Download - Chapter 16
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Chapter 16
Quasars and Active Galaxies
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What do you think?
• What do quasars look like?
• What powers a quasar?
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Galaxy Cygnus A
This galaxy has a red shift corresponding to 6% the speed of light or 750 million light-years away
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Quasars look like stars but have huge redshifts
• object with a spectrum much like a dim star
• highly red shifted
• enormous recessional velocity
• huge distance (ala Hubble’s Law)
• must be enormously bright to be visible at such a great distance
• Quasi-stellar object
• QSO or Quasar
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This object that looks like a star must be
enormously luminous - its redshift indicates it is 4 billion light years
away!!
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3C 273’s spectral lines are greatly redshifted
This change implies a distance of 2 billion light years
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A quasar emits a huge amount of energy from a small volume
Such rapid changes in brightness can only result from changes small objects
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Active Galaxies bridge the energy gap between ordinary
galaxies and quasars• peculiar galaxies (pec)
– appear to be blowing themselves apart
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Active Galaxies bridge the energy gap between ordinary
galaxies and quasars• peculiar galaxies (pec)
– appear to be blowing themselves apart
• Seyfert galaxies– luminous, star-like nuclei with strong emission
lines
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Active Galaxies bridge the energy gap between ordinary
galaxies and quasars• peculiar galaxies (pec)
– appear to be blowing themselves apart
• Seyfert galaxies– luminous, star-like nuclei with strong emission
lines
• BL Lacertae objects (BL Lacs)– featureless spectrum with a brightness that can
vary by a factor of 15 times in a few months.
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Radio image of Cygnus A showing a small but very bright radio galaxy in the middle of the 320,000 ly wide lobes
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Active galaxies lie at the center of double radio sources
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Supermassive black holes lurk at the centers of some galaxies
• High resolution spectroscopy allows astronomers to peak at the motion of gas near centers of galaxies
• Some galaxies exhibit high-velocity jets of material leaving the center
• Observations suggest that the centers of some galaxies are incredibly massive
• All of this suggests the existence of supermassive black holes
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Galaxy (which is actually
quite large)
Intergalactic gas jet
Giant Gas Clouds
(surrounding the galaxy)
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Jets of matter ejected from around a black hole
may explain quasars and active galaxies
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Jets of matter ejected from around a black hole may explain quasars and active galaxies
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From where you observe it might make all the difference ...
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What did you think?
• What do quasars look like?They look like stars but spew out immense
energy
• What powers a quasar?A quasar is a galaxy powered by a supermassive
black hole at its center.
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Self-Check
1: Describe the observable properties of quasars and indicate how they are identified.
2: Explain why intrinsic properties of quasars are so difficult to explain.
3: List the various types of active galaxies and discuss how each might be related to quasars.
4: Discuss the observations that seem to be consistent with the presence of supermassive black holes in most galaxies.