Quasars and Active Galactic Nuclei

• Quasi-stellar objects– Redshifts– Variability– Supermassive black holes

• Active Galaxies

Quasars

• Early radio telescopes found radio emission from stars, nebulae, and some galaxies.

• There were also point-like, or star-like, radio sources which varied rapidly these are the `quasi-stellar’ radio sources or quasars.

• In visible light quasars appear as points, like stars.

Optical picture of a

quasar

Hα unshifted

Quasar optical spectrumRedshift shows this quasar, 3C273, is moving away from us at 16% of the speed of light

3C273The quasar 3C273 is 2.6 billion light years away.

It looks dim, but must be extremely luminous to be visible as such distance.

The luminosity of 3C273 is more than one trillion times the entire energy output of our Sun, or 100 times the luminosity of our entire galaxy.

The distance to PKS 2000-030 is

more than 11 billion light years

Quasars vary

Size places a limit on how fast an object can change brightness.

Conversely, rapid variations place a limit on the size of the emitting object.

Quasar size

Quasars vary

The size of this quasar must be less than about one light year.

Quasar engine

• Quasars can have up to several 1000 times the luminosity of our Galaxy

• The engine powering quasars is only a few light years across or smaller

• The only known engine which is powerful enough and compact enough is a black hole

• Quasars contain supermassive black holes

A quasar varies in brightness by a factor of 2 in 10 days. What does this tell us about the quasar?

1. It has a large magnetic field.

2. It is quite small.

3. It must be highly luminous.

4. It cannot emit radio waves.

Quasar jets

Optical core

Radio jet

Quasars

• object with a spectrum much like a dim star• high red shift• enormous recessional velocity• huge distance (from Hubble’s Law)• enormously luminous• compact physical size• powered by supermassive black hole• often produce huge jets

Quasars are the ultraluminous centers of distant galaxies.

Quasars are often observed to be at the center of distant galaxies. The wispy material is likely gas that has been pulled out of the hot

galaxy by gravitational interactions with nearby galaxies.

Quasars are the most extreme of a class of galaxies known as active galaxies

M87 appears as an elliptical galaxy in visible light, but like a dim quasar in radio. The nucleus of the galaxy contains a weak quasar. This means that the galaxy harbors a supermassive black hole.

Centaurus A

Optical Radio

NGC 1566

Spiral galaxies also sometimes contain active cores.

Active galaxies lie at the center of double radio sources.

Active galaxies lie at the center of double radio sources.

• Quasars• Seyfert galaxies

– luminous, star-like nuclei with strong emission lines.

• BL Lacertae objects (BL Lacs)– featureless spectrum with a brightness that can vary

by a factor of 15 times in a few months.– Most commonly known as a Blazar.

• All contain supermassive black holes

Active Galaxies come in several varieties

In Active Galaxies the disk is surrounded by a dusty torus

Accretion disks

The light from a quasar is emitted from a region 10 light-days across. What does this tell us about the quasar?

1. The brightness will change every day.

2. The brightness will change every 20 days.

3. The brightness will not change in less than 10 days.

4. The brightness will not change in less than 100 days.

• Radio Galaxies – big jets, no obvious BH

• Quasars – jets and BH

• Blazars – mainly see the jet

Varieties of Active Galaxies

Orientation

Black holes in normal galaxiesRotation curves of stars near the centers of most galaxies show the presence of supermassive black holes with mass ranging from 106 to 109 solar masses.

Essentially all galaxies contain supermassive black holes and were likely active galaxies at some point in their lives.

Review Questions

• Why is it thought that quasars are at very large distances?

• How can we place an upper bound on the size of a quasar even if we can’t resolve it with a telescope?

• Where are quasars found?

• Do most galaxies have supermassive black holes?