Quasars and Active Galactic Nuclei

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Quasars and Active Galactic Nuclei

• Quasi-stellar objects
• Redshifts
• Variability
• Supermassive black holes
• Active Galaxies

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If we could see the last few seconds of the
collapse of a star to form a black hole, we would
see the star grow steadily redder. Why?

• The star moves away from us at an increasing
  speed.
  
• The star grows steadily cooler.
• The star's gravitational redshift increases.
• The star becomes obscured by more and more
  interstellar dust.

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Quasars

• Early radio telescopes found radio emission from
  stars, nebulae, and some galaxies.
  
• There were also point-like, or star-like, radio
  sources which varied rapidly these are the
  quasi-stellar radio sources or quasars.
  
• In visible light quasars appear as points, like
  stars.

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Optical picture of a quasar
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Quasar optical spectrum
Redshift shows this quasar, 3C273, is moving away
from us at 16 of the speed of light
Ha unshifted
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3C273
The quasar 3C273 is 2.6 billion light years
away. It looks dim, but must be extremely luminou
s to be visible as such distance.
The luminosity of 3C273 is more than one trillion
times the entire energy output of our Sun, or 100
times the luminosity of our entire galaxy.
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PKS 2000-030 has a distance of more than 11
billion light years

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Quasars vary
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Quasar size
Size places a limit on how fast an object can
change brightness. Conversely, rapid variations p
lace a limit on the size of the emitting object.
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Quasars vary
The size of this quasar must be less than about
one light year.
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Quasar engine

• Quasars have 100 times the luminosity of our
  Galaxy
  
• The engine powering quasars is only a few light
  years across
  
• The only known engine which is powerful enough
  and compact enough is a black hole
  
• Quasars contain supermassive black holes

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A quasar varies in brightness by a factor of 2 in
10 days. What does this tell us about the quasar?

• It has a large magnetic field.
• It is quite small.
• It must be highly luminous.
• It cannot emit radio waves.

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Quasar jets
Optical core ? Radio jet ?
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Quasars

• object with a spectrum much like a dim star
• high red shift
• enormous recessional velocity
• huge distance (from Hubbles Law)
• enormously luminous
• compact physical size
• powered by supermassive black hole
• often produce huge jets

15
Quasars are the ultraluminous centers of distant
galaxies.
Quasars are often observed to be at the center of
distant galaxies. The wispy material is likely
gas that has been pulled out of the hot galaxy by
gravitational interactions with nearby galaxies.
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Quasars are the most extreme of a class of
galaxies known as active galaxies
M87 appears as an elliptical galaxy in visible
light, but like a dim quasar in radio. The
nucleus of the galaxy contains a weak quasar.
This means that the galaxy harbors a supermassive
black hole.
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Centaurus A
Optical
Radio
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NGC 1566
Spiral galaxies also sometimes contain active
cores.
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Active galaxies lie at the center of double radio
sources.
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Active galaxies lie at the center of double radio
sources.
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Active Galaxies come in several varieties

• Quasars
• 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.
  
• Most commonly known as a Blazar.
• All contain supermassive black holes

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Black holes in normal galaxies
Rotation curves of stars near the centers of most
galaxies show the presence of supermassive black
holes with mass ranging from 106 to 109 solar
masses. Essentially all galaxies contain supermas
sive black holes and were likely active galaxies
at some point in their lives.
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Review Questions

• Why is it thought that quasars are at very large
  distances?
  
• How can we place an upper bound on the size of a
  quasar even if we cant resolve it with a
  telescope?
  
• Where are quasars found?
• Do most galaxies have supermassive black holes?