Transcript Active Galactic Nuclei - Pennsylvania State University

The Accelerating Universe
The Hubble Law
According to the Hubble Law, the space between the galaxies is
constantly increasing, with Velocity = H0 D istance
By observing how the expansion rate has changed over time, we
can measure how much effect gravity has had on the universe,
i.e., its deceleration. When we do this with supernovae, we find
The Accelerating Universe!!!
The universe is not slowing
down at all. In fact, it’s
speeding up!!! We live in
an accelerating universe!
It’s as if there’s another
force pushing the universe
apart – a Cosmological
Constant!!!
The Accelerating Universe!!!
Whatever this force is, we think that it is growing stronger as the
universe evolves. The more empty space in the universe, the
greater the acceleration – as if the vacuum of space has pressure!
The Accelerating Universe!!!
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Whatever this force is, we think that it is growing stronger as the
universe evolves. The more empty space in the universe, the
greater the acceleration – as if the vacuum of space has pressure!
The Accelerating Universe!!!
We appear to live in a universe with a flat shape, but which
will go on accelerating forever. The universe is 13.7 billion
years old, and is now dominated by Dark Energy. And it will
only get worse – the more empty space, the more Dark Energy.
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This explains the age mismatch between globular clusters
and the universe. The universal expansion is getting faster!
The Accelerating Universe!!!
We appear to live in a universe with a flat shape, but which
will go on accelerating forever. The universe is 13.7 billion
years old, and is now dominated by Dark Energy. And it will
only get worse – the more empty space, the more Dark Energy.
The Dark Energy even
dwarfs dark matter!
Regular matter is really
insignificant. We really
don’t know anything
about what’s going on!!
What is the Dark Energy?
We’re clueless. There is one “traditional” theory– that particles
and anti-particles are constantly being created and annihilated in
the empty space (due to the uncertainty principle). For the instant
these particles exist, they would act as a repulsive force. But our
estimate of this force is off by a factor of 10122.
History of the Universe
The Big Bang occurred 13.7 billion years ago. Since then
 +9,000,000,000 years: Birth of the Sun
 +2,000,000,000 years: era of galaxy formation/interaction
 +400,000,000 years: Milky Way begins to form
 +100,000 years: release of the microwave background
Helium in the Universe
If the universe began as a high density soup of protons and
neutrons, some of those particles must have undergone fusion.
In the Big Bang,
about 1 of every
10 hydrogen
atoms should have
been changed to
helium. That’s
almost exactly the
helium abundance
we observe for the
universe!
History of the Universe
The Big Bang occurred 13.7 billion years ago. Since then
 +9,000,000,000 years: Birth of the Sun
 +2,000,000,000 years: era of galaxy formation/interaction
 +400,000,000 years: Milky Way begins to form
 +100,000 years: release of the microwave background
 +3 minutes: fusion of hydrogen to helium ends
 +0.00001 seconds: protons, neutrons form
 +10-12 seconds: particles form and annihilate
 +10-35 seconds: quarks form; gravity begins to exist
 +10-43 seconds: ???? Grand unification
In the First 10-35 Seconds
• Why is the universe flat?
• Why does one side of the sky
look like the other side of the
sky? (They were never in
contact with each other.)
• Why are there no monopoles?
(Magnets always have a north
pole and a south pole.)
Inflation
A theory which explains these
puzzles (and others) is that,
very early on (10-35 sec after the
beginning), the universe
expanded much faster than now
(1030 instead of 64). This is
called inflation. The universe
we see now is just a small
region of a “bubble”. It
therefore just looks flat.
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(the observable universe is in red)
Multiverses
Inflation allows that our bubble may not be the only bubble.
Bubbles may be forming all the time in a multi-universe.
(But these other universes can never be observed.)
Multiverses
Inflation allows that our bubble may not be the only bubble.
Bubbles may be forming all the time in a multi-universe.
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(But these other universes can never be observed.)
Active Galactic Nuclei
or
The Monster Within
The Discovery
In 1962, Cambridge University just completed a radio survey of
the sky. Maarten Schmidt took their radio positions and looked
for optical counterparts. He found a few peculiar “radio” stars.
3C 273 looked like an
ordinary, fairly-bright
star (with possibly a
little fuzz). But
ordinary stars do not
emit much in the radio
part of the spectrum.
The Spectrum
The spectrum of the star was odd. It had
 Emission lines instead of absorption lines
 Broad (~10,000 km/s) emission lines, instead of narrow lines
 Emission lines at “strange” wavelengths
The solution: the
emission lines were
those of hydrogen,
but at enormous
redshift. The
object was moving
away at 10% of the
speed of light!
Quasars
Properties of quasi-stellar radio sources (quasars, or QSOs):
 Star-like appearance (with possibly some “jets”)
 Emission-line spectra with internal motions of ~10,000 km/s
 Does not emit as a blackbody (at least, not at a single
temperature). The objects emit light in x-rays, ultraviolet,
optical, infrared, and sometimes microwave and radio
 Irregularly variable on timescales of days/months
 Enormous redshifts (can be more than 90% of the speed of
light)
Stars in the Milky Way cannot move that fast. The only way to
achieve such a redshift is through the Hubble Law. So, through
v = H D, the objects must be incredibly far away. They are
therefore incredibly bright – as bright as 1000 supernovae.
Size and Variability
Since many quasars vary in
brightness we have a crude
way to estimate their size.
 Imagine that there is some
mechanism near the center of
the QSO that controls the
object’s brightness. It says
“get bright”.
 That command goes forth no
faster than the speed of light.
 Within a few months, the
object gets bright.
 Since no signal can go faster
than the speed of light, the
object must be no bigger than
a few light-months across!
The Energy Source
What can outshine ~1000 supernovae for millions of years, and
be just slightly larger than our Solar System? Theoretically, not
much – only a very, very big black hole.
• Start with a 10,000,000,000 M black hole
• Have a star come close enough to be tidally disrupted
• Have the material form into an accretion disk. Energy is
released via the friction in the disk. If you accrete ~ 1 M
per year, the friction you get will produce the luminosity
of a quasar.
Feeding the Monster
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If a star comes too close, the enormous gravity of the black
hole will cause tides on the star and rip it apart. Some of that
material will be trapped in orbit about the hole.
Feeding the Monster
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If a star comes too close, the enormous gravity of the black
hole will cause tides on the star and rip it apart. Some of that
material will be trapped in orbit about the hole.
Explaining a Quasar’s Properties
• Near the event horizon, the
gas is moving close to the
speed of light. Any
emission lines which are
produced will be broad.
• Because of the high speed
of the gas, there is a lot of
friction in the disk. A lot of
light is produced.
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• The temperature of the disk depends on the speed of the gas.
Near the event horizon, the friction produces x-rays. At larger
radii, where the gas revolves more slowly, optical and infrared
light is made.
Black Holes and Jets
As matter accretes onto the black hole, particles can get ejected out
the poles of the system at 99.999% of the speed of light. How this
occurs is almost a complete mystery. But it’s often observed.
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Where are the Quasars Today?
The nearest quasar is 25% of the way across the universe; most
belong to an era when the universe was only 15% of its present
age. If supermassive black holes existed then, where they now?
In the
centers of
galaxies!
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The Quasar-Galaxy Connection
When a supermassive black hole is accreting, it can be thousands
of times brighter than its surrounding galaxy. On the other hand,
if the black hole is not accreting, it will be invisible.
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Active Galactic Nuclei
Many nearby galaxies have some activity in their nucleus:
they may have an extremely bright nucleus, or show a jet
of emission, or have broad emission lines, or emit at radio
wavelengths. These objects (which are probably just
accreting a little mass) are said to have an Active Galactic
Nucleus.
The energy produced by an AGN is still often many times
that of the stars.
Galaxies with Active Galactic Nuclei
Sleeping Monsters
When a black hole is not accreting matter, then it’s invisible.
But its gravitational influence on its surroundings can still be
detected – the stars surrounding the hole must move fast (due
to Kepler’s and Newton’s laws).
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Sleeping Monsters
There’s even a 2,000,000 M black hole at the center of the
Milky Way. We can measure its mass by the motions of stars
which pass close to it.
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AGN and Starbursts
In the present day universe, AGN are rare. However, they are
more common in interacting galaxies. This suggests that the
orbits of some stars have been perturbed enough to pass close
to the black hole. It also suggests that all galaxies possess
supermassive black holes.
AGN and the Universe
Since quasars can be seen 90% of the way across the universe,
they allow us to detect gas throughout the universe. We can
therefore examine galaxies (and proto-galaxies) that we can’t
even see!
Any time the light from a
quasar goes through a galaxy
that has hydrogen gas, there
will be absorption at the
wavelength appropriate to
hydrogen. But remember –
this hydrogen is moving, due
to the Hubble Law. So …
AGN and the Universe
Each absorption is due to hydrogen gas at a different redshift
(i.e., distance). Quasars allow us to probe structure throughout
the universe!
Next time -- REVIEW