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Chapter 17
Galaxies with
Active Nuclei
Guidepost
You can imagine galaxies rotating slowly and quietly
making new stars as the eons pass, but the nuclei of some
galaxies are sites of powerful eruptions that eject highspeed jets in opposite directions. As you study these active
galaxies, you will be combining many of the ideas you
have discovered so far to answer five essential questions:
• What evidence shows that some galactic nuclei are
active?
• What is the energy source of this activity?
• What triggers the nucleus of a galaxy into activity?
• What are the most distant active galaxies?
• What can active galaxies reveal about the history of the
universe?
Guidepost (continued)
There are billions of the galaxies in the sky, and
astronomers can’t study every one. Rather they must use
statistical evidence, and that raises a common question
about the scientific method:
• If statistics isn’t certainty, how can scientists use it to
understand nature?
The active galaxies are the last pieces of evidence you
need before you try to understand the birth and evolution
of the entire universe and the galaxies that fill it. You will
start that journey in the next chapter.
Outline
I. Active Galaxic Nuclei
A. Seyfert Galaxies
B. Double-Lobed Radio Sources
C. Exploring Supermassive Black Holes
D. The Search for a Unified Model
E. The Origin of Supermassive Black Holes
II. Quasars
A. The Discovery of Quasars
B. The Distance to Quasars
C. Evidence of Quasars in Distant Galaxies
D. Superluminal Expansion
E. A Model Quasar
F. Quasars Through Time
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!
The Spectra of Galaxies
Taking a spectrum of
the light from a normal
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
• Emission line spectra from
core region.
• Variability: ~ 50 % in a
few months
Most likely power
source:
Accretion onto a
supermassive black
hole (~107 – 108 Msun)
Interacting Galaxies
Seyfert galaxy NGC 7674
Active galaxies are often
associated with interacting
galaxies, possibly result of
recent galaxy mergers.
Often: gas outflowing at high velocities, in opposite directions
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Radio image
of Cygnus A
Hot spots: Energy in
Material in the jets moves
with almost the speed of
light (“Relativistic jets”).
the jets is released in
interaction
with
surrounding
material
Radio Galaxies
Cygnus A: A giant pair of radio jets.
Jet visible in radio and
X-rays; show bright
spots in similar locations.
Radio Image
Centaurus A
(= “Cen A” =
NGC 5128):
Infrared image
reveals warm
gas near the
nucleus.
Radio Galaxies (2)
NGC 1265: Evidence for
the galaxy moving through
intergalactic material
Radio image
of 3C 75
3C 75: Evidence for
two nuclei  recent
galaxy merger
Radio Galaxies (3)
3C31: Member of a chain of galaxies.
Twisted jets, probably because two
galactic nuclei are orbiting each other.
Formation of Radio Jets
Jets are powered by accretion of matter onto
a supermassive black hole
Black Hole
Accretion Disk
Twisted magnetic fields help to confine the material in
the jet and to produce synchrotron radiation.
The Jets of M 87
M 87 = Central, giant elliptical galaxy in
the Virgo cluster of galaxies
Optical and radio observations detect
a jet with velocities up to ~ 1/2 c.
Evidence for Black Holes in AGNs
NGC 4261: Radio image reveals double-lobed jet structure;
close-up view by Hubble Space Telescope reveals a bright
central source embedded in a dust torus.
NGC 7052:
Stellar velocities
indicate the presence of
a central black hole.
Model for Seyfert Galaxies
Seyfert I:
Strong, broad emission lines from
rapidly moving gas clouds near the BH
Gas clouds
Emission lines
UV, X-rays
Accretion disk
Dense dust torus
Seyfert II:
Supermassive
black hole
Weaker,
narrow
emission
lines from
more slowly
moving gas
clouds far
from the BH
The Dust Torus in NGC 4261
Dust Torus is directly visible with Hubble
Space Telescope
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.
Other Types of AGN and AGN Unification (2)
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”).
Black Holes in Normal Galaxies
X-ray
sources are
mostly
accreting
stellarmass black
holes.
The Andromeda galaxy M 31:
No efficient accretion onto the
central black hole
Bursts of Activity
of Supermassive
Black Holes
A star wandering too close
to a supermassive black
hole can be disrupted and
trigger an X-ray outburst.
Active Galaxies in Galaxy Clusters
The powerful radio lobes of radio galaxies can
push away intergalactic gas in galaxy clusters.
Even hundreds of millions of years after the Galaxy’s
activity has calmed down, there are still “ghost
cavities” in the X-ray emission from intergalactic gas.
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.
The Spectra of Quasars
Spectral lines show
a large red shift of
z = Dl / l0 = 0.158
The Quasar 3C 273
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,
beyond
z~6
z = Dl/l0
This indicates distances of several Gigaparsec
Studying Quasars
The study of high-redshift quasars allows
astronomers to investigate questions of:
1) Large scale structure of the universe
2) Early history of the universe
3) Galaxy evolution
4) Dark matter
Observing quasars at high redshifts:
• distances of several Gpc
• Look-back times of many billions of years
• The universe was only a few billion years old!
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.
Evidence for Quasars in Distant
Galaxies
Quasar 0351+026 at the
same red shift as a galaxy
 evidence for quasar
activity due to galaxy
interaction
Host Galaxies of Quasars
The radio image of the quasar 3C175 shows a
double-lobe jet structure, indicating its association
with an active galactic nucleus.
Gallery of Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies
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
(velocity is
close to c)
Relativity
should be
consider in
the
calculation