An Introduction to Astronomy Part XIII: Black Holes, Quasars and Active Galaxies

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An Introduction to AstronomyPart XIII Black
Holes, Quasars and Active Galaxies

• Lambert E. Murray, Ph.D.
• Professor of Physics

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Einsteins Theory of Special Relativity

• The Special Theory Deals with Reference Frames
  that move at constant velocity.
• Einstein was able to show that
• In order to preserve the notion of cause and
  effect nothing can travel faster than the speed
  of light.
• Because of this fact
• Events that are simultaneous in one reference
  frame are not simultaneous in another.
• Moving objects contract in the direction of
  motion Lawence Contraction.
• Moving clocks run slower The Twin Paradox.
• Objects that move at the speed of light can get
  anywhere in the universe in no time at all.

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Einsteins Theory of General Relativity

• The General theory deals with accelerating
  reference frames and gravity accelerates
  objects.
• Einstein was able to show that mass warps space
  to create the effect of gravity.
• This warping of space causes
• Light to bend (or curve) in the presence of
  massive objects.
• Time to slow near move massive objects.
• A gravitational redshift.

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An Example of Curved Space

• The flat surface shown in (a) represents two
  dimensional space in spacetime. You can think of
  it as a large flat sheet of rubber. In the
  absence of any matter, straight lines are
  straight in our intuitive sense (the sheet is
  flat).
• In the presence of matter, spacetime curves, as
  shown in (b) by the curvature of the sheet when
  mass is laid on it (just like the rubber would
  stretch). Straight lines, defined by the paths
  that light rays take, are no longer straight in
  the usual sense, but follow the curvature of
  the warped spacetime, just as a small marble
  would follow the curvature of the rubber sheet.
• This curvature of space also creates gravity. If
  you place a large mass on the rubber sheet and
  then release a much smaller marble near that
  mass, the stretched rubber sheet will cause the
  marble to roll down hill toward the larger
  mass.

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Curved Spacetime and the Path of Light

• The bending of light by matter was confirmed in
  1919 during a total solar eclipse. Photographs of
  stars in the region of the sky near the sun
  during this eclipse showed the exact displacement
  that Einsteins theory predicted. This is
  illustrated in the diagram above.

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Time Slows Down Near Matter

• If two clocks are synchronized in space (a), and
  then brought near the Earth and the Moon (b), the
  clock nearest to the Earth will slow down more
  that the one nearer the moon.
• This occurs because mass slows down the flow of
  time, and Earth has more mass (and a higher
  density, which adds to the effect) than the Moon.

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Gravitational Redshift

• The color of light from the same object located
  at different distances from a mass appears
  different as seen from far away. The photons that
  leave the vicinity of the massive object lose
  energy causing a redshift. The closer the light
  source is to the mass, the redder the light
  appears, and hence the name gravitational
  redshift. The same argument applies to light
  leaving the surfaces of different stars.

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Trapping of Light by a Black Hole

• (a) The paths and color of light rays departing
  from a main-sequence, giant, or supergiant star
  are affected very little by the stars
  gravitational force.
• (b) Light leaving the vicinity of a white dwarf
  curves and redshifts slightly, whereas
• (c) near a neutron star, some of the photons
  actually return to the stars surface.
• (d) Inside a black hole, all light remains
  trapped. Most photons curve back in. Those that
  move straight upward become infinitely
  redshifted, thereby disappearing.

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Characteristics of Black Holes

• Black Holes retain only three properties that it
  possessed before forming
• Its Mass
• Its Angular Momentum
• Its Electrical Charge
• Black Holes lose their internal magnetic field
  upon collapse.
• Most Black Holes are believed to maintain a
  neutral charge.
• Thus, there are only two types of Black Holes to
  consider
• A Schwarzschild (non-rotating) Black Hole
• A Kerr (rotating) Black Hole

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Structure of a Schwarzschild (Nonrotating) Black
Hole

• A nonrotating black hole has only two notable
  features
• its singularity, and
• its boundary.
• Its mass, called a singularity because it is so
  dense, collects at its center.
• The spherical boundary between the black hole and
  the outside universe is called the event horizon.
• The distance from the center to the event horizon
  is the Schwarzschild radius.

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Structure of a Kerr (Rotating) Black Hole

• The singularity of a Kerr black hole is located
  in an infinitely thin ring around the center of
  the hole. It appears as an arc in this cutaway
  drawing.
• The event horizon is again a spherical surface.
• There is a doughnut-shaped region, called
  ergoregion, just outside the event horizon, in
  which nothing can remain at rest. Space in the
  ergoregion is being curved or pulled around by
  the rotating black hole.

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A Model of a Rotating Black Hole

• Material in a region surrounding a rotating black
  hole would form a swirling accretion disk just
  outside the black hole.

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What is the Evidence for Black Holes?

• Since solitary black holes are not directly
  visible, we can infer their existence only when
  they interact with other matter.
• This may occur in binary star systems.
• It may also occur when large number of stars near
  the center of a galaxy fall into a supermassive
  black hole.
• It may be evident by gravitational lensing.

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X Rays Generated by Accretion of Matter Near a
Black Hole

• Stellar-remnant black holes, such as Cygnus X-1,
  LMC X-3, V404 Cygni, and probably A0620-00, are
  detected in close binary star systems.
• This drawing (of the Cygnus X-1 system) shows how
  gas from the 30 M companion star, HDE 226868,
  transfers to the black hole, which has at least
  11 Solar Masses.
• This process creates an accretion disk. As the
  gas spirals inward, friction and compression heat
  it so much that the gas emits X rays, which
  astronomers can detect.

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Jets Created by a Black Hole in a Binary System

• Some of the matter spiraling inward in the
  accretion disk around a black hole is superheated
  and redirected outward to produce two powerful
  jets of particles that travel at close to the
  speed of light.

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Supermassive Black Holes

• The bright region in the center of galaxy M87 has
  stars and gas held in tight orbits by a black
  hole.
• Doppler shift measurements allow us to calculate
  the central mass of the galaxy.
• M87s bright nucleus (inset) is only about the
  size of the solar system and pulls on the nearby
  stars with so much force that astronomers
  calculate its mass to be a 3-billion-solar-mass
  black hole.

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Photograph of an Accretion Disk Around a
Supermassive Black Hole

• Swirling around a 300-million-M black hole in the
  center of the galaxy NGC 7052, this disk of gas
  and dust is 3700 ly across. The black hole
  appears bright because of light emitted by the
  hot, accreting gas outside its event horizon. NGC
  7052 is 191 million ly from Earth in the
  constellation Vulpecula.

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Image of OJ287 a Supermassive Black Hole
Accretion Disk
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Einstein rings - Light from distant galaxies is
bent into rings around intervening matter
(galaxies or black holes).
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The Discovery of Quasars

• In 1944, Grote Reber detected 3 strong radio
  emission sources with a home-made radio telescope
  in his back yard.
• Though two of these were identified as sources
  within our galaxy (the galactic nucleus and a
  supernova remnant), the third, Cygnus A, was not
  clearly identified until 1951.
• In 1951 a visible source located at Cygnus A was
  observed which looked like an odd-shaped
  elliptical galaxy.
• When examined with a spectrometer, the light from
  this galaxy exhibited unexpected emission lines
  with a relatively large red-shift.

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Cygnus A (3C 405) Today

• Today we know that the red-shifted visible
  spectra comes from the central galaxy and the
  radio signals emanate from two radio lobes
  located on either side of the galaxy.

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Cygnus A (3C 405) Image
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What Sized Red Shift?

• A red-shift as large as that measured for Cygnus
  A would imply (based upon Hubbles Law) that this
  galaxy was nearly 220 Mpc from us farther from
  us that any previously observed galaxy.
• For this radio source to produce a radio signal
  large enough to be detected by a back-yard radio
  telescope, and to be that far away it must be
  emitting a HUGE amount of energy hundreds of
  times the output of the Milky Way.

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Other Strange Stars

• Around 1960 several stars were observed which
  were also strong radio sources unlike most
  stars which are poor radio sources. These were
  3C 48 and 3C 273.
• These stars exhibited strange spectral emission
  lines that no one could identify.
• Initially, scientists believed that these stars
  were in our galaxy. Finally, in 1963, Maarten
  Schmidt realized that the emission lines were
  lines that had been red-shifted by large amounts,
  placing these stars at tremendous distances
  away from our galaxy. Only then did we realize
  that these stars were in fact the same type
  animal as Cynus A.

Note 3C 405 refers to the object number 405 in
the Third Cambridge Catalogue of radio sources.
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In 1963 Maarten Schmidt at CalTech identified the
strange emission lines of 3C 273 as significantly
red shifted lines of ordinary Hydrogen.
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3C273 This Quasi-stellar object also exhibited a
luminous jet
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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!!
Star-like Object 3C 48
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Quasars

• Because these strange stars had looked so much
  like a normal star, while also emitting radio
  waves, they were originally dubbed quasi-stellar
  radio sources, or quasars.
• Since these quasars are so distant and still have
  a large apparent magnitude, we know that no
  single star could emit that much energy these
  objects must be very bright galaxies.
• We have now identified a number of very distant
  very bright objects exhibiting large red-shifts
  some of which are not radio sources. We
  associate the name quasar, however, with both
  types of galaxies.

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Quasars vs. Normal Galaxies

• At first many scientists were reluctant to accept
  the notion of quasars there was too much
  difference between the amount of energy radiated
  by a normal galaxy and by quasars (if they were
  really that far away).
• However, astronomers had observed some peculiar
  galaxies as early as 1900 with strong emission
  line spectra.

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Seyfert Galaxies

• In 1943, Carl Seyfert focused his attention on
  these peculiar galaxies which would later
  become known as Seyfert Galaxies and began
  studying spiral galaxies with bright compact
  nuclei which see med to show signs of violent and
  intense activity (strong sources of Xrays many
  showing significant radio emission).
• Seyfert realized that the brighter of these
  galaxies have about the same luminosity as
  low-luminosity quasars, although they emit only a
  small amount of their energy in the radio region
  of the spectrum.
• About 10 of the most luminous spiral galaxies
  are Seyfert Galaxies.

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A Seyfert Galaxy NGC 1566
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An Indication of the Size of Quasars

• In the mid 1960s it was discovered that some
  quasars change in intensity over periods of
  months, weeks, or even days.
• The variation in brightness of an object gives an
  indication of the size of an object, since an
  object cannot change in brightness faster than
  the speed of light can travel across that object.
  Thus, an object 1 ly in diameter cannot vary in
  brightness with a period of less than a year.
• This means that some quasars must be relatively
  small in size.

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A Quasar Emits a Huge Amount of Energy from a
Small Volume

• These changes in brightness could indicate that
  the quasar cannot be larger than a few light
  years.

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Other Strange Galaxies

• While the spiral Seyfert galaxies resemble dim,
  radio-quiet quasars, certain elliptical galaxies,
  called radio galaxies, appear to resemble dim,
  radio-loud quasars.
• The first of these radio galaxies, M87, was
  discovered in 1918 and exhibited a small jet of
  material streaming away from the nucleus.
• The Hubble Space Telescope and modern day radio
  telescopes give us a better picture of M87 today.

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The Radio Galaxy M87
Visible Elliptical Galaxy
Hubble Image of Core of Galaxy
Radio Image Galaxy is small bright dot in center.
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The Nature of These Active Galaxies

• Many of these active galaxies have double radio
  lobes associated with them.
• These double lobes appear to arise from jets of
  electrons emitted from the galaxy at relativistic
  speeds producing synchrotron radiation in the
  radio part of the spectrum.
• The electrons are emitted along the magnetic
  field lines coming from the poles of the galaxy.

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Computer Enhanced Radio Image of Cygnus A Radio
Emitting Lobes are Clearly Visible (radio lobes
are 320,000 lyrs wide)
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Centaurus A - radio image superimposed on a
visible image...note no light from radio lobes
visible image
Radio lobes
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Active galaxies lie at the center of double radio
sources
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visible radio
visible infrared
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Supermassive Black HolesA Common Solution

• Astronomers believe that supermassive black holes
  at the center of these strange and peculiar
  galaxies may explain what we see
• A relatively small, but massive energy source
• An energy source that radiates energy across the
  spectrum
• An energy source that emits large quantities gas
  in high-speed jets.

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Recent data from center of M87 reveal rapid
rotation of stars about very bright center -
thought to be perhaps a 3 Billion M0 Black Hole
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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
Image of center of NGC 4261 - disk is about 320
ly across
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The location of the observer makes the difference
in what is seen ...
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Model of the center of an Active Galaxy
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End of Part XIII