Transcript Active Galaxies

Active Galaxies
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!
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
Unusual spiral galaxies:
• Very bright cores
NGC 1566
• Emission line spectra.
• Variability: ~ 50 % in
a few months
Most likely power source:
Accretion onto a
supermassive black hole
(~107 – 108 Msun)
Circinus Galaxy
Model for Seyfert Galaxies
Gas clouds
Emission lines
UV, X-rays
Dense dust torus
Accretion disk
Supermassive
black hole
The Dust Torus in NGC 4261
Quasars
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.
Quasar Red Shifts
z=0
z = 0.178
z = 0.240
Quasars have
been detected at
the highest red
shifts, up to
z~6
z = 0.302
z = 0.389
z = Dl/l0
What can astronomers study
when observing high-redshift
quasars?
1.
2.
3.
4.
5.
Large-scale structures in the universe.
The early history of the universe.
Galaxy evolution.
Dark matter.
All of the above.
Probing Dark Matter with High-z Quasars:
Gravitational Lensing
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.
Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies
PG 0052+251
PHL 909
IRAS 04505-2958
0316-346
PG 1012+005
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?
1.
2.
3.
4.
5.
Supernovae.
Pulsars.
Jets.
Gamma-Ray bursts.
Planetary Nebulae.
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Hot spots:
Radio image of
Cygnus A
Material in the jets moves
with almost the speed of
light (“Relativistic jets”).
Energy in the jets is
released in interaction
with surrounding
material
Other Types of AGN and
AGN Unification
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”).
Blazars
BL Lac Objects and quasars: AGN with their jets
directed almost right at us.
Very bright at all wavelengths, from radio to gamma-rays
Over 100 gamma-ray blazars
detected!
Fermi Gamma-Ray
Space Telescope
Blazars
Visible
Radio
Over 1000 times more powerful than an entire galaxy
(~ 100 billion stars);
smaller than our solar system!
Faster than the speed of light?
Superluminal Motion
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
Superluminal Motion
Apparent motion at up to ~ 20 times the speed of light!
Other Types of AGN and
AGN Unification
Cyg A (radio emission)
Radio Galaxy:
Powerful “radio lobes”
at the end points of the
jets, where power in the
jets is dissipated.
Radio Galaxies
M 87: The central galaxy of the Virgo cluster of galaxies
Radio Galaxies
X-rays + Optical
X-rays
Optical
Radio
M 87: The central galaxy of the Virgo cluster of galaxies
Radio Galaxies
Optical image
Radio image
Centaurus A (“Cen A”):
the most nearby AGN.