Quasars

1
Quasars Unsolved mysteries?

• 2 basic things used for studying far away
  galaxies
• the data encoded in the light received
• our creative minds to interpret what is seen
  using the laws of physics.
• Some blue star-like objects appeared to violate
  those rules.

2
Discovery of Quasars

• Stars do not produce much energy in the radio
  band.
• When strong radio emission coming from some blue
  stars was spotted in 1960, astronomers were
  puzzled.
• They quickly took spectra of the stars in the
  visible (optical) band to find out the conditions
  in these strange objects.

3
Quasars Unsolved mysteries?

• Line patterns did not match any of the patterns
  seen in 1000s of stellar spectra gathered over a
  100 years.
• Spectra of the blue radio sources did not have
  absorption lines, but broad emission lines!
• What a mystery!

4
Quasars

• Maarten Schmidt solved the mystery in 1963.
• Constructed an energy level diagram from the
  pattern of the emission lines.
• As a test he compared the spectrum of 3C 273 with
  the spectrum of hydrogen.
• He was shocked because the pattern was the same
  but greatly red-shifted!
• 3C 273 is moving at a speed of 47,400
  kilometers/second (almost 16 the speed of
  light!).

5
Quasars

• The Hubble Law says that this blue radio object
  is far outside the Galaxy.
• Other radio stars were also at great distances
  from us.
• Called quasi-stellar radio sources or quasars.
• Later, other blue star-like objects at large
  red-shifts were discovered to have no radio
  emission, but they are also called quasars.

6
Strangeness of Quasars

• What is strange about the quasars is not their
  great distance, but, rather, their incredible
  luminosities.
• They are 100 to 1000 times more luminous than
  ordinary galaxies.
• Yet, all of this energy is being produced in a
  small volume of space.

7
Strangeness of Quasars

• Their luminosity varies on time scales of a few
  months to as short as a few days.
• The quasars that vary their light output over a
  few months are about the size of our solar
  system.
• This is 10,000 times smaller than a typical
  galaxy!

8
Star/Galaxy Spectra

• The shape of the continuous part of a quasar
  spectrum is also quite unusual.
• Stars are luminous primarily in the visible
  (optical) band of the electromagnetic spectrum.
• Hot stars also emit a significant fraction of
  their light in the ultraviolet band
• Cool stars emit a significant fraction of their
  light in the infrared band.
• Star spectra, and the spectrum of a normal
  galaxy, are thermal, they rise to a peak at a
  wavelength determined by the temperature and
  drops off at shorter or longer wavelengths.

9
Quasar Spectra

• Quasars have a non-thermal spectrum
• luminous in the X-ray, ultraviolet, visible,
  infrared, and radio bands.
• Have same power at all of the wavelengths down to
  the microwave wavelengths (shortwave radio
  wavelengths).
• Spectrum looks like the synchrotron radiation
  from charged particles spiraling around magnetic
  field lines at nearly the speed of light

10
Quasar Mystery

• Quasars are found in clusters of galaxies.
• Galaxies are much fainter than the quasars
• Only the largest telescopes can gather enough
  light to create a spectrum for those far away
  galaxies.
• Their spectra also have the same large redshift
  of the quasars in the cluster.
• Some quasars are close enough to us that some
  fuzz is seen around them.
• Color of the fuzz is like that of normal
  galaxies.
• Spectra show that the light from the fuzz is
  from stars.

11

• Top left core of normal spiral,
• Bottom left core of normal elliptical,
• Top center spiral galaxy hit face-on to make a
  quasarstarburst galaxy,
• Bottom center quasar merging with a bright
  galaxy and maybe another one,
• Top right tail of dust and gas show that the
  host galaxy collided with another one
• Bottom right merging galaxies create a quasar in
  their combined nucleus.

12
Quasars

• Quasars are the exceptionally bright nuclei of
  galaxies!

13
Active Galaxies a Clue to Quasars

• Not all active galaxies blaze with the strength
  of a quasar.
• They do exhibit a non-thermal spectrum that has
  no peak and does not depend on the temperature.
• And the energy is generated in their nucleus.
• Active galaxies are less energetic cousins of the
  quasars.
• Luminosity between that of typical galaxies and
  the powerful quasars.
• Whatever is going on in quasars, is going on in
  active galaxies to a lesser extent.

14
Seyfert Galaxy

• Type of active galaxy named after Carl Seyfert
• first to discover their peculiar spectra.
• Spiral galaxy with compact, very bright nucleus
• produces a non-thermal continuous spectrum with
  broad (fat) emission lines on top.
• Some emission lines produced by multiply ionized
  atoms.

15
Seyfert Galaxy

• Such highly ionized atoms are found only in
  regions of intense energy.
• Many Seyfert nuclei are in disks with distorted
  spiral arms and a companion galaxy nearby that is
  probably gravitationally interacting with the
  galaxy.
• The energy of Seyfert galaxy nuclei fluctuates
  quickly like the quasar fluctuations, so the
  energy generator must be quite small.

16
Seyfert Galaxy (2)

• Broad emission lines are produced by gas clouds
  moving at about 10,000 km/s.
• Doppler shifts of the gas moving around the core
  widens the emission lines.

17
Radio Galaxy

• Yet another type of active galaxy
• Emit huge amounts of radio energy.
• Radio emission from the core AND very large
  regions on either side of the optical part of the
  galaxy called radio lobes.
• Radio lobes can extend for millions of LY from
  the center of the galaxy.

18
Radio Galaxy (2)

• Radio emission from normal galaxies is thousands
  to millions of times less intense and is from the
  gas between the stars.
• Most radio galaxies are elliptical galaxies.
• Spectrum of the radio emission has the same
  non-thermal (synchrotron) shape as the quasars
  and Seyferts.
• Radio lobes produced from electrons shot out from
  the nucleus in narrow beams called jets.
• When the electrons in the beam hit the gas
  surrounding the galaxy, the beam spreads out to
  form the lobes.

19
Examples of Radio Galaxies
quasars can have huge radio lobes also.
20
Power Source for Active Galaxies and Quasars

• Problem
• how does nature produce objects that are luminous
  over a large range of wavelengths and generate
  the energy in a very small volume?
• Number of stars needed to produce the tremendous
  luminosity could not be packed into the small
  region and neither would they produce the
  peculiar non-thermal radiation.

21
Production Scenario - 1

• Production of radiation by hot gas in an
  accretion disk around a black hole.
• Black hole must be super massive.
• The intense radiation from the disk would drive
  the gas outward if the black hole did not have
  enough gravity to keep the gas falling onto the
  black hole.

22
Production Scenario - 2

• In order to keep the gas spiraling in and heating
  up, the mass of the black hole must be hundreds
  of millions to several billion solar masses.
• The accretion disk is a few trillion kilometers
  across (a few light months) but most of the
  intense radiation is produced within a couple of
  hundred billion kilometers from the black hole.

23
(No Transcript)
24
Massive Black Holes

• HST has imaged the nuclei of several active
  galaxies.

• Surrounding the core of the radio galaxy NGC 4261
  is a ring of dust and gas about 400 light years
  in diameter and the jets emerge perpendicular to
  the plane of the dust/gas ring.

The black event horizon of the super-massive
black hole too small to be resolved from our
distance.
25
Core of active galaxy M87 has a disk of hot gas
moving very quickly around the center.

• Doppler shifts of the disk material close to the
  center show that the gas is moving at speeds of
  100 km/s.
• Blueshifted/Redshifted lines produced from
  opposite parts of the disk clear proof of
  rotation.
• Base on the observed speeds and distance the gas
  is from the center, the central object must have
  a mass of 2.5 billion solar masses.
• Only a black hole could be this massivecompact.
• The jet coming from the nucleus (visible in the
  wider-field view at right) is also seen to be
  perpendicular to the plane of the disk.

26
Where Quasars are

• Found at great distances from us
• no nearby quasars.
• We see them as they were billions of years ago.
• Number of quasars increases at greater distances.
• they were more common long ago.
• Number of quasars peaked at a time when the
  universe was about 20 of its current age.
• Back then the galaxies were closer together and
  collisions were more common than today.

27
Where Quasars are (2)

• Also, the galaxies had more gas that had not been
  incorporated into stars yet.
• The number of quasars was hundreds of times
  greater then than now.
• At very great distances the number of quasars
  drops off.
• The light from the most distant quasars are from
  a time in the universe before most of the
  galaxies had formed, so fewer quasars could be
  created.

28
Dead Black Holes

• Predicts there should be many dead quasars
  lurking at the cores of old galaxies.
• Astronomers beginning to find the inactive super
  massive black holes in some galaxies.
• In most galaxies the central BH would have been
  smaller than the billions of solar mass black
  holes for quasars.
• This is why the less energetic active galaxies
  are more common than quasars.
• Our galaxy harbors a super massive black hole in
  its core that has a mass of only 2.5 million
  solar masses.
• Astronomers are studying the cores of other
  normal galaxies to see if there are any signs of
  super massive black holes that are now dead.

29
Whirlpool Galaxy

• X marks the spot in the core of the Whirlpool
  Galaxy!
• Darkest bar may be the dust ring seen edge-on.
• The jet seen in wider fields of view is
  perpendicular to the darkest dust ring.
• The lighter bar may be another disk seen
  obliquely.
• A million solar mass black hole is thought to
  lurk at the center.

30
Some closing comments

• An important implication of the fact that there
  were more quasars billions of years ago than now,
  is that the universe changes over time.
• The conditions long ago were more conducive to
  quasar activity than they are today.
• Also, the sharp drop in the quasar number for the
  earliest times is evidence for a beginning to the
  universe.