The Black Hole That Kills Galaxies
The universe looks like a vast empty ocean sprinkled with the rare islands of galaxies..
But this is an illusion. Just a small fraction of all atoms are found in galaxies,
while the rest is thought to be drifting in between, in the intergalactic medium.
Like the roots of some massive tree, gas spreads out from each galaxy,
gravity funneling fresh mass into this dense, cosmic forest.
Here in the intergalactic medium, are the raw materials of creation: hydrogen and helium,
woven into sheets and filaments that flow into galaxies where they eventually create stars.
But if we look closely, we see who is actually in charge: Quasars, the single most powerful objects
in existence. As small as a grain of sand compared to the amazon river, they reside in the centers
of some galaxies, shining with the power of a trillion stars, blasting out huge jets of matter,
completely reshaping the cosmos around them. They are so powerful that they can kill a galaxy.
What are they, and how do they mold the structure of the universe at their whim?
Everywhere You Look, Weird Things in the Sky
In the 1950s astronomers noticed mysterious loud radio-waves coming from spots all over
the sky. They were named “quasi-stellar radio sources”, or “quasars” because they
were dots like stars, but were seen in radio waves rather than visible light.
Everything about them was strange. Some flickered, others emitted high energy X-rays in addition to
radio waves, but all seemed to be tiny. They also moved extremely fast, as much as over 30%
the speed of light. The only explanation was that they must have been so distant that their
apparent speed was actually the expansion of the universe moving them away from us.
But these enormous distances meant that quasars couldn't just be stars, but the active cores of
galaxies billions of lightyears away! And it gets crazier. To appear so bright and loud,
given these vast distances, they are thousands of times brighter than the entire Milky Way.
Monsters, exploding and screaming into the void with a violence not thought possible before.
As we mapped the sky, we discovered over a million quasars. And they all seemed to be very far away.
Looking into space, far away means very long ago, because their light takes so
long to reach us. Quasars were common in the early universe, having peaked
in number 10 billion years ago when galaxies, and the universe itself was still very young.
Let's go back in time, just 3 billion years after the big bang and see what was going on back then.
The Incredible Power of Quasars
How could an early baby galaxy be so incredibly bright and violent?
All that light and radiation couldn't be stars, as there weren't nearly enough of
them. And since galaxies tend to grow with time by merging,
the starlight from small galaxies shouldn't be far brighter than any galaxy today.
There's only one way to generate the vast amounts of energy a quasar shines with:
feeding supermassive black holes. We still don't know how exactly they formed,
but it seems that every galaxy has one in their center.
But how can the brightest things in the universe be black holes,
which trap anything and everything that crosses their event horizon?
Well the light of a quasar is not coming from inside these black holes. Rather,
it comes from the space around them, a massive orbiting disk of gas called an ‘accretion disk.'
Quasars use the same fuel as stars to shine: Matter. It is just that black holes are the
most efficient engines for converting matter into energy in the universe.
The energy released by matter falling into a black hole can be 60 times
greater than that released by nuclear fusion in the core of a star. Because
the energy released by a black hole comes from gravity, not from nuclear reactions.
Matter falling into a black hole speeds up to almost the speed of light before it crosses
the event horizon, buzzing with an incredible amount of kinetic energy. Of course, once inside
the black hole, it takes that energy with it. You only get to witness this energy if you drop your
matter in the right way. Fall straight down and the outside universe gets nothing. But when you
have a lot of matter, it spirals in incredibly fast towards the event horizon forming a disk.
Collisions between particles and friction heat it up to hundreds of thousands of degrees.
In a space not much bigger than our solar system,
the core of a galaxy can release many times more energy than all its stars combined.
This is what a quasar is, a super massive black hole having a feast.
And these black holes eat a lot. Typical quasars consume one to a hundred Earth masses
of gas per minute! Ten billion years ago, the universe was about a third of its current size,
so the intergalactic medium was much less spread out, meaning the filaments of gas
around quasars could feed them a banquet, making them vomit insane amounts of light and radiation.
The brightest quasars power jets, tangling the magnetic field of the matter around them into
a narrow cone. Like a particle accelerator they launch enormous beams of matter out,
plowing through the circumgalactic medium, forming plumes of matter that grow to hundreds
of thousands of lightyears in size. It's almost unfathomable in scale. A tiny spot
in a galaxy carving out patches of the universe 100,000s of light years long.
But quasars can't eat for long, maybe a few million years,
because their feast ultimately kills their galaxy.
How Quasars Kill Galaxies
Okay, maybe “killing” is a bit of an exaggeration. A galaxy is still there after its quasar turns
off. But it will never be the same again. Quasars, being among the hottest and brightest things in
the universe, break their galaxies by heating them up too much and stopping star formation.
Hot gas cannot form stars. This sounds odd, because Stars are gas that collapsed in
on itself and then got really hot. But in a cloud of gas that is already hot,
atoms are moving quickly. When they collide, they hit hard, exerting pressure that resists
gravity's squeeze – so hot gas can't form stars. Instead, the best gas for
forming stars is already cold, and won't put up a fight when it's time to collapse into a star.
On top of that, quasars push gas out of their galaxies. Not only does this starve
the quasar, but its galaxy loses the raw materials for new stars.
As sad as this sounds, it might be a good thing for life. The alternative
can be far more dangerous: too many stars. New stars forming is usually followed by
massive stars exploding in supernovae, so planets would be burned sterile.
But of course it's more complicated. Like the intricacies of our own planet's biosphere,
every piece of the galaxy is dependent on and influencing every other part of the galactic
environment. While hot things, like quasars and supernovae, tend to push gas out of the galaxy,
shockwaves and quasar jets can also compress gas, making new stars at least for a short
time. And gas that leaves will mix with gas coming back in and recycle it back into the galaxy. But
in general we can say that without things becoming a bit more chill,
we would not exist today. Which brings us to our final question:
Did the Milky Way Have a Quasar in the Past?
It's unclear if every galaxy went through a quasar phase, but understanding distant quasars
may provide clues to the history of the Milky Way. Galaxies don't do a good job of preserving
their history. Like sand on a beach the endless churning mixes away the clues to their past.
It's possible the Milky Way was once a quasar,
which may have allowed our supermassive black hole Sagittarius A star to have
grown to 4 million times the mass of the sun. But sadly we don't know its ancient history.
And as dormant as it is now, Sagittarius A star could turn into a quasar in the future. In a few
billion years the Milky Way will merge with Andromeda. We've seen over a hundred ‘double
quasars' in galaxies smashing together, where fresh gas is provided for the central black holes.
But it won't last for long. When galaxies merge, so do their super massive black holes,
sinking into the center of their new galaxy, kicking up dust and stars in every direction.
We don't know whether this will happen, but it would truly be an incredible sight. Maybe
some beings in the far future are going to witness it and be in awe of what they see.
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