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Finally, let us consider what do these
supermassive black holes that power active

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nuclei come from.
Here is a very old but still a very

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relevant diagram produced by famous
cosmologist, Martin Reese, about how can

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we generate these black holes that power
quasars and other active nuclei.

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And remarkably, a lot of these things are
still being discussed and all of them are

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still considered.
There are some important questions to

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address here.
First of all, where do the original seed

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black holes come from?
Second, how do they grow?

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Then, what can we observe at hierarchies?
And finally, how does this relate to the

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galaxy formation and evolution as a whole?
Some of these things are still at the

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forefront of modern cosmology.
There are three possible mechanisms by

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which we can initiate the seed black holes
which can then grow into the supermassive

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ones.
The two most, more popular ones are that

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you can have stars from Population III,
which you may recall expected to be very

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massive, explode, and generate black hole
remnants, which can be in range of may be

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hundreds of solar masses and then they can
grow by accretion, emerging, and so on.

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An alternative is that you can form, form
black hole through direct gravitational

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collapse of a primordial cloud or gas in
dark matter.

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And you start with a large black hole,
thousands maybe up to million solar

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masses, and, and let that grow.
A third mechanism is collapse of a dense

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star cluster.
We'll come to all those in turn.

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There are pros and cons for each one of
these models.

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There is also a possibility of primordial
black holes that came from Big Bang

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through some sort of instability very
early on in the universe.

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But those are very much hypothetical, and
most cosmologists would not consider that

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as a likely mechanism.
So, we're, it's still worth considering,

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primordial black holes.
They're probably not going, not the, so

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whereas such primordial black holes seeds
are still worth considering and some

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serious scientists have done, so they're
probably not what really happened.

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Certainly, the most likely cause are the
stellar remnants from first massive stars,

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Population [inaudible] stars, that we know
had to exist.

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And if they're like massive stars we know
about, they probably did collapsed in two

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black holes in as they exploded.
Some of them, we may actually detect as

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they've been born through very high
redshift gamma ray burst and people are

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on, all active lookout for those.
It is also possible that a dense star

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cluster, because we know stars do tend to
get formed in clusters from primordial or

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from protostellar clouds, that clusters
can dynamically evolve in a way that

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gravitational collapses run away and they
all form say, black hole, thousands of

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solar masses.
This is an astrophysically plausible

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mechanism but we're not sure that it
actually happens.

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And then, as the dense cores of galaxy
potential wells form, some of them might

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achieve critical density and collapse into
a black hole directly without even making

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any stars.
Of course, more than one of these things

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could be happening.
So, the most plausible mechanism is the

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primordial star formation, Population
[inaudible] stars and has been modeled

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numerically very extensively.
You form high-mass stars, can be hundreds

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of thousands of solar masses or even more.
When they explode, they leave black hole

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remnants with a substantial fraction of
their mass remaining.

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Some of them may actually form binaries,
and then those merge as well.

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Another possibility is that you just have
sufficiently dense cloud of gas and dark

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matter possibly with a cuspy dark halo and
that undergoes gravitational collapse in

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its own.
So, it forms a really large black hole

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without even going through any star
formation.

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This is plausible, although somewhat more
speculative.

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Once you make the seed black hole, you
have to feed it.

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And if it's radiating according to
Eddington luminosity, you may recall that

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this is an exponential process and it can
grow exponentially in time.

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So, it's actually possible to achieve
billion solar mass black holes by about a

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redshift of 6, assuming you turn all the
knobs in one direction, that everything

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goes just right.
It's worth looking at the necessary

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energetics in a little bit more detail.
So, the material that is used to build

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these black holes will come roughly from
kiloparsecs existences, give or take

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factors of 10, and it tends a microparsec
distances.

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So, most of the mass must be radiated of
the order of 10%, as we know.

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However, this is done, if you are
dissipating only 10% of the mass of a

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billion solar mass black hole, that's
still a lot of mc squared.

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And it adds to the order of 10 to the 61
ergs of, and you have to emit that over a

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period of time that's maybe 700 million
years from the formation of the very first

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stars down about redshift of 6 or 6 and a
half, which means, there is going to be a

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really luminous object.
That's good, because then, we can hope to

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detect them.
And this would be something like 10 to the

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13th solar luminosities, as much as most
luminous quasars that we know.

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Interesting thing is that forming such
black holes could actually contribute

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substantially to the reionization.
We said that most of the reionization,

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maybe all of it, is due to the young
star's hot, massive, really bright

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Population [inaudible] stars, but
formation and early growth of black holes

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can be an important contributor.
So here is the challenge.

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If we take the con, concordance cosmology
redshift 30, which is about the early as

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you can start making stars.
You know, this is about a 100 million

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years old.
By about redshift od 6, 6 and a half, it's

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still in the order of 900 million years
old.

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So, you have about 700 million years to do
it.

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00:06:52,104 --> 00:06:57,265
Suppose you want to make a billion solar
mass black hole and actually, those that

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power the most luminous quasars that these
redshifts may be even 10 billion solar

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00:07:02,501 --> 00:07:05,966
masses.
But let's take a billion solar masses,

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00:07:05,966 --> 00:07:11,950
suppose you start with the seed mass of 10
solar masses, a black hole remnant of one

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00:07:11,950 --> 00:07:18,137
of the Pop [inaudible] three stars.
So, that means that you need to go through

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eight orders of magnitude of growth or
about 18 e-folding times.

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00:07:25,238 --> 00:07:30,479
If you start with 100 solar mass black
hole about as massive as plausible for

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stellar remnant, then only 16 e-folding
times.

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00:07:33,893 --> 00:07:40,342
Well, there is a Salpeter timescale which
is e-folding timescale corresponding to

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exponential growth of accreting black
holes that are accreting at Eddington

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luminosity.
That turns out to be of the order of 40 or

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50 million years for an efficiency factor
of 0.1, which is what we've been assuming.

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So, we can just barely fit that.
And that means that you are always ready

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00:08:02,874 --> 00:08:08,858
to think, at Eddington luminosity, there
is an abundant supply of fuel, there are

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00:08:08,858 --> 00:08:12,194
no disruptions, everything has to go
right.

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00:08:12,194 --> 00:08:17,654
So, even though supplying mass maybe
challenging by itself, that's actually

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00:08:17,654 --> 00:08:22,415
probably not the worst problem.
The worst problem is probably getting rid

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of the angular momentum of the fuel that
gets to be absorbed.

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Once you have black holes, they will
accrete material, alright, but they can

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00:08:30,419 --> 00:08:33,414
also merge.
Since there will be one of these core of

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every massive galaxy, galaxies keep
merging, structure forms hierarchically,

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00:08:38,242 --> 00:08:41,345
eventually, black holes will start
merging, too.

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00:08:41,346 --> 00:08:48,867
And this growth by, by merging is probably
just as important for black holes as it is

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00:08:48,867 --> 00:08:53,637
for host galaxies.
Note, however, that this simply means the

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00:08:53,637 --> 00:08:58,834
mass that has already collapsed in black
holes has been rearranged.

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00:08:58,835 --> 00:09:02,737
It does not involve any dissipation of
energy.

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00:09:02,738 --> 00:09:07,588
All of the dissipation, all of the
luminosity comes from the accretion

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00:09:07,588 --> 00:09:10,734
process.
And there are now excellent numerical

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00:09:10,734 --> 00:09:15,893
simulations of black hole mergers and the
hope is to detect these events through

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00:09:15,893 --> 00:09:21,360
gravitational wave astronomy which is now
just beginning with the LIGO observatory

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00:09:21,360 --> 00:09:25,519
and its successors.
We know that such things do happen.

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00:09:25,520 --> 00:09:31,611
Low, you know, low redshifts we've seen,
we've seen very close double active

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00:09:31,611 --> 00:09:35,380
galactic nuclei.
We see double activegalactic nuclear in a

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00:09:35,380 --> 00:09:39,479
broad range of redshifts, but here are a
couple of nearby examples.

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00:09:39,480 --> 00:09:44,629
One is dual radio source.
We can see the jets are actually being

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00:09:44,629 --> 00:09:48,510
distorted.
The other one is a double X-ray source in

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00:09:48,510 --> 00:09:52,347
nearby galaxy.
These are still too far apart, probably

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00:09:52,347 --> 00:09:57,482
take many millions, if not hundred of
millions of years for these to merge, but

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00:09:57,482 --> 00:10:02,064
there could be many more close repairs
that there are, there, out to be

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00:10:02,064 --> 00:10:05,756
unresolved.
And one interesting thing is that the side

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00:10:05,756 --> 00:10:11,228
from predictive of gravitational waves,
what will be electromagnetic signatures of

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00:10:11,228 --> 00:10:16,135
such super massive block hole mergers.
There has been a lot of theoretical

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00:10:16,135 --> 00:10:21,112
decision of this and as of now, it's not
really clear what, if any, will be the

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00:10:21,112 --> 00:10:26,247
observable signature, but we can hope to
detect them someday and thus, see the

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00:10:26,247 --> 00:10:30,164
assembly of supermassive black holes in
action directly.

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And that completes our class.
I hope you found it interesting and you

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learned a few things about cosmology.
Thank you.