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:

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.

2 McE

Suppose AGN has power of 1040 W and lasts for 10 million years.

Then, what masses are needed? Chemical burning… 21016 M.

Nuclear burning… 3109 M.

Accretion… 2108 M.

Matter/anti-matter… 2107 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.