Download Chapter 17

Note that the following lectures include
animations and PowerPoint effects such as
fly ins and transitions that require you to be
in PowerPoint's Slide Show mode
(presentation mode).
Chapter 17
Galaxies With Active Nuclei
Guidepost
This chapter is important for two reasons. First, it draws
together ideas from many previous chapters to show how
nature uses the same basic rules on widely different
scales. Matter flowing into a protostar, into a white dwarf,
into a neutron star, or into the heart of a galaxy must obey
the same laws of physics, so we see the same geometry
and the same phenomena. The only difference is the level
of violence.
Second, this chapter is important because the most distant
objects we can see in the universe are the most luminous
galaxies, and many of those are erupting in outbursts and
are thus peculiar. By studying these galaxies, our attention
is drawn out in space to the edge of the visible universe
and back in time to the earliest stages of galaxy formation.
In other words, we are led to think of the origin and
evolution of the universe, the subject of the next chapter.
Outline
I. Active Galaxies
A. Seyfert Galaxies
B. Double-Lobed Radio Sources
C. Testing The Black Hole Hypothesis
D. The Search for a Unified Model
E. Black Holes and Galaxy Formation
II. Quasars
A. The Discovery of Quasars
B. Quasar Distances
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.
• Variability: ~ 50 % in
a few months
NGC 1566
Most likely power
source:
Circinus Galaxy
Accretion onto a
supermassive black hole
(~107 – 108 Msun)
Interacting Galaxies
Seyfert galaxy NGC 7674
Seyfert galaxy NGC 4151
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
Centaurus A (“Cen A” = NGC 5128): the closest AGN to us.
Jet visible in
radio and Xrays; show
bright spots
in similar
locations.
Infrared image
Radio image superposed
on optical image
reveals warm
gas near the
nucleus.
Radio Galaxies (2)
Radio image
3C129: Evidence
for the galaxy
moving through
intergalactic
material
Radio image
of 3C 75
3C 75: Evidence for two nuclei
 recent galaxy merger
Jet Deflection
(SLIDESHOW MODE ONLY)
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.
M31 at Many Wavelengths
(SLIDESHOW MODE ONLY)
Evidence for Black Holes in AGNs
Elliptical galaxy M 84:
Spectral line shift indicates high-velocity rotation of
gas near the center.
Visual image
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
Seyfert II:
Accretion disk
Dense dust torus
Supermassive
black hole
Weaker, narrow
emission lines from
more slowly moving
gas clouds far from
the BH
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”).
The Dust Torus in NGC 4261
Dust Torus is directly visible with Hubble Space
Telescope
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
Black Holes and Galaxy Formation
Interactions of galaxies not
only produce tidal tails etc.,
but also drive matter
towards the center
 triggering AGN activity.
Such interactions may
also play a role in the
formation of spiral
structures.
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
Quasars have been
detected at the highest
red shifts, up to
z~6
z = Dl/l0
Our old formula
Dl/l0 = vr/c
z = 0.389
is only valid in the
limit of low speed,
vr << c
Quasar Red Shifts (2)
The full, relativistic expression always gives
speeds less than c, but extremely large distance:
Several Gpc.
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
Galaxies Associated with Quasars
Two images of the same
quasar, 1059+730
New source probably a
supernova in the host
galaxy of the quasar
Host Galaxies of Quasars
Host galaxies
of most
quasars can
not be seen
directly
because they
are outshined
by the bright
emission from
the AGN.
Blocking out the light from the center of the quasar 3C
273, HST can detect the star light from its host galaxy.
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
New Terms
radio galaxy
active galaxy
active galactic nucleus
(AGN)
Seyfert galaxy
double-lobed radio
source
double-exhaust model
hot spot
unified model
BL Lac object
blazar
quasar
relativistic Doppler
formula
gravitational lens
superluminal expansion
relativistic jet model
Discussion Questions
1. Do you think that our galaxy has ever been an active
galaxy? Could it have hosted a quasar when it was
young?
2. If a quasar is triggered in a galaxy’s core, what would
it look like to people living in the outer disk of the
galaxy? Could life continue in that galaxy? (Begin by
deciding how bright a quasar would look seen from the
outer disk, considering both distance and dust.)
Quiz Questions
1. Which characterizes the visible part of the spectrum for most
galaxies?
a. They have absorption lines of singly ionized calcium (Ca II).
b. They have absorption lines of neutral atomic hydrogen (H I).
c. They have emission lines of carbon monoxide (CO)
molecules.
d. Both a and b above.
e. All of the above.
Quiz Questions
2. How are the spectra of Seyfert galaxies different from most
galaxies?
a. They have broad absorption lines of highly ionized elements.
b. They have broad emission lines of highly ionized elements.
c. They have narrow absorption lines of highly ionized
elements.
d. They have narrow emission lines of highly ionized elements.
e. They have a continuous spectrum.
Quiz Questions
3. What conditions can create broad emission lines of highly
ionized elements?
a. High-temperature gas must be present.
b. Low-density gas must be present.
c. The gas must be rotating at high speeds.
d. Both a and b above.
e. All of the above
Quiz Questions
4. Seyfert galaxies have a spectrum with broad emission lines
of ionized elements. What other unusual features do Seyfert
galaxies have?
a. They have small, highly luminous nuclei that fluctuate
rapidly.
b. They have small, dark nuclei that fluctuate rapidly.
c. They have large, highly luminous nuclei that fluctuate rapidly.
d. They have large, dark nuclei that fluctuate rapidly.
e. They all have highly red shifted spectral lines.
Quiz Questions
5. What is the difference between type 1 and type 2 Seyfert
galaxies?
a. Type 1 Seyferts are very luminous at ultraviolet wavelengths.
b. Type 1 Seyferts are very luminous at X-ray wavelengths.
c. Type 2 Seyferts have broader emission lines.
d. Both a and b above.
e. All of the above.
Quiz Questions
6. Galaxies in close pairs are three times more likely to be
Seyfert galaxies than are isolated galaxies. What general
conclusion can be drawn from this statistical fact?
a. Most Seyfert galaxies found in galaxy pairs are type 2.
b. Seyfert galaxies in pairs are smaller than isolated Seyfert
galaxies.
c. The majority of Seyfert galaxies in pairs are spiral galaxies.
d. Isolated Seyfert galaxies are most likely to be type 1.
e. Seyfert galaxies are very likely the result of galaxy
interactions.
Quiz Questions
7. What is at the center of Seyfert galaxies?
a. Globular star clusters.
b. Clusters of about one million neutron stars.
c. Supermassive black holes.
d. Dwarf elliptical galaxies.
e. None of the above.
Quiz Questions
8. In the double-exhaust model, how does a double-lobed radio
source form?
a. The high-energy source at the center of the central galaxy is
transforming energy into matter and antimatter that flow out in
opposite directions to form the lobes.
b. The magnetic field of the central galaxy pulls the hot ionized
intragalactic matter toward the two galactic poles and forms
two feeding lobes.
c. The central galaxy experiences gravitational harassment due
to a near collision with another galaxy.
d. Tidal interaction with a nearby galaxy drags matter out into
the two opposing radio lobes.
e. The lobes are inflated by bipolar jets of excited gas emerging
from the central galaxy.
Quiz Questions
9. What observational evidence leads us to believe that AGNs
contain supermassive black holes?
a. Broad emission lines of ionized gases indicate that gas near
the center of an AGN is orbiting at high speeds.
b. Short-duration fluctuations in brightness limit the size of the
object at the center of an AGN.
c. High-resolution imaging reveals dark regions the size of an
event horizon at the center of some AGNs.
d. Both a and b above.
e. All of the above.
Quiz Questions
10. How do blazars (BL Lac objects) differ from type 1 Seyfert
galaxies?
a. Blazars are much more luminous than type 1 Seyfert
galaxies.
b. The luminosity of blazars fluctuates more rapidly than type 1
Seyfert galaxies.
c. The luminosity of blazars fluctuates more slowly than type 1
Seyfert galaxies.
d. Both a and b above.
e. Both a and c above.
Quiz Questions
11. Blazars are more luminous and fluctuate much more rapidly
than both type 1 and type 2 Seyfert galaxies. How does the
unified model of an AGN’s supermassive black holes and
accretion disk explain these differences?
a. Blazars and Seyfert galaxies are different views of the
accretion disk of an AGN.
b. Blazars are the face-on view of the accretion disks of AGNs.
c. Seyfert type 1 galaxies are AGNs with the accretion disk
tipped slightly from face-on.
d. Seyfert type 2 galaxies are AGN accretion disks with an
edge-on view.
e. All of the above.
Quiz Questions
12. Why are galaxies with active nuclei more often found in
close galaxy pairs and in rich clusters of galaxies?
a. Galactic harassment is more likely under these
circumstances.
b. Galactic merger is more likely under these circumstances.
c. Galactic cannibalism is more likely under these
circumstances.
d. Galactic interactions transfer material onto the central
supermassive black holes.
e. All of the above.
Quiz Questions
13. Which of the following are characteristics of quasars?
a. Quasars have bright emission line spectra.
b. The spectra of quasars have large blue shifts.
c. Quasars are larger than the largest spiral galaxies.
d. Quasars are more abundant now than at anytime in the
history of the universe.
e. All of the above.
Quiz Questions
14. What evidence do we have that quasars are small?
a. Some quasars emit strongly at radio wavelengths.
b. They have rapid fluctuations in brightness.
c. They have large red shifts in their spectra.
d. They are very distant.
e. All of the above.
Quiz Questions
15. What evidence do we have that quasars are very far away?
a. Their spectral lines have large red shifts.
b. Some are gravitationally lensed by distant galaxies.
c. Some light from quasars contains less red shifted absorption
lines of distant galaxies.
d. Some quasars have nearby galaxies with similar red shifts in
their spectra.
e. All of the above.
Quiz Questions
16. How do quasars resemble the AGN in Seyfert galaxies?
a. They have jets and pairs of opposing radio lobes.
b. They are small and very luminous.
c. They have new chemical elements never found on Earth.
d. Both a and b above.
e. All of the above.
Quiz Questions
17. How can a quasar jet eject material at apparent
superluminal speed?
a. The accretion disk surrounding the supermassive black hole
at its center can add more than one solar mass per year.
b. The intense magnetic field of a quasar can accelerate ions to
a speed greater than the speed of light.
c. The jet ejects material at nearly the speed of light almost
directly toward Earth.
d. Both a and b above.
e. All of the above.
Quiz Questions
18. What does it mean that quasars are most common at a red
shift of about 2?
a. The average quasar has a recessional speed that is twice
the speed of light.
b. Most quasars cannot be imaged at visible wavelengths.
c. Quasars were more plentiful in the past.
d. Both a and b above.
e. All of the above.
Quiz Questions
19. Where are all the dead quasars?
a. They have dissipated into the ether.
b. They lurk quietly at the hearts of galaxies.
c. They have decayed to become gas and dust.
d. Fortunately, they are many 1000s of Mpc from Earth.
e. The residents of cosmic wormholes have consumed them.
Quiz Questions
20. Why are most quasars so far away?
a. There is a greater volume of the universe that is far away
than is nearby.
b. Quasars were more abundant in the past, when galaxies
were close together.
c. The jets have moved most of them to great distances from
us.
d. Both a and b above.
e. All of the above
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
d
b
e
a
d
e
c
e
d
d
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
e
e
a
b
e
d
c
c
b
d