Active GalaxiesGalaxies with extremely violent energy release in their nuclei (pl. of nucleus).

→ “Active Galactic Nuclei” (= AGN)

Up to many thousand times more luminous than the entire Milky Way;

energy released within a region approx. the size of our solar system!

If you take a spectrum of a normal galaxy, what would

you expect to see?

1) A pure blackbody spectrum from the continua of all the stars in the galaxy

2) A blackbody spectrum with many absorption lines from the stars in the galaxy

3) A spectrum with many emission lines from hot gases

Answer:

2) A blackbody spectrum with many absorption lines from the stars in the galaxy

The light from the galaxy should be mostly star light, and should thus contain many absorption

lines from the individual stellar spectra.

Seyfert Galaxies

NGC 1566

Circinus Galaxy

Unusual spiral galaxies:

• Very bright cores

• Emission line spectra.

• Variability: ~ 50 % in a few months

Most likely power source:

Accretion onto a supermassive black hole

(~107 – 108 Msun)

Model for Seyfert Galaxies

Supermassive black holeAccretion disk

Dense dust torus

Gas clouds

UV, X-rays

Emission lines

The Dust Torus in NGC 4261

Quasars

Active nuclei in elliptical galaxies with even more powerful central

sources than Seyfert galaxies

Also show very strong, broad emission lines in their spectra.

Also show strong variability over time scales of a few months.

Quasar Red Shifts

z = 0

z = 0.178

z = 0.240

z = 0.302

z = 0.389

Quasars have been detected at the highest red

shifts, up to

z ~ 6

z = /

What can astronomers study when observing high-redshift

quasars?

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1. Large-scale structures in the universe.

2. The early history of the universe.

3. Galaxy evolution.

4. Dark matter.

5. All of the above.

Probing Dark Matter with High-z Quasars:Gravitational Lensing

Light from a quasar behind a galaxy cluster is bent by the mass in the cluster.

Use to probe the distribution of matter in the cluster.

Light from a distant quasar is bent around a foreground galaxy

→ two images of the same quasar!

Quasar Host GalaxiesElliptical galaxies; often merging / interacting galaxies

PG 0052+251

PHL 909

IRAS 04505-2958

PG 1012+005

0316-346

IRAS 13218+0522

If quasars and other AGN are powered by accretion disks onto supermassive black holes, which

other phenomenon would you expect to see in many AGNs?

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1. Supernovae.

2. Pulsars.

3. Jets.

4. Gamma-Ray bursts.

5. Planetary Nebulae.

Cosmic Jets and Radio LobesMany active galaxies show powerful radio jets

Radio image of Cygnus A

Material in the jets moves with almost the speed of light (“Relativistic jets”).

Hot spots:

Energy in the jets is released in interaction

with surrounding material

Emission from the jet pointing towards us is enhanced

(“Doppler boosting”) compared to the jet moving in the other

direction (“counter jet”).

Other Types of AGN and AGN Unification

Quasar or BL Lac object (properties very similar to

quasars, but no emission lines)

Observing direction

Blazars

Over 100 gamma-ray blazars detected!

BL Lac Objects and quasars: AGN with their jets directed almost right at us.

Very bright at all wavelengths, from radio to gamma-rays

Fermi Gamma-Ray Space Telescope

Blazars

Over 1000 times more powerful than an entire galaxy (~ 100 billion stars);

smaller than our solar system!

Visible

Radio

Faster than the speed of light?

Superluminal MotionIndividual radio knots in quasar jets:

Sometimes apparently moving faster than speed of light!

Light-travel time effect:

Material in the jet is almost

catching up with the light

it emits

Superluminal Motion

Apparent motion at up to ~ 20 times the speed of light!

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where power in the

jets is dissipated.

Cyg A (radio emission)

Other Types of AGN and AGN Unification

Observing direction

Radio Galaxies

M 87: The central galaxy of the Virgo cluster of galaxies

Radio Galaxies

M 87: The central galaxy of the Virgo cluster of galaxies

Radio

Optical

X-rays

X-rays + Optical

Radio Galaxies

Optical image Radio image

Centaurus A (“Cen A”):

the most nearby AGN.