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Black holes are the densest objects in existence, with gravities so powerful that even light
can’t escape.
As such, it comes as no surprise to find the largest black holes at the centres of galaxies,
where matter has been fed into them for billions of years, continually increase their mass.
The very largest of these supermassive black holes can be billions of times the mass of
our Sun.
However, it may come as a surprise to you to realise that some of the most massive black
holes we know of are actually also the youngest.
You see, when we look at distant galaxies, we are also looking back in time, and the
galaxies billions of light years away often have the largest black holes.
If the universe is only 13.8 billion years old, and light takes billions of years to
reach us, that means the galaxy we are observing can only be a few billion years old at most
from our perspective, pretty young for a galaxy.
Surely though, it should be the case that nearer, and thus older supermassive black
holes are more massive seeing as they’ve had so much extra time to consume matter falling
into them?
So, what’s going on here?
The very largest supermassive black hole we know of is known as TON 618, with an incredible
mass of 66 billion solar masses.
By itself, its mass is comparable to the Milky Way galaxy.
However, TON 618 is exceptionally far away, and it’s taking light emitted by it 10.8
billion years to reach us, meaning we are observing it as it was 10.8 billion years
ago.
This means it can be at most around 2.8 billion years old.
By comparison, our own Milky Way is approximately 13.6 billion years old, yet the supermassive
black hole found at our galaxy’s core, Sagittarius A*, is only 4 million solar masses.
The Andromeda Galaxy’s supermassive black hole, while bigger, is still only 200 million
solar masses.
One of the big factors to consider here is the difficulty in detecting and measuring
black holes.
This is still a really new field of research, as technology has only just allowed us to
start observing black holes in the last few decades.
Even then, we can often only observe the area surrounding black holes, that is, before the
Event Horizon Telescope came along.
But even that telescope takes ages to image just one black hole, so our general understanding
really is still quite limited.
In fact, most of the distant black holes we know about can only be seen because they are
quasars.
TON 618 is a quasar.
Matter is pouring into the black hole’s accretion disk at an incredible rate, and
because of this, it’s erupted into a quasar.
Quasars can only be sustained as long as matter is falling into them, otherwise they revert
back to dark black holes.
It’s hard to fully grasp the physics of the accretion disk, but it is believed that
the friction here is so great, the accretion disk of a quasar by itself can produce thousands
of times more light than entire galaxies combined.
TON 618 produces as much light as 140 trillion Suns, completely outshining the galaxy it
resides in, to the point that we can’t even see it from our perspective.
However, because quasars are the brightest objects in existence, they can be seen from
very far away.
So, one reason for the largest black holes being far away is down to something known
as Malmquist Bias.
This is where brighter objects further away appear more plentiful, when in reality, we
simply can’t see the dimmer objects at that distance, implying there may be an argument
that the largest supermassive black holes are actually distributed fairly evenly throughout
the universe.
If a galaxy has a very large black hole but it’s not a quasar, it means we won’t see
it after a certain distance because a galaxy is much dimmer than a quasar.
Another reason we don’t see the biggest black holes close to us is due to the nature
of the universe itself shortly after the Big Bang.
As you may know, the universe is ever expanding, and during the early universe, matter was
a lot closer together.
Quasars were more common back then because they need extreme amounts of matter falling
into them to give off light, and there was a lot more gas around during the early stages
of the universe.
Not only has the universe expanded, but over time, gas gets converted into stars.
Some of the largest types of stars eventually turn into neutron stars and black holes themselves,
meaning that they can never get recycled back into gas.
Less available gas means less gas will fall into a supermassive black hole.
One of the theories for the fate of the universe is actually based on this, called the Big
Freeze, where after some trillions of years, all the gas in the universe is eventually
converted to black holes.
Even now, we see some galaxies where their gas has been completely used up, meaning no
new stars can form.
These are called elliptical galaxies.
Spiral galaxies still have gas and dust structures and thus can still produce new stars.
It is interesting that most of the largest supermassive black holes appear to be in elliptical
galaxies, where there is no gas left.
Gas needs to lose angular momentum to fall into the galaxy’s central supermassive black
hole, and if that happened, then the supermassive black hole is likely to be much bigger because
of all the infalling matter.
With elliptical galaxies, this has already happened, whereas with spiral galaxies, this
hasn’t happened to the same extent.
One such trigger for gas losing angular momentum could be the gravitational influence of nearby
galaxies, or even collisions with other galaxies.
In addition, there is less gas available in the universe now than there was during the
early universe, so black hole growth probably occurred rapidly then , but has slowed down
now.
This might be why there is no quasar within 500 million light years of us.
As the universe ages and things become less chaotic and more spread out, the number of
active quasars has decreased.
Which means the only quasars we see, some of which are the largest black holes we know
of, are the ones that happened a long time ago.
So, why are the largest supermassive black holes often the youngest?
Well, although it may appear that way, it might not actually be the case at all.
We can measure distant bright quasars simply because we can see them.
Older and closer black holes may also be large, but because of Malmquist Bias, we haven’t
found them yet.
As studies continue and technology improves, we’ll start to get a more complete picture
of the universe around us.
Now, there is actually a reasonably big supermassive black hole not too far from us, known as M87*.
It was the Event Horizon Telescope’s recent subject of observation, giving us the first
ever look at a black hole’s accretion disk.
There’s a documentary on Curiosity Stream called space phenomena, black holes, which
talks about the significance of the image, how they took it, and how there may be a connection
between black holes and the existence of life.
Curiosity Stream is a streaming platform with thousands of documentaries about a variety
of topics, like history, nature, technology and science, including plenty of really great
ones on space.
With my link in the description, you can watch the black hole documentary and get access
to their entire catalogue of award winning documentaries for only $14.99 for the whole
year, so I highly recommend checking them out.
Thanks for watching!
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