There’s a supermassive black hole at the center of almost every galaxy in the Universe.
How did they get there?
What’s the relationship between these monster black holes and the galaxies that surround
them?
Every time astronomers look farther out in the Universe, they discover new mysteries.
These mysteries require all new tools and techniques to understand.
These mysteries lead to more mysteries.
What I’m saying is that it’s mystery turtles all the way down.
One of the most fascinating is the discovery of quasars, understanding what they are, and
the unveiling of an even deeper mystery, where do they come from?
As always, I’m getting ahead of myself, so first, let’s go back and talk about the
discovery of quasars.
Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a
class of bizarre objects in the distant Universe.
They were very bright, and incredibly far away; hundreds of millions or even billion
of light-years away.
The first ones were discovered in the radio spectrum, but over time, astronomers found
even more blazing in the visible spectrum.
The astronomer Hong-Yee Chiu coined the term “quasar”, which stood for quasi-stellar
object.
They were like stars, shining from a single point source, but they clearly weren’t stars,
blazing with more radiation than an entire galaxy.
Over the decades, astronomers puzzled out the nature of quasars, learning that they
were actually black holes, actively feeding and blasting out radiation, visible billions
of light-years away.
But they weren’t the stellar mass black holes, which were known to be from the death
of giant stars.
These were supermassive black holes, with millions or even billions of times the mass
of the Sun.
As far back as the 1970s, astronomers considered the possibility that there might be these
supermassive black holes at the heart of many other galaxies, even the Milky Way.
In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation.
It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the
“excited atoms” perspective.
This would match the emissions of a supermassive black hole that wasn’t actively feeding
on material.
Our own galaxy could have been a quasar in the past, or in the future, but right now,
the black hole was mostly silent, apart from this subtle radiation.
Astronomers needed to be certain, so they performed a detailed survey of the very center
of the Milky Way in the infrared spectrum, which allowed them to see through the gas
and dust that obscures the core in visible light.
They discovered a group of stars orbiting Sagittarius A-star, like comets orbiting the
Sun.
Only a black hole with millions of times the mass of the Sun could provide the kind of
gravitational anchor to whip these stars around in such bizarre orbits.
Further surveys found a supermassive black hole at the heart of the Andromeda Galaxy,
in fact, it appears as if these monsters are at the center of almost every galaxy in the
Universe.
But how did they form?
Where did they come from?
Did the galaxy form first, and cause the black hole to form at the middle, or did the black
hole form, and build up a galaxy around them?
Until recently, this was actually still one of the big unsolved mysteries in astronomy.
That said, astronomers have done plenty of research, using more and more sensitive observatories,
worked out their theories, and now they’re gathering evidence to help get to the bottom
of this mystery.
Astronomers have developed two models for how the large scale structure of the Universe
came together: top down and bottom up.
In the top down model, an entire galactic supercluster formed all at once out of a huge
cloud of primordial hydrogen left over from the Big Bang.
A supercluster’s worth of stars.
As the cloud came together it, it spun up, kicking out smaller spirals and dwarf galaxies.
These could have combined later on to form the more complex structure we see today.
The supermassive black holes would have formed as the dense cores of these galaxies as they
came together.
If you want to wrap your mind around this, think of the stellar nursery that formed our
Sun and a bunch of other stars.
Imagine a single cloud of gas and dust forming multiple stars systems within it.
Over time, the stars matured and drifted away from each other.
That’s top down.
One big event that leads to the structure we see today.
In the bottom up model, pockets of gas and dust collected together into larger and larger
masses, eventually forming dwarf galaxies, and even the clusters and superclusters we
see today.
The supermassive black holes at the heart of galaxies were grown from collisions and
mergers between black holes over eons.
In fact, this is actually how astronomers think the planets in the Solar System formed.
By pieces of dust attracting one another into larger and larger grains until the planet-sized
objects formed over millions of years.
Bottom up, small parts coming together.
So which is it?
Astronomers think they know the answer now, and we’ll get to it in a second, but first
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Shortly after the Big Bang, the entire Universe was incredibly dense.
But it wasn’t the same density everywhere.
Tiny quantum fluctuations in density at the beginning evolved over billions of years of
expansion into the galactic superclusters we see today.
I want to stop and let this sink into your brain for a second.
There were microscopic variations in density in the early Universe.
And these variations became the structures hundreds of millions of light-years across
we see today.
Imagine the two forces at play as the expansion of the Universe happened.
On the one hand, you’ve got the mutual gravity of the particles pulling one another together.
And on the other hand, you’ve got the expansion of the Universe separating the particles from
one another.
The size of the galaxies, clusters and superclusters were decided by the balance point of those
opposing forces.
If small pieces came together, then you’d get that bottom up formation.
If large pieces came together, you’d get that top down formation.
When astronomers look out into the Universe at the largest scales, they observe clusters
and superclusters as far as they can see - which supports the top down model.
On the other hand, observations show that the first stars formed just a few hundred
million years after the Big Bang, which supports bottom up.
So the answer is both?
No, the most modern observations give the edge to the bottom up processes.
The key is that gravity moves at the speed of light, which means that the gravitational
interactions between particles spreading away from each other needed to catch up, going
the speed of light.
In other words, you wouldn’t get a supercluster’s worth of material coming together, only a
star’s worth of material.
But these first stars were made of pure hydrogen and helium, and could grow much more massive
than the stars we have today.
They would live fast and die in supernova explosions, creating much more massive black
holes than we get today.
The first protogalaxies came together, collecting together these first monster black holes and
the massive stars surrounding them.
And then, over millions and billions of years, these black holes merged again and again,
accumulating millions and even billions of times the mass of the Sun.
This was how we got the modern galaxies we see today.
There was a recent observation that supports this conclusion.
Earlier this year, astronomers announced the discovery of supermassive black holes at the
center of relatively tiny galaxies.
In our own Milky Way, the supermassive black hole is 4.1 million times the mass of the
Sun, but accounts for only .01% of the galaxy’s total mass.
But astronomers from the University of Utah found two ultra compact galaxies with black
holes of 4.4 million and 5.8 million times the mass of the Sun respectively.
And yet, the black holes account for 13 and 18 percent of the mass of their host galaxies.
The thinking is that these galaxies were once normal, but collided with other galaxies earlier
on in the history of the Universe, were stripped of their stars and then were spat out to roam
the cosmos.
They’re the victims of those early merging events, evidence of the carnage that happened
in the early Universe when the mergers were happening.
We always talk about the unsolved mysteries in the Universe, but this is one that astronomers
are starting to puzzle out.
It seems most likely that the structure of the Universe we see today formed bottom up.
The first stars came together into protogalaxies, dying as supernova to form the first black
holes.
The structure of the Universe we see today is the end result of billions of years of
formation and destruction.
With the supermassive black holes coming together over time.
Once telescopes like James Webb get to work, we should be able to see these pieces coming
together, at the very edge of the observable Universe.
This was a fun episode, and I know you’re fascinated by black holes.
Were there any other topics that you’d like me to dig into?
Let me know your thoughts in the comments.
In our next episode we look at the Deep Space Gateway, NASA’s plans to put a space station
out at the Moon, which will serve as a stepping stone to the rest of the Solar System.
It’s time for a playlist, all about supermassive black holes.
First, I’d like to direct you to an interview I did with Dr. Andrea Ghez, who found the
supermassive black hole at the heart of the Milky Way.
Followed by a TED talk she gave.
A response from Michio Kaku about this puzzling mystery.
SciShow Space video about the black hole.
Finally, a public lecture about supermassive black holes.