Do I have your attention…?

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Do I have your attention?
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Galaxies with Active Nuclei
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Active Galaxies
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Galaxies with extremely violent energy released
in their nuclei (pl. of nucleus).
? active galactic nuclei ( AGN)
Up to many thousand times more luminous than the
entire Milky Way energy released within a region
approx. the size of our solar system!
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Line Spectra of Galaxies
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Taking a spectrum of the light from a normal
galaxy
The light from the galaxy should be mostly star
light, and should thus contain many absorption
lines from the individual stellar spectra.
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Seyfert Galaxies
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Unusual spiral galaxies

• Very bright cores

• Emission line spectra

• Variability 50 in a few months

Most likely power source Accretion onto a
supermassive black hole (107 108 Msun)
NGC 1566
Circinus Galaxy
NGC 7742
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Interacting Galaxies
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Seyfert galaxy NGC 7674
Seyfert galaxy 3C219
Active galaxies are often associated with
interacting galaxies, possibly result of recent
galaxy mergers.
Often gas outflowing at high velocities, in
opposite directions
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Cosmic Jets and Radio Lobes
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Many active galaxies show powerful radio jets
Hot spots Energy in the jets is released in
interaction with surrounding material
Radio image of Cygnus A
Material in the jets moves with almost the speed
of light (relativistic jets).
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Radio Galaxies
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Centaurus A (Cen A NGC 5128) the closest AGN
to us.
Jet visible in radio and X-rays show bright
spots in similar locations.
Infrared image reveals warm gas near the nucleus.
Radio image superposed on optical image
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Radio Galaxies (II)
Visual radio image of 3C31
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Radio image of 3C75
Radio image of NGC 1265
Evidence for the galaxy moving through
intergalactic material
3C75 Evidence for two nuclei ? recent galaxy
merger
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Formation of Radio Jets
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Jets are powered by accretion of matter onto a
supermassive black hole.
Black Hole
Accretion Disk
Twisted magnetic fields help to confine the
material in the jet and to produce synchrotron
radiation.
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The Jets of M87
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Jet 2.5 kpc long
M87 Central, giant elliptical galaxy in the
Virgo cluster of galaxies
Optical and radio observations detect a jet with
velocities up to 1/2 c.
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The Dust Torus in NGC4261
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Dust torus is directly visible with Hubble Space
Telescope
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Model for Seyfert Galaxies
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Seyfert I Strong, broad emission lines from
rapidly moving gas clouds near the black hole
Gas clouds
Emission lines
UV, X-rays
Seyfert II Weaker, narrow emission lines from
more slowly moving gas clouds far from the black
hole
Supermassive black hole
Accretion disk
dense dust torus
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Other Types of AGN and AGN Unification
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Observing direction
Cyg A (radio emission)
Radio Galaxy Powerful radio lobes at the end
points of the jets, where power in the jets is
dissipated.
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Other Types of AGN and AGN Unification
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Quasar or BL Lac object (properties very similar
to quasars, but no emission lines)
Emission from the jet pointing towards us is
enhanced (Doppler boosting) compared to the jet
moving in the other direction (counter jet).
Observing direction
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The Origin of Supermassive Black Holes
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Most galaxies seem to harbor supermassive black
holes in their centers.
Fed and fueled by stars and gas from the
near-central environment
Galaxy interactions may enhance the flow of
matter onto central black holes
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Quasars
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Active nuclei in elliptical galaxies with even
more powerful central sources than Seyfert
galaxies.
Also show strong variability over time scales of
a few months.
Also show very strong, broad emission lines in
their spectra.
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The Spectra of Quasars
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The Quasar 3C273
Spectral lines show a large redshift of
z Dl / l0 0.158
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Quasar Red Shifts
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Quasars have been detected at the highest
redshifts, up to z 6
z 0
z 0.178
z Dl/l0
z 0.240
Our old formula Dl/l0 vr/c is only valid in the
limit of low speed, vr ltlt c
z 0.302
z 0.389
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Studying Quasars
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The study of high-redshift quasars allows
astronomers to investigate questions of
1) Large scale structure of the universe
2) Early history of the universe
3) Galaxy evolution
4) Dark matter
Observing quasars at high redshifts ? distances
of several Gpc

• Look-back times of many billions of years
• ? Universe was only a few billion years old!

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Probing Dark Matter with High-z
QuasarsGravitational Lensing
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Light from a distant quasar is bent around a
foreground galaxy
? two images of the same quasar!
Light from a quasar behind a galaxy cluster is
bent by the mass in the cluster.
Use to probe the distribution of matter in the
cluster.
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Gravitational Lensing of Quasars
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Gallery of Quasar Host Galaxies
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Elliptical galaxies often merging / interacting
galaxies
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• What evidence suggests that the energy source in
  a double-lobed radio galaxy lies at the center of
  the galaxy?
• Firstly, the geometry suggests that the lobes are
  inflated by gas jets emerging from the central
  galaxy.
• This is supported by the presence of synchrotron
  radiation which suggests magnetic fields that
  confine the emitted jets to narrow beams, and hot
  spots which suggest gas is being pushed into the
  surrounding gas causing the heating. Also, we
  know that matter falling onto a massive compact
  object (e.g., a black hole) can cause these jets.

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2. How does the peculiar rotation of NGC5128 help
explain the origin of this active galaxy? There
is a spherical cloud of stars orbiting about an
axis which is perpendicular to the axis of
rotation of the disk. ? This strongly hints that
this is the result of a merger.
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3. What statistical evidence suggests that
Seyfert galaxies have suffered recent
interactions with other galaxies? They are three
times more common in interacting pairs of
galaxies than in isolated galaxies. 25 have
shapes that suggest tidal interactions with other
galaxies.
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4. How does the unified model explain the two
kinds of Seyfert galaxies? It all depends on how
the accretion disk is tipped WRT your line of
sight. Tipped slightly ? you are able to observe
the hot, fast moving gas in the central galaxy,
thus the x-rays and higher Doppler shifts produce
the smeared, broad spectral lines. Not tipped at
all ? the disk blocks the radiation from the
central galaxy. Plus, this gas is moving slower
and explains the narrow spectral lines.
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5. What observations are necessary to identify
the presence of a supermassive black hole at the
center of a galaxy? Observations of size and
motion the short time period it takes to
fluctuate in brightness ? small Motion of stars
near the center allow for use of Kepler 3 and
hence the mass. Thus Doppler shifts combined
with other observations that allow for the
distance to be calculated (e.g., Cepheids).
Basically everything to allow for us to use
Kepler 3.
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6. How does the unified model implicate
collisions and mergers in triggering active
galaxies? Tidal interactions with other galaxies
not only can rip matter from a galaxy, but also
can throw matter inward, toward the center of the
galaxy. In this case, you would have a flood of
matter falling into the black hole increasing the
intensity of the bipolar flow.
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7. Why were quasars first noticed as being
peculiar? How could quasars be so luminous that
they emit 10 to 1000 times the energy of a
galaxy, yet reside in a region only the size of
our solar system?
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8. How do the large redshifts of quasars lead
astronomers to conclude they must be very
distant? ,
if z is large, then d must also be large.
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9. What evidence suggests that quasars are
ultraluminous but must be very small? They are
very distant, yet easily photographed ? very
luminous. The small time to fluctuate in
brightness (a few days) ? they must be smaller
than a few light days in diameter.
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10. How do gravitational lenses provide evidence
that quasars are distant? The spectra of quasars
are similar, yet each are as unique as
fingerprints. In 1979, the object 0957561 was
observed. It consists of two quasars separated
by 6 of arc. These two objects share the exact
same spectra, which implies that they are the
same object! Yet the closer lensing galaxy is so
far away that it is difficult to detect.
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11. What evidence is there that quasars occur in
distant galaxies? Astronomers recorded the
spectra of objects near quasars. Those objects
share the same spectra of normal galaxies, and
they have the same redshift as the quasar.
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12. How can our model quasar explain the
different radiation received from quasars? The
two kinds of radiation are a continuous spectrum
and some emission lines. The continuous spectra
fluctuates rapidly, which suggests that the
object emitting it is small probably a central
black hole with an accretion disk. The emission
lines dont fluctuate rapidly, suggesting that
they emanate form a larger region many
light-years in diameter probably clouds of gas
excited from the synchrotron radiation from the
central black hole. Also, as with AGNs, how the
disk is tipped will affect what kind of quasar
youll see.
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13. What evidence is there that quasars must be
triggered by collisions and mergers? Galaxies
were closer when the universe was young. They
would have collided more often, and we know that
interactions can cause matter to flow inward.
Also, quasars are often located in distorted
galaxies which suggests they interacted with
other galaxies.
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14. Why are there few quasars at low redshifts
and at high redshifts but many at redshifts of
about 2? A redshift of 2 corresponds to a time
in the universe when galaxies were most actively
forming, colliding, and merging.