Good afternoon, ladies and gentlemen. Can I bring you to order?
Thank you. So welcome to this the 11th Hennessy lecture.
A particular welcome to the first school or in the fourth row at the back there.
Thank you for coming from Maidenhead. So it's my great pleasure to introduce the speaker today.
Meg Urie is the Israel Moonstone Professor of physics and Astronomy at Yale.
She's also the director of the Yale Centre for Astronomy and Astrophysics.
And at the moment she is the 45th president of the American Astronomical Society.
So here we have somebody of enormous eminence and distinction.
Her research interests, your research interests that you also find in this department on accreting objects,
black holes and active galactic nuclei in galaxies.
Among the many distinctions, she's one the only jump canon award, the American Astronomical Society, and more recently, the Von Beaverbrook Prize.
She's a fellow of the American Society of Arts and Sciences.
She has, for the whole of her career, been a champion of women in science.
And in fact, she was the first woman tenured at the Yale Physics Department.
That was in the year 2000.
She's also played a lot of major roles in astronomy policy in the United States for the National Research Council of the Academy,
the American Physical Society, and, of course, NACI.
It is my privilege to introduce Professor McGorry to give the 11th himself lecture Growing black holes over 12 billion years.
Well, thank you for the lovely introduction, and thank you to Mr. Hennessy for endowing the lecture so I can actually be here.
I've had a wonderful week visiting the department, talking to the students, the post-docs, the faculty.
It's a really a very lively place, and I could probably spend several weeks here fully engaged at the same rate,
but I don't know if I'd survive it physically, so I think I'll just go home instead anyway.
Thank you. I'm really pleased to be talking to you about black holes.
My research group studies when and where they grow across cosmic time and how they affect the galaxies that they live within.
So that's what the topic is today. And let's go.
So first I want to set the context of what the universe, what the history of the universe is in one slide.
This is like, you know, the history of the British Empire in one microgram or something.
Time is moving to the right in this cartoon. The size of the bell shaped thing is the size of the universe modulo a ballistic expansion,
which astronomers describe in terms of something called redshift,
which increases from zero today to higher values in the past and the size of the universe.
That Redshift Z is one over one plus Z times that size today.
So, for example, when the cosmic microwave background radiation is seen,
which is at a few hundred thousand years after the Big Bang, the universe was about a thousand times smaller in each dimension.
So the universe starts with a big bang. There's a very sudden, very large inflation.
But then after that, it's kind of coasting. And that's why you see this is almost a straight line edge, if you like,
in terms of the amount of expansion over the ballistic expansion when the universe is very small.
Those of you who've taken physics know that gravity goes inversely as the square of the distance between two particles or so, said Newton.
Einstein said it differently to particles of mass with mass attract each other, and the force is stronger when they're close to each other.
So when the universe is small, gravity is a pretty good, pretty strong force.
And as it gets bigger, the grip, the overall gravity, pulling it back together weakens.
And other things like dark energy cause it, in fact, expansion to accelerate.
I'm not talking about that today, but that's just to explain to the people who like details why this curve is flared at the end.
Okay. So what I'm talking about today has to do with what happens after structures form
gravitationally bound structures like stars or the galaxies that are bound,
gravitationally bound collections of stars. We think so.
After the Big Bang, the universe is very hot.
As it expands, it cools the way an ideal gas cools when you expand it.
There's a period when the universe is dark after the cosmic right, let's say.
Sorry, I skipped a step. Once it cools enough for atoms to remain stable, electrons and protons recombine to form hydrogen, then we can see photons.
They don't scatter off the electrons.
So that's why the CMB is visible at this particular roughly redshift of a thousand epic, because after that time,
the photons can stream to us and we can see them coming directly from where they have been emitted.
But there's a dark ages before stars form and sometime around a redshift of, I don't know, 2015 or something like that.
The first stars form the first stars are unusual because they form out of hydrogen and helium and not much else.
There are very few metals initially in the universe, metals meaning heavier elements than helium,
so they form unusually large stars and those stars of all very quickly and they burn very brightly
and then they explode and perhaps leave behind remnant black holes around which galaxies will form.
You see, the stars have formed at the densest parts of the universe,
and the rest of the 13 some billion year history of the universe is basically a story of the dense
parts getting denser as matter is attracted to them and the vacant parts getting more vacant.
So the contrast is being enhanced now.
Yeah. So redshifts convenient for astronomers, but it doesn't mean much in terms of time.
All of the time is between redshift zero and one or two, and.
But most of the action is around redshift. One, two, two, two, three.
That's when most of the stars are forming in galaxies and where most of the mass appears to be accreting in black holes.
So that's an epic that we try to focus on a black hole.
Growing means matter is going into it. As the matter approaches it, it heats up and it glows.
And the amount of light emitted from its accretion process before particles get into
the black hole is actually quite substantial and can outshine the galaxy itself.
So when that's happening, when the accretion rate is sufficiently high, we call it an active galactic nucleus or AGM for short.
So any time the accretion onto the black hole is high, that means it's growing rapidly.
Then we have an AGM. And my job today is to tell you what those look like to an astronomer and to how we measure them.
Okay. So the first thing to know is that there are lots of different kind of galaxies in the universe.
Here's a little sampling of different shapes and sizes. They're all bound collections of stars.
They have different ages, different stellar content. But one thing pretty much every galaxy has is a supermassive black hole at its core.
We haven't known this for all that long. I see that some of the people in the room are nearly as old as I am.
And when we were kids, we didn't know black holes existed.
We knew they existed in theory, but we hadn't ever found observational evidence that they exist in nature.
And now it's just commonplace for all of us to understand that there's a supermassive black hole at the centre of every galaxy.
But that's really since the Hubble Space Telescope and other observations that have happened over the last 15 to 20 years.
So the best black hole is in our own galaxy. That is the best measured one.
And. I think this movie is the next thing.
Yeah. So our our galaxy, the Milky Way galaxy is a collection of about 100 billion stars.
There's a black hole at the centre that weighs about three or 4 million times the mass of our sun.
It's actually a very unremarkable black hole. It's modest in size.
It's modest in activity. It is not in again, but it's nearby.
So we can actually study it exquisitely.
Well, and this is a movie produced by Andrea Ghez and her group at UCLA showing the orbits of stars at the centre of our galaxy as they orbit.
The centre of the galaxy. At the centre of these stellar orbits is the mass that is attracting them, that is making them go in a circle.
It's the centripetal force of gravity that allows them to have these orbits.
I'm going to make this guy go again. Hang on. When it stops.
Let's just do it this way. The cheap way. Okay. So now when you watch the stars,
notice that the ones that are close to the centre are moving rapidly because they feel the strongest
force because they're closest and the ones that are far away are actually moving quite slowly.
And if you are studying Kepler's laws, as I believe some of our audience is.
You can explain this with proposed laws. But any one of these orbits tells us an enormous amount about the mass of the the mass interior to the orbit.
When you have the combination of orbits, you have incredible constraints on everything,
not just the mass, but also how far it is to the centre of our galaxy and many other parameters.
And you also know that the mass has to be in a very small volume because the stars are, you know, coming quite close to it in different dimensions.
And when you put all that together, the least exotic explanation of this, of these orbits is a black hole.
Anything else is beyond weird. So we're pretty sure there's a black hole at the centre of our galaxy.
Okay. Now, when I was a youngster, I started out working on it again.
Nobody else did. It was kind of passé. Now it's exciting again because it has something to do with galaxies.
So, in fact,
we now think or theorists now think that AGM energy that AGM radiate into the Galaxy has a profound effect on the future evolution of the galaxy.
And I'll be talking more about that. These are the four arguments for why we think again are important to galaxy evolution.
One Every black every every galaxy has to go through a stage where its black hole is growing and therefore energy is being deposited in the galaxy.
They also happen contemporaneously.
That is the peak of star formation in cosmic history coincides roughly with the peak of black hole growth in cosmic history.
And this is this is a plot that shows that as a function of redshift, you see in the data points,
the star formation rate density, how much, how many stars are forming per unit mass.
And then in the grey curve that well, that's the error on the black curve.
That's a rough estimate of how much mass is being accreted on to black holes at the same time.
So you see that they both have a history. Remember, the left is redshift, zero is today.
And if you go to the right, a redshift one is about 6 billion years after the Big Bang, and rich of two is about 3 billion years after the Big Bang.
So over that period is when they both peak or appear to peak.
Well, that could be coincidence. It doesn't have to happen in individual objects.
So a better argument perhaps is this amazing correlation called the Sigma relation, which was reported about in 2000.
And that is the following that the do I show this plot? I can't remember if I have it on here or not.
Nope. Okay. I'm just going to tell you about it. So I didn't want to outdo overdo the plots.
So this correlation is that the mass of the black hole at the centre of a galaxy is correlated with the effectively the stellar mass of the galaxy.
It's actually the velocity dispersion, but it's proportional to mass.
Now, you might think because you studied Kepler's laws, that that makes sense, that the stars are responding to the gravity of the black hole.
So okay, they should be correlated. But the black hole, although very, very massive, is actually much smaller in mass than the whole galaxy.
So it's about in the case of our galaxy. Three some three point something million solar masses, black hole and 100 billion solar masses of stars.
So black hole is tiny compared to the stars.
And as soon as you go any distance away from it, that encloses more mass in stars than in black.
In the black hole, its gravity is irrelevant. So the star's way far away from the black hole somehow know about the mass of the black hole they move.
How fast they move is related to the mass of the black hole. And that suggests that in individual objects there's a correlation between how much
mass goes into the black hole and how much new stars are forming in the galaxy.
And that's so that's where the really the big hint that we need it for galaxy evolution.
And then the last reason is, while theorists need black holes, it's so nice to be needed.
So the idea here is that if gravity is really simple, at least not black hole gravity maybe, but Newtonian gravity, it's simple gravity.
So we know from the cosmic microwave background measurements what the density contrast was at its epic of about 380,000 years after the Big Bang.
And we know how much time there is until today.
And in computers, we can run the universe from those initial conditions forward and form galaxies.
That has helped us understand that there is a lot more dark matter than than baryonic matter in the universe and other other insights.
But what happens is you get the wrong kind of galaxies. If you don't add some physics, you get too many small galaxies and too many big galaxies.
The small galaxies are a problem for another day. But the big galaxies, the idea is that because, again, inject energy into the galaxy,
they can heat the interstellar medium or give it energy in some way that prevents new stars from forming.
So you're sort of artificially restricting the number of stars that form, and that makes the galaxy appear smaller, less, less luminous.
There's a beautiful movie of this made by the team that did the illustrious.
This is a computer simulation. This is what I should turn the lights out for.
Okay, don't walk around because you fall down. But here is the one.
Yeah. You really have to get a good look at this one.
So what you're looking at is the dark matter distribution in the lower left is the time since the big bang.
So right now it's at 1.5 billion years since the Big Bang.
That's probably hard for you to read.
And you're looking at the blue is just where is most of the dark matter and that is where the baryonic matter goes.
Also, it follows the same, essentially the same.
It responds to the gravity of the dark matter. Let's say it that way. In a moment, we're going to change colour.
And now what you're looking at is temperature. Okay.
In the same simulation, just looking at the temperature of the gas,
that means the atoms of material that are making up these filaments restructures, which is where galaxies form.
And now I guess you can see explosions. Those are pretty clear.
And those are the ones at the centres of galaxies. Those are injections of energy from the black hole, accretion from the black hole growth.
There's also supernovae in this in this simulation.
So some of the energy injection comes from supernovae, which are the end points of stellar evolution.
Evolution of some stars. And that energy makes the galaxies we live in today different from what they would otherwise look like.
Oh, yeah. Let me just tell you, the last colour is the last colour is metallicity.
So as supernovae go off, the elements that have been forged in the interiors of stars are distributed throughout space.
And this must happen very quickly, because we see in very distant objects some of the earliest AGM that can be detected.
We see lots of signs of of heavy elements like carbon or other nitrogen, oxygen, etc.
And so the purple is showing the metallicity of the gas as a function of time.
And right now, just to keep you current, we are at 12 billion years after the big bang.
And today's age of the universe is 13.7 billion years after the big bang.
Okay. And there are the credits.
So I don't I don't think it was obvious from that computer simulation, but after galaxies form, they frequently collide and merge.
Collide is maybe the wrong term because galaxies are mostly empty space.
They're made up of stars. But the the distance between stars is much bigger than the size of the stars.
So you can think of them as sort of like tiny grains of sand, each of which are miles apart.
One way to think about this is the sun is very close to us.
It's a big, bright star and then the next brightest stars.
You know, those things at night that you look at that you can see maybe at twilight.
So there's a big space between stars. So when two galaxies collide, they literally pass through each other.
The stars don't ever hit each other. So it's not a collision like your car hitting another car.
But but they, of course, are exerting gravity on one another and they undergo what we call well,
these are different snapshots of interactions between different pairs of galaxies and the whole collision of a galaxy,
which, again, we can simulate in computers, will take something like 200 million years.
So we only see what they look like today.
So we'll see a particular pair of galaxies at one stage of its evolution of a merger.
We don't ever get to see the whole 200 million years for a given pair of galaxies.
But what we can do is simulate mergers of two galaxies.
And what I'm showing here is what we call a major merger of two disk, like gas rich galaxies going in a computer.
And then every so often the simulation is going to pause and show you an actual pair of galaxies that looks like the simulation.
So, again, let me make this dark. Cause it's really pretty.
As the galaxies merge, they they pull on each other.
And we call these tidal forces because the the near side of one galaxy pulls
hardest on the near side of the other galaxy and less hard on the far side.
So they stretch. Each galaxy stretches the other and you get these amazing tidal tails,
which are stars that have basically been ripped out of the galaxy in a big tidal stream.
And we actually see this frequently in the images of galaxies.
If the two galaxies are sufficiently massive and and or not moving too fast, they will actually end up merging, as this simulation does.
And in fact, these were equal mass galaxies with slightly different angular momentum.
I think there's one more of these guys. Okay.
So so we can do that in the computer and we know that happens in nature.
And we can compare our observations, too, to what we see in these snapshots.
So all of those movies are kind of telling you this story, this bedtime story about how galaxies evolve.
The idea is that that mergers play a significant role.
Let me just say, this is the standard scenario. And at the end, I'm tell you how, we don't quite substantiate this whole thing yet.
But you have some event happen, like a major merger, and that stimulates stars forming,
which happens almost right away because there are shocks when the galaxies pass the gas and the galaxies collide and then you
get an enhanced star formation rate and some of the material because you now have a very different gravitational potential,
it helps funnel material down toward the black hole. Now.
How many of you have had a physics mechanics class?
It should be most of you. Of course, there's a lot of physicists in here.
Okay. Thank you. Yeah. Okay. So the reason it takes the black hole a long time to start accreting is because of angular momentum.
You have to get a proton from a scale of many orders of magnitude, larger size down into the size of a black hole,
which is, you know, maybe a typical 10 to 100 million solar mass black hole might be ten light seconds across.
And you're trying to get gas in from light years out in the galaxy.
So there's a huge amount of scale difference and it takes a long time to lose the angular momentum for protons actually to get into the black hole.
But once it turns on, it should supply energy to the interstellar medium in the galaxy.
That is the cold gas out of which stars would otherwise form.
It can heat that gas, turn off star formation. And now this needs a little explanation.
Stars are formed at all masses and with some distribution, but the more massive stars are hotter and bluer and they live a shorter amount of time.
They evolve faster. So you start with some distribution of stars, and as if you stop forming new stars,
the bluest ones go away first and the reddest ones are left behind.
So that means the whole population ages from blue to red. So we should see the Aegean turn on, and then we should see the galaxy age from blue to red.
And yeah, so we kind of see that, but there's some stuff we don't see.
So let me give you a roadmap for where we're going. I'm going to tell you about three topics.
One, how do we know when and where black holes grew, and in particular,
how multi wavelength surveys have shown us that much of black hole growth is actually shrouded from view at optical wavelengths.
So many of our earliest surveys of Aegean and and luminous Aegean were very incomplete.
Second, I'm just going to review what we know.
I've showed you that mergers are thought to be important, and I'll review what we actually know about mergers.
Very briefly. And then I'll talk about a really cool recent result we have on a particular Aegean called Side 947.
We don't we should have really good names like, you know, the cool one, but we don't.
So it's the ID 947 to tell you what we learned about it. Okay, so first the multi wavelength surveys.
Before moving to Yale, I worked for the Space Telescope Science Institute, which runs the Hubble Space Telescope for NASA.
And just before I left there, I designed a survey called Good's, which was supposed to which did, in fact,
answer this question of how much black hole growth is obscured as opposed to sort of freely visible in the optical.
So now I'm going to explain what that means. This cartoon shows not to scale sort of what we think the geometry of the inner regions of AGN look like.
So the black thing in the middle is the black hole and the pink doughnut around it is the the little pink doughnut is an accretion disk because again,
matter trying to fall in the black hole will settle into a disk because the vertical motions can be cancelled out,
but the angular momentum is maintained in the material and so it slowly moves in.
A particle will move in as another particle moves out because you have to conserve angular momentum.
And so for stuff to get into the black hole, you have to have some other stuff going out.
So that all happens in an accretion disk.
We see emission lines from clouds of gas that are photo ionised by the hot UV and X-ray radiation from the accretion disk.
And then the key point of this picture is the big orange doughnut,
which is the obscuring gas and dust that keeps you from seeing the hot accretion disk and the emission line radiation from the centre of of the AGM.
So if you're looking from the side you see a very faint again, not U.V. or soft X-ray bright.
So if you're looking from the side, but if you look from the poles,
you'll see the hot accretion disk and the broad emission lines broad because the gas is moving fast near the black hole.
And for this purposes of this talk, you can forget the jets that are staring at you.
They're not. Yeah, I'm not talking about them in this particular talk.
So this is the picture of what we think, again, look like locally.
We're pretty sure that's correct locally.
And that means that in most cases, we don't get a good view of the accretion process and the energy that's going into the the surrounding galaxy.
We also know that in the past when the universe was smaller and the star formation rate was higher, the density of gas and dust is higher.
So it's more likely that in the past there was even more obscuration.
And indeed, we eventually proved that to be true. And finally, let me be a little bit I don't want to say parochial.
I was trained as an X-ray astronomer, meaning A focusing on the X radiation coming from stars and galaxies and so on.
And black holes primarily are rather than optical light and X-ray.
Astronomers have actually known for 30 years that most black hole growth is obscured.
But since most astronomers are optical astronomers, it wasn't a fully known thing.
So let me say it that way. That's kind of rude, but you're filming me, so.
Okay, I'm really trouble with my colleagues. The idea here is the X-ray background is the name given to the radiation that was seen with
the very first rockets that flew above the atmosphere so they could detect cosmic x rays.
But fortunately, x rays do not penetrate through our atmosphere because we would be we would have shorter lives if they did.
We evolve faster if they did. But but you have to go above the atmosphere.
So you have to have a rocket or you have to have a satellite to see x rays.
So the very first detectors, when they found a few sources, but they found a sort of all sky glow, pretty uniform around the sky,
which they called the X-ray background because it was background to the sources they actually detected.
However,
we now know from improving the X-ray detector capabilities that the radiation is actually the sum of all the again in the universe emitting X-rays.
And so so we can measure what their energy distribution is.
In other words, x rays can be admitted at a range of energies.
And if we look at how much light is emitted at the different X-ray energies, that tells us something about the process that's producing the x rays.
So let me explain that. First, I'm showing you the X-ray background.
This is a plot of intensity. Basically, the amount of energy coming out at each energy from one to a few hundred kilowatts in energy.
So that's a broad x ray band. And you see that it's peaked.
That's all I want you to see here. That's a curved spectrum.
The kind of AGM that optical surveys see the UN obscured cases would be a horizontal line in this plot.
In these units, that means you can't add them up and get this shape, but you can if there's some obscuring material in the line of sight.
And the reason for that is something called the photoelectric effect,
which is just an atom can absorb an X-ray and kick out an electron and that energy disappears.
So if you have atoms around that can absorb X-rays, you absorb the soft X-ray radiation.
And what you see here is an input spectrum with no absorption and then increasing column densities in AD.
So, so sorry. The left hand line is ten to the 20 atoms per centimetre squared of column density.
That's more or less the column density through our galaxy that we have to look through to get,
you know, to look at things that are outside our galaxy. So that's sort of the minimum we tend to see.
And as you increase by a factor of ten, each column density, you you absorb many more of the soft X-rays, the low energy X-rays.
So you can see how if you have a bunch of things that are very obscured, you can get a peaked spectrum.
And indeed, that's how we form this this X-ray spectrum.
It's from absorb mainly absorbed AGM.
Okay. So I'm not going to show a lot of the modelling we did, but let me just say that goods,
the survey that I designed when I was at Space Telescope was one of the first was probably the
first of these deep multi wavelength surveys with x rays that could find again much more fuzzy.
How to that? Yeah, optical surveys get a tiny fraction of the action an x ray surveys get almost all of them because the hard,
the very energetic x rays can penetrate the gas and dust in exactly the same way that they penetrate your skin but are stopped by your bones.
So when you get an x ray from a doctor, you know you can see your skeleton.
So the x rays can get through the stuff. That's why they're good.
Some of them are soft x rays and especially the UV, the ultraviolet light is absorbed and it gets re radiated by the dust.
So we see that in the infrared and then we could use Hubble to separate the
emission at the very centre of a galaxy from the nucleus that is from this,
this again part in the centre, from the starlight in the rest of the galaxy.
And I see my former graduate student, Brooks Simmons, whose thesis was on that topic.
So good, this is okay, now we have to be dark again.
This slide shows the goods survey. Goods stands for Great Observatories Origins Deep Survey Great Observatories refers to
what NASA it's what NASA calls its three Hubble Space Telescope Optical Observatory,
the Spitzer Space Telescope, Infrared Observatory and the Chandra X-ray Observatory.
So Great Observatories is using all three together and deep survey because it was the largest, deepest survey ever done to that time and origins,
because we were looking at the origins of galaxies and of course,
but also because if we didn't have a letter, we were afraid we wouldn't get our telescope time.
So you're looking at an image which is a two dimensional projection of a three dimensional picture.
Almost everything in this picture is a galaxy, and we can figure out the distance to many of these galaxies.
Really, most of them. Let me show you. Did I skip the movie?
I did skip the movie. Okay. Let me show you what we found.
So we have this is now a tiny portion.
Sorry, I should have said this. This picture is the full good survey and now I just zero in on less than 1% of the area.
What you're looking at is the Hubble image. So optical light at different wavelengths, so added together for sort of a realistic colour.
And then the three circles show you where the X-ray sources are in this particular part of the field.
But you also see that two of them are yellow and one is white.
The yellow circles are X-ray sources that have a lot of soft photons missing.
So the yellow ones are probably absorbed, and the white one is the one where it appears unobserved.
The soft photons are not affected. So it's interesting that in the optical you see the optical image kind of corresponds to that, right?
Because the yellow circles, the optical image is very, very faint. In fact, in the bottom one, it's undetected.
And in the white circle, the absorbed and obscured X-ray source has a pretty bright optical image.
So the this is showing you exactly what I was saying is that the optical surveys find the on obscured objects easily,
but they don't find the obscured ones easily, whereas the x rays do. Now I'm going to show you an image with the infrared data superimposed.
This is from the Spitzer telescope. And you see, the first thing you see maybe is that there are a lot more sources.
That's because stars emit a lot of infrared light. And so all of the galaxies appear as infrared sources.
But you but very importantly, what I want you to notice here is that the two yellow circles have pretty bright infrared sources in them,
meaning they're X-ray weak, not because they're intrinsically weak, but because they're absorbed.
And that energy is re radiated in the infrared and they're bright infrared sources.
So this was the crux of our experiment. We did. Obviously, we did a very quantitative analysis.
This is the Ph.D. thesis of my student, Ezekiel Thruster.
And he took I'm just showing you one cartoon to kind of illustrate.
He made a model of what a gene should look like that matched local objects that we had studied very well.
And that model is basically a clumpy Taurus of gas clouds and dust that the model specifies how those clouds radiate in the infrared.
It also specifies what the column density is in any direction.
So he fixes the model to fit local again.
It predicts a distribution of column densities for the whole population.
And then from then on, it's just about the column densities and the underlying luminosity distribution.
Okay so he made artificial spectral energy distributions populate a simulated a universe sampled it
at these survey select with the survey selection functions and then asked the question do these fit,
you know, have we understood the population? And the answer was more or less yes.
We were able to explain the distribution of source intensities, how many bright sources there are, how many medium, how many faint.
We were able to explain the redshift distribution. That means how many we should see as a function of redshift.
Um, let me yeah, let me try to explain why this is really significant because.
People have looked at the x ray surveys and seen that there were not many objects between Redshift one and of two,
and there were papers about how our understanding of again had to be incorrect because we should have seen these objects.
What it turns out is we did see them,
but they're optically so faint that no telescope on earth at the moment can measure their redshifts they're just too faint.
And when you correct for that effect, which our model was able to do,
and it was the only the first time anyone had made such a such a model linking the optical and x radiation.
We were able to show it precisely mimic the effect.
That is, the objects that are between Redshift one and redshift two that are obscured are simply too faint in the optical to get a redshift.
So they're not in this plot. You know, they're not in the plot of the redshift distribution.
So that was a nice discovery. Quantitatively, three quarters of all AGM are obscured by very thick column densities of gas and dust.
So they're really not visible in the optical at all. What else?
Don't say we have trouble saying too much about the the the biggest column densities in the last thing we did was to fit the X-ray background.
So remember I told you that the x ray background is really the sum of all the emission from all the.
And so we can just we could do that with our model. We just summed up all the emission and now I'm going to show you how that.
Yeah. So this is the same plot you saw before with the data at the top and then with the lines.
The red, blue and black lines are the components that that fit essentially.
Let me a first look at the grey line things grey line, that's the total model fit with no adjustable parameters.
Right?
We just add up all the action that explain the infrared data, the optical data in the x ray data, and they fit the x ray background pretty well.
What the coloured lines show is if I split it in two,
so the more obscured objects are in red and the less obscured or blue that shows you that
you'd need the obscured ones to make the the peaked shape and the Compton's thick ones.
Those are objects for for the experts. Those are objects whose common density is means an optical depth greater than one to Thomson's scattering
so they really absorb they it's hard to see x rays from that column densities that high in higher.
Okay. And a little more detail is just that they come from moderate luminosity.
That's what this decomposition is and low redshifts. And again, that's because the optical light is extinguished.
Okay. So let me let me touch on the two other topics.
I'm going to talk about triggering again with mergers.
It does not happen. What is the role?
So this is a cartoon that shows that the idea that two galaxies merging could trigger an AGM and cause a lot of accretion and also be detectable.
Remember when I showed you from the movie When Galaxies Merge, you can detect that they've merged because they have these weird shapes.
So we can tell when a merger has happened and we can look at what the content is and what one thing we can do is just image AGM.
These are quasars. These are the first quasars that were imaged with the Hubble Space Telescope.
Actually, these are before it's optics were corrected.
So these aren't the best images. And then here are some nicer images of galaxies that are have AGM in them.
And you see that mergers look common. But if you do a careful analysis, you find that really mergers are common only at the highest luminosity.
So this is a plot of the fraction of AGM that shows signs of having been in
a merger versus the luminosity logarithmic scale and and at low luminosity,
basically very few of them are in mergers and at high luminosity, most of them are in mergers are merger remnants.
So we think that mergers that this picture, the standard picture of mergers triggering AGM is true at the high luminosity end,
but not for most, not for most AGM or most galaxies.
Okay. Quasars are what we call high luminosity again.
And I told you that optical surveys only see the and obscured ones.
That's also true in soft X-rays. The hard X-ray survey goods that I mentioned are small,
and they don't survey quasars because the volume they sample the volume of the universe they sample is
so small that it's extremely unlikely you would find a rare high luminosity object in that volume.
So the goods and Cosmos and other surveys are look, small surveys are looking at lower luminosity again.
And the optical surveys, the big optical stories, they're looking at high luminosity again.
So we have a we have a matching up problem. So we need an x ray survey to get the quasars.
That means we need a large survey. And that's something that I and my former postdoc have been doing.
It's called the Strike 82 X Survey. I won't go into details, but it's basically trying to fill in this missing, you know,
trying to get the apples and oranges to look like grapefruits, I guess, and and be able to compare them.
And we're going to do a bunch of science with it. I won't. We haven't done it yet, though, so I will skip that.
But we have published the catalogues.
And if any of you are on the Exam Time Allocation Committee, this is a very important project that really needs time.
Thank you. I hope they're not watching the movie.
I don't know. Maybe they're watching. Okay. So last topic is this weird object we just found.
So this is kind of fun to end with.
One of the side projects we're doing with these survey multi weather surveys is to try to estimate black hole masses in some of these.
Again, how do we do that? We do it using in a similar way conceptually as the movie I showed you of stars orbiting the centre of our galaxy.
By watching the velocity of the stars, you could deduce the mass of the black hole.
Now, we don't have sufficient spatial resolution to do the exact same experiment, but we can.
We can look at the mean velocity of stars in a galaxy around around a black hole and and figure out the mass of the black hole specifically.
And this is aimed at the high school students we use.
Oh, sorry. I forgot to say we're using the Cosmos survey. This is the this is showing you an X-ray field of one of the bigger surveys,
but not it's about 20 times the size of goods, but about 1/50 the size of Stripe 82.
And I'm just zooming in to show you that these are thousands of X-ray sources and there are 4000 in all, and we are identifying all of them.
That's work that's been done by my postdoc, Francesca Charbonneau.
And out of these we've looked at about a dozen at Redshift, three and a half using mass fire, the Keck mass fire infrared instrument.
We've bene tracked and brought. Who is leading the project? He's from ET cetera.
And the idea is to use infrared spectroscopy to measure black hole mass.
And the way we do that is Newton's laws F equals A, we equate the gravitational attraction between, say,
a star and the black hole, except we're looking at ensembles of stars and we equate that to a mass of the Star Times.
It's a centripetal acceleration which is v squared over ah it's velocity square over R.
Did I already do that. No, there it is. And you can solve that.
The mass of a satellite cancels out so you can solve it for the mass little black
hole in terms of the velocity of stuff orbiting it and the location of that stuff,
you can get the first from the velocity dispersion in the spec, the width of the emission line essentially because it's Doppler broadened,
it's broadened by the velocity of the gas that's emitting the light.
And then you can get the size from some scaling relations, which I won't describe.
So here is a mass fire, a spectrum, by the way, mass fire is an amazing instrument.
Here's a mass fire spectrum of six nine, four seven.
What you're looking at is intensity of light versus wavelength and the spiky things you see, not the one with the plus,
which means it's a observational artefact, but the the ones in the middle, those are emission lines.
The two thinner lines on the right are oxygen, three double it and the very fat.
I'm going to show you the fit so you can see it.
The very broad, very broad feature is an alpha hydrogen line broadened by very high velocity gas emitting it.
And at the bottom, the bottom of the top plot, you can see dashed lines that show you the components going into that.
So there's two narrow Gaussian components for the oxygen, three lines, and then the broader component for the hydrogen line.
And we can in this plot that I showed first, we varied how to say this.
We did a mind Carlo's simulation to make sure that we bracketed sort of all of the allowed widths to try to make sure
we weren't overestimating the width that we looked at what was the narrowest it could be and still be acceptable.
It's actually still enormously broad. So when you plug that in to the simple formulae, a little bit more complicated than what you saw,
but more or less the same, you get a 7 billion mass solar mass black hole that's very high.
You know, it's more than. 2000 times the mass of the sun in our galaxy.
It's not the biggest ever seen. There are black holes this big seen in the Sloan survey, but it surveys a much, much larger volume.
And we didn't expect to see anything nearly this massive in the volume that we have surveyed.
So it is very surprising. There are also black holes this big in the local universe, but they've had 13 billion years to grow.
This one has only had about, you know, a billion, maybe 2 billion years to grow.
So it's kind of surprising in that way. And the other thing to say is, well, I'll get to that.
Okay. And then we also looked at the galaxy around this AGM part and it's under massive.
According to the rate, the typical ratio between a black hole mass and a galaxy mass locally is about 1 to 500.
And this thing is 1 to 10. So the galaxy is way under massive compared to what it would be in the normal case locally at that.
So the key point here is that Cosmos.
Cosmos is remember I said it's a little bit bigger than good, but it's still a small survey and that means it samples only common things.
You can only see common things. It can't see rare things. So huge black holes like this.
The implication is they ought to be common. And.
And the mismatch between galaxy and black hole mass should be common.
So those are weird things. And then. But it's only one object, so never extrapolate from one, especially for the students.
Never extrapolate from one. But. But we definitely have to go looking for more of these.
If there are more objects like this, then that means that the black hole in the galaxy.
Going back to my original theme, do not evolve in lockstep.
In this case, the black hole grew enormously first.
And then the galaxy takes 12 billion years to catch up to be this so that the portion that we see today is achieved.
So this is really intriguing. This is this kind of data will give us insight into how the black holes and the galaxies evolved together.
Maybe it's not, as the theorists think it is, but is tightly locked.
Okay. So let me recap where we are.
I get you know, I think I never turn the lights back on. I totally ruin your film for you.
Um, so the first part I told you about how we've surveyed black hole growth.
We've done a census of black hole growth that is much more complete than previous censuses,
and about three quarters of all black holes would have been missed in optical surveys of the same object.
We still have some work to do on the most obscured again.
And there's a satellite called NuStar, which is a NASA satellite that is sensitive to the highest energy,
much higher energy X-rays, and therefore can detect some of these objects that still are obscured from the lower energy x rays.
So stay tuned for more from NuStar. We've done some surveys and we're analysing them now.
The second chapter I told you about is about the role of mergers and triggering accretion.
I remind you this is a favourite theoretical construct for how star, how galaxies evolve and how they are triggered.
But it seems to be relevant to only a minority of galaxies.
So few galaxies go through major mergers. They don't all do this.
It's just some.
And we are doing a new survey to try to match up the luminosity and redshift sensitivity of our X-ray surveys to what has been done in the optical,
so that we can see right now that the two populations appear completely different.
The optically selected quasars evolve very early on and they peak at Redshift two, and they're very little is happening after that.
And the X-ray selected ones are peaking at Redshift one and below that is much more locally.
So is that a luminosity effect? Is that what is that?
Why are they different? And so on. And we'll figure that out with stability to X.
And lastly, we found this weird object CD 8947, which if it isn't a weirdo, if it's common,
then it's a sign that black holes and galaxies do not evolve synchronously.
They first the black hole got big, and then the galaxy caught up.
And is that always the case? Or in some cases, is the galaxy big first and then the black hole catches up?
Who knows? So let me end with my going back to this paradigm.
There it is. Which is the standard model of Black Hole Galaxy Coevolution.
The idea of a common trigger. That's okay in a minority of galaxies.
So probably that's out or half out. The enhanced star formation rate.
That's probably okay. You see stars that of galaxies that have been in a merger that have enhanced star formation rates.
Black hole accretion delayed from the stars. I don't think we've measured that at all.
So I gave that a graded that with a question mark because it's very hard to.
Yeah, you're always making a statistical question.
You can't actually watch a galaxy go through 100 million years of merger.
And then, I mean, some graduate students spend a long time on their thesis, but that's too long.
So we haven't really done that.
Whether the AGM feedback thing is happening, I think all the signs are that from what we've been able to see so far, not really.
It's not having it. We can't we haven't found symptoms of a direct effect on the galaxy.
We definitely have found signs that star formation has to turn off something, makes it stop,
and the guesses are that either it has to do with the halo mass being large enough that matter can't continue to accrete.
So you've cut off the supply of gas to the galaxy. I favour that explanation.
Or again, might might launch a wind that affects the galaxy instead of the radiation affecting the galaxy, which I think we can rule out.
So I give that kind of half a no. And then the stellar population ageing from blue to red.
Yes, that happens. But we're still back. We're still back trying to answer the basic question of why that happens.
And and I although I love again and have profited from studying them, I don't think they play a larger role as we once thought.
Thank you.