Good afternoon, everybody. Thank you very much for coming on this.
Not terribly pleasant afternoon and thank you for not attending the alternative event, the procession,
the open top bus through Broad Street by Oxford United, which is starting at this very minute.
So it's my privilege to introduce our speaker today, Professor Rob Kennicott.
He's the premium professor in Cambridge at the Institute of Astronomy.
And his research interests include the structure and evolution of galaxies
and star formation in galaxies in which he's made some seminal contributions.
In fact, he's held some of the most influential positions in contemporary astronomy for a long time.
In the late 19th and early twenties, he was the editor in chief of the Astrophysical Journal.
This is the principal American Journal for publishing in.
And so the editor in chief of that journal, of course, carries a great deal of weight and not too little or amongst the community.
He has been widely recognised for his contributions to astronomy.
A few examples are the award of the American Astronomical Society Dannie Heineman Prize in 2007 and a share in the
Gruber Prize in 2009 for one of the most important pieces of work that's been done with the Hubble Space Telescope,
namely a precision measurement of the Hubble constant.
He was made a fellow of the Royal Society in 2011. But even this work on the Hubble constant is not actually the work for which he is best known.
He is his his most prominent piece of work is on the measurement of the empirical
relation between the gas density and the star formation rate in galaxies,
a relationship which is fundamental to understanding galaxy evolution and is known as the Kennicott Schmidt Law.
It is my great pleasure to introduce Professor Rob Kennicott to deliver the 12th GMC Lecture unveiling the birth of Stars and Galaxies.
Robert. Time to put my mark on.
Okay. Should work. You're right. I can't tell. So thank you.
And for all that flattery. And just let me add water.
Despite all jokes about Cambridge and Oxford, what a tremendous honour to be here.
And Michael, for for for making it all possible.
So I should begin, I guess, by explaining what this unveiling is all about.
Good. You know, astronomy proceeds in fits and starts.
And I think if you follow astronomy mainly, you know, through the the media, the economist and so on,
often there are breakthroughs, just moments of inspiration where we leap ahead.
The most recent example, doubtless the detection of gravitational waves a couple of months ago,
announcement a couple of months ago by the advanced Lego team.
And you can think of many other examples, I'm sure. But the story I want to tell today is of a different kind.
There are other advances in astronomy that occur where progress is almost measured,
like the motion of the minute hand on a clock where steady progress over a period of years.
The story I'm going to tell today, over 15 or 20 years have resulted in results that are every bit as profound.
And the one that I'm going to talk about today is something that often in the U.S. we call the origins problem.
It's where did everything come from? How were galaxies formed?
When how how was the Hubble sequence of shapes and sizes and masses and galaxies?
How did that come to be where it is? How did stars and planets form?
Much of my research efforts since working on the distant scale has been on the problem of star formation in the universe and on the large scales.
And part of the story I want to tell today is how, although we're making remarkable advances on every scale from woops,
sorry, I've got to I'm going to make that mistake a few times. This is an armour, a millimetre image of dust, a forming star,
a solar system where these rings may be protoplanets around a nearby star, just as the theories of the last decades have predicted.
And of course, you've seen iconic images like this of stars forming in our own Milky Way.
But there's another story to be told. It's star formation.
When you look on the grander scales in the universe within nearby galaxies, like in 94 here,
where each of those red blobs is a cluster of several thousand stars forming to the Hubble ultra deep field,
where we're peering out actually the extreme deep field where we're peering back to within, you know,
the first billion years of the age of our universe and detecting stars forming at the
very beginning some of the first stars and galaxies to have appeared in our universe.
And so the unveiling I'm going to talk about is actually has a literal meaning and some metaphorical meetings all rolled into one.
It's a lot of discoveries.
The literal story that I'll tell along the way is how the opening of a new region of electromagnetic space, infrared observations in particular,
and submillimeter observations of galaxies from space have on an unveiled 50% of one half of the star formation that
had previously been hidden and a 50% that includes the most violent regions of star formation in our universe.
And then three Fig. unveilings along the way, learning about the incredible array of diversity,
of how galaxies are transforming the raw material from stars, gas in galaxies into stars.
How we can now reconstruct that history over cosmic time observationally.
And to me, something I'll have the least time to talk about.
In fact, if my talks go as usual, not much time at all.
How a theory, a theoretical paradigm, a cold,
dark matter based paradigm which was originally designed to explain large scale structure of cosmic filaments,
free cosmic structure that was seed seeded by quantum fluctuations in the early universe.
How that's the computer simulations in the state of the Earth today can reproduce not only the large scale structure of the universe,
but also the properties of galaxies, the Hubble sequence and so on.
They can't quite get down to planets yet, but they're working on it and they're getting there.
And it's one theory without fine tuning that does it all.
Now, in saying that as an introduction, my colleagues.
Upfront would have every reason to get worried because on those four bullets, there's easily material for one hour of talk on each.
And I know you're sort of aiming to get out of here six ish rather than nine ish.
So am I. So and that's going to be part of the point.
But that's the method to this, is to show you how this all goes together.
As Roger said, I work on this problem on the large scale.
And so in describing I'll be describing where that, you know, that relation he talked about Schmidt law comes from.
And it turns out that although what I've done is a tiny piece of this essentially generational accomplishment that's ongoing,
the piece I work on actually occurs at the intersection of these four threads or the cusp of all of this.
And so in some sense, to explain the implication requires going through all of this.
So anyway, that's the design. Let's see how it goes.
So to begin, I'm going to at the beginning sort of speaker,
I'll start at the sort of most general level and we'll move on to the plots that none of you will understand as we get deeper into the talk,
even, Roger, I hope at some point. So I'll go through the context of all four of these.
Let's begin with galaxies with a few things,
hopefully almost all of you know about of what you're seeing as an infrared image in three colours of the nearby spiral.
M1 or one. In Ursa major. I've chosen it.
It's a galaxy, as you see, about 23 million light years away.
We measured the distance as part of this Hubble project. We got the Gruber Prize for.
So it's a pretty good number. And it is chosen because we, of course, we can't see the Milky Way from the outside yet anyway.
But it's pretty close to what the Milky Way would probably look like if we could see it on the outside.
So you're looking at 100 billion stars very roughly across a disk that's about 100,000 light years across.
These are mind boggling numbers if you begin to think about it when you ask what it's made of in this case of the baryonic matter of the matter,
we know about about two thirds is stars and about one third is gas.
Raw material gas you think of is fuel for a future star formation.
So this galaxy has managed to use up about two thirds of its gas tank.
And of course, 85% is a dark matter that we don't know.
We know we can measure how much of it is with incredible precision.
But what it is? No, not yet. So that's another talk.
There. I will go through this quickly.
Again, I think, you know, just as there are about 100 billion stars in a typical large galaxy like the Milky Way or M1 or one there,
but by coincidence, about 100 billion galaxies like that within the visible limits of the universe.
As far back out as we can see, this is a map of those within the brightest,
within about a billion light years, and there are thousands of galaxies there.
And, you know, they're not arranged randomly. They are in some sort of spiderweb like filament tree structure.
That's real. That's not your eyes. Those are not Martian canals. That that's real structure that you're seeing.
Again, a part of the basics. And 101 Hubble that are our nomenclature.
The old school way of describing galaxies was in terms of what's called the Hubble sequence.
Edwin Hubble did many. He established what the nature of galaxies were, the distance to Andromeda, the first one.
And he proposed a classification scheme that was still used today, that of elliptical galaxies, rather steroidal galaxies,
as we'll see that are largely devoid of young stars going through the spirals and then beyond irregular galaxies.
And what no one would be sitting somewhere here in the diagram?
No. As I say, we can't produce such an image for our own galaxy, the Milky Way.
But this is an artist's visualisation based on the best data that we have of roughly what it would look like a barred.
We're certainly on the lower part of Hubble's tuning for the system, actually a very interesting system of stars in a bar.
We are about two thirds of the way out. And one thing I didn't say in our one hour, one, if you do an age census of the stars in the spiral galaxies,
that you can find almost a uniform range of ages from forming as we speak to stars almost as old as the universe itself, up to about 13 billion years.
And that's the case in the Milky Way. And there are three examples.
Our solar system has been accurately age dated for meteorites and such to about four and a half, 4.46 billion years.
And the oldest example of a globular, a star cluster approach, 13 billion years.
And then if you look at active nebulae, you see stars with negative ages.
Essentially, they're in the process of forming as we speak, and it's typical of galaxies of that type.
So that's sort of the introduction to galaxies.
Now, before this unveiling, before we started studying the universe across the electromagnetic spectrum,
there's a lot that you could learn simply from visible starlight.
And so, again, from just basic introduction, here's an example of one of the nearest very active regions of star formation, the Orion Nebula.
Most of many of you will recognise this object here.
It's actually visible, maybe not from Oxford on a typical certainly not last night.
It's in, I guess, officially the Sword of Orion.
What I taught undergraduate astronomy in Minnesota. Some female students tried to convince me this was rather anatomical feature of Orion,
and this explained why he was immortalised in the stars for a while.
But either way, and and within this is a this is a cloud of gas that's been ionised.
This glowing, fluorescein heated. Actually, this organisation is produced by just a so-called trapezium for very massive stars.
30 times the mass of our sun at up in the centre and a cluster of several hundred stars.
And that's typical of the star forming units in the Milky Way.
Likewise, if you study, for example, with the Hubble telescope Orion at higher resolution and look very carefully around,
you will see evidence of other interesting phenomena on smaller scale.
These objects are called probe lids and what they are now believed to be are individual stars.
A solar system's basically in the process and forming sometimes a front lip where you see some sort of circumstellar material,
perhaps a disk and sometimes backlit where, you know, the essentially sitting in front of a bright light source.
So so we we've gathered a lot of information over the last few decades on star formation in our own neighbourhood,
in great detail in selected regions. But if you restrict yourself to the visible, you begin running up against a figurative brick wall very quickly.
So, for example, if I wanted to make a map of all of the Orions in the Milky Way or all of the stars.
Anything you like. You come up against this problem.
So here is a I think, an all sky camera photograph made in the southern sky of the southern Milky Way.
This beautiful band in the sky. Most of you are familiar with.
But you see, of course, cutting across the stars, all of these dark bands.
These are holes in the Milky Way. They are clouds of interstellar gas and dust particles that are blocking the light to the point that,
in fact, even though I said the Milky Way is out of order,
100,000 light years around in most sightlines, especially in the disk of the Milky Way, you don't see much farther than a few thousand light years.
That, in addition to this, clumped us, there's more diffuse dust that is creating just a smog,
basically a fog preventing us from looking at this dust.
Interesting stuff. We just checked. Yeah, got it. It's particles roughly the size of a wavelength of light and smaller.
Everything from nanoparticles up to, like, the soot particles.
Smoke particles, you know, from a cigarette or so on, up to small silicate, small rocks, but micron size.
And it in terms of concentration is incredibly concentrated in the interstellar medium.
I don't know. Does anybody happen to know what the. If you're a coal miner,
what the concentration in parts per million of coal dust can be in that mine before you're not allowed in without reading the equipment.
I had to look it up. I didn't know no coal miners in the other.
It's about at least in the U.S., it's one, one part per million.
Any more than that, you need breathing apparatus.
And of course, that applies to the people in the construction site, outside and and everything else.
Interstellar space. It's one part, 410,000 parts per million.
And of course, much more rarefied in space that the typical density of gas particles even is sort of a particle per cubic centimetre.
So does even less than that. But you're looking through hundreds of trillions of miles of it.
So so it's very opaque. In fact, if the air in this room had dust at 10,000 parts per million, those in the back room would not be seeing me.
And when it condensed out, you know, it would leave far more than a little stain of dust on your shoes.
You know, it would be you know, it would be microscopically deep. So the interstellar medium, the gas in galaxies is is very dusty.
And that. So that has prevented us not only from making a census of, say, where the stars form or Milky Way,
it prevents us from looking into peering into the dense disks of those individual star forming regions.
And in other galaxies, as you'll see later,
it blocks out the very most interesting that the star formation in the most intense and violently star forming regions in the universe.
And again, the reason is that the size of this dust is about peaks, just very much so invisible in ultraviolet light.
So the solution, it's easy to say, but difficult to do technologically is to observe the universe in wavelengths that are longer,
wavelengths of light, that are longer than the dust grains.
And then they just diffracted around or simply shine through the dust and penetrate right through.
And so this is the infrared just mentioned anecdotally.
Infrared radiation was discovered by William Herschel, the discoverer of the planet Uranus.
20 years later,
he discovered infrared radiation and measured the amount of it in the sun with a very ingenious experiment where he set up thermometers,
used a prism to disperse sunlight and and used thermometers to show that there was heat he picked up far beyond,
both in the red part of the solar spectrum, but then far beyond in regions where there was no light falling that you could see following on the table.
And Herschel Space Observatory was named after him in honour of that.
Sorry. There we go. And so the penetrating power of the infrared, even in near infrared wavelengths, is is is amazing.
And there's just there's a terrestrial example, I think, from the 1950s.
This is Silicon Valley a long time ago when it was an empty valley, invisible light on a hazy day.
And and then the same time and infrared.
Here's Orion again. Visible and infrared.
And then quickly, here's the Milky Way. So here is the visible view, another photograph of what you see in visible light.
When you look, try to look through the Milky Way from where we're located in the solar system.
Centre of galaxy in this direction. And here in the near infrared is what you see.
You penetrate the dust, the obscuration goes away, and voila.
There's this beautiful, flat, bright, bulgy bar.
Is the bar actually seen edge on a galaxy there?
And in fact, if for not for the dust, I hope most of you have seen the Milky Way.
But unless you're in a really dark site, it's you know, it usually surprises people.
It's not an obvious thing to you. But believe me, if if that dust weren't there, there would be no doubt it would just dominate the sky.
Very bright at night. Infrared has a second advantage,
and that is if we go to even longer wavelengths that are still called infrared rather than a few times the wavelength of, say, red light.
If you go two of order tens to 200 times brighter than those wavelengths.
Then the boots, the dust glows.
And that's just a basic property of physics that all objects in the universe emit light at some particular wavelength.
The cooler they are, the longer the wavelength at which it glows.
And it turns out that the dust in our galaxy or other galaxies is heated by the starlight to temperatures of anywhere
from a few degrees above absolute zero to about 100 on average about 20 or 30 degrees centigrade above absolute zero.
And at those wavelengths, they glow characteristically at these wavelengths.
And by the same token, objects on the earth, which are, of course, at near 270 degrees Kelvin ish, also glow in the infrared.
And these are just some infrared pictures, you know, hairdryer, dog, penguin.
You can go online and find all sorts of these things.
Now again, what I this the dust.
This is no trivial matter. Even though that dust only makes up .000 2% of the amount of mass in the universe, if you count the dark matter,
the dark inner resonance of the dark energy, it actually over the whole universe absorbs half of all the starlight that there is.
And it isn't just any half, it turns out. It's it's where the dust tends to concentrate, where the stars are.
That's the kind of Schmidt law. It's basically the there's a very tight correlation where stars form and where the gas and the dust concentrate.
So it's year until we went to the infrared.
We not only needed it to recover that the half of the picture we were missing, we got a completely different picture when we did it.
Progress in this field had to wait, however,
was technology limited that needed the development of infrared sensors and the ability to put them on cold telescopes and launch them well into space.
The reason it doesn't work on the ground for a couple of reasons.
For one thing, a lot of this infrared radiation doesn't penetrate the Earth's atmosphere.
It's the greenhouse effect. And reverse greenhouse effect is the CO2 and the methane and so on, let visible light in.
But when the earth glows, the ground gets warm and heats back.
The heat tries to escape the infrared wavelengths.
The atmosphere traps it. So that same trapping effect keeps infrared radiation from space, by and large, from penetrating to the ground.
And the other problem is, since everything it room temperature glows brightly in the infrared.
Of course, it's difficult to see the sky.
By contrast, one of my colleagues from the University of Arizona, where I used to work, George Rekhi,
has written a book about this, and he likens observing the sky from the ground in the infrared.
You can try. He likens it to try to observe with a conventional telescope in broad daylight, with all of the lights on in the observatory.
And with shining, you know, holding a blowtorch down over your detector while you're doing it.
I actually Jorge didn't have the blowtorch prior to that, but it just is correct.
So the way to escape all of this is to put your telescope in space.
And there have been a series of missions, all sky surveys on the left and pointed telescopes on the right.
This is an area in which Europe and the UK in particular have played leading roles to reveal that part.
So when you do that that you unveil basically.
And so just a couple of examples before we move to galaxies.
Here again is the famous Eagle Nebula, part of a complex called M16 in Sagittarius.
Here's the famous Hubble photo. What you see in the visible, you see this dark obscuring material which is hosting the young stars,
but you don't see the stars themselves, observe the same region in the infrared,
and you see that in fact, the stars themselves are forming actually at the interfaces between these fingers of molecular material,
basically in dust and the rest of the nebula.
And when you you can now map the entire Milky Way and make the first full inventories of star formation in the galaxy.
I'll come back to talk about that later. One thing I won't have time to talk about today, but is a key feature of studies is that this part,
the infrared, is also a very rich part of the electromagnetic spectrum.
Of all the species of material that sit between stars radiate at these wavelengths ionised gas, the accretion disks around black holes.
In this case, molecular molecules from the cold gas, atomic gas to just about any temperature.
And even the dust grains themselves, they don't emit continuously.
They have a band structure that can be used to trace the solid phase of the interstellar medium.
Some of what I'll talk about the diagnostics came using some of this, but I won't have time to go through it explicitly.
So that's the Milky Way.
When you look at other galaxies in the infrared, depending on what wavelength you tune your telescope to, you will get very different pictures.
This is M81, another favourite, a sort of amateur telescope project.
Here's what it looks like in the visible. Here's an infrared composite.
And basically at long wavelengths you're looking at different components, different species of interstellar dust.
It is tracing in exquisite detail the distribution of cold gas, the raw, the fuel for the star formation.
And as you go to the longer wavelengths, you begin to see you see that gas, but you're going to see concentrations.
Each of these as are being heated by newly formed stars.
And you can also measure the star light.
That has been essentially the start of the forming stars, the light from them that has been blocked by the dust at other wavelengths.
And what really the the emphasis of work in the past decade is on multi wavelength studies.
The infrared shows the hidden star formation. But of course not all of the the star formation is blocked by dust.
And so the way you can pick that up is by observing the same regions in either the ultraviolet.
Part of the spectrum, just short word of visible light where young stars we met.
Most of the mass of young stars emit most of their energy or in line sensitive to the nebulae.
The clouds of gas like Orion, for example, the boom or alpha line in the red.
This is a composite of both. And so any of these can be calibrated.
You can take the brightnesses of galaxies or region subregions in any of those wavelengths,
but by combining the obscured and the obscured get very precise measurements of the star formation rates.
One of the things I've spent a lot of time in my career is calibrating this toolbox.
Boring bore you to tears by showing you the details. So I won't.
But I will show one slide for the experts.
This just shows what these multi wavelength techniques enable you to do.
These are plots of measuring the number of stars forming in a galaxy in terms of luminosity in different ways.
On the top plots, it's an infrared tracer of a very one way to eight and 24 microns versus an optical tracer dust.
Correct. At each alpha.
And you get general before you if you only look at the dust emission and you only look at the unknown obscured, you get a general correlation.
But the scatter is more than a factor of two and and the total scatter of our galaxy.
Individual galaxies can deviate by over a factor of 100.
Either either the they have they're incredibly obscured.
So you see nothing in the visible or you can have the other extreme.
They have no dust at all for various reasons, and the dust is missing it, whereas there are fairly straightforward schemes,
actually linear combinations of the two, which then when you calibrate this measure, the star formation rates in different ways give you consistency.
That's good too, less than 10%. So we've gone from factors of a few errors, down 2% to 10% or less.
Not as good as the precision of some of the cosmological measurements or what a
physicist in the field would consider good but superb for this sort of application.
Real progress. So those are the tools.
And if we start now applying these measurements to galaxies around us, what do we learn?
So here is a Hubble tuning fork.
Let me just make sure I've got your Hubble's tuning fork.
But now from one of the major surveys from these, in addition to surveying the Milky Way,
there have been a number of projects targeting nearby galaxies and distant galaxies at a range of infrared,
visible, optical, actually radio X-ray wavelengths. This is one of them from the Spitzer Infrared Nearby Galaxies Survey project I led in the U.S.
And the galaxies we've arranged the galaxies under a long Hubble sequence.
And you may notice a trend here. The regulars down here, as you go left to right, galaxies here are blue.
This blue is from starlight, actually, infrared starlight, ironically, old stars.
Whereas on the right you see more lumpiness and more red.
The red is 24 micron emission from active star forming regions.
And so the first thing we learned, we saw this in the visible is the Hubble sequence.
One of the things that defines the Hubble sequence is that it's a star forming sequence.
Now, the one way I can illustrate that is just with the case study taking two of the galaxies closest to the Milky Way.
On the left you should recognise some many. You should recognise these.
On the left, the Andromeda Galaxy.
It's the nearest large neighbour to the Milky Way, a very similar in mass to ours, about 50% more massive than the Milky Way.
2 million, half million light years away. You can see it has a few satellite galaxies.
On the right is is the large Magellanic Cloud.
This is the nearest satellite of the Milky Way, much closer, 160,000 light years away.
And much, much smaller than either Milky Way or Andromeda, about less than actually 10% of the mass of the Milky Way.
There's two images are roughly two, two angular size.
So Milky Way on the Andromeda enlargement closer have comparable angular size.
You have to be in the southern hemisphere to see Magellanic Clouds.
If I put them on the same absolute scale, here's the LMC you see, it's roughly it's actually small compared to some of the dwarf galaxy companions.
So huge difference in number of stars, a total mass of stars formed.
On the other hand, if I measure these galaxies in wavelengths that are sensitive to the star formation.
This is I'm afraid it's mixed media here.
We've got ultraviolet here for Andromeda and H alpha for LMC.
Nevertheless, the visual impression is clear even though this galaxy is more than 15 times less massive.
If you add up the number of stars forming today,
they're comparable between the two galaxies that those low mass gas rich galaxies on the right hand side of
the Hubble sequence are forming a relatively large fraction of their stars today relative to Andromeda.
That those differences even extend down to the smaller scale.
If you wanted a for reference, you know what would Orion if Orion were placed in a large Magellanic cloud, what it would look like?
What would be some of these almost unresolved dots? Maybe that object there, that object there, maybe?
Well, not that that's several Orions, but yeah.
Look at this monster over here.
This in the LMC, there's a region called 30 DORADUS called the Tarantula Nebula, 30 DORADUS means it's the 30th brightest star in the Constellation.
Right? It's one like Orion. You can see this star formation region without binoculars, but this one's 160,000 light years away.
If we zoom in in emission lines, you can see the reason for the tarantula name.
An incredible complex of bubbles and loops. And this thing.
Whoops, sorry. Is. Well, where's my laser?
Laser here.
If we zoom in on the centre, whereas the Orion Nebula was powered by four massive stars, or this one is powered by more than 2000 of those stars.
In fact, a couple dozen stars of more than 100 solar mass times the mass of the sun we used to think is the mass limit for stars.
And in total, over 40,000 stars, I think, forming in this region.
And so the Hubble sequence turns out to be a sequence, not only the total number of stars,
but also in the mass spectra of the masses of the brightest individual complexes.
And that has implications for feedback is all talk about later.
It's difficult in the public talk. I'll show a couple of graphs in a minute, but this diversity extends in all directions,
just as there are galaxies which have much more star formation per given volume mass than the galaxy.
The big one. The extremes go down thousands of times in the other direction.
These are four examples of very low density star forming environments.
This is an example of an early type galaxy,
what we call a lenticular S0 galaxy that used to be called this class of Gaussian is called red and dead thought to not be forming any stars today.
But when you look closely, often are forming very tiny amounts, but star formation still going on.
We now know that star formation extends to the outermost edges of many galaxies.
Here's an example of a dwarf galaxy. Even more than a factor of ten less massive than Orion, I think 100 times less.
Oh, sorry. Ten times loss of the LMC that has a very of sporadic staffers, but it's ubiquitous.
It's difficult to find galaxies with any gas in it that is not forming stars.
But this particular subject is one where a lot of work has been done here.
Both studies in the ultraviolet or the integral field spectrographs where they're
studying galaxies were thought to be gas free and devoid of star formation.
And what you find is typically in a third of the cases, in fact, nature is still finding a way to form stars.
So it's a ubiquitous phenomenon, far more widespread than we had thought it was.
It's just one example in the extreme will connect to something we'll talk about in about five, 10 minutes.
We even see star formation in environments which by any prediction, would have been totally hostile to the formation of stars.
This is amateur image of the Perseus cluster of galaxies, a cluster you see about a quarter of a billion light years away.
There are a couple of hundred galaxies in this region of the cluster.
Galaxies often congregate in these dense clusters.
There are the most massive structures found in the universe and you probably can't see from where you sit.
But the centre of mass of this cluster is NGC 1275, and it's got a little bit of a peculiar appearance to it.
And if I show you, I'm afraid orientation has been flipped. But here is the galaxy imaged in H alpha.
And if you look close up, it's, it looks like to be an elliptical galaxy,
but it's surrounded by these filaments of ionised gas, which you will also have dust associated with them.
This is material from what is called the cooling flow of the the dominant mass in
these clusters is a huge cloud of hot gas temperatures of of millions of degrees,
sometimes tens of millions of degrees, more mass in that gas than in all of the stars in these galaxies.
And in Perseus, the gas in the centre is cooling and raining down onto the central galaxy.
And if you imaged this in the ultraviolet with Hubble, you find a lot of behold a cluster of stars forming all over the place.
So this, you know, what started out as a million degree gas is finding a way to cool and it's still forming stars today.
So it's happening everywhere. You can quantify these trends in various ways, and I'll show you two ways in which we do it.
I've been showing you Hubble sequence, but of course Hubble type is a rather non quantitative variable.
It turns out, although the Hubble sequences, the sequence in star formation and of obviously disk structure versus spheroid and so on,
but also turns out to be a mass sequence in the sense that the elliptical galaxies
tend to attain masses at least ten times more massive than the brightest.
The most massive spirals in galaxies like the Magellanic clouds tend to be less massive yet.
So this is a measure from the Sloan Digital Sky Survey showing for a sample of about a half a million such galaxies.
The star the rate at which stars are forming normalised to the massive stars that have already formed.
So it's a measure, it's essentially a way to say how active taking out the size effect,
the fact that bigger galaxies have more of everything as a logarithmic measure mass.
Milky Way would be somewhere here, roughly here. What you see is you see these are the spiral galaxies, basically.
And they form a tilted but rather narrow sequence, rather remarkable that when you divide out the mass,
most spirals of the universe today are forming about the same amount of fraction of their stellar mass out of of raw material into stars.
This is a narrow sequence that isn't fully understood, but it's a hint that there some sort of thermostat.
There are some regulators that switch star formation on and off in galaxies.
And here are the the nearly red and dead, elliptical and lenticular galaxies up here.
You see, they're reforming about a hundred times fewer. So it's not zero often, but much less.
Another way to look at it that's important for understanding the Senate law that I'll be
introducing in a minute is just showing the star formation properties in another way.
Oops, I keep having trouble up here is for a sample of galaxies in the nearby universe.
Mainly normal galaxies, the distribution and the x axis of how many stars form every year.
Purple is us. Milky Way. Well, Milky Way's forming about two solar masses.
A star like the sun, about two such stars formed every year in the Milky Way,
and the maximum in the local universe is about ten, ten or 15 such things.
And then, but the rates as you go to these rendered galaxies,
so it goes down essentially to zero to you know, well, one part of ten to the five times less than that.
Also notice and I want to describe the sequence then there what's the vertical axis?
It's also the star formation rate. But now I have normalised it to the the area where the projected area in which the stars form in the disk.
So it's not per unit mass, it's per per unit area.
I'll explain why we do that later. And you see, there's also a rather tight sequence.
Well, first of all, there's extraordinary diversity, right?
There's the range in star formation rate, no matter how you measure it, is over a factor of 10 million, an either axis at any one fixed absolute rate.
The dispersion in star formation rate per unit area or vice versa is over a factor of 10,000.
So there's, you know, there's tremendous diversity.
But among normal galaxies, they also tend to cluster in this diagram significance that to be explained in a minute.
Now. Here we go. So up to now,
I've been talking about star formation along the normal Hubble sequence of the extended
Starfleet like that in Orion like that we've been seeing in the images you saw.
But with the advent of opening up the infrared part of the spectrum,
we realise there's a whole second mode of star formation that had been hinted at previously but not fully understood.
And so here's what one of the few times you can get a hint of it in the visible.
Here's a nearby spiral NGC 1097 Spiral like the Milky Way barred spiral imaged with
Hubble and the blow up shows that if you look in the centre of the disk of this galaxy,
there's actually a spiral within a spiral, a very tight ring of about a thousand,
couple thousand light years across where you have intense number of clusters of stars.
Now, when you look at this in the visible, that looks well, that's interesting.
But, you know, I'm not going to write home or write to nature about it because that looks like, well, maybe there's 10% excess of star formation.
And that's what we all thought until about 15 years ago.
But if I look in the infrared at the same galaxy, suddenly the picture changes.
This thing only looks like a 10% perturbation because dust is actually, in this case, obscuring something that 97% of the star formation.
And we look in the infrared, actually, this ring saturated the detector of the instrument on the satellite.
And in fact, something like two thirds of all the star formation in the galaxy is being emitted by this very compact region.
In the centre of just this little region alone has nearly an order of magnitude more star formation than the entire Milky Way.
The poster child for the such galaxies.
The nearest example of an extreme of this is a companion to M81 Ursa major M82 in this beautiful, immature white field image again.
And you notice M82 is rather oddball.
Hubble's classification works about 98% of the time.
You can classify galaxies as elliptical spiral or irregular, meaning irregular with at least some semblance of uniformity.
But this one looks more like a train wreck. Something is going on right in here.
And so we call it peculiar, or there's a class called irregular to to explain it.
And if you go around the sky, it's only maybe it's far even less than 1%.
You see a whole class of galaxies back again.
When we study these in the visible, we could see what some of these guys were unusual,
but it's only in the infrared when suddenly we realised there was something extraordinary about them.
Here's M82 in a closeup and it's an example of an infrared luminous starburst galaxy.
Whereas in M81 excuse me, or Milky Way, the interstellar dust is blocking out about half of the starlight in this case when we corrected.
So in other words, if you correct for the obscuration of the dust, you double the star formation rate.
When you do it for m82, you increase the star formation rate by more than a factor of 100.
Over 99% of that light is being blocked.
And in fact, that galaxy's forming more stars than all the Milky Way and all of that stuff,
which is coming from a tight little disk, a few hundred, well, about 1000 light years across.
And it's a class of what are called infrared luminous or an ultra luminous galaxies.
The most extreme examples of this one is a nondescript galaxy in the optical called ARP 220 is actually as luminous as quasars.
It's putting out pumping out more than ten times the energy of the entire Milky Way galaxy.
And it's being powered by and large by young stars, whereas the Milky Way is forming two stars like the sun every year.
This one's forming about 500 solar masses per year.
And all of that star formation is going on in a region that is 100 times smaller than the disk of the Milky Way.
Sort of extraordinary. And in fact, objects like this one and this one are, if you work it out,
are forming stars at the fastest rate that Mother Nature will allow the cause of the bursts.
You can see that these all have the train wreck appearance and that in this case, appearances are.
Do explain it. They're all the result of some sort of collision or interaction between galaxies.
In each of these cases, two galaxies have or are merging together.
And basically, in the case of our 222 galaxies, the mass of the Milky Way, a little bigger, have merged together.
All of their gas is being funnelled into a little disk.
A couple thousand. Well, several hundred light years across in that gas.
Rather than forming stars over 13 billion years,
it's essentially converting entirely or almost entirely to stars instantly and limited by the time
it took to bring that gas together about a hundred thousand years with nearly 100% efficiency.
So nature turning the dial all the way up.
In the case of Emily, too, it's a very complicated waltz.
It wasn't clear. It wasn't merging with M81.
But if you image this part of the sky in cold gas, which is the fuel for the star formation,
there you see the train wreck is there's a very complicated dance going around
involving at least six galaxies of where clearly M82 and this is NGC 30,
77 have made close passages to M81 taking essentially accreted gas,
which is falling onto these and and creating the immense starburst that we see here.
If I put a bring up the diagram I showed you earlier with the normal galaxies in the Milky Way and add these various peculiar galaxies,
the centres of galaxies like 37 objects, like ARP 220.
You see, they extend the diversity of star formation even more radically.
Now these galaxies form up to actually over a larger part of the universe,
up to a few thousand solar masses per year, with concentrations over a million times higher.
And so you have an extraordinary phenomenology that you want to try to understand.
Running a little short on time. So I'm going to skip through a few things.
The. I can't leave without, of course, trying to explain the structure you're seeing here,
the stars, young stars forming or are buried even in the infrared, they're highly obscured.
We're at a very tight cluster buried in the centre, and everything else you're seeing here is an outflow.
It's the star formation has been so intense that the the star formation is an extra thermal process.
The stars pour more energy into their birth clouds than they consume in being formed.
Stars explode massive stars as supernovae. They inject radiation into the interstellar.
They inject gas and winds. And it's actually, in this case, blowing the gas that fell in and blowing completely out of the the galaxy.
In fact, this will be a self terminating process. You're in the end,
we look at what the main power of supernovae like the supernova of 1054 that made the Crab Nebula
but on a grand scale in 30 doradus we see these are X-ray bubbles powered by individual supernovae.
But in a case like M82,
what you're seeing are the collective effects of literally thousands of supernovae that have occurred over the last few tens of thousands of years.
Blowing that away on this feed. This so-called feedback has profound effects on galaxies.
The the if you look at the distribution of masses of galaxies in this case, in the local universe, it has a very characteristic shape.
It has power law over a restricted range.
But it but galaxies are very high mass and very low mass tend to be suppressed relative to the distribution of dark matter particles.
If, if, if there was no feedback from these processes,
we think galaxies of the distribution masses would follow a very similar power law to that of the dark matter that sees the formation of galaxies.
But something is suppressing the very most massive and lowest mass galaxies.
And here's a little cartoon picture actually from a previous civilian,
Professor Joe Silk, that sort of explains that in the very most mass of galaxies,
the process that's limiting is feedback from quasars and supermassive black holes regularly.
If you came to the last lecture in the series, talked about that, I'm sure it's related to what we looked at in Perseus,
but it's feedback like that you see in M82 that actually not only limits the number and size of the size of these galaxies,
we think in the lowest masses actually disperses galaxies before they can completely of form.
Here's the part I will not have much time to say, but I can give you the punch line.
Everything I've said up to now characterises star formation in the current present day universe.
But as we Hubble and other instruments, Herschel and Spitzer allow us to look out in the universe.
Of course, when you look out it further out, you look in the universe.
You're seeing the universe as it was further back in time.
And we have through work of Andy Bunker and many other people,
we're putting a picture together, a time lapse movie of how these galaxies came together.
As you go further back, the objects look more and more primitive.
We're actually seeing the assembly process of galaxies going on, at least in a statistical sense, in these.
And if you measure the amounts of star formation as you go back, you see lots of changes as you go back.
You maintain this for a while back in time.
This is today.
If you go back to the universe and was about 40% of its current age, you still have the Hubble sequence of elliptical galaxies and spirals.
But two things happen is over time the proportion of the number of elliptical
and lenticular galaxies decreases because even they were actively star forming.
This is back when the universe was about 15 20% of the present age.
And if you look at the amount of star formation that was going on, it was 10 to 100 times higher than it is in those galaxies today.
Those objects would populate this part of the diagram and actually upwards and onwards and.
You can actually measure the total amount of star formation in the air over a very large volume of cosmic time over the history of the universe.
Here we are today at the present time.
And as you can see, about 10 billion years ago, star formation peaked at a level averaged over the whole universe.
That was higher. It was higher by over a factor of ten.
Essentially what you're seeing is back then the andromeda's, which are relatively quiet today,
the mass of galaxies of they're quiet today because they converted their gas to stars much more rapidly in the past than they do today.
Finally, I can't let you go. I'm going to have to run two or 3 minutes.
So you know what the law is. I don't like talking about my own work.
You probably have gotten that idea. I always save it for the end.
Can we understand this incredible diversity?
Fair to say there's no theory. I can. You know, this is a difficult problem, multi scale problem.
No one, either on a computer or in an analytical calculation, can explain this diversity of properties.
But we do have a we've uncovered some empirical patterns and we can begin to understand some of the underlying physics.
And the key to doing it is in addition to measuring the star formation, just to make maps of the cold gas,
molecular gas and atomic gas and correlate that with the star formation, either on an integrated basis or on a point by point basis in galaxies.
So you've seen this plot many times. If I, for example, look at the properties of this incredible, diverse array of nearby galaxies,
star formation, rate per unit area, and look at as a function of, say, absolute star formation rate.
Even though this number is the denominator of this number, there are orders of magnitude dispersion.
However, for these galaxies, those where you can make the measurements, if I plot on the x axis rather than the surface density of star formation,
the surface density of cold gas, not the mass, but the surface density of that cold gas using the same essentially y axis as this.
The plot tightens up to this,
that among among the normal galaxies you have a very tight power law correlation between concentration of gas and concentration of star formation.
And if you make a plot for the two twenties and maybe twos, you get another power law relation that sits on the extrapolation of this.
And if you add the circum nuclear regions, they fit here, the dwarf starburst galaxies, they fit on it and you get a relation.
Now you might be thinking, okay, more gas, more stars.
Trivial, who cares? What makes this significant is it's not a linear relation.
It isn't ten times more gas, ten times more stars.
It's got a slope of about one and a half and power law, which means 100 times more concentration of gas,
about a thousand times or several hundred times more star formation.
So the more you concentrate the gas, it's a very nonlinear nature.
Nature, you squeeze the gas a little, you get more star formation out.
In the end, don't have time. But it's this is actually when you look at a spatially resolved basis is a combination of
a steep power life threshold at low density and a more linear relation at high density.
We'll leave it there. I'm not going to go through these in detail, but this law,
the reason people make a fuss about this law is, you know, it explains all kinds of things at once.
It explains why when you bang two galaxies together, you can get off where you doubled the gas mass.
You can get 100 times more star formation. It's climbing up this nonlinear relation that explains why high redshift.
There was much more star formation today because there was more fuel and if there was ten times more fuel,
you'd get a you'll get, you know, 30, 40 times more star formation.
That explains why massive galaxies evolved more quickly than low mass galaxies, like the Magellanic Clouds.
You even can explain some of the size of evolution and growth, and I don't have time,
but actually explains why all the most active regions of star formation were completely invisible in the.
I think invisible in the in the optical, because the intense star formation only occurs where you have high concentrations of gas.
And that's also if you express that in concentrations of dust.
This line is essentially where 98% or 99% of the jet of the light starlight is absorbed.
And so it all sorts of of hangs together.
And for the cargo cente you begin putting this together,
you can begin to understand this whole restricted space of, of, of why galaxies are following the locus they have.
It's a combination of cosmology which sets a maximum size for galaxies and then fundamental timescales of galaxies.
The only observational limit and the one is that diagonal, which is just the resolution limit of the instruments we have today.
Finally, less like 5 minutes over.
So as the field is evolving, I hope you want this is progress made.
You can see many, many unanswered questions. You may think that's just as I went through things too fast and you didn't understand half of it?
Nope. There's lots of mystery even for the experts, but there is great promise for pushing this.
The is already being advanced transformational lead by the Atacama Large millimetre array and in a
millimetre array that can map the molecular gas and dust in galaxies with a better resolution than Hubble,
the successor to Hubble, the James Webb Space Telescope,
that will allow these kinds of measurements to be carried out to the highest redshifts, the very first stars and galaxies.
And I'm afraid, I hope a dream sort of. Well, Leicester City.
So the Leicester City is equivalent for us astronomers.
There's a mission called Spike, a Japanese led mission that would be the next infrared, the successor to Spitzer and Herschel.
Apologies to run over, but I hope I have given you some sense of this beautiful convergence of understandings that is happening.
And please stay tuned, because much more to be learned in the coming years.
Thank you.