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Active Galactic Nuclei
Astronomy 315Professor Lee
CarknerLecture 21
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Strange Galaxies Some galaxies have a compact,
powerful source of energy at their core Among the most energetic objects in
the universe
Need multiwavelength observations to
understand them
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M87
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Jet From M87
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Seyfert Galaxies Seyfert galaxies are spirals with
optically bright, concentrated nuclei
Can change in brightness very quickly
Small, but very bright
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Other Seyfert Properties Some Seyferts have broad emission
lines
These same Seyferts also are bright in high energy X-ray and UV radiation
Called Type 1 Seyferts
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Seyfert Clues
Are there any common properties shared by Seyfert galaxies?
Are Seyferts caused by gravitational interactions?
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Radio Galaxies
Some galaxies are flanked on either side by a pair of radio lobes
Some central galaxies are also bright radio sources and some are not
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Cygnus A Radio Galaxy
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Mapping the Lobes Radio lobes often show hot spots of
enhanced emission
Lobes are material ejected from the star and impacting the intergalactic medium
Radio galaxies produce bipolar jets
(like young stars)
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Source of Radio Waves
Radio emission is due to synchrotron radiation
Lobes must have magnetic field and galaxy must be ejecting electrons
Total energy stored in lobes is huge
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Radio Galaxy Properties Central galaxy is often giant elliptical
and in a crowded cluster Often deformed
Jets sometimes are twisted
Some radio galaxies have broad and
narrow lines, some just narrow lines
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BL Lac Objects BL Lac objects look like stars but show
rapid variations
They don’t show the broad or narrow lines we see in Seyfert galaxies
Don’t have radio lobes
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Quasars Some sources of radio galaxies look like
stars
Quasars have very large red shifts and very large distances
Since quasars are billions of light years away, we are seeing what they looked like billions of years ago
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Quasar Properties In most cases you can’t see the host galaxy
Core must be brighter than regular AGN
Quasars are younger than “normal” AGNs Type 1 quasars have broad emission lines,
Type 2 quasars do not
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AGN Power Source
We have two questions about AGNs
Different types of AGNs are due to viewing matter falling into a black hole from different angles
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Massive Black Holes AGN black holes are a million to a billion
times the mass of the Sun Why do we think they have black holes?
Computed densities indicate black hole
AGNs vary so rapidly that the emitting region must be very small (small+massive=BH)
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Structure of the Core Black hole pulls matter into an accretion disk
Outer disk is thick can block view of center The moving material twists up the magnetic
field creating a magnetic flux tube that the jets follow out the poles
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Unified Model
How does this model account for the basic properties of AGNs?
We thus see the jet and disk regions
in different ways, producing the observed type of AGN
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Case 1 -- Face on
Can see the radio jets, but no lobes Can’t see broad or narrow lines
Type of AGN
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Case 2 -- Inclined Can see radio emission and lobes
Can also see broad and narrow lines
Types of AGN:
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Case 3 -- Edge on Can see radio emission and lobes
Only see narrow lines
Types of AGN
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Unified Model for AGNs
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Quasars and AGNs Quasars act very much like extra
powerful versions of radio galaxies or BL Lac objects
Quasars may be young active AGN,
low red shift active galaxies may be AGN that have been refueled
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AGN and Non-AGN Why are some galaxies active and others
not? We think all galaxies have black holes
What makes a galaxy active is matter falling
into it
Collisions and tidal forces may disrupt the center of galaxies and move material into the black hole
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