Active Galaxies

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Active Galaxies
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Galaxies 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!
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Question
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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
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What kind of spectrum would you expect to see?

• A featureless continuum
• A blackbody continuum with absorption lines
• Only emission lines.

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4
Answer
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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.
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Seyfert Galaxies
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Unusual spiral galaxies

• Very bright cores

• Emission line spectra.

NGC 1566

• Variability 50 in a few months

Most likely power source Accretion onto a
supermassive black hole (107 108 Msun)
Circinus Galaxy
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Model for Seyfert Galaxies
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Gas clouds
Emission lines
UV, X-rays
Supermassive black hole
Accretion disk
Dense dust torus
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The Dust Torus in NGC 4261
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Quasars
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Active nuclei in elliptical galaxies with even
more powerful central sources than Seyfert
galaxies
Also show strong variability over time scales of
a few months.
Also show very strong, broad emission lines in
their spectra.
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Quasar Red Shifts
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z 0
Quasars have been detected at the highest red
shifts, up to z 6
z 0.178
z 0.240
z 0.302
z Dl/l0
z 0.389
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What can astronomers study when observing
high-redshift quasars?

• Large-scale structures in the universe.
• The early history of the universe.
• Galaxy evolution.
• Dark matter.
• All of the above.

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Observing quasars at high redshifts ? distances
of several Gpc

• Look-back times of many billions of years
• ? Universe was only a few billion years old!

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Probing Dark Matter with High-z
QuasarsGravitational Lensing
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Light from a distant quasar is bent around a
foreground galaxy
? two images of the same quasar!
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.
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Quasar Host Galaxies
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Elliptical galaxies often merging / interacting
galaxies
0316-346
PG 0052251
IRAS 04505-2958
IRAS 132180522
PHL 909
PG 1012005
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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?

• Supernovae.
• Pulsars.
• Jets.
• Gamma-Ray bursts.
• Planetary Nebulae.

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Like in so many other accreting systems
(protostars, X-ray binaries, gamma-ray bursts?),
the accretion disk might be associated with a
relativistic jet.
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Cosmic Jets and Radio Lobes
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Many active galaxies show powerful radio jets
Hot spots Energy in the jets is released in
interaction with surrounding material
Radio image of Cygnus A
Material in the jets moves with almost the speed
of light (Relativistic jets).
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Other Types of AGN and AGN Unification
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Quasar or BL Lac object (properties very similar
to quasars, but no emission lines)
Emission from the jet pointing towards us is
enhanced (Doppler boosting) compared to the jet
moving in the other direction (counter jet).
Observing direction
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Blazars
Light curves of the BL Lac object 3C66A
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Highest luminosities of all active galaxies
Variable on very short time scales (a few hours)
Sources of very strong X-ray and gamma-ray
emission
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Blazars
Light curves of the BL Lac object 3C66A
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Highest luminosities of all active galaxies
Variable on very short time scales (a few hours)
Sources of very strong X-ray and gamma-ray
emission
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Blazars
Light curves of the BL Lac object 3C66A
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Highest luminosities of all active galaxies
Variable on very short time scales (a few hours)
Sources of very strong X-ray and gamma-ray
emission
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Blazars
Radio images of the BL Lac object 3C66A
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Highest luminosities of all active galaxies
Variable on very short time scales (a few hours)
Sources of very strong X-ray and gamma-ray
emission
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Other Types of AGN and AGN Unification
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Observing direction
Cyg A (radio emission)
Radio Galaxy Powerful radio lobes at the end
points of the jets, where power in the jets is
dissipated.
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Radio Galaxies (I)
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Optical image
Radio image
Centaurus A (Cen A) the most nearby AGN.
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Radio Galaxies (II)
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Optical jet
Optical image
Radio image
M 87 The central galaxy of the Virgo cluster of
galaxies
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Superluminal Motion
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Individual 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