The Largest Black Holes in the Universe


Uploaded by SpaceRip on 26.09.2012

Transcript:
We’ve never seen them directly, yet we know they are there, lurking within dense star
clusters or wandering the dust lanes of the galaxy, where they prey on stars or swallow
planets whole.
Our Milky Way may harbor millions of black holes, the ultra dense remnants of dead stars.
But now, in the universe far beyond our galaxy, there's evidence of something far more ominous,
a breed of black holes that has reached incomprehensible size and destructive power.
Just how large, and violent, and strange can they get?
A new era in astronomy has revealed a universe long hidden to us.
High-tech instruments sent into space have been tuned to sense high-energy forms of light
– x-rays and gamma rays – that are invisible to our eyes and do not penetrate our atmosphere.
On the ground, precision telescopes are equipped with technologies that allow them to cancel
out the blurring effects of the atmosphere.
They are peering into the far reaches of the universe, and into distant caldrons of light
and energy.
In some distant galaxies, astronomers are now finding evidence that space and time are
being shattered by eruptions so vast they boggle the mind.
We are just beginning to understand the impact these outbursts have had on the universe:
on the shapes of galaxies, the spread of elements that make up stars and planets, and ultimately
the very existence of Earth.
The discovery of what causes these eruptions has led to a new understanding of cosmic history.
Back in 1995, the Hubble space telescope was enlisted to begin filling in the details of
that history.
Astronomers selected tiny regions in the sky, between the stars. For days at a time, they
focused Hubble’s gaze on remote regions of the universe.
These Hubble Deep Field images offered incredibly clear views of the cosmos in its infancy.
What drew astronomers’ attention were the tiniest galaxies, covering only a few pixels
on Hubble’s detector.
Most of them do not have the grand spiral or elliptical shapes of large galaxies we
see close to us today. Instead, they are irregular, scrappy collections of stars.
The Hubble Deep Field confirmed a long-standing idea that the universe must have evolved in
a series of building blocks, with small galaxies gradually merging and assembling into larger
ones.
You can see evidence of this pattern by looking out into the universe. Many galaxies are gyrating
around one another. Some are crashing together, others ripping each other apart.
Gravity calls the tune as these galaxies draw together, exchanging stars and gas, and, over
time, merging to form larger composite galaxies.
This came to be known as the hierarchical picture of cosmic history, in which the universe
evolved from the ground up, with its structures growing larger and larger over time.
A team operating at the Subaru Observatory atop Hawaii’s Mauna Kea volcano examined
one of the deepest galaxies known, whose light has taken nearly 13 billion years to reach
us.
It was a messenger from a time not long after the universe was born.
This object is known as a quasar, short for “quasi-stellar radio source.” It offered
a stunning surprise. A small region in its center is so bright that astronomers believe
its light is coming not from a collection of stars, but from a single object of at least
a billion times the mass of our sun.
This beacon is generated by gas falling onto the object and heating up to extreme temperatures.
The only thing known to generate this much power is a swirling caldron, where space suddenly
turns dark as it merges into a giant black hole.
For astronomers, the question was: how did this black hole get so big so early in the
history of the universe?
It likely got its start in an early generation of stars, often known as population 3 stars.
Made up of hydrogen, they are thought to have been hundreds of times the mass of the sun.
These giant stars burned hot and fast, and died young.
A star is like a cosmic pressure-cooker. In its core, the crush of gravity produces such
intense heat that atoms are stripped and rearranged. Lighter elements like hydrogen and helium
fuse together to form heavier ones like calcium, oxygen, silicon, and finally iron.
When enough iron accumulates in the core of the star, it begins to collapse of its own
weight.
That can send a shock wave racing outward that literally blows the star apart in a supernova.
At the moment the star dies, if enough matter falls into its core, it can collapse to a
point, forming a black hole.
The first generations of stars and black holes burst onto the cosmic scene in a time of incredible
turbulence.
Within primordial gas clouds, stars were being born in dense knots. They gave rise to black
holes that began to swallow more and more matter.
A computer simulation of the early universe shows just how quickly these voracious monsters
were able to grow.
The project, by scientists at Carnegie Mellon University, was designed to recreate a region
in the early universe that measured over a hundred million light years on a side.
It shows what took place in the first one billion years of cosmic history.
This virtual universe is set in motion by equations describing the properties of gas,
the energy released in star birth and the outward motion of time and space.
The result: an intricate cosmic web, with gravity drawing matter into filaments and
knots like a vast tangle of interconnected spiders’ webs.
Inside the densest regions is where the largest galaxies, and black holes, grew. Here, circles
indicate the appearance of black holes deep in the data.
As they gain weight, by eating up their surroundings, the circles grow larger. A few, in the largest
galaxies, reach ultra massive proportions, billions of times the mass of the sun.
These black holes were not just swallowing gas.
The orbiting Chandra X-Ray Observatory was dispatched to look into distant galaxies for
black holes on growth spurts.
Scientists looked for pockets of gas and stars glowing hotly in X-ray light.
What Chandra found was that the core of some distant galaxies countained hot pairs, twin
supermassive black holes drawn together by gravity.
Black holes by nature resist this dark marriage. As the two approach each other, they go into
an orbit that could last virtually forever.
To learn what allows them to merge, we go back to the ideas developed by Albert Einstein.
He said that when massive bodies accelerate or whip around each other, they literally
disturb the fabric of space, causing it to ripple like a disturbance on a pond.
When these ripples move outward, they carry with them the energy of the pair’s orbit,
causing them to spiral closer.
When this dance of death comes to an end, that’s when the pair joins together to form
a larger black hole.
That moment may be approaching for a quasar called OJ-287, at 3.5 billion light years
away.
Flareups in the surrounding region have led astrophysicists to conclude that another black
hole is flying around it.
By measuring the giant's gravitational hold on its companion, astronomers estimate its
mass at 18 billion solar masses.
For a time, OJ-287 was the largest black hole ever detected. It no longer is.
Deep in the heart of the Coma galaxy cluster, a mere 321 million light years away, lies
a giant eliptical galaxy known as NGC 4889.
Astronomers used several large telescopes to measure the speed at which stars are orbiting
around the center. They used that data to calculate the mass of the central object,
a whopping 21 billion solar masses, give or take a few billion.
Theoretically there are no limits to how much weight a black hole can gain.
And yet even the largest black holes, and their host galaxies, seem to obey limits.
What holds them back has to do with the way clusters of galaxies evolve, a pattern long
noted by scientists.
This computer simulation shows the evolution of a galaxy cluster in the early universe.
The gravity of the entire region draws small galaxies by the thousands, along with great
streams of gas, into the center.
So why doesn’t the central galaxy, and the black hole that resides within it, capture
all this matter? Why don’t they swallow the entire cluster?
You can see the answer in a region called MS0735. At two and a half billion light years
away, it appears in visible light to be a typical galaxy cluster.
In X-ray light, you can see that it’s enveloped in a cloud of hot gas, measured at nearly
50 million degrees.
Hollowed out of this cloud are two immense cavities up to 600,000 light years across.
That’s enough room in each to stuff 600 galaxies the size of our Milky Way.
Now add in a radio image of the cluster. You can see two vast streams of matter pushing
out from the center.
That’s a give-away that the cavities were formed by an eruption in the core of the giant
central galaxy. Two jets, shooting out of a central black hole, have launched blast
waves that plowed through the gas that makes up the inter-galactic medium.
The energy it took to carve out these Xray cavities is remarkable, the equivalent of several billion
supernovae, according to one calculation.
In fact, this has been referred to as the largest single eruption recorded since the
big bang.
It was generated by a black hole that weighs in at around 10 billion solar masses.
Black hole jets like this have been seen all around the universe, including in our own
cosmic neighborhood.
This is the famous M87 galaxy, at the center of the Virgo galaxy cluster, around 50 million
light years away
Astronomers have been intensively studying the black hole that lurks in its heart, and
recently estimated its mass at 6.6 billion solar masses.
It powers a pair of high-powered jets that are plowing through the galaxy.
But how does a black hole, a creature famous for hiding in the dark, emit this much energy?
Think of a black hole as the eye of a cosmic hurricane, kept rotating by all the stars,
gas, and other black holes that fall into it.
As this matter flows in, it forms a spinning donut-like feature called an accretion disk,
which works like a dynamo.
The spinning motion of the disk generates magnetic fields that twist around and channel
some of the inflowing matter out into a pair of high-energy beams, or jets.
How much energy depends on the black hole’s gravity, and how much matter has already crashed
through the event horizon.
Is this just another frightening spectacle of Nature? Or is it part of a more profound
process at work? It shows that a monster black hole will not be forcefed.
The largest black holes in the universe probably rose between 10 and 12 billion years ago,
the age of the quasars. By releasing energy in the form of jets, they heated up their
surrounding regions. This prevented gas from collapsing into the central galaxy, and allowed
smaller galaxies on the periphery to form and grow.
But the impact the black holes did not stop there. This Chandra image of the Hydra A galaxy
cluster shows the same immense hot cavities, glowing in X-ray light, as well as a jet blasting
out of its central galaxy.
Gas along the edge of the jet was found to contain high levels of iron and other metals
probably generated by supernova explosions in the center.
By pushing these metals into regions beyond, a black hole seeded more distant galaxies
with the elements needed to form stars and solar systems like ours.
The black holes in these more remote galaxies then seeded their own environments. This is
what might be happening in Centaurus A, also known as the “hamburger galaxy.”
Peering through the dense dust lanes that dominate our line of sight, astronomers have
come to believe that it’s actually two galaxies in the act of colliding.
In X-rays, you can see a jet erupting from the center.
This computer simulation shows the effect of such a merger on black holes. As the two
galaxies pass by each other, the pull of gravity disrupts their spiral shapes, forcing huge
volumes of gas into their cores.
As the black holes begin to feed, they emit blast waves that push much of the loose gas
out beyond the boundaries of the new galaxy.
In the final steps of this cosmic dance, the two black holes merge, and emit one final
blast.
To think that our Earth, our solar system, ourselves are the beneficiaries of these far-away
monsters. The largest black holes have played dual roles in a great cosmic struggle. They
are the product of gravity’s relentless inward pull, the force that has drawn matter
into galaxies, clusters, and the structures they form.
But with their incredible power, they emit energy that pushes back on gravity. In so
doing, these strange and powerful objects have become the master architects of space
and time.
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