is actually moving faster
  than the speed of light.
• A light-travel-time effect tricks us into
  believing that the material in the jet has
  travelled much farther than it actually has.
• A light-travel-time effect tricks us into
  believing that the material in the jet has
  travelled a much shorter distance than it
  actually has.
• A light-travel-time effect tricks us into
  believing that the material in the jet has been
  on its way for a much shorter time than it
  actually has.
• A light-travel-time effect tricks us into
  believing that the material in the jet has been
  on its way for a much longer time than it
  actually has.

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Superluminal Motion
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Individual radio knots in quasar jets
Sometimes apparently moving faster than speed of
light!
Light-travel time effect
Material in the jet is almost catching up with
the light it emits