Transcript Active Galactic Nuclei

Quasars and Active Galactic Nuclei
• Quasi-stellar objects
– Redshifts
– Variability
– Supermassive black holes
• Active Galaxies
Quasars
• Early radio telescopes found radio emission
from stars, nebulae, and some galaxies.
• There were also point-like, or star-like, radio
sources which varied rapidly these are the
`quasi-stellar’ radio sources or quasars.
• In visible light quasars appear as points, like
stars.
Optical
picture of a
quasar
Quasar optical spectrum
Hα unshifted
Redshift
shows this
quasar,
3C273, is
moving
away from
us at 16%
of the speed
of light
3C273
The quasar 3C273 is
2.6 billion light years
away.
It looks dim, but must
be extremely
luminous to be visible
as such distance.
The luminosity of
3C273 is more than
one trillion times the
entire energy output of
our Sun, or 100 times
the luminosity of our
entire galaxy.
The distance to
PKS 2000-030 is
more than 11
billion light years
Quasars vary
Quasar size
Size places a limit on how fast
an object can change brightness.
Conversely, rapid variations
place a limit on the size of the
emitting object.
Quasars vary
The size of this quasar must be less than about one
light year.
Quasar engine
• Quasars can have up to several 1000 times
the luminosity of our Galaxy
• The engine powering quasars is only a few
light years across or smaller
• The only known engine which is powerful
enough and compact enough is a black hole
• Quasars contain supermassive black holes
A quasar varies in brightness by a factor of 2 in
10 days. What does this tell us about the quasar?
1.
2.
3.
4.
It has a large magnetic field.
It is quite small.
It must be highly luminous.
It cannot emit radio waves.
Quasar jets
Optical core 
Radio jet 
Quasars
•
•
•
•
•
•
•
•
object with a spectrum much like a dim star
high red shift
enormous recessional velocity
huge distance (from Hubble’s Law)
enormously luminous
compact physical size
powered by supermassive black hole
often produce huge jets
Quasars are the ultraluminous centers of
distant galaxies.
Quasars are often observed to be at the center of distant galaxies.
The wispy material is likely gas that has been pulled out of the hot
galaxy by gravitational interactions with nearby galaxies.
Quasars are the most extreme of a class
of galaxies known as active galaxies
M87 appears as an elliptical galaxy in visible light, but like a
dim quasar in radio. The nucleus of the galaxy contains a
weak quasar. This means that the galaxy harbors a
supermassive black hole.
Centaurus A
Optical
Radio
NGC 1566
Spiral
galaxies
also
sometimes
contain
active
cores.
Active galaxies lie at the center of
double radio sources.
Active galaxies lie at the center of double radio sources.
Active Galaxies come in several varieties
• Quasars
• Seyfert galaxies
– luminous, star-like nuclei with strong emission lines.
• BL Lacertae objects (BL Lacs)
– featureless spectrum with a brightness that can vary
by a factor of 15 times in a few months.
– Most commonly known as a Blazar.
• All contain supermassive black holes
In Active Galaxies the disk is surrounded
by a dusty torus
Accretion disks
The light from a quasar is emitted from a
region 10 light-days across. What does this
tell us about the quasar?
1. The brightness will change every day.
2. The brightness will change every 20 days.
3. The brightness will not change in less than 10
days.
4. The brightness will not change in less than 100
days.
Varieties of Active Galaxies
• Radio Galaxies – big jets, no obvious BH
• Quasars – jets and BH
• Blazars – mainly see the jet
Orientation
Black holes in normal galaxies
Rotation curves of stars
near the centers of most
galaxies show the
presence of
supermassive black
holes with mass ranging
from 106 to 109 solar
masses.
Essentially all galaxies
contain supermassive
black holes and were
likely active galaxies at
some point in their lives.
Review Questions
• Why is it thought that quasars are at very large
distances?
• How can we place an upper bound on the size of
a quasar even if we can’t resolve it with a
telescope?
• Where are quasars found?
• Do most galaxies have supermassive black
holes?