“Do I have your attention…?”
Galaxies with Active Nuclei
Active Galaxies
Galaxies with extremely violent energy
released 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!
Line 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:
Accretion onto a supermassive black hole
(~107 – 108 Msun)
Circinus Galaxy
NGC 7742
Interacting Galaxies
Seyfert galaxy NGC 7674
Seyfert galaxy 3C219
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
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
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.
Radio image superposed
on optical image
Infrared image
reveals warm gas
near the nucleus.
Radio Galaxies (II)
Visual + radio
image of 3C31
Radio image
of 3C75
Radio image
of NGC 1265
Evidence for the
galaxy moving
through
intergalactic
material
3C75: Evidence for two
nuclei  recent galaxy
merger
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 M87
M87 = Central, giant elliptical galaxy in
the Virgo cluster of galaxies
Optical and radio observations detect
a jet with velocities up to ~ 1/2 c.
The Dust Torus in NGC4261
Dust torus is directly visible with Hubble Space Telescope
Model for Seyfert Galaxies
Seyfert I:
Gas clouds
Strong, broad emission
lines from rapidly moving
gas clouds near the black
hole
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 black hole
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
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 Origin of Supermassive
Black Holes
Most galaxies seem to harbor
supermassive black holes in their
centers.
Fed and fueled by stars and gas
from the near-central
environment
Galaxy interactions may
enhance the flow of matter onto
central black holes
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
The Quasar 3C273
Spectral lines show
a large redshift of
z = Dl / l0 = 0.158
Quasar Red Shifts
z=0
z = 0.178
z = 0.240
z = 0.302
Quasars have been
detected at the highest
redshifts, 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
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
 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.
Gravitational Lensing of Quasars
Gallery of Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies
1. What evidence suggests that the energy source in a double-lobed radio
galaxy lies at the center of the galaxy?
Firstly, the geometry suggests that the lobes are inflated by gas jets
emerging from the central galaxy.
This is supported by the presence of synchrotron radiation which suggests
magnetic fields that confine the emitted jets to narrow beams, and hot
spots which suggest gas is being pushed into the surrounding gas
causing the heating. Also, we know that matter falling onto a massive
compact object (e.g., a black hole) can cause these jets.
2. How does the peculiar rotation of NGC5128 help explain the origin of this
active galaxy?
There is a spherical cloud of stars orbiting about an axis which is
perpendicular to the axis of rotation of the disk.  This strongly hints
that this is the result of a merger.
3. What statistical evidence suggests that Seyfert galaxies have suffered
recent interactions with other galaxies?
They are three times more common in interacting pairs of galaxies than in
isolated galaxies.
25% have shapes that suggest tidal interactions with other galaxies.
4. How does the unified model explain the two kinds of Seyfert galaxies?
It all depends on how the accretion disk is tipped WRT your line of sight.
Tipped slightly  you are able to observe the hot, fast moving gas in the
central galaxy, thus the x-rays and higher Doppler shifts produce the
smeared, broad spectral lines. Not tipped at all  the disk blocks the
radiation from the central galaxy. Plus, this gas is moving slower and
explains the narrow spectral lines.
5. What observations are necessary to identify the presence of a
supermassive black hole at the center of a galaxy?
Observations of size and motion… the short time period it takes to fluctuate
in brightness  small
Motion of stars near the center allow for use of Kepler #3 and hence the
mass. Thus Doppler shifts combined with other observations that allow
for the distance to be calculated (e.g., Cepheids). Basically everything to
allow for us to use Kepler #3.
6. How does the unified model implicate collisions and mergers in triggering
active galaxies?
Tidal interactions with other galaxies not only can rip matter from a galaxy,
but also can throw matter inward, toward the center of the galaxy. In this
case, you would have a flood of matter falling into the black hole
increasing the intensity of the bipolar flow.
7. Why were quasars first noticed as being peculiar?
How could quasars be so luminous that they emit 10 to 1000 times the
energy of a galaxy, yet reside in a region only the size of our solar
system?
8. How do the large redshifts of quasars lead astronomers to conclude they
must be very distant?

c

Dl
l
zd 
cz
H0
, if z is large, then d must also be large.
9. What evidence suggests that quasars are ultraluminous but must be very
small?
They are very distant, yet easily photographed  very luminous. The small
time to fluctuate in brightness (a few days)  they must be smaller than
a few light days in diameter.
10. How do gravitational lenses provide evidence that quasars are distant?
The spectra of quasars are similar, yet each are as unique as fingerprints.
In 1979, the object 0957+561 was observed. It consists of two quasars
separated by 6” of arc. These two objects share the exact same spectra,
which implies that they are the same object! Yet the closer lensing
galaxy is so far away that it is difficult to detect.
11. What evidence is there that quasars occur in distant galaxies?
Astronomers recorded the spectra of objects near quasars. Those objects
share the same spectra of normal galaxies, and they have the same
redshift as the quasar.
12. How can our model quasar explain the different radiation received from
quasars?
The two kinds of radiation are a continuous spectrum and some emission
lines. The continuous spectra fluctuates rapidly, which suggests that the
object emitting it is small – probably a central black hole with an
accretion disk. The emission lines don’t fluctuate rapidly, suggesting that
they emanate form a larger region many light-years in diameter –
probably clouds of gas excited from the synchrotron radiation from the
central black hole. Also, as with AGNs, how the disk is tipped will affect
what kind of quasar you’ll see.
13. What evidence is there that quasars must be triggered by collisions and
mergers?
Galaxies were closer when the universe was young. They would have
collided more often, and we know that interactions can cause matter to
flow inward. Also, quasars are often located in distorted galaxies which
suggests they interacted with other galaxies.
14. Why are there few quasars at low redshifts and at high redshifts but
many at redshifts of about 2?
A redshift of 2 corresponds to a time in the universe when galaxies were
most actively forming, colliding, and merging.