Transcript 20agn1s
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
Need multiwavelength observations to
understand them
M87
Jet From M87
Seyfert Galaxies
Seyfert galaxies are spirals with
optically bright, concentrated nuclei
Can change in brightness very quickly
Small, but very bright
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
Seyfert Clues
Are there any common properties
shared by Seyfert galaxies?
Are Seyferts caused by gravitational
interactions?
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
Cygnus A Radio Galaxy
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)
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
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
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
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
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
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
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)
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
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
Case 1 -- Face on
Can see the radio jets, but no lobes
Can’t see broad or narrow lines
Type of AGN
Case 2 -- Inclined
Can see radio emission and lobes
Can also see broad and narrow lines
Types of AGN:
Case 3 -- Edge on
Can see radio emission and lobes
Only see narrow lines
Types of AGN
Unified Model for AGNs
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
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
Next Time
Read 24 .3, 25.1-25.4