15.4 Quasars and Other Active Galactic Nuclei

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15.4 Quasars and Other Active Galactic Nuclei

• Our Goals for Learning

• What are quasars?
• What is the power source for quasars and other
  active galactic nuclei?
• Do supermassive black holes really exist?

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What are quasars?
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If the center of a galaxy the galaxys nucleus
is unusually bright we say that the galaxy
hosts an active galactic nucleus or AGN. Quasars
are the most luminous examples of AGN.
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The highly redshifted spectra of quasars indicate
large distances From brightness and distance we
find that luminosities of some quasars are more
than a trillion times the Suns Variability
shows that all this energy comes from region
smaller than the solar system
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What can you conclude from the fact that quasars
usually have very large redshifts?

1. They are generally very distant.
2. They were more common earlier in the universe.
3. Galaxy collisions might switch them on.
4. Nearby galaxies might harbor quasar remnants.
5. All of the above.
6. None of the above.

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Galaxies around quasars sometimes appear
disturbed by collisions Quasars fueled by gas
concentrated in nucleus by galaxy collisions
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Radio galaxies contain active nuclei generating
vast, straight jets of plasma. The plasma
(ionized gas) emits radio waves that we can
detect.
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Jets of radio galaxies moving through gas in
galaxy clusters can be deflected
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What is the power source for quasars and other
active galactic nuclei?
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Accretion of gas onto a supermassive black hole
appears to be the only way to explain all the
properties of quasars
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Energy from a Black Hole

• Matter falling towards a black hole gains speed
  (turns gravitational potential energy into
  kinetic energy)

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Energy from a Black Hole

• Matter falling towards a black hole gains speed
  (turns gravitational potential energy into
  kinetic energy)
• Friction in an accretion disk turns kinetic
  energy into thermal energy (heat light)

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Energy from a Black Hole

• Matter falling towards a black hole gains speed
  (turns gravitational potential energy into
  kinetic energy)
• Friction in an accretion disk turns kinetic
  energy into thermal energy (heat light)
• Hot disk emits optical and ultraviolet light

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Energy from a Black Hole

• Matter falling towards a black hole gains speed
  (turns gravitational potential energy into
  kinetic energy)
• Friction in an accretion disk turns kinetic
  energy into thermal energy (heat light)
• Hot disk emits optical and ultraviolet light
• Matter spiralling into a black hole can radiate
  energy equal to 10-40 of the matter's
  mass-energy E mc2 ... much more efficient than
  fusion (1 of mc2)

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An active galactic nucleus (left-hand spot) can
eject a blob of plasma (right-hand spot) moving
at nearly the speed of light Speed of ejection
suggests that a black hole is present
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Jets are thought to come from twisting of
magnetic field in the inner part of accretion disk
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Jets may be produced only when black hole is
rapidly spinning (not all black holes spin
rapidly, so not all quasars have jets).
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Radio galaxies dont appear as quasars because
dusty gas clouds block our view of accretion disk
(but some radio quasars do exist)
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Quasar feedback

• Jets and other feedback from quasars affect the
  host galaxy
• Gas is heated, ionized, pushed away from nucleus,
  eventually stopping quasar from shining and
  galaxy from forming stars
• To do all that in a more massive galaxy requires
  a more massive black hole

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Quasar feedback

• Jets and other feedback from quasars affect the
  host galaxy
• Gas is heated, ionized, pushed away from nucleus,
  eventually stopping quasar from shining and
  galaxy from forming stars
• To do all that in a more massive galaxy requires
  a more massive black hole
• Quasar can even affect the surrounding galaxy
  cluster, if the host galaxy is in a cluster see
  black hole animations

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Do supermassive black holes really exist?
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Orbits of stars at center of Milky Way stars
indicate a black hole with mass of 4 million MSun
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Orbital speeds and distances of gas orbiting in
the center of M87 indicate a black hole with mass
of 3 billion MSun
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Black Holes in Galaxies

• Many nearby galaxies perhaps all large ones
  have supermassive black holes at their centers
• These black holes seem to be dormant active
  galactic nuclei
• All large galaxies probably passed through a
  active galactic nucleus stage earlier in time

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What have we learned?

• What are quasars?
• Some galaxies have unusually bright centers which
  are called active galactic nuclei.
• A quasar is a particularly bright active galactic
  nucleus. Quasars are generally found at very
  great distances, telling us that they were much
  more common early in the history of the universe,
  when galaxy collisions were more common.

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What have we learned?

• What is the power source for quasars and other
  active galactic nuclei?
• Supermassive black holes are thought to be the
  power sources for active galactic nuclei. As
  matter falls into a supermassive black hole
  through an accretion disk, its gravitational
  potential energy is transformed into thermal
  energy and then into light with enormous
  efficiency.

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What have we learned?

• Do supermassive black holes really exist?
• Observations of orbiting stars and gas clouds in
  the nuclei of galaxies suggest that all large
  galaxies may harbor supermassive black holes at
  their centers.

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Activity 39, pages 139-140Superluminal
(faster-than-light)motion in quasar jets?

• A quasar ejects a blob of gas on day zero.
• Blob travels at (13/14) times the speed of light
• Light from blob on day zero heads towards Earth,
  but blob travels in a slightly different
  direction.

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1. On day 14,the blob is at position C and the
light that the blob emitted on day zero is
located at

1. A
2. B
3. C
4. D
5. E
6. F
7. G
8. H

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2. On day 28, where is the light the blob emitted
on day zero and day 14?

1. Day zero light at E, Day 14 light at G
2. Day zero light at G, Day 14 light at I
3. Day zero light at H, Day 14 light at G
4. Day zero light at F, Day 14 light at H
5. Day zero light at H, Day 14 light at I
6. Day zero and Day 14 light both at I

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3. If the blobs light from day zero (location F)
is seen on Earth on day 10 million, the blobs
light from day 14 (location H) will be seen on
Earth

1. On Earth day 10 million and 1
2. On Earth day 10 million and 2
3. On Earth day 10 million and 12
4. On Earth day 10 million and 13
5. On Earth day 10 million and 14

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4. How far will the blob appear to move on the
sky in those 2 days?

1. 1 light-day
2. 2 light-days
3. 5 light-days
4. 7 light-days
5. 10 light-days
6. 28 light-days

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5. We know the blob is travelling at 13/14ths of
lightspeed, but from Earth it looks like the blob
travels at anapparent speed of

1. Lightspeed (1 light-day per day)
2. 2.5 lightspeed (2.5 light-days per day)
3. 13 lightspeed (13 light-days per day)

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Apparent superluminal (faster-than-light)motion
in quasar jets!

• A quasar ejects a blob of gas on blob day zero.
• Blob travels at 13/14ths of the speed of light
  c
• Light from blob on day zero heads towards Earth,
  but blob travels in a slightly different
  direction.
• Blob moving slower than light almost in Earths
  direction almost catches up with the light it
  emitted earlier, so blob appears to move faster
  than light.