Class 25 : Active galactic nuclei




Discovery of AGN (3C 273).
What are AGN?
Radio galaxies.
AGN and colliding galaxies.
I : The discovery of AGN

Early 1960s…





Radio astronomers started to survey the sky.
Found many mysterious radio point sources.
Quasi-stellar radio sources (quasars for short).
Very difficult to identify… radio images were too fuzzy
to allow quasars to be localized on sky.
Cyril Hazard…




Used “lunar occultation” to localize 3C273.
I.e., measure the precise time when the radio signal
from the quasar is blocked by the Moon…
… then use knowledge of the Moon’s position to
determine position of quasar.
Hazard could localize 3C273 to about 1 arcsec.
Parkes Radio Telescope, Oz
3C273 in
visible…
3C273 in
radio…
3C273 in
X-ray…

M.Schmidt




Took Hazard’s position and observed it with “normal”
(optical) telescope… in particular, he measured its
spectrum and computed the cosmological redshift.
Redshift implied huge speed (50,000 km/s).
Hubble’s law  huge distance (700 Mpc+).
Thus, 3C273 must be very powerful to still
appear bright when it’s so far away!



About 1000 luminosity of our whole galaxy.
Source was variable  must be “small”, about size of
our solar system (WHY? See next slide…).
What could cause this??
Size limits from variability
Photons from A
arrive one year
before photons
from C – any
more rapid
changes in
brightness are
washed out; so
variability gives
maximum size
D < ctvar.
There are now many many
examples of AGN…
MCG-6-30-15
II : The theory of AGN


What powers AGN?
Define efficiency of an AGN:
E   Mc

2
Think about efficiency of different
processes…




Chemical burning,   10-9.
Nuclear fusion,   0.007.
Accretion onto a black hole,   0.1.
Matter/anti-matter annihilation,  = 1.

Suppose AGN has power of 1040 W and lasts
for 10 million years.

Then, what masses are needed?





Chemical burning… 21016 M.
Nuclear burning… 3109 M.
Accretion… 2108 M.
Matter/anti-matter… 2107 M.
Masses tend to argue for accretion (there are
not large amounts of anti-matter in space!)


Now believe that AGN are indeed accreting
supermassive black holes.
Matter can’t fall straight in… forms an
accretion disk.
III : Radio galaxies
Cygnus A

About 1-10% of AGN are radio-galaxies.

Radio galaxies.



Possess jets that transport energy away
from center.
Processes associated with jets then give
powerful radio emission.
Material in the jets is moving very close to
the speed of light… how do we know this?

Superluminal motion




Jets have “blobs.”
Some blobs appear to move
faster than light!
Motion faster than light is not
allowed by the Special Theory
of Relativity.
What’s going on?


Superluminal motion is optical
illusion – blobs partially “catch
up” with the light they emit.
But jet needs to be traveling
close to the speed of light.
A last word on collisions…
Recall that galaxy collisions can
trigger star formation. Similarly,
collisions can funnel gas toward
supermassive black holes and
thereby power AGN.