Introduction%20To%20Modern%20Astronomy%20II

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Introduction To Modern Astronomy II
ASTR 113 003

Spring 2006 Lecture 11 April 12, 2006
Review (Ch4-5) the Foundation

1. Sun, Our star (Ch18)
2. Nature of Stars (Ch19)
3. Birth of Stars (Ch20)
4. After Main Sequence (Ch21)
5. Death of Stars (Ch22)
6. Neutron Stars (Ch23)
7. Black Holes (Ch24)

Star (Ch18-24)

1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)

Galaxy (Ch 25-27)

1. Evolution of Universe (Ch28)
2. Early Universe (Ch29)

Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
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Quasars, Active Galaxies,and Gamma-Ray Bursters
ASTR 113 003

Spring 2006 Lecture 11 April 12, 2006

• Chapter Twenty-Seven

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Guiding Questions

1. Why are quasars unusual? How did astronomers
   discover that they are extraordinarily distant
   and luminous?
2. What evidence showed a link between quasars and
   galaxies?
3. How are Seyfert galaxies and radio galaxies
   related to quasars?
4. How can material ejected from quasars appear to
   travel faster than light?
5. What could power the incredible energy output
   from active galaxies?
6. Why do many active galaxies emit ultrafast jets
   of material?
7. What are gamma-ray bursters? How did astronomers
   discover how far away they are?

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Quasars Discovery

• Quasars, or quasi-stellar radio sources, look
  like stars but have huge redshifts.
• They were first discovered in radio wavelength
  they were strong radio sources in the sky, e.g.,
  Cygnus A

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Quasars Distance

• The redshifts (gt0.05 to gt 5) indicate that
  quasars are at least several hundred Mpc away,
  and often several thousand Mpc away

3C 273 Z0.158 d682 Mpc (or 2 billion ly)
PKS 2000-039 Z3.773 d3810 Mpc (or 12.4 billion
ly)
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Quasars Luminous Objects

• A quasars luminosity can be calculated from its
  apparent brightness and the distance using the
  inverse-square law
• Even though small, the luminosity of a quasar
  (1038 to 1042 Watts) can be very larger, i.e.,
  several thousand times more than the entire Milly
  Way Galaxies (1037).
• A quasar has emission spectrum, not the
  absorption spectrum of ordinary stars or
  galaxies.
• We now know that about 10 of all qauasars are
  strong sources of radio emission and are
  therefore called radio-loud
• The remaining 90 are radio-quiet, or
  quasi-stellar objects, or QSOs

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Quasars Distribution

• Quasars are most populated in 1 to 4 billion
  years after the Big Bang.
• There are no nearby quasars (gt250 Mpc)

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Quasars are centers of active galaxies

• A quasar is not a star
• A quasar is the ultra-luminous center of an
  active galaxy

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Missing Links

• Quasar are extreme galaxies.
• What are the missing links between normal
  galaxies and quasars
• Seyfert Galaxies
• Radio Galaxies

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Seyfert galaxies

• Seyfert galaxies are spiral galaxies with bright,
  compact nuclei that show intense radiation and
  strong emission lines in their spectra.
• The nucleus of Seyfert galaxies resembles a
  low-luminosity quasar nearby

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Radio galaxies

• Radio galaxies resemble low-luminosity,
  radio-loud quasars
• Radio galaxies are often elliptical galaxies with
  a nucleus of intense activities. Including jets

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Radio Galaxy Centaurus A

• In visible light, it is a elliptic galaxy about
  4 Mpc away
• In radio wavelength, it shows a central source
  and two lobes
• In x-ray, it shows a jet
• It looks similar to a quasar in the radio and
  X-ray wavelengths

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Jet

• Jets are from the synchrotron radiation of
  relativistic particles that are ejected from the
  nucleus of a radio galaxy along two oppositely
  directed beams
• Jets are collimated by the twisted magnetic field
  lines along the rotational axis of the central
  object

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Synchrotron Radiation

• Synchrotron radiation
• Produced by relativistic electrons spiraling
  around magnetic field lines
• is non-thermal radiation
• Is polarized radiation
• Blackbody radiation
• Produced by the random thermal motion of the
  atoms that make up the emitting object
• Is thermal radiation
• Is un-polarized radiation

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Super-luminous Motion of Jets

• Some jets appeared to move faster that the speed
  of light, the super-luminous motion
• For example, the blob seems moving 10 times
  faster than the speed of light

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Super-luminous Motion of Jets

• Super-luminous motion is a projection effect
• Because the blob is moving toward us close to the
  speed of light, the signals from the blob always
  reach us earlier, which makes any lateral motion
  appear faster.

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Blazar

• Similar to quasar, a blazar is an extraordinary
  luminous, compact star-like object that is the
  core of distant galaxies
• But unlike quasar, the spectrum of a blazar is
  featureless, without emission line or absorption
  line
• A blazar is dominated by synchrotron radiation

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AGN Active Galactic Nuclei

• Because the similar properties among quasars,
  blazars, Seyfert Galaxies, and radio galaxies,
  they are now collectively called active galaxies
• Active galaxies possess active galactic nuclei,
  which cause intense radiations, fast variations,
  jets, lobes, et al.

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AGN Variation and Size

• One common property of all AGN is variability
• Variability place strict limit on the maximum
  size of a light source

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AGN Variation and Size

• A principle an object can not vary in brightness
  faster than light can travel across the object
• E.g., flash from an object 1 ly across reaches us
  over I yr period

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Super-massive black holes the central engines
of AGN

• AGN is powered by the accretion of galaxy
  material onto a super-massive black hole at the
  center
• The energy for AGN is the gravitational energy
  converted to radiation
• Material in an accretion disk spirals inward
  toward the black hole

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Super-massive black holes the central engines
of AGN

• The fast orbital motion of stars at the core
  indicates the presence of a central object
• Calculations show the object to be 3 X 107 solar
  mass
• Super-black hole exists in the nucleus of almost
  every galaxy, including Milky Way

Rotation Curve of Andromeda Galaxy (M31)
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Super-massive black holes the central engines
of AGN

• Estimate the mass of the central black hole for
  3C273
• The luminosity is 3 X 1013 Ls
• Assuming the luminosity is at the Eddington limit
• Eddington limit radiation pressure, the pressure
  produced by photons streaming outward from the
  in-falling material, is equal to the
  gravitational force.
• The minimum mass of black hole in 3C273 is 109 Ms
• If BH mass were smaller than this number, the
  in-falling material would be pushed away from the
  radiation pressure

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Jets from a Super-Massive Black hole

• The rotation of the accretion disk surrounding a
  super-massive black hole twists the disks
  magnetic field lines into a helix.
• Relativistic subatomic particles are channeled
  along the field lines

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A Unified Theory of Active Galaxies

• Blazars, quasars, and radio galaxies may be the
  same type of object, viewed at different angles
• The same object is consisted of a super-massive
  black hole, its accretion disk and its
  relativistic jets

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A Unified Theory of Active Galaxies

• Why there are no nearby quasars
• Because of the strong accretion, over time, most
  of the available gas and dust surrounding a
  quasars central engine is accreted onto the
  black holes the central engine becomes less
  active
• The collision of galaxies transfer gas and dust
  from one galaxy to another, providing more fuel
  for the super-massive black

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Gamma-ray Bursters

• Short (in seconds), intense bursts of gamma rays
  are observed at random times coming from random
  parts of the sky

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Optical Counterparts of Gamma ray Burster

• Tracking the Afterglow, indicating Gamma X-ray
  bursters are from distance galaxies
• E.g.,optical object z3.418, 12 billion light
  years away

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Origins of Gamma Ray Bursters

• Supernova explosion
• Collision between two neutron stars, or between a
  neutron star and a black hole, or two black holes

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Key Words

• accretion disk
• active galactic nucleus (AGN)
• active galaxy
• blazar
• collapsar
• double radio source
• Eddington limit
• gamma-ray burster
• head-tail source
• nonthermal radiation
• polarized radiation
• quasar
• radio galaxy
• radio lobes
• Seyfert galaxy
• superluminal motion
• supermassive black hole
• thermal radiation