Download Goal: To understand the expansion of our universe.

Goal: To understand the most
energetic stars in the universe,
quasars
Objectives:
1) To understand Quasars
Quasars
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Quasars are QUASi-stellAR radio sources
They were first observed in the 1950s.
They looked like stars on an image.
You can tell stars from galaxies because
stars appear as a point (and have
diffraction patterns if bright) but galaxies
are more spread out.
• However, when they looked at the
spectrum, they found something very
strange!
These are Quasars (HST)
No known elements!
• The emission lines matched no known element!
• Why would this be?
• A) Quasars are made out of materials unknown
in the 1950s.
• B) They made a mistake with the observations.
• C) The lines were Doppler shifted by a factor of
a few.
• D) They forgot to take relativity into account.
They are far away!
• Once it was realized what the emission line
difference was they quickly realize that Quasars
are very far away!
• In fact some are on the other side of the
observable universe!
• A red flag should go up now – if they are that far
away and we can see them – how bright are
they?
• You have a star which is brighter than any
galaxy – how is that possible?
Variable
• Furthermore Quasars change in brightness!
• They can do so very quickly (in hours).
• The shortest a time span an object can fluctuate
its brightness is related to its size.
• The time is the light speed time across the
object.
• So, these are extremely bright objects located in
a small area (size of a solar system).
• How is this possible?
Debate
• In the 1970s there was a lot of debate over this
puzzling matter.
• Some suggested antimatter (the only way they
could think of to convert that much energy).
• Some suggested that the redshift wasn’t due to
distance but due to light coming out of a huge
gravitational well.
• However the gas emitting the light was hot and
diffuse (known by the emission lines) – so could
not be.
The solution
• The light doesn’t even come from the “star”.
• The light is coming from an accretion disk.
• As gas and dust orbits friction causes the gas
and dust to heat up, emit light, and fall slowly
down towards the object in the center.
• If the object in the center is massive you can
liberate 10% of the mass energy of the dust and
gas as energy (as opposed to 0.7% from fusion).
Not the full story
• Still, how do we get a region of space the
size of a solar system to radiate more light
than a galaxy?
• Also, this light is radiated in all type of light
from gamma rays to radio!
• How is this possible?
Black Holes!
• Quasars are super massive black holes!
• Quasars are billions of times the mass of
our sun.
• Oddly enough their density is about the
same as our sun.
• The amount of light they emit is dependant
on the rate they accrete matter.
More bizarre news
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Most quasars are > 5 billion light years away.
Very few are < 3 billion.
Why do they seem to shut off?
Some may have to do with them no longer
having gas to eat.
• Also, as a black hole gets bigger, its density
decreases.
• If the density gets too low it no longer plays with
its food, it eats it whole (although black holes are
NEVER vacuum cleaners).
Active galaxies
• The closest to quasars we get in the
present is active galaxies.
• These are galaxies that for some reason
have materials falling into the center so
that the center is bright.
• Often times this is a result of a galactic
merger.
• 1% of galaxies are like this.
4.6
billion
light
years
away
• The light still
travels out from
the universe
even after the
quasar shuts off
Jets
What else we can learn from
Quasars
• Quasars are light bright search lights shining
though our universe.
• They shine not just through space but through
time.
• Since the universe expands, the light from the
searchlight expands with time too.
• This creates what is known as a Lyman-alpha
forest
• (called this because the Hydrogen emission it
follows is called the Lyman-alpha line)
How Lyman-alpha forest works
• Each time the light from the Quasar passes a
cloud or galaxy the light from the Quasar is
shifted to a different wavelength.
• The gases in the cloud will emit and absorbed
(based on the properties of the cloud or galaxy)
at a specific wavelength that is not shifted.
• So, each cloud adds its fingerprint or signature
to the spectrum.
• This allows us to know what everything is like
along the line of sight (and in some way how the
gas in the universe has evolved with time).
spectrum
w/o normalization
Conclusion
• Some of the most distant
observable objects are Quasars
which are super massive and
super bright black holes that are
eating large amounts of matter.