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One of the most enigmatic of all astrophysical phenomena
is the mighty quasar.
They're also a subject of my own research,
and so are close to my heart.
Let's talk about what happens when the largest black holes
in the universe start to feed.
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Space stuff is awesome.
Take stars-- 100 billion megaton per second thermonuclear
explosions that just don't stop exploding.
Pulsars-- city-size atoms that beam
deathrays through the galaxy.
Giant molecular clouds-- beautiful and tranquil,
but also screaming vortices spitting stars into the cosmos.
Of course, everyone knows that quasars are the most awesome
of all.
They have everything.
They're like the fire-breathing bat-winged vampire rainbow
zebra unicorns of astrophysical phenomena.
They don't just have a black hole.
They have a supermassive black hole,
millions to billions of times the mass of the sun.
That's surrounded by a solar system-sized whirlpool
of superheated plasma that shines
brighter than an entire galaxy.
Sometimes they even have jets of near light speed
particles filling the surrounding universe
with giant radio plumes.
Yep, quasars are clearly the most metal
of all the space things.
This is one reason why I study them myself.
But it's not just that they're cool.
Quasars helped shape our universe.
In fact, without these most violent
of all astrophysical phenomena, we
might not be here to think about them.
Let me start with a bit of history.
When the very first radio telescopes
pointed to the heavens, they saw fat blobs
of radio light, whose sources were unknown.
Those blobs were only blobby because those early radio
antennae had some pretty bad spatial resolution,
making it difficult to pinpoint exactly where on the sky
they were located.
Then, in 1962, astronomers caught a break.
In an event known as an occultation,
the moon passed right in front of one of the brightest
of these radio blobs.
It was object number 273 in the brand new 3rd Cambridge Radio
Catalog--
3C273, for short.
The Parkes radio telescope in Australia
was trained on the occultation and
registered the exact instant that the radio signal vanished
behind the moon.
That timing allowed astronomers to identify
a tiny star-like point of bluish light
as the source of the radio emission.
Astronomers turned their optical telescopes
on this strange star, and split the light into a spectrum.
It looked nothing like the spectrum of any star ever seen.
And so the name quasi stellar radio source was born.
Later, to become quasar.
But what was so different?
For one thing, its spectrum was redshifted,
the wavelength of its light stretched out
as those photons traveled through the expanding universe.
That put 3C273 very far away.
Its light must have been traveling from two billion
light years away to acquire the observed redshift.
Yet, to be as bright as it appeared at that distance,
the weird object had to be emitting
many galaxies worth of light from a seemingly impossibly
small region of space.
A hysterical flurry of hypothesizing followed--
swarms of neutron stars, an alien civilization
harnessing their entire galaxy's power,
bright, fast-moving objects being ejected
by our own galaxy's core.
But by the 1980s, we were converging on the most awesome
explanation.
It goes a little like this.
Take a black hole of millions to billions of times the mass
of the sun.
Where from?
Well, it turns out that every decent sized
galaxy has one at its core.
Now, drive gas into the galactic core.
One way this can happen is when galaxies merge and grow.
That gas descends into the waiting
black hole's gravitational well and gains incredible speed
on the way.
It is swept up into a raging whirlpool around the black hole
that we call an accretion disk, where its energy of motion
is turned into heat.
The heat glow of the accretion disk
is so bright that we can see quasars
to the ends of the universe.
Some gas is swallowed, causing the black hole to grow.
However, a lot of it never makes it below the event horizon.
Some is converted directly into energy and radiated as light.
And this same light drives powerful winds of gas back out
into the surrounding galaxy.
In some cases, for reasons we don't fully understand,
some of that gas can also be swept up and collimated,
channeled into jets that erupt from the poles of the quasar.
This may be due to the magnetic field
of a rapidly rotating black hole,
but the jury is still out.
The exact appearance of this phenomenon
depends enormously on our viewing angle.
Looking down onto a bright accretion disk,
we see a quasar in all of its glory.
But viewed side on, that disk is obscured
by a thick ring of dusty gas.
Then, we only see hints of the central monster
because it lights up gas in the surrounding galaxy.
However, if such an edge-on quasar has powerful jets,
we see them blasting through the galaxy
and even filling intergalactic space with beautiful radio
plumes.
We call these radio galaxies.
Oh, and if one of these jets happens to be pointed directly
at us, then we see strange effects
due to the near light speed motion of the jet material.
In an effect called relativistic beaming, the light from the jet
is vastly magnified.
These rare cases are called blazars.
So when a supermassive black hole feeds and blasts energy
into the universe, what we see depends on its orientation,
whether or not it has a jet, the power of the accretion disk,
and a few other properties besides.
However, the family name for any type
of accreting supermassive black hole
is active galactic nucleus.
This is a simplified description of our modern understanding
of quasars and active galactic nuclei.
But it was a hard won understanding.
Most of the energetic craziness happens
on a size scale similar to our solar system, or even smaller.
We're talking, at most, a few light days across.
But when viewed from halfway across the observable universe,
that is impossibly tiny.
Even for 3C273, the nearest bright quasar,
the accretion disk falls into a region less than 100,000
times smaller than a single pixel on the Hubble Space
Telescope.
Over half a century after their discovery,
we're still hard at work on this puzzle,
and not just for the fun of it.
Anything as energetic as a quasar
must have had an influence on the universe.
The first quasars turned on in a very young universe
that was still thick with the raw hydrogen
gas produced in the Big Bang.
As the first galaxies coalesced from this gas,
the universe entered a long period
of violent star formation.
As galaxies coalesced, they went through starburst phases,
producing new stars at insane rates.
The birth of large numbers of new stars
is always quickly followed by the explosive deaths
of the most massive, shortest lived of those stars.
Waves of star formation, followed
by waves of supernovae.
These forming galaxies were continuously blasted
with energetic radiation and cosmic rays.
If life did manage to evolve during this earlier epoch,
it would have been quickly obliterated.
However, the same rich gas supplies
that fueled those starbursts also
gave rise to the epoch of quasars.
As some of this gas found its way
into the nuclei of galaxies, it encountered
there the supermassive black holes
that had been growing since the beginning of the universe.
Accretion disks formed, and many knew quasars were born.
Each burst of quasar activity in a given galaxy
probably only lasts 10 million years or so.
However, that's enough to heat gas throughout the galaxy.
Hot gas doesn't collapse into stars,
and so the extreme starburst activity was shut down.
A few billion years after the Big Bang,
when the universe was around a quarter of its current age,
both starbursts and quasars started to dwindle.
Galaxies had formed, but were no longer wracked by supernovae.
Life finally had a chance.
We are now well out of the quasar epoch.
Active galactic nuclei still do fire up in the modern universe,
although usually they are at full quasar power.
The much weaker, Seyfert galaxies are more common.
But good old 3C273 is a full blown quasar.
In fact, it's one of the most luminous known.
Although it's far away, its light
comes to us from a time long after the peak of the quasar
epoch.
It's a late relic from a more violent time.
But it's not the last.
Perhaps in a few billion years, when
the Andromeda galaxy and the Milky Way
inevitably collide and their supermassive black holes merge,
the violence will deliver one last wave of fuel
to the combined galactic core, and a new quasar
will shine forth, illuminating this little patch of spacetime.
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Hey, guys, I want to give a big thank
you to all our Patreon supporters,
and to all our about to be Patreon supporters.
Links to follow.
And a very, very big thank you to Tambe Barsbay,
who's supporting us at the quasar level.
Tambe, your own personal spacetime quasar
is in the mail.
Expect it in two to four billion years.
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