Transcript Quasars and Active Galactic Nuclei

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.
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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.
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Quasars – Unsolved mysteries?
• Line patterns did not match any of the patterns seen
in 1000’s 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!
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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!).
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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.
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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.
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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!
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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.
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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
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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.
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Top left: core of normal spiral,
Bottom left: core of normal elliptical,
Top center: spiral galaxy hit face-on to make a quasar+starburst 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.
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Quasars
• Quasars are the exceptionally bright nuclei of
galaxies!
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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.
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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.
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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.
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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.
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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.
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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.
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Examples of Radio Galaxies
quasars can have huge radio lobes also.
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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.
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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.
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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.
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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.
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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 massive+compact.
• 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.
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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.
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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.
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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”.
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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.
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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.
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