So it's my very great pleasure to introduce Professor James Binney to give us the 10th definition Memorial Lecture Lecture.
He is sponsored by the Department of Physics and by also his college.
And so, James, I know James for a very long time since I was a graduate student.
In fact, when Dennis Sharman proudly introduced me to the rare visits, then to Oxford.
And meeting James, even at that young age, it was clear he had a brilliant future in front of him.
And it's quite remarkable to look at the many fields that he has worked in.
These include everything from stars to galaxies to galaxy clusters.
Cosmology. So he's made a major imprint in astrophysics, areas of physics, galaxy dynamics.
But more than that, his books textbooks have become legion in the field.
I think books like Binney and Tremayne are known to just about every astrophysics graduate student throughout the world.
I would say has, as you know, the the handbook for commencing their research and to have, you know,
combined both writing these major books along with a brilliant research career, I think is a wonderful achievement.
So James has, after his Ph.D. in Oxford, he spent a postdoctoral stint at Princeton and then was on the faculty for a couple of years.
And then in 1981, he moved to Oxford as a university lecturer and has been here ever since and has been professor of physics since 1996.
And ever since he's been Oxford has been a fellow of American college.
So his his various honours include being a fellow of the Royal Society.
So before and also the Maxwell Prize and being true to physics and other prizes from the American Stroke Society.
So before I introduce James, let me just mention that he was a student of Dennis Sharma, this lecture series,
this is the 10th lecture, annual lecture, and the series is in honour of Dennis's brilliant success in generating.
Students have gone on to really wonderful things in the world of astrophysics and in related disciplines and in other areas too.
And so it's a very great pleasure that I introduce James to tell us about galaxies and the intergalactic medium.
Thank you, Joe. So it's just about 42 years since I came to work with Dennis as his part of this second generation of graduate students.
And just in the three years that I was in Dennis's hut, as it was called,
it was a prefabricated building provided by the Radcliffe Trust that has since been knocked down and replaced by some part of physiology.
There were five people I mean, there were six people in that hut who went on to have already gone gone on to be fellows of the Royal Society.
And that, I think, is stupid. He had an amazing record as a research supervisor in Cambridge Martin Rees,
Stephen Hawking and so on were his students and this is beginning of his Oxford collection.
What made Dennis such an amazing the many factors obviously made Dennis an amazing supervisor, director of research.
He had an infectious enthusiasm, a very wide knowledge of physics,
a love of everything to do with physics, an enormously generous and outgoing and stuffy character.
So a talent for friendship and tremendous loyalty to the people who'd worked with him.
But he also had, I think, good taste in his choice of problems.
And he was interested in quantum gravity long before anybody much was interested, or very few people were interested in quantum gravity.
And the topic of my talk today, I felt I had to talk about this topic because it was something that Dennis was working on.
As this nature paper indicates, when I when I came to join his group in 1972, so this was with Roland Hunt,
who had been his research student in Cambridge and was a post-doc in Oxford at that time.
So here's a sort of outline of what I want to talk about. I want to know about the cosmic baryon budget and star formation,
what we know about something that's a little bit of what we know about star formation,
something about the phenomenon of extra plane, ah1 that's neutral hydrogen, that doesn't lie in the equatorial plains of just galaxies like our own.
And about a fountain model that can explain that too, has been built to explain that and does so well,
and then talk about the role that the exchange of gas between galaxies,
the disk, the star forming disks of galaxies like our own in the intergalactic medium plays in the big picture of galaxy evolution and cosmology.
Of course, in science. What?
What we achieve, what's done, what's worthwhile is largely it's very largely done by one's collaborators, is always done with collaborators.
And what I'm going to talk about today is a programme of work which started when Philip Alfreton early was came to Oxford as a marie Curie fellow,
and we've worked together on this programme.
Well, we, I say, but largely Filippo has with his research students in Bologna because he's returned to Bologna and is on the faculty there.
But I'm also going to mention so influential in this has been work that Colin Potty who's also on the faculty in Bologna has done with me and to.
And towards the end I'll mentioned stuff ideas which emerged out of work that I did with two students,
Gavin de Boer nearly 20 years ago and Henri Goma about ten years ago.
So the Baryon budget well from the cosmic microwave background analysis of the fluctuations on the sky in the microwave background,
we know what the contribution is of baryons to the to the overall mass budget,
the overall energy budget of the universe and what the contribution from dark matter is.
So in, in cosmology people use this symbol. Omega was introduced, I think by the Russians to mean the ratio of the density contributed.
The average density contributed by some component to the closure density of the universe,
the critical density that would, in the absence of cosmological constant lead to the expansion drifting to a halt.
So an edge here is the Hubble constant in units of 100 kilometres a second per megaparsec.
So h squared is essentially a half, just call it a half. So the point is that Baryons contribute maybe four or 5% of the closure energy density of
the universe and dark matter contributes maybe 22% of the closure density of the universe,
something like that. In other words, there are about five times as much of dark matter is of light matter from Galaxy Redshift surveys,
particularly the Sloan Digital Sky Survey.
The study says we know what the how much luminosity in some band the band is emitted on average per unit of matter density.
The answer is about 220 solar masses of matter density are needed to produce one solar luminosity of radiation.
But given that most that there's five times as much dark matter as there is baryonic matter,
that means that there are something like 40 solar masses of ordinary matter required to
produce one solar luminosity of radiation on the average and the universe as a whole.
If you count stars in the Milky Way or you do population synthesis, let's say you do construct in a computer you stellar models,
you you you make models of stars of every different size and appropriate metallicity and ages and so on.
You can you can come up with some kind of estimate of how much mass in stars you need to emit.
From a typical population of stars. You need to produce the solar luminosity of radiation.
And the answer is numbers that vary but are around around three.
Here is the actual plot of that. It's the, it's the, it's the mass required.
Uh, sort this colour down here and this is the mass light ratios is this massive rate of luminosity.
This is slightly different band, the g band used by the SS, but it makes little difference.
And the individual galaxies the dots here and you can see this is a function of colour.
Red galaxies need rather more mass per unit luminosity blue galaxies need rather less mass per unit luminosity,
but your typical value is the square root of ten i.e. three.
So that's, that's about how much mass you need to produce luminosity once you get going with some collection of stars.
But actually in the universe as a whole, as I've said, we know that the the average number is something like 40,
even though in individual galaxies it's it's more like three. And in the Milky Way, we can do much more exact work.
And it's currently something that my group is working on quite vigorously at the moment.
We can say that the mass in in in baryons is less than something like five by
ten to the ten solar masses and the total mass is something like ten to the 12.
So the, it's a very small portion of the galaxy which is made up of baryons and is producing the relevant luminosity.
So there's a really big problem of the missing baryons in the sense that there are 4% of luminosity,
there are something like 40 baryons out there, the microwave background and galaxy redshift surveys tell us.
But actually in the galaxies there are numbers more like three.
There's a there's a very large factor of missing burials.
When in galaxy clusters, which are the largest relaxed structures in the universe, the missing barrier islands are not missing.
The barriers that are not in galaxies and not in stars are present and correct and have been and been known for a long time.
So enrich here is the coma cluster, one of the richest, most magnificent clusters of galaxies.
These are all these fuzzy things are all galaxies, two really huge galaxies, a load of satellites around them and so on.
And this space around the galaxies is filled with plasma at the variable temperature,
at the temperature at which there's enough pressure for the gas to resist the enormous gravitational field of the whole cluster of galaxies.
And in if you look in X-rays so this is from ROSAT, quite an ancient German satellite, you see a surface brightness distribution like this.
If you look in Planck, you look at the influence which the fast moving electrons,
thermal electrons associated with all this hot gas have on the cosmic microwave background photons.
You can detect the gas and get a plot like this. So we see this gas and it has an enormous mass.
It has a mass which is many times several times the mass of what's in the galaxies, just like cosmology says it should be.
A. Yeah, I think I've said those things.
And another very important fact is that this plasma that lies between the galaxies emits spectral lines associated with iron,
silicon and other elements that still have some bound electron at these enormously high temperatures,
the temperatures being ten to the 7 to 1038 degrees Kelvin,
which when you do the accounting on this,
you conclude that on the order of half of the heavy elements that that meaning everything from carbon oxygen right up to ion uranium,
whatever half of the elements are in hydrogen helium which have been synthesised by stars, which are presumably more or less in these galaxies don't.
These heavy elements are not in the galaxies. They're outside the galaxy, the galaxies in this intergalactic medium.
Right. So what do we. Sorry.
Backwards. Backwards.
If you look in groups of galaxies, so you go from an enormous cluster like the coma cluster down to smaller groups of galaxies and do the same thing.
Surveying with X-ray. You find an interesting plot.
This is from the group of Trevor Polman in Cambridge sorry in Birmingham as a function of temperature along here.
So ten KV so ten to the seven K up there, ten to the six K or so down here.
Logarithmic scale. You have different groups and estimates of the fraction of the mass of the group which is contained in the intergalactic gas.
And there is clearly a tendency for this fraction to increase as you go to higher temperature.
And that in practice means more massive groups which have stronger gravitational potential.
Well, that's why the temperature is higher. The variable temperature is higher because you need more kinetic energy in the in the
in the plasma in order to to be extended in the gravitational field of the system.
So. So there are two conclusions from this kind of study.
One is that a smaller fraction of the of the mass of the system is contained in intergalactic gas as you go to lower mass clusters groups.
And also that the another important fact which is not evident from this diagram,
is that the central entropy, the the the heat oddness of the of the gas sits at the centre of the group,
lies more and more above the value that you would estimate if this plasma have been heated
simply by falling downhill into the deep into the gravitational potential well of the group.
So in the simplest picture, you imagine that there's gravitational clustering of all this dark matter, and so gravitational potential will arise.
And the baryons, the the ordinary gas falls in there and gets compressed and shock heated when it gets as it's brought to a halt by other gas,
which is already taking up the space in the middle of the group.
So from that model you can compute how what the entropy of the gas would be, which is being heated in this way at the middle of the group.
And the X-ray observations indicate a discrepancy between that value of the entropy
and the entropy you actually see in this gas in the middle of these groups,
which increases becomes larger and larger as you go to lower and lower temperatures so that there's
evidence what that what that what that the interpretations are here that the intergalactic medium is not,
in fact, only heated by stuff falling down into the gravitational potential wells of these,
of these, uh, gatherings of these, these, these gatherings of galaxies and dark matter.
It's also heated by the energy, by the energy of ultimately of thermonuclear origin, which is with which the gas is flung violently out of galaxies.
And so we've had two pieces of evidence that that heating by gas being flung out of galaxies is important.
One is that the heavy elements produced in the galaxies, in clusters like coma to a large measure,
not any longer in the galaxies, but are in the intergalactic gas outside the galaxies, they must have been pushed out.
And the other is this indication that that when you're looking at relatively low mass clustering of galaxies,
which wouldn't, by gravitational landfall, heat the gas to a very high temperature,
you can notice that the actual temperature of the gas is higher than it would be if it was only being gravitationally heated.
And you can infer that it's also, as it were, from a nuclear heated.
So the key points of my argument so far are that forming galaxies have ejected large quantities of gas and
that most of the baryons that were are in the universe actually lie between galaxies and not in galaxies.
So something a little bit about star formation.
So the start, these diagrams and others show that the star formation rate is very strongly correlated with the surface density of cool gas.
It's not exactly surprising since stars have to form out of cool gas.
But what these diagrams show. So this is this is this is from this paper, from orientale as a function of galactic centric radius.
So this is a fraction this is our 25, which is a very large radius,
is the radius of which the surface brightness of the galaxy is 25 magnitudes per square second.
In sum, in some bands, probably the blue band. The point is that it's a very large radius.
So as you go, what you're doing is you're going here as you're going out and you're going out in a lot of different galaxies.
And what you've got up here are logarithmic scale is the star formation efficiency.
The star formation efficiency is defined to be the surface density of star formation.
That's the number of stars formed per unit area per unit time divided by the surface density of gas, obviously the amount mass of gas per unit area.
So this the star formation efficiency has units of one over time.
And you can see that it's it's fairly constant in the inner regions and then tails off and the different colours
here indicate that this is a region where we're talking about where most of the gas is in molecular form,
whereas the further out most of the gas is in atomic form.
So this is already an indication that it's the density of molecular gas that's important.
And this is in. These are these are small, low mass galaxies.
These are large mass galaxies. And in low mass galaxies, there's much less molecular gas.
Much less of the gas is molecular. The important point,
one important point is that is that the efficiency is fairly constant in the region that's dominated by molecular gas rather than by atomic gas.
So that's a relatively central, more dense region. And and the other point is the numerical value that the inverse of this.
This thing here, which is the time if the star formation continued, at this rate,
it would use up that gas in the time, which is the inverse of this star formation efficiency.
So it would use up the available gas according to that in a in three, four giga years in less than a Hubble time.
Now, actually, as we've said, the gap the box isn't closed.
Gas star formation definitely pushes gas out of galaxies.
There's a great deal. I've spoken of a little evidence of that, but there's much other evidence that.
So one conclusion of this is perhaps we should just go over.
Sorry, I keep going backwards is look at this diagram here which shows the staff
measured efficiency in terms of just using molecular gas and then it's really,
really flat as a function of a narrow range.
It's true. It lacks a centric radius, so the existing gas will be exhausted in 2 to 3 Giga Years because even if you didn't blow any gas out,
you'd run out at present rates of star formation in in four or five Giga Years.
And also, you would indicate that if if there was if what if the way galaxies worked was they started off with a fixed mass of gas and they
started to turn them into into stars at a rate which depended upon the which was proportional to the surface density of gas.
They would have had much higher surface densities of gas in the past and much higher star formation rates.
And we really would be at very much at the end of the star formation process in galaxies like ours.
So the star formation rate in the past would have been much higher if if the galaxy started with a fixed supply of gas,
which they then steadily turned into stars. But if you look at the mix of stars near the sun,
you can you can form an opinion about how many how how many relatively young stars there are vis a vis how many old stars there are.
You conclude that the star formation rate has declined by maybe a factor of three, something like that,
only in the entire ten Giga Years or so that it's taken to form the disk of our galaxy.
So. So I think the bottom line from all of this is that galaxies don't start off with a fixed supply of gas.
They start off with some gas. They form some stars, they eject some gas and they accrete some gas.
That this exchange two ways is an important both the they have to both accrete gas to sustain star formation.
Well if they do it Greek gas to sustain star formation and this keeps their
their gas densities so that the surface density of gas is falling only slowly.
Therefore, the therefore the level of star formation rate is falling only slowly because they almost balance their income
coming from accretion and their expenditure locking gas up in stars and ejecting gas into the intergalactic medium.
So now what to do about the phenomenon of extra plain a H1.
So this is a galaxy that's really fairly similar to our own galaxy, NGC 891, which we happen to see almost perfectly edge on.
So it's the classic galaxy to study. If you want to study an edge on galaxy, it's nearby.
It's very similar to the Milky Way. It has a slightly more vigorous star formation than the Milky Way.
And. This is these.
These are contours of the surface density of neutral hydrogen seen in the 21 centimetre hyper fine line of hydrogen as measured in 1979.
Now, of course, as the electronic engineers have gone on. They have got more and more sensitive.
They built more and more sensitive radio telescopes. And so the the extent people.
This is all done in Westerbork in the Netherlands, in fact. The the the limiting surface density has gone from five by 20,
the ten to the 20 atoms per square centimetre through seven by ten to the 19 is that through 1.7 times ten to the 19.
So there's been a drop by almost no magnitude and a half in the surface density that can be detected.
And in that and in that technical progress, what has happened is that this galaxy has grown an h one halo.
It's it's we're seeing here gas, which is not in the plane of the galaxy.
Now, you might worry that what you're seeing is a disk of gas rotating around the galaxy,
which isn't seen edge on which you can rule out that possibility based on the kinematics of this gas, because this gas is detected in line radiation.
So you know very precisely how fast it's moving up and down the line of sight.
And that kinematic information tells you that this gas is in fact,
this galaxy is enveloped in gas which lies well off its galactic plane, which you see here.
Edge on the Galaxy has grown in this direction as a result of the increased sensitivity, much more than it's grown in this direction.
Of course, it's growing in every direction, but it really has been growing away from the plane.
So this galaxy like this keeps something like a quarter of its supply of neutral hydrogen, more than a killer parsec off its disk.
That's an amazing feat, because the temperature, the characteristic temperature of neutral hydrogen is less than ten to the four degrees K.
The variable temperature of this gravitational potential well is over a million degrees K.
So this gas has less than a 10th, less than 100th of the temperature required for pressure to to resist,
to be in effect, to provide effective resistance to the gravitational field of this galaxy.
And yet it keeps this gas up there. And that's what I want to talk about next.
So this is another galaxy, a galaxy seen face on, also seen from Westerbork in the Netherlands.
This is what this galaxy, NGC 6946 looks like in the optical.
This is what it looks like in 21 centimetres. So it's one of these galaxies which has a very huge extended disk of neutral hydrogen.
And if you look in this disk,
there are various bright spots where there's lots of 21 centimetre emission and there
are a few very small dark spots where there's very little 21 centimetre emission.
And if you look at the distribution of neutral hydrogen along this line, you get you see a plot like this.
This is from blacksmith's theses. So what's plotted here is line of sight velocity.
So the point about 21 centimetre measurements or measurements in a in a in a spectral line of gas like 21 centimetre is that it generates a data cube.
You have two dimensions on the sky and then you have a frequency dimension which you interpret as a velocity one because Doppler shifts and all that,
because you're looking at a particular spectral line.
So as you scan along this along this line, that means moving along here from the middle of this line,
you see emission at a velocity, which shifts because the galaxy is rotating.
And you see places like this where there's an absence of gas, that's one of these holes.
And then you see material here, which is at velocities, which are different from the velocities of gas that lies in the plane.
So the interpretation here is that some cluster of supernovae occurring at this point in the disk.
So somewhere here have gone off in a very short time interval.
One after another created a blast wave which has swept up the the coolest gas and flung it off the disk.
And here you're seeing some of it flag flung off the disk.
So personally and I set out to model this process in the following with the following very naive model.
So here is our disk. Here is some point on the disk where star formation takes place here.
In the case of our own Milky Way, here is the sun and we were able to stir up.
So we'll do some of that. But for the moment we're applying this model to NGC 891 and some other external galaxies.
And in that case, the sun isn't here. The sun's over there somewhere or out here, out in the lecture room.
So then. So supernovae go off here, they blast a cloud of gas upwards.
Here is the cloud of cool gas moving upwards through an ambient atmosphere of temperature of gas, which is at the very low temperature of the galaxy.
This cloud is rather cool and as it flies through the ambient medium, it there's a flow of ambient medium across its top.
The ambient medium turbulent mix is in. I'll show you a simulation of this.
This is just a cartoon.
The flow it flows over this thing you you get mixing of the of the intergalactic medium and this cool thing of gas and you get a wake here forming.
And I want to show you some evidence that this model works very well.
So this is this is called a galactic fountain, not surprisingly, because the gas goes up,
reaches some maximum height and falls back down again at some other radius.
Some other point. So we we built a very simple model of this of this process.
And what were the what were the headline conclusions from this?
So what we were doing was we were making a model of the complete data cube.
This is this is this is why it's a little bit difficult to show you the, uh,
the way in which the degree to which the model fits the data because the data of three dimensional models.
Three dimensional. And so it's, it's. You rely on numerical tools to tell you whether you're doing well or not.
But anyway, the Cube's required that the h one clouds as they flew through the intergalactic medium,
as they went on their passage up from the disk and back down to the disk.
These clouds had to grow in mass.
And that was not what we were expecting, because it's intuitively obvious that what happens is that these clouds suffer ablation,
that on their surfaces material is swept off and and mixed in with the hot gas and lost to 21 centimetre observation.
And because they grew in mass as they flew through the intergalactic medium, when they landed,
they had more mass than they set off, which meant that as a result of this process, the disk was gaining mass.
And how fast was it gaining mass? Very much at the same rate as the rate at which the one sex peaked as ready,
which the observations indicate that gas was being used up in star formation.
So.
So that was the inclusion of those two studies of two galaxies, the galaxy I've shown you NGC eight, nine, one, and another galaxy which is down here.
This is NGC 2403, which is a very different sort of galaxy from this.
It's lower in mass, it's seen much less edge on.
So the the nature of the observational material is very different.
But one in the same model could make it would could do a very good account of both of the datasets,
the data cubes belonging to both of these galaxies.
Then Fred Phillips from Tenaris graduate student Federico marinucci simulated the hydrogen recall flow over of gas over such a gas cloud.
So, so in our phenomenological model we just said the cloud moved through the intergalactic medium and it gains mass or it loses mass in some way.
Parameterised And then we in this way fit it to some, to some, to some data.
Cube We've got parameters of our model out.
But, but, but Marinus you actually did a hydrogen chemical simulation of the flow of hot gas over cold gas.
And that study led to the conclusion there's a critical line in the in the the metallicity pressure and t and is the number density t is
the temperature is 20 is the pressure in appropriate units and f over h is a is the is stands for the metal abundance in the medium.
So there's a in the in the Corona's plane of possible metal metal abundances and pressures.
There is a plane above which when the metallicity is high and the pressure is high and all the pressure is high,
the mass of cold gas increases as the cloud flies.
And if you're below this critical surface, the mass of your clouds decreases as it flies through.
The reason for this is simply that if you increase the metallicity,
you increase the ability of the gas to cool through spectral lines of the heavy elements.
Heavy elements are very good at enabling plasma to cool.
And also if you increase the pressure, you increase the density.
That also increases the rate at which cooling occurs because cooling occurs at a
rate that's a rate per ASM that is proportional to the number density of atoms,
the number of things there are for electrons to bump into. So here is some evidence for this.
Here is one of Marinol, which is simulations of the cloud which started off spherical,
has been flattened by the by the pressure as it ploughs into the into the coronal gas.
That's the variable temperature gas up here. It's been flattened.
And then this wake is turbulent, wake downstream is developed.
And in this turbulent weight downstream, one of two things can happen.
Well, what necessarily happens is that the cold gas mixes with the hot gas in all those vertical rings.
Then the hot gas can possibly evaporate.
The cold gas quickly raised the cold gas to high temperatures where cooling is inefficient or the cold gas can succeed in cooling the
hot gas by mixing with it and and lower its temperatures to two to values at which it's dense enough that it cools very rapidly.
And if you so if you if you study the mass of cold gas,
this is the difference between the mass at time t in the mass originally in the cloud of cold gas.
That's gas with a temperature of less than five by ten to the five k as a function of time.
Then it declines like this. This is just a convergence study showing the result of using different numerical resolutions.
Whereas if you if you take the higher the medium resolution,
so the medium resolution simulation and do the computation with a more metal rich and a more dense coronal plasma,
you find that the mass increases over time. So that's that critical.
There's that critical thing of so flushed with success for four months.
Standing what was happening in the in the explaining modelling successfully the data cubes of external galaxies.
We turn to the data cube of our own galaxy,
which is very different in nature from the data cubes of external galaxies, because we sit right inside our galaxy.
And then when you look out, you have absolutely no idea.
You see a mission. And as a function of angle, the direction you point your telescope at various velocities,
but you have no idea where along the line of sight this emission lies.
So the early 21 centimetre surveys around 1960 found that there was 21 centimetre emission coming from very high galactic latitudes,
which was which surprised people at the time.
And what's more, you saw emission happening at velocities that you simply couldn't explain if you thought in terms of gas,
the cold gas in the galaxy rotating in a nice orderly flow around the galactic centre.
That's the so-called forbidden gas or gas of seen it forbidden velocities.
And the nature of this of the gas that gave rise to this emission,
this high velocity emission, as it was called, was was fiercely debated for decades.
So as late as 1999, Blix and CO created something of a stir by arguing that this gas was at megaparsec distances.
And if this gas was at very large distances because it produces a certain amount of flux at the telescope,
it had to have you needed an enormous mass of gas.
The further away you put this gas to produce the given flux at your telescope that you measure the mass of the gas has to increase.
So if this stuff was at megaparsec distances, it would have it would be present in quantities that were suitable for forming whole galaxies.
Whereas if the gas is very close to you, we are talking about rather trace amounts of gas.
So this was hotly debated for a very long time.
But in the in the in the not in the naughties, as it were in the last decade, this question was finally resolved.
That by ultraviolet absorption measurements.
It turns out that this gas lies at distances of less than the killer particle, so some of it's much closer than that.
And that means that it's in red. It represents relatively small amounts of gas.
So it it was an interesting exercise to ask whether this model of extra playing a gas
that we had developed for these external galaxies would could account for the data cube,
this data cube for our own galaxy, which is very different in nature.
And so that so another student of Antonio Morasco, another student of Fratantoni,
fitted the model that we built for these external galaxies to the neutral hydrogen observed in the light, an Argentine bond survey.
So a recent reworking of the data cubed of our own galaxy.
And here are some diagrams. This is what we're seeing here is gas at some particular line of sight velocities,
this gas that's coming towards us at about 66 kilometres a second in sum.
And here we have galactic longitude. So that's that's angled around the galactic plane.
And here we have galactic latitude, 20 degrees, 40 degrees, etcetera.
So this is quite a big piece of the sky in some sense. This is the data.
This is this is what you actually see. And this is the model.
This down here is, broadly speaking, the allowed gas.
This is is gas belonging to that that we interpret as being long belonging to the galactic fountain.
So, of course, it's not a perfect match here because this fountain involves individuals, groups of supernovae which send out particular plumes of gas.
It's a noisy process you can't expect to get. You can only talk about the average.
If we would look at this at some 200, 300 mega years later, this plot would be rather different.
But I think this is a reasonable representation of it.
An indication, though, that we have got the physics right is, I think, given by these pictures up here.
What these are showing is the. The.
Well, this is showing the court.
Read it from here. Yet.
Sorry. Yeah. The fraction of condense mass and the plot.
Exactly. So I want the bottoms of the bottoms mass. So this is showing for this issue.
So you fit this model to the to the data cube.
The model is a phenomenological model and it involves things like how rapidly does a
cloud gain mass as it sweeps through the intergalactic medium or through this corona,
all this hot gas, very low temperature gas?
How how much does it slow down as it moves through the medium because it's sweeping up material, etc., etc., etc.
So you, you the the, the phenomenological model fitted that provides this fit to the data route produces these
lines because numbers come out of it which enable you to produce those straight lines from,
from Mariana, which is hydrodynamic simulation, which I showed you earlier.
This one one's like this. Here you get the data points.
And these are two enormously different computations.
One is,
is the mariner actually calculation involves numerical hydrodynamics and detail cooling functions with atomic physics and all that sort of thing.
The other thing is a phenomenological model fitted to a data cube produced by ready observers and is completely ignorant of atomic physics or a.
Yeah, that kind of thing. And hydrogen and hydrogen, that mix.
And yet we get this degree of agreement, and this, I think, is an indication that we're doing the right thing.
Another indication that we're doing the right thing is you can you can say.
So let's go back to that cartoon. This cartoon.
So when you if you are line of sight to some some ultraviolet emitting source up here,
which might be a quasar or might be some some very hot star far out in the galaxy.
If your line of sight passes through this turbulent wake, you should be able to see absorption lines because this turbulent wake contains
gas that on the order of ten to the fifth K and it will contain ions of carbon,
oxygen, silicon, etc. which which have characteristic absorption lines and when.
And you will have a very sensitive probe, you'll be very sensitive to quite small amounts of gas containing these, these things.
So. And indeed, when you look out to these ultraviolet sources with space telescope, you see that that's the data points here.
Could you move away from the sorry, could you move away from the projector?
Yeah. You see, so these these dots here are actual observations.
These are lines of sight down to quasars or ultraviolet emitting stars, which you can get absorption spectra with HST.
This is the work. Work of Lana et al. Yeah, there we are.
There. And the colour indicates the, the, the velocity of a kind of an absorption system.
Right.
This is, the colours indicate velocity from from minus to the deep blue is -200 kilometres a second to two red is is plus 200 kilometres a second.
And the that's sort of patchwork all over the place, all the predictions of our model.
So in our model we've got a number of we've got a number of clouds moving around and we can, which is noisy,
of course, and then we can, uh, we can in each of these wakes, we have a certain amount of turbulence,
so we can say how much gas we expect to find in this pixel here at because because of the because of which clouds
which have a turbulent wake which might scatter which might scatter a small volume of gas into that velocity.
So the so the the, the, except when you're outside one of these circles, you're looking at one of our model.
When you're inside one of these circles, you're looking at the actual data.
And I think you have to agree that there's pretty decent agreement. I think that's all I have to say about that.
You can also ask down which lines of sight do you expect to see?
Absorptions, these black dots are these are plots of velocity versus galactic longitude in different a different galaxy.
Each plot is belongs to a different galactic latitude.
This is, uh. So this is -50 degrees, -20 degrees plus 50 degrees, I think, plus 20 degrees.
Anyway, the point is that the model gives you probabilities which are, which is signalled by the,
by the coloured contours and the black dots where you actually detect the absorption.
And again, there's reasonable agreement, you can show that statistically pretty decent thing.
So the key points of my discussion so far are that we have a model of this fountain
process that yields a coherent picture of a very large body of disparate data,
the 21 centimetre data cubes for external galaxies, the very different 21 centimetre data cube for our own galaxy,
and these measurements of ultraviolet absorption line systems in our own galaxy.
And it implies that the star formation driven fountain draws gas out of the corona,
out of the the gas at the very old temperature that fills intergalactic space at a rate
that's roughly equal to the rate at which gas is being used up by star formation.
And therefore it replaces losses by star formation, and it requires that star forming galaxies have to be enveloped in a corona, in the corona,
which above the disk is almost but not quite cool enough and dense enough to to
to make the transition from the very old temperature down to low temperatures.
So it needs a little help and it gets this help from the cold gas sent up.
But if the corona were much hotter than it is or than we think it is,
this process wouldn't work and the gas you sent out would become evaporated and less would come down.
You'd run out of gas on a fast rate. So I want to connect this now to the global picture of galaxy evolution.
Here is a very famous diagram from from ten years ago when from analysis of the enormous statistics gathered by the Sloan Digital Sky Survey.
So what this is showing is colour with blue colours at the bottom and red colours at the top.
And what we're seeing here is absolute magnitude.
So these are blue. This is high, very high luminosity at this end of a galaxy.
This is medium to low luminosity at this end.
And what you've got. So this survey saw millions of galaxies and was able to assign these galaxies luminosity is it was able to assign them colours.
And now what's plotted here is the density, the number of galaxies, the number density of galaxies in this luminosity colour plot.
And what you see is a by modality. There's, there's what's called a blue cloud here.
A relatively low luminosity is in bluish colours. And there's the red sequence here at higher luminosity is and red colours.
So the general. So in a huge amount of work has been going on over the last decade to try and understand this in a constructive way.
Uh. Right. Another interesting fact is that the red galaxies tend to occur in denser regions,
regions like clusters, rather than in the in the voids between clusters.
And it's generally thought that galaxies evolve from the blue to the red,
because the reason these galaxies are relatively blue is because they're still forming stars.
They and because the stars that are blue are typically of the importance the blue colours are given to these galaxies by relatively massive,
therefore short lived stars, and therefore only galaxies that are forming stars are capable of being this blue.
So the general idea is that's accepted by everybody that these galaxies,
that galaxies at some level migrate up in this diagram and they possibly migrate sideways as a result of merging,
but they certainly migrate upwards from blue to red as their star formation peters out.
And you're just left with the old low mass stars that will go on glowing for a very long time, but then they're red.
And I think the basic reason why stars move from blue to red is that star galaxies start to move with respect to their intergalactic medium.
So they haven't spoken much at all yet about about the motion of a galaxy with respect to the intergalactic medium that it finds itself in.
But if you're in a cluster of galaxies, there is only one galaxy which can be stationary with respect to the intergalactic medium,
and that's the galaxy at the middle of the cluster, because it's the only because the intergalactic medium will be stationary.
With respect to the cluster, well, it might conceivably be rotating, but we don't expect it to be rotating very much.
And the other galaxies are zipping around at random. And so they're moving.
With respect to the intergalactic medium,
it is very difficult for the processes that I've been discussing to function if you're moving fast with respect to the intergalactic medium.
But there is a paradox in this in this in all this,
which is that where the intergalactic medium is densest and most ready and cooling and radiating fastest,
namely in the middle of rich clusters of galaxies.
Remember, we knew we've known for 30 years,
we've had vivid evidence that the clusters of galaxies are full of enormous masses of very low temperature gas,
because it emits so strongly in X-rays that even the X-ray telescopes of 1970 could detect.
So so in these dense these regions is exactly where the galaxies are reddest and dearest and not forming stars.
And the explanation for that, I think, is a.
Is that the cooling rate of the intergalactic medium is highest in a cluster of galaxies.
It is indeed highest at the very centre of the cluster of galaxies where the pressure is largest.
And the, the, the, the accretion rate onto the central black hole.
So. So this is this is in fact, a radio continuum picture of a Galaxy M87 that sits at the middle of a relatively small cluster, the Virgo cluster.
And in the middle there, there is an enormous black hole with a mass of some by ten to the nine solar masses.
And the rate at which this thing accretes is a very sensitive function of the temperature.
So as the temperature goes down, the accretion rate on the black hole goes up and the black hole accretes.
It emits jets which here impact on the we see impacting on the intergalactic medium.
This is this is a blow up of the middle of this. So here is the this is a much smaller scale picture of what you see in the middle here.
So a jet comes out, it it impacts on the ambient gas and re heats it.
So if the temperature in the middle of the cluster drops significantly, you get a major energy outburst from the from the engine in the middle,
which reads the system and keeps this this variable temperature gas from falling to the kinds of temperatures less than 100 degrees K,
maybe 30 degrees K required to form significant numbers of stars.
And this is something on which I worked with Tibor and Henri Goma in the past.
In our own galaxy, we have a rather wimpy black hole.
Sagittarius a star which you can't see but sits there.
This is an X-ray picture of the middle of the galaxy. I showed you previously a radio continuum picture of the middle of the Virgo cluster.
And I think the argument would be that the role of Sagittarius a star is to prevent the
corona gas of our own galaxy from cooling in the place where it's most ready to cool,
which is right in the middle of the galaxy where the pressure is highest.
So the question because the question I'm addressing is why is it that the star formation in our
galaxy that occurs outside of is predominantly occurs while away from the centre of the galaxy,
although it must be in the centre of the galaxy.
Even though I argue that what sustained star formation in the Milky Way is accretion of gas from this corona of a very low temperature gas,
and this corona,
a variable temperature gas, may not be ready to cool where we are, but surely where it's densest in the middle of the galaxy is ready to cool.
And the answer is the answer I would give you is Yes it does.
It is keen to cool the air, but there sits the monster which is capable of reheating it very,
very promptly in the event that it allows its temperature there to drop.
So so that's the picture one has that the middle of the corona is prevented from cooling by the black hole.
But out here, the the fountain, which is driven by star formation, reaches up into the corona and grabs gas, brings it down to the turns.
Hot gas into cold gas brings it into the the disk of cold gas that circulates around the galaxy within which star formation happens.
And this is what sustains. Star formation is only very slowly falling right over the life of the galaxy.
But consider what what's likely to happen otherwise.
If you have a major merger and our galaxy is falling together with the Andromeda Nebula M31 and we will tumble together
in a very interesting orgy of stellar dynamics and hydrodynamics and star formation in about three giga years.
I think that's the right number and when that happens, there will be a burst of star formation.
This is something Jo Silk has worked on a great deal because the gas will be vigorously shot and so on and supernova
will will quickly heat much of the gas and quite likely will will will succeed in heating all of the gas,
all of the cold gas. So there's no cold gas left in that case.
With no cold gas left, there'll be no star formation.
There will be no fountain to reach up into the corona at distances safely removed from the black hole.
They'll just be this nice, smooth atmosphere, a very low temperature gas,
densest and in danger of cooling and forming stars only right on top of the new improved black hole that will
form by the merging of Sagittarius Star and the black hole which sits at the middle of the Andromeda Nebula.
The black hole will periodically thrash the variable temperature gas and keep make sure it doesn't get any ideas about cooling too much.
And that will be it.
And our galaxy, well, the new improved galaxy formed by us and and the Andromeda Nebula will grow red and dead like those things that we have seen.
So here are my conclusions. Most of the baryons lie between galaxies rather than in galaxies.
We've only a partial understanding of why of how it comes.
It's a it is. It's still a little bit puzzling that star formation is so efficient at pushing gas out of galaxies.
But anyway, it's a fact we have to bear in mind. Galactic Centre Black holes prevent substantial accretion of taking place in the natural place,
which is at the centre of a corona of hot gas where the density is highest.
But in blue galaxies are the blue clouds such as stars.
Star formation has been sustained for the lifetime of the universe,
essentially by accretion from the IG m sustained through the through this fountain process that I've described to you.
We have a model of the impact of this star formation on the insular medium that was structured to explain data from
external galaxies but does a super job of of of of the of the data for the very different data for our own galaxy.
So I'm completely convinced that it's right. And galaxies.
The conclusion from all this is that what makes galaxies red and dead is when some event, particularly a merger.
Well, some event could be two things. One can be a merger, which is probably our fate when we fall together with with with the Andromeda Nebula.
The other thing that can kill your star formation is that you fall in to a rich cluster of galaxies,
and then you you fall through the very temperature gas of that cluster of galaxies, which blows away very quickly.
Your own corona. Of of of thermal temperature.
Gas which was stationary with respect to you. And from now on you're moving with respect to the variable temperature grass that surrounds you.
And in those circumstances, if you are foolish enough to put up a cloud of cold gas into the corona,
hoping to harvest some of this life sustaining coronal gas for your own purposes, you'll.
Cold packet will simply be blown away and incorporated in the in the bigger entity's pool of variable temperature gas.
And that's how galaxies become red and dead. Thank you.