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					Black Holes and Active Galaxies




  Doug Roberts, Ph. D.
  Adler Planetarium & Astronomy Museum
  Northwestern University

  Adler Planetarium & Astronomy Museum
  http://www.adlerplanetarium.org
Basic concepts of gravity

  • Gravity is created by mass.
  • Gravity is always attractive.
  • Gravitational attraction is proportional
    to the sum of the masses of both
    objects.
  • Gravitational attraction increases as two
    objects come closer to each other.

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All due to Isaac Newton

  • In 1665, the plague had shut down
    Cambridge University where Newton
    had been working.
  • He subsequently worked from home on
    circular motion and other ideas.
  • When the university reopened two
    years later, Newton used Kepler‟s laws
    and his own observations to derive the
    Universal Law of Gravitation.
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Newton‟s Universal Law of
Gravitation

              F =GMm/r                   2


  F: Gravitational attraction (Force)
  G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2
  M: Mass of central object
  m: Mass of smaller object
  r: Distance between the objects
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Dark stars (a.k.a black holes)

  • English geologist Rev John Michell realized that it
    would be theoretically possible for gravity to be so
    overwhelmingly strong that nothing – not even light
    could escape.
  • In his 1783 paper to the Royal Society Michell wrote:
        If the semi-diameter of a sphere of the same density as the
        Sun in the proportion of five hundred to one, and by
        supposing light to be attracted by the same force in
        proportion to its [mass] with other bodies, all light emitted
        from such a body would be made to return towards it, by its
        own proper gravity.



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Dark stars (a.k.a black holes)

• At the time, the necessary conditions for “dark stars”
  (as Michell called them) seemed physically impossible.
• In 1796, the great French mathematician, Pierre
  Laplace proposed similar ideas to those of Michell in
  his famous paper „Exposition du Systeme du Monde‟.
• In the early 1800‟s experiments on optical interference
  led to the predominance of the wave theory of light
  and the end of the corpuscular theory. Since light
  waves were thought to be unaffected by gravitation,
  interest in the hypothetical “dark stars” ceased.


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General relativity

  • 1915 Einstein published his General
    Theory of Relativity.
  • The General Theory was a new theory of
    gravitation and one of its fundamental
    predictions was the effect of gravity on
    light.




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General relativity

  • According to the theory, matter causes
    space-time to curve.
  • The paths followed by light rays or
    matter is determined by the curvature
    of the space-time and allowed a
    modern scientific proof of Mitchell‟s
    hypothesis.



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General relativity

  • Soon after Einstein developed general
    relativity, Karl Schwarzschild discovered a
    mathematical solution to the equations of the
    theory that described such an object.
  • It was only much later, with the work of such
    people as Oppenheimer, Volkoff, and Snyder
    in the 1930‟s that people thought seriously
    about the possibility that such objects might
    actually exist in the universe.

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General relativity

  • Einstein himself vigorously denied their
    reality, believing, as did most of his
    contemporaries, that black holes were a
    mere mathematical curiosity.
  • He died in 1955, before the term “black
    hole” was coined or understood and
    observational evidence for black holes
    began to mount.
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General relativity

  • Near a black hole, this distortion of
    space-time is extremely severe and
    causes black holes to have strange
    properties.
  • In particular, a black hole has an event
    horizon, which is a spherical surface
    that marks the boundary of the black
    hole.
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Event horizon

  • You can think of the horizon as the place
    where the escape velocity equals the velocity
    of light.
  • Outside of the horizon, the escape velocity is
    less than the speed of light.
  • But if you find yourself inside the horizon, the
    escape velocity would be larger than the
    speed of light, thus there is no escape.



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Black holes have no hair

     Black holes, unlike most objects
     can only have three characteristics
        •Mass
        •Angular momentum or spin
        •Electric charge


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The event horizon and the
Schwarzschild radius
• For a nonrotating black hole, the horizon is
  located at the Schwarzschild radius (Rs)

              Rs =2GM/c                  2

  G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2
  M: Mass of the black hole
  c: Speed of light = 3 × 108 km s-1



  Adler Planetarium & Astronomy Museum
The event horizon and the
Schwarzschild radius

   • For a mass as small as a human
     being, the gravitational radius is of
     the order of 10-23 cm, much smaller
     than the nucleus of an atom.
   • For a typical star such as the Sun, it is
     about 3 km (2 miles).



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Black hole classifications

  • Black holes are theorized to come in
    three different sizes
      – Small (“mini” or “primordial”)
      – Medium (“stellar”)
      – Large (“supermassive”).




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Observations of black holes

  • How can you check whether something
    is a black hole or not?
  • The first thing you‟d like to do is
    measure how much mass there is in
    that region.
  • If you've found a large mass
    concentrated in a small volume, and if
    the mass is dark, then it's a good guess
    that there's a black hole there.
  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • One class of black-hole candidates are
    stellar-mass black holes, which are
    thought to form when a massive star
    ends its life in a supernova explosion.




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Stellar black holes

  • Stellar evolution: low mass stars end up
    as white dwarves
  • Moderate mass stars end up as neutron
    stars and pulsars
  • The highest mass stars become black
    holes



  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • Another possibility is
    that black holes
    might form as a
    merger of two
    neutron stars.



                                    Credit: Wai-Mo Suen, Malcolm Tobias,
                                    Mark Miller, et. al.



  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • Merger from loss of
    gravitational
    radiation




                                     Credit: J. Faber & F. Rasio,
                                     Northwestern U.




  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • If such a stellar black hole were to be
    off somewhere by itself, we wouldn't
    have much hope of finding it.
  • However, many (probably most) stars
    come in binary systems – pairs of stars
    in orbit around each other.



  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • If one of the stars in such a binary
    system becomes a black hole, we might
    be able to detect it.
  • In particular, in some binary systems
    containing a compact object such as a
    black hole, matter is sucked off of the
    other object and forms an “accretion
    disk” of stuff swirling into the black
    hole.
  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • The matter in the
    accretion disk gets
    very hot as it falls
    closer and closer to
    the black hole, and
    it emits large
    amounts of
                                    Accretion from companion onto
    radiation.                      compact object.
                                    Credit: Gamma-Ray Astronomy Program
                                    Working Group, NASA.


  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • Because of the intense heat created as
    the mass falls into the accretion disk,
    most of the radiation we observe is in
    the X-ray part of the spectrum.
  • Many such “X-ray binary systems” are
    known, and some of them are thought
    to be likely black-hole candidates.


  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • In order to determine if an unseen compact object is
    a black hole, you need to do is to estimate its mass.
  • By measuring how fast the visible companion orbits
    the center of mass of the system (together with a
    few other things), you can figure out the mass of the
    invisible companion.
  • The technique is quite similar to the one for
    supermassive black holes in galactic centers: the
    faster the star is moving, the stronger the
    gravitational force required to keep it in place, and so
    the more massive the invisible companion.


  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • If the mass of the compact object is found to
    be very large very large, then there is no kind
    of object we know about that it could be
    other than a black hole.
  • An ordinary star of that mass would be
    visible. A stellar remnant such as a neutron
    star would be unable to support itself against
    gravity, and would collapse to a black hole.


  Adler Planetarium & Astronomy Museum
Observations of stellar black holes

  • The combination of such mass estimates and
    detailed studies of the radiation from the
    accretion disk can supply powerful
    circumstantial evidence that the object in
    question is indeed a black hole.
  • Many of these “X-ray binary” systems are
    known, and in some cases the evidence in
    support of the black-hole hypothesis is quite
    strong.

  Adler Planetarium & Astronomy Museum
Stellar black holes: Cygnus X-1

  • Cygnus X-1 was the name given to a
    source of X-rays in the constellation
    Cygnus, discovered in 1962 with a
    primitive X-ray telescope flown on a
    rocket.
  • By 1971, the location of the X-ray
    source in the sky had been measured
    more precisely, using rocket and
    satellite observations.
  Adler Planetarium & Astronomy Museum
Stellar black holes: Cygnus X-1

•    A faint star appears to be the companion
     to Cygnus X-1. Astronomers studying the
     light of this companion star have found
     two important facts:
    1. HDE 226868 is a blue supergiant star – a
       massive, normal star near the end of its life
    2. the star is orbiting another massive object in a
       5.6-day orbit.

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Stellar black holes: Cygnus X-1

•    The explanation or “model” which best fits
     these facts is that the companion is a black
     hole of about 10 solar masses – the corpse
     of a massive star which was once the
     companion of the observed star.




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Stellar black holes: Cygnus X-1

•    The X-rays are produced as gas from the
     atmosphere of the blue supergiant star
     falls into the collapsed object and is
     heated.
•    The collapsed object cannot be a white
     dwarf or neutron star, because these
     objects can‟t have masses greater than 1.4
     and 3 solar masses, respectively.
    Adler Planetarium & Astronomy Museum
Similar stories for other X-ray binary
systems

•    LMC X-3
•    Nova Muscae 1991
•    V616 Mon (A0620-00)




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Supermassive black holes

  • Some point like sources of radio
    emission are not related to any optical
    star or galaxy.
  • In 1963, the location of the radio
    source (called 3C273) was identified
    and coincided with a very distant star-
    like object in the visual wavelengths.


  Adler Planetarium & Astronomy Museum
   Supermassive black holes
                                                                       CHANDRA X-RAY




(Credit: NASA/HST, Jodrell Bank Observatory, & NASA/CXC/SAO/H. Marshall et al.)


        Adler Planetarium & Astronomy Museum
Supermassive black holes

  • They called these objects quasi-stellar
    radio sources – quasars, for short –
    because they looked like stars, and
    produced large amounts of radio waves
    as well as light.




  Adler Planetarium & Astronomy Museum
Supermassive black holes




 If you had radio eyes you would see these quasars and the centers of active
 galaxies all over the sky in stead of stars. Credit: image courtesy of NRAO/AUI

  Adler Planetarium & Astronomy Museum
Supermassive black holes

  • Astronomers also realized that,
    although quasars were rare, there were
    many other objects – apparently
    galaxies of stars – which showed less
    extreme versions of the same
    phenomenon: very large power from a
    very small volume.


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Supermassive black holes

  • These objects
    shared another
    remarkable
    property: jets of
    high-energy
    particles emitted
    from their cores.

                                  Credit: NASA.


  Adler Planetarium & Astronomy Museum
Supermassive black holes




                 Credit: C.M. Urry & P. Padovani

  Adler Planetarium & Astronomy Museum
Supermassive black holes

  • More recent studies have confirmed that
    QSO‟s lie at the hearts of galaxies which are
    themselves too dim to be visible.
  • QSO‟s are thought to be of the order of size of
    our solar system, but radiate more than 1000
    times as much energy as our entire galaxy.
  • The current explanation is that they result
    from a supermassive black hole which is
    consuming matter from its surrounding galaxy.

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Supermassive black holes

  • The observed power output could be
    explained if material the mass of our Sun
    were to fall into the black hole each
    year.
  • This amount of material which could
    easily come from the orbiting gas and
    winds from massive stars near the core
    of the galaxy.
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Supermassive black holes

  • The jets of particles in active galactic nuclei
    are produced by material spiraling into a disk
    around the black hole; jets are emitted from
    the top and bottom of the disk.

                                         Cygnus A radio galaxy taken
                                         at the Very Large Array.
                                         Credit: Image courtesy of
                                         NRAO/AUI




  Adler Planetarium & Astronomy Museum
Supermassive black holes

  • This explanation for the “central engine”
    in an active galactic nucleus has been
    strongly supported by images obtained
    by the Hubble Space Telescope, Chandra
    X-ray observatory and radio telescopes
    such as the Very Large Array.



  Adler Planetarium & Astronomy Museum
Supermassive black holes




 2D MHD Simulation of Jet (Mach number=10, Jet density=0.01 of external
 medium), Top: Gas Density; Bottom: Magnetic Pressure. Credit:
 I. L. Tregillis, T. W. Jones & Dongsu Ryu.

  Adler Planetarium & Astronomy Museum
Supermassive black holes




 3D MHD Simulation of Jet (Mach number=6), Synchrotron (radio) surface
 brightness. Credit: I. L. Tregillis, T. W. Jones & Dongsu Ryu.


  Adler Planetarium & Astronomy Museum
How do supermassive black holes
form?

 • Some theories hold that the first
   generation of stars included a large
   proportion of very massive stars, all of
   which formed black holes which somehow
   merged.
 • Other theories hold that a single “seed”
   black hole accreted stars and gas, growing
   more and more massive with time.
  Adler Planetarium & Astronomy Museum
Observations of supermassive black
holes

  • Many galaxies have been observed to
    contain such massive dark objects in
    their centers.
  • The masses of the cores of these
    galaxies range from one million to
    several billion times the mass of the
    Sun.


  Adler Planetarium & Astronomy Museum
Observations of supermassive black
holes
  • The mass is measured by observing the speed which
    stars and gas orbit around the center of the galaxy:
    the faster the orbits, the stronger the gravitational
    force required to hold them in their orbits.
  • This is the most common way to measure masses in
    astronomy. For example, we measure the mass of
    the Sun by observing how fast the planets orbit it,
    and we measure the amount of dark matter in
    galaxies by measuring how fast things orbit at the
    edge of the galaxy.



  Adler Planetarium & Astronomy Museum
Observations of supermassive black
holes

  • These massive dark objects in galactic
    centers are thought to be black holes
    for at least two reasons.
  • First, it is hard to think of anything else
    they could be: they are too dense and
    dark to be stars or clusters of stars.



  Adler Planetarium & Astronomy Museum
Observations of supermassive black
holes

  • Secondly, the only promising theory to
    explain the enigmatic objects known as
    quasars and active galaxies suggests that
    such galaxies have supermassive black holes
    at their cores.
  • If this theory is correct, then a large fraction
    of galaxies – all the ones that are now or
    used to be active galaxies – must have
    supermassive black holes at the center.

  Adler Planetarium & Astronomy Museum
Exotic technique: gravitational
radiation

   • The existence of curved spacetime opens
     up the possibility that ripples or waves
     can exist in the spacetime continuum.
   • These ripples are called gravitational
     waves. Gravity waves could be detected
     from colliding black holes, supernova
     explosions and the black hole at the core
     of our Galaxy.


  Adler Planetarium & Astronomy Museum
Supermassive black holes: M87

  • Hubble measurements show the disk at the
    center of M87 is rotating very rapidly.
    Scientists believe it contains a massive black
    hole at its hub.
  • Though the black hole weigh as much as 3
    billion of our Suns, it is concentrated into a
    space no larger than our solar system.
  • A brilliant jet of high-speed electrons that
    emits from the nucleus is believed to be
    produced by the black hole‟s “engine.”

  Adler Planetarium & Astronomy Museum
Supermassive black holes: M87




         Credit: Image courtesy of NRAO/AUI.


  Adler Planetarium & Astronomy Museum
 Supermassive black holes: M87

• Hubble Space
  Telescope image of
  a spiral-shaped disk
  of hot gas in the
  core of active galaxy
  M87.




                               Credit: STScI WFPC2.

     Adler Planetarium & Astronomy Museum
 Supermassive black holes: NGC 4261
A composite image of the
active galaxy NGC 4261,
showing jets of radio-
emitting particles spurting
from the core of the galaxy.
A false-color image (right)
from the Hubble Space
Telescope, shows a dark,
doughnut-shaped structure
surrounding a possible
supermassive black hole.             Credit: Walter Jaffe, Leiden Observatory;
                                     Holland Ford, STScI, NASA


     Adler Planetarium & Astronomy Museum
 Supermassive black holes: NGC 4258
NGC 4258 was found to have a system
of “water masers” near its nucleus.
Using the technique of very-long-
baseline [radio frequency]
interferometry, researchers were able
to determine the motion of the gas
very accurately. From this they can
conclude that the massive object at the
center of this galaxy is less than half a
light-year in radius. It is hard to
imagine anything other than a black
hole that could have so much mass
concentrated in such a small volume.

                                            Credit: Harvard Smithsonian CfA, NRAO/AUI.
      Adler Planetarium & Astronomy Museum
Supermassive black holes: The Milky
Way

  • There has been
    growing evidence
    that our own Galaxy
    harbors a black hole
    in its center.




  Adler Planetarium & Astronomy Museum   Credit: Image courtesy of NRAO/AUI
Supermassive black holes: The Milky
Way




                 Credit: Image courtesy of NRAO/AUI

  Adler Planetarium & Astronomy Museum
Supermassive black holes: The Milky
Way




                 Credit: Image courtesy of NRAO/AUI

  Adler Planetarium & Astronomy Museum
Supermassive black holes: The Milky
Way




                 Credit: Image courtesy of NRAO/AUI

  Adler Planetarium & Astronomy Museum
Proper motion of hot gas in the
Galactic Center




                 Credit: D. Roberts, F. Yusef-Zadeh & W.M Goss

  Adler Planetarium & Astronomy Museum
Proper motion of stars in the
Galactic Center




              Credit: R. Genzel, A. Eckart, T. Ott, MPE.


  Adler Planetarium & Astronomy Museum
Proper motion of stars in the
Galactic Center




                 Credit: UCLA Galactic Center Group.

  Adler Planetarium & Astronomy Museum
Supermassive black holes: The Milky
Way
• Research that
  investigated the
  motion of gas and
  stars around the
  center has shown
  that the enclosed
  mass is constant to
  within a few times
  the size of our solar
  system.

  Adler Planetarium & Astronomy Museum
Supermassive black holes: The Milky
Way

  • The only known object that could produce
    this effect is a supermassive black hole
    (although this is the smallest supermassive
    one known) that is about 2 million times
    more massive than the sun.
  • The big mystery in our Galaxy is why; with
    such a large black hole in the center don‟t we
    see our core as an active galactic nucleus.


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