Finally, let us consider what do these supermassive black holes that power active
nuclei come from. Here is a very old but still a very
relevant diagram produced by famous cosmologist, Martin Reese, about how can
we generate these black holes that power quasars and other active nuclei.
And remarkably, a lot of these things are still being discussed and all of them are
still considered. There are some important questions to
address here. First of all, where do the original seed
black holes come from? Second, how do they grow?
Then, what can we observe at hierarchies? And finally, how does this relate to the
galaxy formation and evolution as a whole? Some of these things are still at the
forefront of modern cosmology. There are three possible mechanisms by
which we can initiate the seed black holes which can then grow into the supermassive
ones. The two most, more popular ones are that
you can have stars from Population III, which you may recall expected to be very
massive, explode, and generate black hole remnants, which can be in range of may be
hundreds of solar masses and then they can grow by accretion, emerging, and so on.
An alternative is that you can form, form black hole through direct gravitational
collapse of a primordial cloud or gas in dark matter.
And you start with a large black hole, thousands maybe up to million solar
masses, and, and let that grow. A third mechanism is collapse of a dense
star cluster. We'll come to all those in turn.
There are pros and cons for each one of these models.
There is also a possibility of primordial black holes that came from Big Bang
through some sort of instability very early on in the universe.
But those are very much hypothetical, and most cosmologists would not consider that
as a likely mechanism. So, we're, it's still worth considering,
primordial black holes. They're probably not going, not the, so
whereas such primordial black holes seeds are still worth considering and some
serious scientists have done, so they're probably not what really happened.
Certainly, the most likely cause are the stellar remnants from first massive stars,
Population [inaudible] stars, that we know had to exist.
And if they're like massive stars we know about, they probably did collapsed in two
black holes in as they exploded. Some of them, we may actually detect as
they've been born through very high redshift gamma ray burst and people are
on, all active lookout for those. It is also possible that a dense star
cluster, because we know stars do tend to get formed in clusters from primordial or
from protostellar clouds, that clusters can dynamically evolve in a way that
gravitational collapses run away and they all form say, black hole, thousands of
solar masses. This is an astrophysically plausible
mechanism but we're not sure that it actually happens.
And then, as the dense cores of galaxy potential wells form, some of them might
achieve critical density and collapse into a black hole directly without even making
any stars. Of course, more than one of these things
could be happening. So, the most plausible mechanism is the
primordial star formation, Population [inaudible] stars and has been modeled
numerically very extensively. You form high-mass stars, can be hundreds
of thousands of solar masses or even more. When they explode, they leave black hole
remnants with a substantial fraction of their mass remaining.
Some of them may actually form binaries, and then those merge as well.
Another possibility is that you just have sufficiently dense cloud of gas and dark
matter possibly with a cuspy dark halo and that undergoes gravitational collapse in
its own. So, it forms a really large black hole
without even going through any star formation.
This is plausible, although somewhat more speculative.
Once you make the seed black hole, you have to feed it.
And if it's radiating according to Eddington luminosity, you may recall that
this is an exponential process and it can grow exponentially in time.
So, it's actually possible to achieve billion solar mass black holes by about a
redshift of 6, assuming you turn all the knobs in one direction, that everything
goes just right. It's worth looking at the necessary
energetics in a little bit more detail. So, the material that is used to build
these black holes will come roughly from kiloparsecs existences, give or take
factors of 10, and it tends a microparsec distances.
So, most of the mass must be radiated of the order of 10%, as we know.
However, this is done, if you are dissipating only 10% of the mass of a
billion solar mass black hole, that's still a lot of mc squared.
And it adds to the order of 10 to the 61 ergs of, and you have to emit that over a
period of time that's maybe 700 million years from the formation of the very first
stars down about redshift of 6 or 6 and a half, which means, there is going to be a
really luminous object. That's good, because then, we can hope to
detect them. And this would be something like 10 to the
13th solar luminosities, as much as most luminous quasars that we know.
Interesting thing is that forming such black holes could actually contribute
substantially to the reionization. We said that most of the reionization,
maybe all of it, is due to the young star's hot, massive, really bright
Population [inaudible] stars, but formation and early growth of black holes
can be an important contributor. So here is the challenge.
If we take the con, concordance cosmology redshift 30, which is about the early as
you can start making stars. You know, this is about a 100 million
years old. By about redshift od 6, 6 and a half, it's
still in the order of 900 million years old.
So, you have about 700 million years to do it.
Suppose you want to make a billion solar mass black hole and actually, those that
power the most luminous quasars that these redshifts may be even 10 billion solar
masses. But let's take a billion solar masses,
suppose you start with the seed mass of 10 solar masses, a black hole remnant of one
of the Pop [inaudible] three stars. So, that means that you need to go through
eight orders of magnitude of growth or about 18 e-folding times.
If you start with 100 solar mass black hole about as massive as plausible for
stellar remnant, then only 16 e-folding times.
Well, there is a Salpeter timescale which is e-folding timescale corresponding to
exponential growth of accreting black holes that are accreting at Eddington
luminosity. That turns out to be of the order of 40 or
50 million years for an efficiency factor of 0.1, which is what we've been assuming.
So, we can just barely fit that. And that means that you are always ready
to think, at Eddington luminosity, there is an abundant supply of fuel, there are
no disruptions, everything has to go right.
So, even though supplying mass maybe challenging by itself, that's actually
probably not the worst problem. The worst problem is probably getting rid
of the angular momentum of the fuel that gets to be absorbed.
Once you have black holes, they will accrete material, alright, but they can
also merge. Since there will be one of these core of
every massive galaxy, galaxies keep merging, structure forms hierarchically,
eventually, black holes will start merging, too.
And this growth by, by merging is probably just as important for black holes as it is
for host galaxies. Note, however, that this simply means the
mass that has already collapsed in black holes has been rearranged.
It does not involve any dissipation of energy.
All of the dissipation, all of the luminosity comes from the accretion
process. And there are now excellent numerical
simulations of black hole mergers and the hope is to detect these events through
gravitational wave astronomy which is now just beginning with the LIGO observatory
and its successors. We know that such things do happen.
Low, you know, low redshifts we've seen, we've seen very close double active
galactic nuclei. We see double activegalactic nuclear in a
broad range of redshifts, but here are a couple of nearby examples.
One is dual radio source. We can see the jets are actually being
distorted. The other one is a double X-ray source in
nearby galaxy. These are still too far apart, probably
take many millions, if not hundred of millions of years for these to merge, but
there could be many more close repairs that there are, there, out to be
unresolved. And one interesting thing is that the side
from predictive of gravitational waves, what will be electromagnetic signatures of
such super massive block hole mergers. There has been a lot of theoretical
decision of this and as of now, it's not really clear what, if any, will be the
observable signature, but we can hope to detect them someday and thus, see the
assembly of supermassive black holes in action directly.
And that completes our class. I hope you found it interesting and you
learned a few things about cosmology. Thank you.