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Good afternoon, ladies and gentlemen. Can I bring you to order?

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Thank you. So welcome to this the 11th Hennessy lecture.

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A particular welcome to the first school or in the fourth row at the back there.

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Thank you for coming from Maidenhead. So it's my great pleasure to introduce the speaker today.

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Meg Urie is the Israel Moonstone Professor of physics and Astronomy at Yale.

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She's also the director of the Yale Centre for Astronomy and Astrophysics.

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And at the moment she is the 45th president of the American Astronomical Society.

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So here we have somebody of enormous eminence and distinction.

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Her research interests, your research interests that you also find in this department on accreting objects,

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black holes and active galactic nuclei in galaxies.

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Among the many distinctions, she's one the only jump canon award, the American Astronomical Society, and more recently, the Von Beaverbrook Prize.

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She's a fellow of the American Society of Arts and Sciences.

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She has, for the whole of her career, been a champion of women in science.

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And in fact, she was the first woman tenured at the Yale Physics Department.

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That was in the year 2000.

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She's also played a lot of major roles in astronomy policy in the United States for the National Research Council of the Academy,

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the American Physical Society, and, of course, NACI.

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It is my privilege to introduce Professor McGorry to give the 11th himself lecture Growing black holes over 12 billion years.

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Well, thank you for the lovely introduction, and thank you to Mr. Hennessy for endowing the lecture so I can actually be here.

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I've had a wonderful week visiting the department, talking to the students, the post-docs, the faculty.

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It's a really a very lively place, and I could probably spend several weeks here fully engaged at the same rate,

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but I don't know if I'd survive it physically, so I think I'll just go home instead anyway.

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Thank you. I'm really pleased to be talking to you about black holes.

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My research group studies when and where they grow across cosmic time and how they affect the galaxies that they live within.

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So that's what the topic is today. And let's go.

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So first I want to set the context of what the universe, what the history of the universe is in one slide.

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This is like, you know, the history of the British Empire in one microgram or something.

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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,

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which astronomers describe in terms of something called redshift,

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which increases from zero today to higher values in the past and the size of the universe.

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That Redshift Z is one over one plus Z times that size today.

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So, for example, when the cosmic microwave background radiation is seen,

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which is at a few hundred thousand years after the Big Bang, the universe was about a thousand times smaller in each dimension.

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So the universe starts with a big bang. There's a very sudden, very large inflation.

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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,

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in terms of the amount of expansion over the ballistic expansion when the universe is very small.

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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.

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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.

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So when the universe is small, gravity is a pretty good, pretty strong force.

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And as it gets bigger, the grip, the overall gravity, pulling it back together weakens.

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And other things like dark energy cause it, in fact, expansion to accelerate.

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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.

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Okay. So what I'm talking about today has to do with what happens after structures form

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gravitationally bound structures like stars or the galaxies that are bound,

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gravitationally bound collections of stars. We think so.

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After the Big Bang, the universe is very hot.

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As it expands, it cools the way an ideal gas cools when you expand it.

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There's a period when the universe is dark after the cosmic right, let's say.

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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.

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They don't scatter off the electrons.

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So that's why the CMB is visible at this particular roughly redshift of a thousand epic, because after that time,

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the photons can stream to us and we can see them coming directly from where they have been emitted.

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But there's a dark ages before stars form and sometime around a redshift of, I don't know, 2015 or something like that.

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The first stars form the first stars are unusual because they form out of hydrogen and helium and not much else.

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There are very few metals initially in the universe, metals meaning heavier elements than helium,

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so they form unusually large stars and those stars of all very quickly and they burn very brightly

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and then they explode and perhaps leave behind remnant black holes around which galaxies will form.

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You see, the stars have formed at the densest parts of the universe,

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and the rest of the 13 some billion year history of the universe is basically a story of the dense

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parts getting denser as matter is attracted to them and the vacant parts getting more vacant.

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So the contrast is being enhanced now.

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Yeah. So redshifts convenient for astronomers, but it doesn't mean much in terms of time.

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All of the time is between redshift zero and one or two, and.

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But most of the action is around redshift. One, two, two, two, three.

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That's when most of the stars are forming in galaxies and where most of the mass appears to be accreting in black holes.

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So that's an epic that we try to focus on a black hole.

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Growing means matter is going into it. As the matter approaches it, it heats up and it glows.

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And the amount of light emitted from its accretion process before particles get into

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the black hole is actually quite substantial and can outshine the galaxy itself.

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So when that's happening, when the accretion rate is sufficiently high, we call it an active galactic nucleus or AGM for short.

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So any time the accretion onto the black hole is high, that means it's growing rapidly.

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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.

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Okay. So the first thing to know is that there are lots of different kind of galaxies in the universe.

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Here's a little sampling of different shapes and sizes. They're all bound collections of stars.

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They have different ages, different stellar content. But one thing pretty much every galaxy has is a supermassive black hole at its core.

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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.

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And when we were kids, we didn't know black holes existed.

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We knew they existed in theory, but we hadn't ever found observational evidence that they exist in nature.

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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.

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But that's really since the Hubble Space Telescope and other observations that have happened over the last 15 to 20 years.

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So the best black hole is in our own galaxy. That is the best measured one.

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And. I think this movie is the next thing.

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Yeah. So our our galaxy, the Milky Way galaxy is a collection of about 100 billion stars.

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There's a black hole at the centre that weighs about three or 4 million times the mass of our sun.

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It's actually a very unremarkable black hole. It's modest in size.

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It's modest in activity. It is not in again, but it's nearby.

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So we can actually study it exquisitely.

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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.

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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.

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It's the centripetal force of gravity that allows them to have these orbits.

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I'm going to make this guy go again. Hang on. When it stops.

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Let's just do it this way. The cheap way. Okay. So now when you watch the stars,

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notice that the ones that are close to the centre are moving rapidly because they feel the strongest

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force because they're closest and the ones that are far away are actually moving quite slowly.

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And if you are studying Kepler's laws, as I believe some of our audience is.

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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.

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When you have the combination of orbits, you have incredible constraints on everything,

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not just the mass, but also how far it is to the centre of our galaxy and many other parameters.

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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.

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And when you put all that together, the least exotic explanation of this, of these orbits is a black hole.

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Anything else is beyond weird. So we're pretty sure there's a black hole at the centre of our galaxy.

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Okay. Now, when I was a youngster, I started out working on it again.

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Nobody else did. It was kind of passé. Now it's exciting again because it has something to do with galaxies.

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So, in fact,

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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.

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And I'll be talking more about that. These are the four arguments for why we think again are important to galaxy evolution.

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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.

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They also happen contemporaneously.

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That is the peak of star formation in cosmic history coincides roughly with the peak of black hole growth in cosmic history.

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And this is this is a plot that shows that as a function of redshift, you see in the data points,

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the star formation rate density, how much, how many stars are forming per unit mass.

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And then in the grey curve that well, that's the error on the black curve.

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That's a rough estimate of how much mass is being accreted on to black holes at the same time.

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So you see that they both have a history. Remember, the left is redshift, zero is today.

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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.

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So over that period is when they both peak or appear to peak.

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Well, that could be coincidence. It doesn't have to happen in individual objects.

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So a better argument perhaps is this amazing correlation called the Sigma relation, which was reported about in 2000.

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And that is the following that the do I show this plot? I can't remember if I have it on here or not.

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Nope. Okay. I'm just going to tell you about it. So I didn't want to outdo overdo the plots.

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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.

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It's actually the velocity dispersion, but it's proportional to mass.

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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.

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So okay, they should be correlated. But the black hole, although very, very massive, is actually much smaller in mass than the whole galaxy.

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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.

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So black hole is tiny compared to the stars.

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And as soon as you go any distance away from it, that encloses more mass in stars than in black.

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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.

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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

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mass goes into the black hole and how much new stars are forming in the galaxy.

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And that's so that's where the really the big hint that we need it for galaxy evolution.

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And then the last reason is, while theorists need black holes, it's so nice to be needed.

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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.

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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.

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And we know how much time there is until today.

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And in computers, we can run the universe from those initial conditions forward and form galaxies.

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That has helped us understand that there is a lot more dark matter than than baryonic matter in the universe and other other insights.

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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.

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The small galaxies are a problem for another day. But the big galaxies, the idea is that because, again, inject energy into the galaxy,

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they can heat the interstellar medium or give it energy in some way that prevents new stars from forming.

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So you're sort of artificially restricting the number of stars that form, and that makes the galaxy appear smaller, less, less luminous.

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There's a beautiful movie of this made by the team that did the illustrious.

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This is a computer simulation. This is what I should turn the lights out for.

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Okay, don't walk around because you fall down. But here is the one.

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Yeah. You really have to get a good look at this one.

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So what you're looking at is the dark matter distribution in the lower left is the time since the big bang.

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So right now it's at 1.5 billion years since the Big Bang.

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That's probably hard for you to read.

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And you're looking at the blue is just where is most of the dark matter and that is where the baryonic matter goes.

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Also, it follows the same, essentially the same.

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It responds to the gravity of the dark matter. Let's say it that way. In a moment, we're going to change colour.

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And now what you're looking at is temperature. Okay.

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In the same simulation, just looking at the temperature of the gas,

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that means the atoms of material that are making up these filaments restructures, which is where galaxies form.

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And now I guess you can see explosions. Those are pretty clear.

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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.

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There's also supernovae in this in this simulation.

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So some of the energy injection comes from supernovae, which are the end points of stellar evolution.

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Evolution of some stars. And that energy makes the galaxies we live in today different from what they would otherwise look like.

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Oh, yeah. Let me just tell you, the last colour is the last colour is metallicity.

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So as supernovae go off, the elements that have been forged in the interiors of stars are distributed throughout space.

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And this must happen very quickly, because we see in very distant objects some of the earliest AGM that can be detected.

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We see lots of signs of of heavy elements like carbon or other nitrogen, oxygen, etc.

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And so the purple is showing the metallicity of the gas as a function of time.

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And right now, just to keep you current, we are at 12 billion years after the big bang.

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And today's age of the universe is 13.7 billion years after the big bang.

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Okay. And there are the credits.

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So I don't I don't think it was obvious from that computer simulation, but after galaxies form, they frequently collide and merge.

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Collide is maybe the wrong term because galaxies are mostly empty space.

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They're made up of stars. But the the distance between stars is much bigger than the size of the stars.

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So you can think of them as sort of like tiny grains of sand, each of which are miles apart.

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One way to think about this is the sun is very close to us.

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It's a big, bright star and then the next brightest stars.

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You know, those things at night that you look at that you can see maybe at twilight.

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So there's a big space between stars. So when two galaxies collide, they literally pass through each other.

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The stars don't ever hit each other. So it's not a collision like your car hitting another car.

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But but they, of course, are exerting gravity on one another and they undergo what we call well,

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these are different snapshots of interactions between different pairs of galaxies and the whole collision of a galaxy,

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which, again, we can simulate in computers, will take something like 200 million years.

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So we only see what they look like today.

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So we'll see a particular pair of galaxies at one stage of its evolution of a merger.

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We don't ever get to see the whole 200 million years for a given pair of galaxies.

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But what we can do is simulate mergers of two galaxies.

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And what I'm showing here is what we call a major merger of two disk, like gas rich galaxies going in a computer.

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And then every so often the simulation is going to pause and show you an actual pair of galaxies that looks like the simulation.

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So, again, let me make this dark. Cause it's really pretty.

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As the galaxies merge, they they pull on each other.

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And we call these tidal forces because the the near side of one galaxy pulls

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hardest on the near side of the other galaxy and less hard on the far side.

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So they stretch. Each galaxy stretches the other and you get these amazing tidal tails,

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which are stars that have basically been ripped out of the galaxy in a big tidal stream.

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And we actually see this frequently in the images of galaxies.

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If the two galaxies are sufficiently massive and and or not moving too fast, they will actually end up merging, as this simulation does.

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And in fact, these were equal mass galaxies with slightly different angular momentum.

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I think there's one more of these guys. Okay.

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So so we can do that in the computer and we know that happens in nature.

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And we can compare our observations, too, to what we see in these snapshots.

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So all of those movies are kind of telling you this story, this bedtime story about how galaxies evolve.

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The idea is that that mergers play a significant role.

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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.

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But you have some event happen, like a major merger, and that stimulates stars forming,

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which happens almost right away because there are shocks when the galaxies pass the gas and the galaxies collide and then you

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get an enhanced star formation rate and some of the material because you now have a very different gravitational potential,

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it helps funnel material down toward the black hole. Now.

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How many of you have had a physics mechanics class?

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It should be most of you. Of course, there's a lot of physicists in here.

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Okay. Thank you. Yeah. Okay. So the reason it takes the black hole a long time to start accreting is because of angular momentum.

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You have to get a proton from a scale of many orders of magnitude, larger size down into the size of a black hole,

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which is, you know, maybe a typical 10 to 100 million solar mass black hole might be ten light seconds across.

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And you're trying to get gas in from light years out in the galaxy.

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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.

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But once it turns on, it should supply energy to the interstellar medium in the galaxy.

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That is the cold gas out of which stars would otherwise form.

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It can heat that gas, turn off star formation. And now this needs a little explanation.

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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.

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They evolve faster. So you start with some distribution of stars, and as if you stop forming new stars,

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the bluest ones go away first and the reddest ones are left behind.

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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.

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And yeah, so we kind of see that, but there's some stuff we don't see.

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So let me give you a roadmap for where we're going. I'm going to tell you about three topics.

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One, how do we know when and where black holes grew, and in particular,

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how multi wavelength surveys have shown us that much of black hole growth is actually shrouded from view at optical wavelengths.

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So many of our earliest surveys of Aegean and and luminous Aegean were very incomplete.

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Second, I'm just going to review what we know.

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I've showed you that mergers are thought to be important, and I'll review what we actually know about mergers.

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Very briefly. And then I'll talk about a really cool recent result we have on a particular Aegean called Side 947.

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We don't we should have really good names like, you know, the cool one, but we don't.

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So it's the ID 947 to tell you what we learned about it. Okay, so first the multi wavelength surveys.

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Before moving to Yale, I worked for the Space Telescope Science Institute, which runs the Hubble Space Telescope for NASA.

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And just before I left there, I designed a survey called Good's, which was supposed to which did, in fact,

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answer this question of how much black hole growth is obscured as opposed to sort of freely visible in the optical.

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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.

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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,

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matter trying to fall in the black hole will settle into a disk because the vertical motions can be cancelled out,

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but the angular momentum is maintained in the material and so it slowly moves in.

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A particle will move in as another particle moves out because you have to conserve angular momentum.

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And so for stuff to get into the black hole, you have to have some other stuff going out.

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So that all happens in an accretion disk.

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We see emission lines from clouds of gas that are photo ionised by the hot UV and X-ray radiation from the accretion disk.

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And then the key point of this picture is the big orange doughnut,

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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.

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So if you're looking from the side you see a very faint again, not U.V. or soft X-ray bright.

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So if you're looking from the side, but if you look from the poles,

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you'll see the hot accretion disk and the broad emission lines broad because the gas is moving fast near the black hole.

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And for this purposes of this talk, you can forget the jets that are staring at you.

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They're not. Yeah, I'm not talking about them in this particular talk.

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So this is the picture of what we think, again, look like locally.

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We're pretty sure that's correct locally.

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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.

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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.

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So it's more likely that in the past there was even more obscuration.

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And indeed, we eventually proved that to be true. And finally, let me be a little bit I don't want to say parochial.

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I was trained as an X-ray astronomer, meaning A focusing on the X radiation coming from stars and galaxies and so on.

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And black holes primarily are rather than optical light and X-ray.

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Astronomers have actually known for 30 years that most black hole growth is obscured.

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But since most astronomers are optical astronomers, it wasn't a fully known thing.

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So let me say it that way. That's kind of rude, but you're filming me, so.

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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

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the very first rockets that flew above the atmosphere so they could detect cosmic x rays.

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But fortunately, x rays do not penetrate through our atmosphere because we would be we would have shorter lives if they did.

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We evolve faster if they did. But but you have to go above the atmosphere.

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So you have to have a rocket or you have to have a satellite to see x rays.

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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,

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which they called the X-ray background because it was background to the sources they actually detected.

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However,

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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.

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And so so we can measure what their energy distribution is.

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In other words, x rays can be admitted at a range of energies.

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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.

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So let me explain that. First, I'm showing you the X-ray background.

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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.

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So that's a broad x ray band. And you see that it's peaked.

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That's all I want you to see here. That's a curved spectrum.

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The kind of AGM that optical surveys see the UN obscured cases would be a horizontal line in this plot.

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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.

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And the reason for that is something called the photoelectric effect,

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which is just an atom can absorb an X-ray and kick out an electron and that energy disappears.

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So if you have atoms around that can absorb X-rays, you absorb the soft X-ray radiation.

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And what you see here is an input spectrum with no absorption and then increasing column densities in AD.

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So, so sorry. The left hand line is ten to the 20 atoms per centimetre squared of column density.

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That's more or less the column density through our galaxy that we have to look through to get,

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you know, to look at things that are outside our galaxy. So that's sort of the minimum we tend to see.

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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.

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So you can see how if you have a bunch of things that are very obscured, you can get a peaked spectrum.

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And indeed, that's how we form this this X-ray spectrum.

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It's from absorb mainly absorbed AGM.

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Okay. So I'm not going to show a lot of the modelling we did, but let me just say that goods,

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the survey that I designed when I was at Space Telescope was one of the first was probably the

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first of these deep multi wavelength surveys with x rays that could find again much more fuzzy.

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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,

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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.

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So when you get an x ray from a doctor, you know you can see your skeleton.

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So the x rays can get through the stuff. That's why they're good.

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Some of them are soft x rays and especially the UV, the ultraviolet light is absorbed and it gets re radiated by the dust.

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So we see that in the infrared and then we could use Hubble to separate the

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emission at the very centre of a galaxy from the nucleus that is from this,

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this again part in the centre, from the starlight in the rest of the galaxy.

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And I see my former graduate student, Brooks Simmons, whose thesis was on that topic.

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So good, this is okay, now we have to be dark again.

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This slide shows the goods survey. Goods stands for Great Observatories Origins Deep Survey Great Observatories refers to

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what NASA it's what NASA calls its three Hubble Space Telescope Optical Observatory,

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the Spitzer Space Telescope, Infrared Observatory and the Chandra X-ray Observatory.

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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,

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because we were looking at the origins of galaxies and of course,

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but also because if we didn't have a letter, we were afraid we wouldn't get our telescope time.

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So you're looking at an image which is a two dimensional projection of a three dimensional picture.

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Almost everything in this picture is a galaxy, and we can figure out the distance to many of these galaxies.

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Really, most of them. Let me show you. Did I skip the movie?

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I did skip the movie. Okay. Let me show you what we found.

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So we have this is now a tiny portion.

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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.

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What you're looking at is the Hubble image. So optical light at different wavelengths, so added together for sort of a realistic colour.

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And then the three circles show you where the X-ray sources are in this particular part of the field.

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But you also see that two of them are yellow and one is white.

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The yellow circles are X-ray sources that have a lot of soft photons missing.

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So the yellow ones are probably absorbed, and the white one is the one where it appears unobserved.

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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?

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Because the yellow circles, the optical image is very, very faint. In fact, in the bottom one, it's undetected.

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And in the white circle, the absorbed and obscured X-ray source has a pretty bright optical image.

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So the this is showing you exactly what I was saying is that the optical surveys find the on obscured objects easily,

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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.

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This is from the Spitzer telescope. And you see, the first thing you see maybe is that there are a lot more sources.

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That's because stars emit a lot of infrared light. And so all of the galaxies appear as infrared sources.

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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,

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meaning they're X-ray weak, not because they're intrinsically weak, but because they're absorbed.

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And that energy is re radiated in the infrared and they're bright infrared sources.

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So this was the crux of our experiment. We did. Obviously, we did a very quantitative analysis.

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This is the Ph.D. thesis of my student, Ezekiel Thruster.

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And he took I'm just showing you one cartoon to kind of illustrate.

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He made a model of what a gene should look like that matched local objects that we had studied very well.

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And that model is basically a clumpy Taurus of gas clouds and dust that the model specifies how those clouds radiate in the infrared.

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It also specifies what the column density is in any direction.

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So he fixes the model to fit local again.

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It predicts a distribution of column densities for the whole population.

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And then from then on, it's just about the column densities and the underlying luminosity distribution.

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Okay so he made artificial spectral energy distributions populate a simulated a universe sampled it

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at these survey select with the survey selection functions and then asked the question do these fit,

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you know, have we understood the population? And the answer was more or less yes.

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We were able to explain the distribution of source intensities, how many bright sources there are, how many medium, how many faint.

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We were able to explain the redshift distribution. That means how many we should see as a function of redshift.

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Um, let me yeah, let me try to explain why this is really significant because.

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People have looked at the x ray surveys and seen that there were not many objects between Redshift one and of two,

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and there were papers about how our understanding of again had to be incorrect because we should have seen these objects.

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What it turns out is we did see them,

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but they're optically so faint that no telescope on earth at the moment can measure their redshifts they're just too faint.

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And when you correct for that effect, which our model was able to do,

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and it was the only the first time anyone had made such a such a model linking the optical and x radiation.

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We were able to show it precisely mimic the effect.

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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.

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So they're not in this plot. You know, they're not in the plot of the redshift distribution.

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So that was a nice discovery. Quantitatively, three quarters of all AGM are obscured by very thick column densities of gas and dust.

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So they're really not visible in the optical at all. What else?

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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.

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So remember I told you that the x ray background is really the sum of all the emission from all the.

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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.

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Yeah. So this is the same plot you saw before with the data at the top and then with the lines.

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The red, blue and black lines are the components that that fit essentially.

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Let me a first look at the grey line things grey line, that's the total model fit with no adjustable parameters.

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Right?

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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.

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What the coloured lines show is if I split it in two,

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so the more obscured objects are in red and the less obscured or blue that shows you that

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you'd need the obscured ones to make the the peaked shape and the Compton's thick ones.

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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

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so they really absorb they it's hard to see x rays from that column densities that high in higher.

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Okay. And a little more detail is just that they come from moderate luminosity.

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That's what this decomposition is and low redshifts. And again, that's because the optical light is extinguished.

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Okay. So let me let me touch on the two other topics.

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I'm going to talk about triggering again with mergers.

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It does not happen. What is the role?

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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.

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Remember when I showed you from the movie When Galaxies Merge, you can detect that they've merged because they have these weird shapes.

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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.

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These are quasars. These are the first quasars that were imaged with the Hubble Space Telescope.

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Actually, these are before it's optics were corrected.

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So these aren't the best images. And then here are some nicer images of galaxies that are have AGM in them.

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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.

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So this is a plot of the fraction of AGM that shows signs of having been in

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a merger versus the luminosity logarithmic scale and and at low luminosity,

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basically very few of them are in mergers and at high luminosity, most of them are in mergers are merger remnants.

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So we think that mergers that this picture, the standard picture of mergers triggering AGM is true at the high luminosity end,

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but not for most, not for most AGM or most galaxies.

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Okay. Quasars are what we call high luminosity again.

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And I told you that optical surveys only see the and obscured ones.

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That's also true in soft X-rays. The hard X-ray survey goods that I mentioned are small,

386
00:41:59,110 --> 00:42:06,069
and they don't survey quasars because the volume they sample the volume of the universe they sample is

387
00:42:06,070 --> 00:42:12,430
so small that it's extremely unlikely you would find a rare high luminosity object in that volume.

388
00:42:13,150 --> 00:42:21,400
So the goods and Cosmos and other surveys are look, small surveys are looking at lower luminosity again.

389
00:42:21,940 --> 00:42:26,139
And the optical surveys, the big optical stories, they're looking at high luminosity again.

390
00:42:26,140 --> 00:42:35,300
So we have a we have a matching up problem. So we need an x ray survey to get the quasars.

391
00:42:35,690 --> 00:42:40,280
That means we need a large survey. And that's something that I and my former postdoc have been doing.

392
00:42:40,640 --> 00:42:48,980
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,

393
00:42:48,980 --> 00:42:53,810
trying to get the apples and oranges to look like grapefruits, I guess, and and be able to compare them.

394
00:42:54,800 --> 00:42:59,750
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.

395
00:42:59,750 --> 00:43:01,190
But we have published the catalogues.

396
00:43:01,190 --> 00:43:09,410
And if any of you are on the Exam Time Allocation Committee, this is a very important project that really needs time.

397
00:43:09,440 --> 00:43:13,220
Thank you. I hope they're not watching the movie.

398
00:43:14,540 --> 00:43:20,570
I don't know. Maybe they're watching. Okay. So last topic is this weird object we just found.

399
00:43:20,580 --> 00:43:21,950
So this is kind of fun to end with.

400
00:43:22,820 --> 00:43:32,270
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.

401
00:43:32,450 --> 00:43:44,990
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.

402
00:43:45,800 --> 00:43:49,610
By watching the velocity of the stars, you could deduce the mass of the black hole.

403
00:43:50,030 --> 00:43:56,570
Now, we don't have sufficient spatial resolution to do the exact same experiment, but we can.

404
00:43:57,080 --> 00:44:08,090
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.

405
00:44:08,090 --> 00:44:12,930
And this is aimed at the high school students we use.

406
00:44:12,950 --> 00:44:22,790
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,

407
00:44:22,790 --> 00:44:28,670
but not it's about 20 times the size of goods, but about 1/50 the size of Stripe 82.

408
00:44:29,330 --> 00:44:38,239
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.

409
00:44:38,240 --> 00:44:40,970
That's work that's been done by my postdoc, Francesca Charbonneau.

410
00:44:42,500 --> 00:44:51,350
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.

411
00:44:52,400 --> 00:44:57,020
We've bene tracked and brought. Who is leading the project? He's from ET cetera.

412
00:44:57,350 --> 00:45:02,690
And the idea is to use infrared spectroscopy to measure black hole mass.

413
00:45:02,690 --> 00:45:10,700
And the way we do that is Newton's laws F equals A, we equate the gravitational attraction between, say,

414
00:45:10,910 --> 00:45:19,459
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.

415
00:45:19,460 --> 00:45:24,170
It's a centripetal acceleration which is v squared over ah it's velocity square over R.

416
00:45:25,670 --> 00:45:29,840
Did I already do that. No, there it is. And you can solve that.

417
00:45:30,080 --> 00:45:33,920
The mass of a satellite cancels out so you can solve it for the mass little black

418
00:45:33,920 --> 00:45:38,750
hole in terms of the velocity of stuff orbiting it and the location of that stuff,

419
00:45:39,470 --> 00:45:47,180
you can get the first from the velocity dispersion in the spec, the width of the emission line essentially because it's Doppler broadened,

420
00:45:47,180 --> 00:45:49,850
it's broadened by the velocity of the gas that's emitting the light.

421
00:45:50,330 --> 00:45:53,960
And then you can get the size from some scaling relations, which I won't describe.

422
00:45:54,890 --> 00:45:58,520
So here is a mass fire, a spectrum, by the way, mass fire is an amazing instrument.

423
00:45:59,120 --> 00:46:03,020
Here's a mass fire spectrum of six nine, four seven.

424
00:46:03,440 --> 00:46:10,309
What you're looking at is intensity of light versus wavelength and the spiky things you see, not the one with the plus,

425
00:46:10,310 --> 00:46:15,560
which means it's a observational artefact, but the the ones in the middle, those are emission lines.

426
00:46:16,340 --> 00:46:21,829
The two thinner lines on the right are oxygen, three double it and the very fat.

427
00:46:21,830 --> 00:46:23,690
I'm going to show you the fit so you can see it.

428
00:46:24,200 --> 00:46:36,680
The very broad, very broad feature is an alpha hydrogen line broadened by very high velocity gas emitting it.

429
00:46:37,100 --> 00:46:44,839
And at the bottom, the bottom of the top plot, you can see dashed lines that show you the components going into that.

430
00:46:44,840 --> 00:46:50,930
So there's two narrow Gaussian components for the oxygen, three lines, and then the broader component for the hydrogen line.

431
00:46:52,220 --> 00:46:57,770
And we can in this plot that I showed first, we varied how to say this.

432
00:46:58,130 --> 00:47:05,420
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

433
00:47:05,420 --> 00:47:10,790
we weren't overestimating the width that we looked at what was the narrowest it could be and still be acceptable.

434
00:47:10,790 --> 00:47:19,759
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,

435
00:47:19,760 --> 00:47:26,720
but more or less the same, you get a 7 billion mass solar mass black hole that's very high.

436
00:47:27,830 --> 00:47:32,130
You know, it's more than. 2000 times the mass of the sun in our galaxy.

437
00:47:34,220 --> 00:47:43,940
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.

438
00:47:44,450 --> 00:47:50,450
And we didn't expect to see anything nearly this massive in the volume that we have surveyed.

439
00:47:50,900 --> 00:47:58,400
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.

440
00:47:58,610 --> 00:48:03,590
This one has only had about, you know, a billion, maybe 2 billion years to grow.

441
00:48:04,070 --> 00:48:11,390
So it's kind of surprising in that way. And the other thing to say is, well, I'll get to that.

442
00:48:11,420 --> 00:48:19,370
Okay. And then we also looked at the galaxy around this AGM part and it's under massive.

443
00:48:20,560 --> 00:48:27,790
According to the rate, the typical ratio between a black hole mass and a galaxy mass locally is about 1 to 500.

444
00:48:28,180 --> 00:48:38,830
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.

445
00:48:39,250 --> 00:48:42,490
So the key point here is that Cosmos.

446
00:48:43,330 --> 00:48:50,830
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.

447
00:48:51,610 --> 00:48:56,170
You can only see common things. It can't see rare things. So huge black holes like this.

448
00:48:56,770 --> 00:49:00,280
The implication is they ought to be common. And.

449
00:49:00,280 --> 00:49:04,510
And the mismatch between galaxy and black hole mass should be common.

450
00:49:05,540 --> 00:49:16,140
So those are weird things. And then. But it's only one object, so never extrapolate from one, especially for the students.

451
00:49:16,150 --> 00:49:20,740
Never extrapolate from one. But. But we definitely have to go looking for more of these.

452
00:49:20,980 --> 00:49:25,750
If there are more objects like this, then that means that the black hole in the galaxy.

453
00:49:25,780 --> 00:49:30,310
Going back to my original theme, do not evolve in lockstep.

454
00:49:31,750 --> 00:49:35,200
In this case, the black hole grew enormously first.

455
00:49:35,740 --> 00:49:43,030
And then the galaxy takes 12 billion years to catch up to be this so that the portion that we see today is achieved.

456
00:49:43,510 --> 00:49:52,239
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.

457
00:49:52,240 --> 00:49:56,950
Maybe it's not, as the theorists think it is, but is tightly locked.

458
00:49:58,630 --> 00:50:01,870
Okay. So let me recap where we are.

459
00:50:02,380 --> 00:50:07,360
I get you know, I think I never turn the lights back on. I totally ruin your film for you.

460
00:50:07,810 --> 00:50:13,360
Um, so the first part I told you about how we've surveyed black hole growth.

461
00:50:13,360 --> 00:50:18,400
We've done a census of black hole growth that is much more complete than previous censuses,

462
00:50:18,910 --> 00:50:25,030
and about three quarters of all black holes would have been missed in optical surveys of the same object.

463
00:50:26,020 --> 00:50:29,290
We still have some work to do on the most obscured again.

464
00:50:29,590 --> 00:50:35,530
And there's a satellite called NuStar, which is a NASA satellite that is sensitive to the highest energy,

465
00:50:35,830 --> 00:50:44,140
much higher energy X-rays, and therefore can detect some of these objects that still are obscured from the lower energy x rays.

466
00:50:45,480 --> 00:50:49,560
So stay tuned for more from NuStar. We've done some surveys and we're analysing them now.

467
00:50:50,760 --> 00:50:55,740
The second chapter I told you about is about the role of mergers and triggering accretion.

468
00:50:56,070 --> 00:51:05,010
I remind you this is a favourite theoretical construct for how star, how galaxies evolve and how they are triggered.

469
00:51:05,340 --> 00:51:09,660
But it seems to be relevant to only a minority of galaxies.

470
00:51:09,990 --> 00:51:16,200
So few galaxies go through major mergers. They don't all do this.

471
00:51:16,230 --> 00:51:17,130
It's just some.

472
00:51:19,130 --> 00:51:29,300
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,

473
00:51:29,300 --> 00:51:34,640
so that we can see right now that the two populations appear completely different.

474
00:51:35,360 --> 00:51:43,670
The optically selected quasars evolve very early on and they peak at Redshift two, and they're very little is happening after that.

475
00:51:44,090 --> 00:51:49,159
And the X-ray selected ones are peaking at Redshift one and below that is much more locally.

476
00:51:49,160 --> 00:51:53,360
So is that a luminosity effect? Is that what is that?

477
00:51:53,360 --> 00:51:56,990
Why are they different? And so on. And we'll figure that out with stability to X.

478
00:51:58,010 --> 00:52:04,880
And lastly, we found this weird object CD 8947, which if it isn't a weirdo, if it's common,

479
00:52:05,330 --> 00:52:10,340
then it's a sign that black holes and galaxies do not evolve synchronously.

480
00:52:10,700 --> 00:52:14,810
They first the black hole got big, and then the galaxy caught up.

481
00:52:15,200 --> 00:52:22,370
And is that always the case? Or in some cases, is the galaxy big first and then the black hole catches up?

482
00:52:22,370 --> 00:52:27,170
Who knows? So let me end with my going back to this paradigm.

483
00:52:27,860 --> 00:52:34,070
There it is. Which is the standard model of Black Hole Galaxy Coevolution.

484
00:52:35,060 --> 00:52:40,129
The idea of a common trigger. That's okay in a minority of galaxies.

485
00:52:40,130 --> 00:52:45,990
So probably that's out or half out. The enhanced star formation rate.

486
00:52:45,990 --> 00:52:51,810
That's probably okay. You see stars that of galaxies that have been in a merger that have enhanced star formation rates.

487
00:52:52,590 --> 00:52:57,899
Black hole accretion delayed from the stars. I don't think we've measured that at all.

488
00:52:57,900 --> 00:53:02,420
So I gave that a graded that with a question mark because it's very hard to.

489
00:53:03,510 --> 00:53:06,629
Yeah, you're always making a statistical question.

490
00:53:06,630 --> 00:53:11,520
You can't actually watch a galaxy go through 100 million years of merger.

491
00:53:11,670 --> 00:53:16,350
And then, I mean, some graduate students spend a long time on their thesis, but that's too long.

492
00:53:18,720 --> 00:53:19,860
So we haven't really done that.

493
00:53:21,210 --> 00:53:28,530
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.

494
00:53:28,860 --> 00:53:33,600
It's not having it. We can't we haven't found symptoms of a direct effect on the galaxy.

495
00:53:34,590 --> 00:53:39,900
We definitely have found signs that star formation has to turn off something, makes it stop,

496
00:53:40,170 --> 00:53:46,709
and the guesses are that either it has to do with the halo mass being large enough that matter can't continue to accrete.

497
00:53:46,710 --> 00:53:50,550
So you've cut off the supply of gas to the galaxy. I favour that explanation.

498
00:53:51,000 --> 00:54:01,830
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.

499
00:54:02,100 --> 00:54:06,780
So I give that kind of half a no. And then the stellar population ageing from blue to red.

500
00:54:06,780 --> 00:54:12,479
Yes, that happens. But we're still back. We're still back trying to answer the basic question of why that happens.

501
00:54:12,480 --> 00:54:21,600
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.

502
00:54:21,960 --> 00:54:22,350
Thank you.