Active Galaxies
Post on 01-Jan-2016
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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
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
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
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
Optical
X-rays
X-rays + Optical
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