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Chapter 17
Quasars and Active Galaxies and Other
Ultahigh Energy Sources
What are quasars?
Radio Astronomy began in 1936 when amateur
astronomer Grote Reber built a crude radio
telescope in his back yard.
By 1944 Reber had detected 3 very strong radio
sources
Sagittarius A (Sgr A)
Cassiopeia A (Cas A)
Cygnus A (Cyg A)
Both Sgr A and Cas A are in Milky Way.
Sgr A is the nucleus of our galaxy,
Cas A was a supernova remnant
Cyg A was another situation - no
easy identification
Galaxy Cygnus
A
This galaxy has a red shift corresponding to 6% the speed of
light or 600 million light-years away
3C 405 refers to the object number
405 in the Third Cambridge
Catalogue of radio sources
When its overall radio intensity was
determined, it was found to be more
intense (about 1011 times) than an
entire galaxy - such as M31
(Andromeda Galaxy).
21 cm radio image of Cygnus A source taken by VLA in 1994
Image of radio lobes spans about 500,000 light years
Using Palomar 200” telescope a visible
spectrum of Cyg A was taken. The
observed redshifts gave a speed of 17,000
km/sec which implied (using Hubble’s
Law) that the source was 750 Mly away.
Cygnus A (3C 405) in a visible image
It was astonishing that a source stronger
than an entire galaxy such as M31 could be
so far away....… This implied that Cyg A was
an extraordinary object.
Astronomers began to examine other 3C
objects
1960 Alan Sandage at Palomar discovered
a “star” at the location of source 3C 48.
This “star” was very unusual in the fact
that it had emission lines that could not be
identified.
It was also a strong radio emitter while
normal stars are not strong radio sources.
3C 48 visible image : Believed by astronomers to be simply a
very unusual star.
1962
another
3C273
“star” was
The luminous
discovered
at
jet 273
can be
3C
seen and an
This
“star” also
enlarged
had
image as well
unidentifiable
emission lines
and also had a
“jet” of bright
gas streaming
from one side
of the “star”
Then in 1963 Maarten Schmidt at CalTech identified
the strange emission lines of 3C 273 as significantly
red shifted lines of ordinary Hydrogen.
3C 273’s spectral lines are
greatly redshifted
This change implies
a distance of 2
billion light years
3→2 4→2 5→2 6→2 7→2 8→2 9→2 transition Name
Hα
Hβ
Hγ
Hδ
Hε
Hζ
Hη
Wavelength (nm)
656.3 486.1 434.1 410.2 397.0 388.9 383.5
Hβ (lab) = 486.1 nm
Hβ (meas) = 565 nm
Δλ = 78.9 nm
v = [Δλ/λ]c = 4.87 x 107 m/sec
D = v/Ho = 4.87 x 104 km/sec ÷ 25 km/sec/Mly
= 1,948 million light years ~ 2 billion lyrs
The observed red shift for 3C 273
corresponded to a distance of 2 Billion light
years.
A similar analysis was done for object 3C 48
and the results indicated a distance of 4
billion light years!
Star-like Object 3C 48
This object that
looks like a star
must be
enormously
luminous - its
redshift
indicates it is 4
billion light
years away!!
Since it would be absolutely impossible to
see even the brightest possible star at a
distance this large, these objects could not
be “stars” and were dubbed QUASARS
(quasi-stellar-radio-objects).
In fact, most Quasars are NOT strong radio sources! But
the name has stuck!
 = 66 nm for the 410 nm line!
If  = 66 nm, then by Doppler Shift Eq.

v
c
 or written as v =

c

(3x108 m / sec)(66 x109 )
v
9
410 x10
Then v = 4.8 x 107 m/sec or 48,000 km/sec
By Hubble’s Law
v = HoD where Ho = 75 km/sec/Mpc
This gives a distance of
D = v/Ho
D = (48,000 km/sec) / 75 km/sec/Mpc
D = 640 Mpc or 2090 Mly or 2.1 Billion Light Yrs
Quasars look like stars but have
huge redshifts
•
•
•
•
•
object with a spectrum much like a dim star
highly red shifted
enormous recessional velocity
huge distance (ala Hubble’s Law)
must be enormously bright to be visible at
such a great distance
• Quasi-stellar object - QSO or Quasar
Galaxies are bright and very big....
The Milky Ways shines with the
light of about 10 Billion suns.
The largest elliptical galaxies have
brightnesses about 10 or 100X
this brightness.
Quasars have brightnesses this large
and larger!
Light Variations
Quasars have been observed to fluctuate in brightness
with periods ranging from a few years to a few hours.
Recall that this light variation places an upper limit on
the size of the quasar’s energy source.
If they are as distant as Hubble’s Law indicates then
some mechanism must be producing energies greater
than 100s of galaxies in a region about the size of our
solar system.
A quasar emits a huge amount of
energy from a small volume
Quasar 3C 279
Such rapid changes in brightness can only
result from small objects (~ 5 light years)
Quasar Brightness
•Most galaxies cannot be seen beyond
4 Billion light years
•With a few of the very brightest
barely visible at 8 Billion light yr.
•Since more distant quasars are clearly
visible
•They must be much brighter than
even the brightest galaxies.
When imaged by telescopes, Quasars appear
very unimpressive...just points of light, as
can be seen on the next image
This “dot of light” has a luminosity of 1000 Milky Way
galaxies.
It looks unimpressive because it is located at about 2 Billion
light years distance
The Quasar 3C48 we
saw earlier is located
at 4.2 Billion light years
Today thousands of quasars have been observed
with redshifts corresponding to speeds as high as
92% of the speed of light, giving distances of 10 13 Billion light years (ALL quasars lie at least 800
Million light years from the Milky Way).
Since looking OUT in distance = looking BACK in
time.....
When we look at quasars we are seeing objects as
they existed when the universe was very young.
This Quasar is the most distant object ever imaged. It is
about 11 – 13 Billion light years distant.
If the center of a
galaxy is
unusually bright
we call it an
active galactic
nucleus or
(AGN)
Quasars are the
most luminous
AGN Examples
Active Nucleus in M87
The highly redshifted spectra of quasars indicate large distances
From brightness and distance we find that luminosities of some
quasars are >1012 LSun
Variability shows that all this energy comes from region smaller
than solar system
Thought Question
What can you conclude from the fact that quasars usually
have very large redshifts?
A. They are generally very distant
B. They were more common early in time
C. Nearby galaxies might hold dead quasars
D. Galaxy collisions might turn them on
Galaxies
around
quasars
sometimes
appear
disturbed by
collisions
Radio galaxies contain active nuclei shooting out vast jets of
plasma that emits radio waves
Centaurus A - radio image superimposed on a visible
image...note no light from radio lobes
visible image
Radio lobes
An active galactic
nucleus can shoot
out blobs of plasma
moving at nearly
the speed of light
Speed of ejection
suggests that a
black hole is
present
Radio
galaxies don’t
appear as
quasars
because
dusty gas
clouds block
our view of
accretion disk
What is the power source for quasars and
other active galactic nuclei?
Accretion of gas onto a supermassive black hole appears to be
the only way to explain all the properties of quasars
This Quasar is the one of the most distant objects ever
imaged. It is about 11 – 13 Billion light years distant.
The current record for the most distant object imaged is
the gravitationally lensed galaxy in this image ~ 13Bly
Active Galaxies bridge the energy
gap between ordinary galaxies and
quasars
• Many different types of so-called “active” or
“peculiar” galaxies have been discovered in the
last 30 years
• Besides Quasars, 3 interesting types are:
• Radio Galaxies
• Seyfert Galaxies
• Peculiar galaxies (pec)
– appear to be blowing themselves apart
Radio Galaxies
•Very bright in radio emissions (usually ellipticals)
• Emissions come from core and gas “lobes”
• Radio spectrum is “synchotron radiation”
•
high speed electrons moving in
magnetic fields
• Electron emission often in “jets”
21 cm radio image of Cygnus A source taken by VLA in 1994
Image of radio lobes spans about 500,000 light years
Centaurus A - radio image superimposed on a
visible image...note no light from radio lobes
visible image
Radio lobes
Seyfert Galaxies
• Spiral galaxies (mostly) with abnormally
“bright” nucleus
• Highly energetic core that is very small and
emits more energy than entire milky way. With
emissions in radio, optical, infrared, uv and Xray. Some also emits “jets” of fast moving gas.
• Core radiations often fluctuate rapidly ~
minutes.
• Gas clouds moving very fast about core (104
km/sec.
Energy Output Variation of Seyfert 3C 84
(Note the variations over about 1 year intervals)
Seyfert NGC 1566 (50 Mly) This galaxy’s luminosity varies
over a range of 700 million L0 in a few weeks, and has a
strong source of radiation showing emission lines of highly
ionized atoms.
Seyfert Galaxy NGC 5728
This galaxy has extraordinary infrared
luminosity
Seyfert galaxy NGC 1275 is actually two galaxies in collision. The HST image
(right) shows about 50 small, bright, young globular clusters formed as a result
of the collision. NGC 1275 is also a strong source of X rays and radio waves
(bottom).
Active Galaxy NGC 1275 (3C 84)
HST
Ground based image
Rosat X-Ray Image
Other Active Galaxies
• BL Lacertae objects (BL Lacs)
– featureless spectrum with a brightness that
can vary by a factor of 15 times in a few
months.
BL Lacertae (900 Mly),
brightness varies 15X in
few months
BL Lac objects
appear to be
giant elliptical
galaxies with
bright quasarlike
nuclei. BL Lac
objects contain
much less gas
and dust than
Seyfert galaxies.
Active galaxies lie at the center of most
double radio sources
NGC 1265
NGC 1265 is an active elliptical galaxy moving at a
high speed through the intergalactic medium.
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a
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Some forms of “Peculiar” galaxies are also
“Starburst” galaxies
Many of these galaxies have large regions where intense
star birth seems to be occurring.
Examination of these (and other) types of galaxies can
demonstrate the power of multi-spectral imaging for
analyzing the properties of astronomical objects.
As an example, we will look at M82, a galaxy in the
constellation Ursa Major located 12 Million light years
from us. It is a satellite galaxy of the much larger M81.
Radio Image of M81 and its satellite M82. The image
reveals the lanes of Hydrogen gas between these galaxies
Visible
radio
A visible, long-exposure (~40 min.)
from the Digital Sky Survey. The
center is saturated to bring out the
wispy halo around the galaxy.
A true color image, short
exposure <10 min. The diff.
in color of the center and
outer parts can be seen as
well as dust/gas in galaxy
A very short exposure (< 1 min.) clearly shows
the bands of dust/gas throughout the galaxy.
The red color of the center is due to scattering
away the blue light by large dust/gas clouds.
This is a composite image (4 different wavelengths
combined) showing in much greater detail the dust/gas
lanes throughout the galaxy.
An interferometric 21 cm radio
image ( the VLA in Az. and a UK
radio telescope). The image
shows the chaotic pattern of
radiation emitted by the galaxy.
This far infrared image shows
the intense IR radiation from
the immense dust/gas clouds
in this galaxy.
This galaxy is one of the
brightest IR sources in the
Universe.
An ultraviolet image. UV is given
off by large bright stars (must be
young stars) indicating that this
part of the galaxy is a region of
intense starbirth activity.
This x-ray image (ROSAT) shows
that these emissions are localized
to the center of this galaxy which
is a very strong x-ray source
Is there a single mechanism that can explain
the intense energy emissions of Quasars,
Radio Galaxies, BL Lacs, and Seyfert
Galaxies?
• small energy sources
• ejecting jets of gas at tremendous
speeds
• radiates strongly at many wavelengths
Giant Gas
Clouds
(surrounding the
galaxy)
Intergalactic
gas jet
Galaxy-M87
(which is actually
quite large)
Black Holes
• A common choice is to assume the energy is
emitted by the rapidly spinning accretion disk
around a massive black hole.
• Black Hole about size of earth’s orbit
containing 100 Million or more solar masses
• As long as it was “fed” more matter, radiation
would continue
Supermassive black holes lurk at the
centers of some galaxies
• High resolution spectroscopy allows
astronomers to peak at the motion of gas
near centers of galaxies
• Some galaxies exhibit high-velocity jets of
material leaving the center
• Observations suggest that the centers of
some galaxies are incredibly massive
• All of this suggests the existence of
supermassive black holes
Image of center of NGC 4261 - disk is about 320 ly across
radio
visible
Jets of matter ejected from around a black hole may
explain quasars and active galaxies
Jets of matter ejected from around a black hole may
explain quasars and active galaxies
From where you observe it might make all the
difference ...
Small black hole 4 - 6 MO perhaps created in supernova
near center of the galaxy
•Hole grows as dust/gas within galaxy falls into it
•If large enough, the Black Hole could swallow entire stars
and grow very massive, maybe millions of MO
•If galaxy massive enough, or through encounters with
other galaxies, could grow even more massive
•As galaxy ages, available mass drops and activity
diminishes
Expect more such energetic cases in younger
galaxies
•Quasars are very distant, therefore we see them
when very young
•Black hole model is only consistent general
explanation for most active galaxies
•Consistency does NOT guarantee its correct!
Model of the center of an Active Galaxy
Gravitational lensing
• Einstein’s General Relativity predicted that light
rays can be bent by intense gravitational field.
• Examination of distant quasars provided the
first direct experimental evidence that such could
occur in space.
location of one
image
quasar
location of
second image
Earth
intervening
galaxy
First Observed Gravitational Lens : English Radio Telescope
from 1972. Visual confirmation in 1979
HST Infrared Image of
Einstein Ring
Equivalent Radio
Image showing
multiple images of
distant quasar
Einstein Cross - gravitational lensing produces 4 images of a
quasar 8 billion lys distance, the imaging galaxy is 400 million
lys away
Next slide shows 10 different gravitational lenses
imaged by Hubble Space Telescope
A gravitational lens distorts our view of things behind it
A gravitational lens distorts our view of things behind it
All methods of measuring cluster mass indicate similar
amounts of dark matter
Einstein rings - light from a distant quasar is bent
into rings around the intervening galaxy
A Gravitational Lensed Cloverleaf
Measuring the timing variations for
changes in the different images in a
gravitationally lensed image has
provided a powerful experimental
confirmation of the validity of
Einstein’s General Theory of Relativity.
Dark matter map.
Optical view of a small galaxy
cluster (center of image).
Analysis of the distortions this
galaxy produces on images of
more distant galaxies allows
an estimate of the presence of
dark matter around the galaxy.
Lensing simulation
Does dark matter really exist?
Knowing the orbital speed, one can calculate the force of
gravity necessary...which in turns tells how much mass is
necessary to keep the sun in orbit
This mass is about 1011 M0
inside the solar system’s orbit
force of gravity
solar system
Consider a galaxy like our own.......
Assuming most stars are
smaller than the sun,
gives about 400 billion
stars inside the orbit of
the sun.
Measuring the mass of a galaxy is done as was
discussed earlier, using the sun’s motion in the
Milky Way.
If we can measure the velocities of stars or gas
within a galaxy (optical or radio wavelength
Doppler shifts), then the mass can be estimated
using Kepler’s Laws and the Law of Gravity.
When this is done, the calculated mass ALWAYS
exceeds the observable mass…..DARK MATTER!
The existence of Dark matter was predicted in the 1930s by
astronomer Fred Zwicky (who also predicted the existence of
neutron stars)
Zwicky had an eccentric
personality and his ideas
were not accepted by the
astronomy community
despite the careful detail
with which the work was
done. No one else was
willing to work on the
idea to independently
confim/deny his results.
The existence of Dark Matter is further
suggested by looking at Galaxy Rotation
Curves for all spiral galaxies, as we shall see
next.
In many galaxies the calculated mass exceeds
the visible mass by a factor of 10. What can
be seen constitutes only ~ 10% of the actual
mass found in the galaxy.
Using the measured doppler shifts, one can
determine the rotation velocities of different
parts of a galaxy forming a:
galaxy rotation curve
Using this data and a modified Kepler’s Third
Law, one can estimate the mass of a galaxy
If all the mass were uniformly
distributed in the disk
Rotation_merrygoround.htm
If all the mass were concentrated in the
center of the disk
Actual curve for
the Milky Way
Galaxy
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
Dark Matter associated with a Spiral galaxy like ours
Elliptical galaxies pose a problem for such rotation
curve measurements since they have little hydrogen
gas and do not produce detectable 21 cm radiation.
The velocities of individual stars are disorganized and
randomly oriented (unlike stars in spiral arms). The
Doppler shifts measured are collective averages of many
stars.
The widths of the Doppler shifted peaks is determined
by the speeds of the individual stars. The faster the
stars, the broader the peaks
These galaxies also
have dark matter
Doppler broadening peaks for Elliptical galaxies
Our Options
1. Dark matter really exists, and we are
observing the effects of its gravitational
attraction
2. Something is wrong with our understanding
of gravity, causing us to mistakenly infer the
existence of dark matter
Because gravity is so well tested, most
astronomers prefer option #1
What might dark matter be made of?
How dark is it?
… not as bright as a star.
Two Basic Options
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
Two Basic Options
• Ordinary Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
The
Best
Bet
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
… but not
enough
lensing
events to
explain dark
matter
Why Believe in WIMPs?
• There’s not enough ordinary matter. WMAP
results puts ordinary matter at 4% of universe
• WIMPs could be left over from Big Bang
• Models involving WIMPs may explain how
galaxy formation works
Thought Question
What would you conclude about a galaxy whose
rotational velocity rises steadily with distance
beyond the visible part of its disk?
A.
B.
C.
D.
Its mass is concentrated at the center
It rotates like the solar system
It’s especially rich in dark matter
It’s just like the Milky Way
What is the evidence for dark
matter in clusters of galaxies?
We can
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
The mass
we find
from galaxy
motions in a
cluster is
about
50 times
larger than
the mass in
stars!
Clusters contain
large amounts of
X-ray emitting hot
gas
Temperature of
hot gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing, the bending of light rays by
gravity, can also tell us a cluster’s mass
Cluster
CL0025+1654
4.5 Bly
Dark matter is
Blue
Isolated neutron star -1st detected by its x-ray emissions has been imaged by the Hubble telescope, ~ 18 lys and
traveling at about 100 km/sec.
The End!