Transcript Hubble Science Briefing: The Real World: Black Hole Edition

Hubble Science Briefing:
The Real World:
Black Hole Edition
Chandra/CXC
Dr. Eileen Meyer
Space Telescope Science Institute
4 September 2014
Outline
Part I: What is a Black Hole, Really?
Part II: Black Holes in the Universe
Part III: Not-so-Black Holes (Outbursts, Outflows,
and Jets from super-massive Black Holes)
Part IV: The Big Picture: Why do we care about
Black Holes?
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Part I: What is a Black Hole, really?
There are many black
holes in this picture, even
if they don’t appear like
the artist’s conception
above. The ones we’re
most sure about lurk in
the centers of massive
galaxies. (Image: Hubble
Deep Field, NASA/ESA)
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A black hole is a region of spacetime which is so dense
that nothing, not even light, can escape (once inside
the point of no return, called the event horizon).
Many people associate black holes with Einstein and the Theory of General
Relativity (“GR”, published in 1915).
But, the idea actually existed as far back as the 18th century, using only a
Newtonian understanding of Gravity:
For a large enough mass (M) and small
enough radius (R), you can get an
escape velocity greater than the speed
of light, the universal speed limit. In
other words, a very compact object
could be impossible to escape.
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It is not an exaggeration to say that GR changed our
entire view of the Universe.
Under GR, space and time form a 4-dimensional
construct called spacetime, which is curved in the
presence of mass.
We usually depict it as two-dimensional because it is easier to visualize.
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GR is a theory that accurately predicts the behavior of the
large-scale Universe.
However, it says nothing at all about electromagnetism, or
atoms, or anything on the very small scale (the scales on
which we use quantum field theory (QFT) to understand
what is going on).
To put it most simply: we have a theory for large, heavy
things (where QFT is not important). We have another
theory for small, light things (where gravity is not
important). We don’t have a theory for small, heavy
things.
The major problem in physics is to unify GR and QFT into a
“grand unified theory” (GUT).
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GR is incomplete precisely because it predicts black holes.
Or more accurately, because it predicts singularities.
Under GR, it is possible for a massive star to run out of the
fuel needed to balance out gravity, and collapse. In fact, GR
predicts that the star collapses to an infinitely small point
with infinite density.
Many people think this is true in
real life, but it is not. The truth is,
singularities (infinite physical
quantities) tend to show up in
physical regimes where the theory
doesn’t apply anymore.
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All physical theories have limits.
Example: An opera singer can sing at a
frequency that causes a glass to
vibrate. The simplest theory, which
describes this very well up to a point,
says that these oscillations of the glass
will grow to an infinite size, as the
acoustic waves reinforce them. Of
course, in real life, the glass will break.
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Do black holes really exist? Yes. (see Part II)
We expect black holes to exist, though we are in
ignorance of precisely the description of matter
inside them.
Image credit: Ute Kraus
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How to produce a black hole
Normal stars exist in a state of equilibrium.
Every point inside a star is
balanced perfectly so
that the gas and radiation
pressure balance the selfgravity of the star. Any
imbalances will cause the
star to re-adjust (either
shrink or puff up) until
equilibrium is reached.
Stars live for millions to billions of years (depending on their mass) in this way,
slowly using up their Hydrogen and Helium through Nuclear Fusion.
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How to produce a black hole
But, eventually, all stars run out of “fuel”.
When that happens, suddenly the inward force of gravity is
unopposed, which causes the star to shrink in size and become
denser and denser (“stellar collapse”).
“Stellar Mass” Black Holes are those which are formed from stars a little
more massive than our sun (say 5 to 15 times the mass of our sun).
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A Common Misconception
Black holes do not “suck”. Their gravitational force on
any object is simply proportional to their mass, like
every other object in the Universe.
Thought experiment: if you replaced the sun with a black hole of
the same mass, we would not be sucked in! Our orbit would not
change at all. (Though we would freeze to death.)
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(End of Part I)
Next…
Part II: Black Holes in the Universe
Part III: Not-so-Black Holes (Outbursts, Outflows,
and Jets from super-massive Black Holes)
Part IV: The Big Picture: Why do we care about
Black Holes?
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Part II: Black Holes in the Universe
Massive stars become black holes at the end of
their lives (usually in quite spectacular fashion:
gamma-ray bursts and supernovae). These are
about 5 – 15 times the mass of our sun.
Supernova N49 in the Large Magellanic Cloud
Supernova N63A
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Evidence for Stellar-Mass Black Holes
Our theories predict black
holes will form at the end of
the life of a massive star, and
we have observed the
explosions that we believe
accompany them.
Supernova 1987A
But, direct observation is difficult (after all, light cannot escape a
black hole)!
However, we can observe indirect signatures that point to a
black hole. One case of this are systems called X-ray binaries.
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X-ray Binaries
Stellar-mass
black hole
X-ray binaries are extremely bright: just
one can outshine all the other stars in a
galaxy in X-rays. Where does all that
energy come from?
Normal
companion
star in close
binary
Powered by Accretion: The infalling
matter releases gravitational
potential energy, which ultimately
heats up the accreting material to
extremely high temperatures, so
hot that they produce huge
amounts of X-rays.
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On a different scale entirely:
Super-Massive Black Holes
Most astronomers had no reason to suspect black holes more
massive than several times our sun existed.
Until we started noticing something odd about certain galaxies.
At left, the optical spectrum
for a normal galaxy. It is
fairly smooth with no big
spikes and only a few dips.
Amount of
Light
(Brightness)
These spikes and dips tell
us about the chemical
compositions of the stars in
the galaxy, among other
things.
Increasing wavelength
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The “strange” galaxies, first observed in the early 1900s, had very
“spikey” spectra! We call these spikes “emission lines” – they
indicate the presence of very hot gas in large quantities.
Amount of
Light
(Brightness)
Increasing wavelength
These strange cases showed
emission lines like you see in
nearby Nebulae  became
known as “Active Galaxies” or
AGN
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Active Galaxies (AGN)
AGN also have extremely luminous ‘Nuclei’.
In some cases, the nuclei
are so bright, we can’t even
see the rest of the galaxy!
Even more strange: they can “flicker”, changing
brightness by a factor of 2 – 10 in less than a year.
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Super-Massive Black Holes (SMBH)
AGN must be powered by something extremely compact.
After a lot of effort, we now know that AGN can only be powered
by a super-massive black hole, with a mass of 1 million to
10 billion times the mass of the sun.
The power source for
AGN is the same as in
X-ray binaries: accretion
onto a black hole.
But because the black
hole is a million to a
billion times bigger, the
brightness is much more
extreme.
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How to detect a super-massive Black Hole
How do you go
about “weighing a
black hole”?
The most direct
evidence is
actually from the
center of our own
galaxy.
Our SMBH is
“only” about 4
million times the
mass of the sun.
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How to detect a super-massive Black Hole
We also use Gas
Dynamics to look for
the signature of
extremely fast-moving gas
near the black hole. The range of
speeds observed (dispersion) is
directly proportional to the central
mass. If you can show that the mass
must be contained in a small enough
region, then it must be a supermassive black hole.
Image credit: NASA/ESA
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(End of Part II)
Next…
Part III: Not-so-Black Holes (Outbursts, Outflows,
and Jets from super-massive Black Holes)
Part IV: The Big Picture: Why do we care about
Black Holes?
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Part III: Not-so Black Holes
As should be clear by now, the main way we
detect black holes is from the radiation coming
from matter very close to them.
We rely on the fact that the “extreme
environment” near black holes are unique
enough that we can thus identify them.
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Accretion in AGN
The main reason AGN are so bright is because of their
accretion disks.
A common misconception is that these black holes are
swallowing enormous amounts of gas all the time. In fact
it’s probably only a solar mass per year, or even a lot less.
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AGN are still observed from far away
Here is the typical
artist’s picture of
an AGN
However, MOST of the components here cannot be
directly imaged. They are inferred based on looking
at different types of radiation.
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Jets from AGN
However, in about 10% of all AGN, there are some
very, very big associated structures that we can see
quite clearly.
It should also be mentioned that the artist’s
conception is quite out of scale!
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In fact, we have even made movies of
them.
This is a Hubble image of the 1700-parsec long jet
from an AGN known as “M87”. It is one of the
nearest jets to Earth.
Using over 400 images taken of this jet over nearly 13
years, we were able to actually measure the motions
of the plasma as it extends out into space.
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In fact, we have even made movies of
them.
(please see accompanying video file)
Or visit http://hubblesite.org/newscenter/archive/releases/exotic/2013/32/
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The most extreme recyclers of energy
in the Universe?
In addition to the jets, it is now understood that many (probably all) AGN
produce outflows of gas from the central part of the galaxy.
These can be huge (3000 solar masses per year, which can clear a galaxy of the
cold gas needed to support forming new stars) and also very, very fast (e.g., 3030
40% of the speed of light).
A common misconception (partly due to scientific
shorthand):
AGN jets and winds are often described as coming
“from the black hole”.
This is technically incorrect! It is still true that
nothing escapes a black hole once it has fallen in.
The extreme gravity of the black hole is essential to
the whole system, but the outflows are not coming
out of the black hole event horizon. It is more
accurate to say these jets and winds come from the
accretion disk around the black hole.
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(End of Part III)
Next…
Part IV: The Big Picture: Why do we care about
Black Holes?
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Part IV: The Bigger Picture (and some Questions)
Currently, Astronomers are trying to put together the big picture:
how the Universe evolved from the time of the Big Bang to the
galaxies and clusters of galaxies we see today.
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(Credit: NASA/WMAP
Science Team)
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Did AGN reionize the Universe?
The redshift of an AGN tells us how far
away it is. In Astronomy, light from
distant objects was released earlier and
earlier in the Universe.
At a redshift of 10-20, the Universe was reionized by something very energetic,
that existed everywhere.
One possibility is AGN, another is the very first massive stars.
We have only observed AGN (aka quasars) as far back as redshift 7,
so we have to extrapolate to the epoch of reionization.
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ESO/M. Kornmesser
How did the SMBH form?
We have evidence of extremely super-massive black holes (10 billion
solar masses) at very high redshifts – when the Universe was only a few
hundred million years old!
You can’t grow a 10 billion solar mass BH in that time using accretion.
1.
2.
Collapse of super-massive early stars (100-1000 times mass of the
sun), followed by accretion?
Direct Collapse of gas in the very early Universe (200-500 Million
years old) into a “quasistar”
This is an area of active research!
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Do all galaxies host an AGN?
Image Credit: Galaxy Zoo
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How do jets impact their
environment?
NASA/ESA/STScI
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Summary
Black holes exist! They can be stellar-mass, up to billions
of times the mass of the sun (and we’re looking for
intermediate ones now).
Black holes that we know about (except in our own
galaxy) are usually accreting matter which radiates very
brightly.
Black holes are associated with some of the most
energetic phenomena around, producing huge
luminosities, particularly in the centers of galaxies.
The SMBH at the centers of galaxies probably played a
key role in how the Universe evolved into what we see
today.
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