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Active Galactic Nuclei
Astronomy 315
Professor Lee Carkner
Lecture 19
Strange Galaxies
Some galaxies have a compact,
powerful source of energy at their core
Among the most energetic objects in
the universe
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Need multiwavelength observations to
understand them
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M87
Jet From M87
Seyfert Galaxies
Seyfert galaxies are spirals with
optically bright, concentrated nuclei
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Can change in brightness very quickly
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Small, but very bright
Other Seyfert Properties
Some Seyferts have broad emission lines
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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?
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Are Seyferts caused by gravitational
interactions?
Radio Galaxies
Some galaxies are flanked on either
side by a pair of radio lobes
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Some central galaxies are also bright
radio sources and some are not

Cygnus A Radio Galaxy
Mapping the Lobes
Radio lobes often show hot spots of
enhanced emission
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Lobes are material ejected from the star and
impacting the intergalactic medium
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Radio galaxies produce bipolar jets (like
young stars)
Source of Radio Waves
Radio emission is due to synchrotron
radiation
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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
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Jets sometimes are twisted
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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
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Quasars have very large red shifts and
very large distances
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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
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Core must be brighter than regular AGN
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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
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Different types of AGNs are due to
viewing matter falling into a black hole
from different angles
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
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Outer disk is thick can block view of center
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The moving material twists up the magnetic
field creating a magnetic flux tube that the jets
follow out the poles
Unified Model
How does this model account for the
basic properties of AGNs?
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We thus see the jet and disk regions in
different ways, producing the observed
type of AGN
Case 1 -- Face on
Can see the radio jets, but no lobes
Can’t see broad or narrow lines
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Type of AGN
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Case 2 -- Inclined
Can see radio emission and lobes
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Can also see broad and narrow lines
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Types of AGN:
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Case 3 -- Edge on
Can see radio emission and lobes
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Only see narrow lines
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Types of AGN
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Unified Model for AGNs
Quasars and AGNs
Quasars act very much like extra
powerful versions of radio galaxies or
BL Lac objects
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Quasars may be young active AGN,
low red shift active galaxies may be
AGN that have been refueled
AGN and Non-AGN
Why are some galaxies active and
others not?
We think all galaxies have black holes
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What makes a galaxy active is matter
falling into it
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Collisions and tidal forces may disrupt
the center of galaxies and move
material into the black hole
Next Time
Read 24 .3, 25.1-25.4