- Good evening and welcome to UW Space Place.
Tonight is one of our guest speaker nights here.
And tonight our speaker is Professor Christy Tremonti
who is with the UW Astronomy Department
and is involved with an important instrument
development project here that's going
to lead to interesting work on, as her title says,
cosmic chemistry.
So, Christy.
(applause)
- Thank you, I'm really delighted to be here tonight
and to tell you about this fascinating topic.
So I wanted to start off with a quote from Carl Sagan
that I thought was both funny and particularly apropos.
And it's this idea that if you want to make something
from scratch, you really gotta think about
where all your ingredients ultimately come from.
And that's what we're going to be doing tonight.
We're going to be thinking about that apple pie
might be full of carbon and oxygen and hydrogen,
and we wanna think about where all
of those ingredients ultimately came from.
So one thing I should note is that what we're going
to be talking about tonight is the ordinary matter
in the universe, things that are made of protons,
neutrons, electrons, things that are familiar to you.
This is only 5% of the total universe.
When we think about the mass and energy budget
in the universe, the stuff that we know and love
is a mere 5% of it.
But we're gonna concentrate on that for this evening,
and I hope to show you that even among the stuff
we think we know well,
there are plenty of intriguing mysteries.
So tonight I hope to touch on a couple
of different questions.
We're gonna first think about
where the chemical elements come from.
This is something we actually know pretty well.
Then we're gonna think about how the chemical elements
are distributed in the universe.
And this is something we have a broad brush picture of,
but we'll talk about how we're learning more about this.
And then finally we'll touch on a real emerging field,
how life in the universe, or the potential for life,
at least, could be shaped by this distribution
of chemical elements in the universe.
Okay, so Jim warned you, I'm a professor,
so I thought it would be appropriate
to begin this discussion with a quiz.
(audience laughing)
So if you can think for a minute about what the top
four most abundant elements are on Earth,
and let's confine ourselves to Earth's crust,
so this is, again, the stuff you're familiar with,
and if we're gonna be a little technical,
let's say we want the top four by mass.
So which of the elements on Earth
contributes the most mass to the crust?
So what do you guys think?
Does anybody want to offer a suggestion?
Yeah, back there.
- [Voiceover] Water.
- [Christy] Well, water, but what's water made of,
what's inside of water?
- [Voiceover] Hydrogen and oxygen.
- [Christy] Yeah, yeah, so, okay.
So you did pretty good.
Anybody else want to suggest something?
- [Voiceover] Silicon.
- [Christy] Silicon, yeah, that's up there too.
- [Voiceover] Carbon.
- Carbon, yeah, all good choices.
In fact, the answer's a little bit surprising.
Oxygen makes up nearly half,
which really does seem surprising when you think about it.
But I think a lot of that oxygen is locked up into rocks
and various oxides.
There's a lot of silicon, we heard that.
Aluminum and iron, carbon would be down just a little bit
lower on our list, I forget exactly where it clocks in,
but it's not too much further below.
So that's the crust of the Earth.
If we decided to average over the entire Earth
we would have gotten the core as well,
and then three and four would have been
iron and magnesium instead.
But the Earth is kind of a special place
in the universe, right?
So let's enlarge our box a little bit,
and let's think about what the top four
most abundant elements are in a slightly bigger volume.
In this case, let's take our own Milky Way Galaxy.
So what kind of elements are the most abundant
in our Milky Way?
- [Voiceover] Hydrogen.
- Hydrogen, oh, okay, you guys are on this one.
Very good, we heard hydrogen and helium.
So hydrogen makes up 74% of the Milky Way's mass.
If we had instead decided to count number of atoms
it would be something like 90%, right?
So 90% of the atoms would by hydrogen,
74% of the mass, 'cause hydrogen is light.
Helium making up 24%, and oxygen,
which was so important on Earth, almost 50% of Earth,
is now a mere 1%.
And in fact, if you add up all of the other elements,
it's 2% total.
So all of the stuff what we think of as our lives
being made of is actually really, really rare
when we think about the Milky Way.
So that, in itself, is something interesting
to take note of.
Now, largely because of this dominance of hydrogen
and helium, astronomers have maybe an unusual way
of looking at the Periodic Table.
So here is our Periodic Table of the Elements.
And again, the numbers there just indicate
the relative abundances.
But because hydrogen and helium are so dominant,
and they're such a tiny fraction of everything else,
we tend to label that everything else metals.
So to you a metal might be gold or iron,
to me a metal is anything that's not hydrogen or helium.
So I'm gonna lapse into using that terminology,
so I thought I'd just introduce that up front.
So when I talk about metals,
predominantly we mean oxygen, actually.
Okay, so now we can start to think about
where these chemical elements come from.
And so before we do that, we should pause for a moment
and just remind ourself what we mean by chemical element,
what defines a chemical element.
And of course, that is the number of protons in the nucleus.
So a hydrogen atom has one proton,
a helium atom has two protons,
a lithium atom has three.
You can have different numbers of neutrons
and that can give you some different isotopes,
but the basic definition of what is a chemical element
is defined by the number of protons.
And we can go right across the Periodic Table,
and as we go across we're just going up
in terms of the number of protons in the nucleus.
Okay, so how do we make these chemical elements
with their various numbers of protons?
Well, it turns out the first important step happens
in the first three minutes of the universe's history.
So we think that our universe began
in what we call a hot Big Bang,
when the universe was incredibly hot, infinitely hot,
infinitely dense, and then it began to expand and cool.
Now, it didn't have to do this for very long
before things started to resemble our normal universe.
So initially, right after what we call cosmic inflation
ends, the universe is this soup of subatomic particles,
so things like quarks and other exotic particles
and their antiparticles.
- [Voiceover] Excuse me.
- Yeah?
- [Voiceover] By universe,
we only mean Milky Way or everything else?
- Here we're talking about, so the question was,
"By universe, do we mean just the Milky Way or everything?"
And so here we're really stepping back
and thinking about everything.
It turns out the Milky Way is a pretty typical galaxy
in the universe, so it's not a bad representation
of what much of the universe is like.
- [Voiceover] And we're also ending
this with visible matter.
- Right, so so far that's correct.
So so far, we're only talking about the visible matter,
although of course, all that dark matter and dark energy
is also expanding along with the rest of the universe
during the Big Bang.
But we're gonna focus on the visible matter for tonight.
Okay, so initially we have this soup of subatomic particles
bathed in very, very hot radiation.
We have very energetic particles of light
bouncing all over.
But by one millionth of a second,
the universe has become surprisingly normal.
Those subatomic particles have bound themselves
into protons and neutrons, and the universe
is still very, very hot, but in many ways
it resembles the universe we know today.
It's made of the kind of matter we're familiar with.
But we're still not, in a millionth of a second,
we're still not ready to form any chemical elements
because the universe is so hot that we have all
of these energetic particles of light that we call photons,
and were you to bring two protons together to try
to make a helium atom, those energetic particles
of light would at first just knock them apart.
So we need the universe to expand some more,
the light becomes less energetic,
the light is expanding along with the universe,
the wavelengths are becoming longer,
the energies are becoming lower.
And then, at a certain point, hydrogen,
a proton can begin to bond with a neutron to form deuterium,
which is an isotope of hydrogen.
And then deuterium atoms can combine to form helium
and in a few cases we can also form lithium.
So at about 100 seconds we begin this era
of what we call nucleosynthesis.
Now, the temperature of the universe at this point,
and this is initiated, is about one billion degrees Celsius,
and that's just right for this sort of nucleosynthesis
we want to do.
Temperature turns out to be pretty important
because what we're trying to do is
to get these protons to combine.
And the thing about protons is
that they're positively charged, and positive charges,
any like charges will repel.
Just like magnets, try to jam two north poles together,
they'll repel, try to bring two positive charges together
and they will repel.
So what are you gonna do?
Well, you need to apply some more force
if you want to bring them close.
And...
Temperature is important
because temperature is really a measure
of how quickly particles are moving.
So at low temperatures, particles zip around
kind of sluggishly, but at high temperatures
the particles are moving quickly.
And so at the high temperatures of a billion degrees
or so, the particles are moving fast enough
that you can overcome some of that repulsion
if you have a high-speed collision
between some of these protons.
Okay, but the universe, of course, is still cooling.
And eventually it's no longer hot enough
to form any more elements.
So we kinda stop once we've formed deuterium
and helium and a little bit of lithium.
Essentially, nucleosynthesis comes to a halt.
So at this point we've made the first three elements
on the Periodic Table, but we've got a lot more work to do.
Now, this work can't continue in the universe at large
because it's too cool.
So we have to wait for about 100 million years
for the very first stars to form.
And once those stars begin to form,
the cores of the stars, not the whole star,
you could think of most of you probably know fusion
is happening in our sun, hydrogen is being turned
into helium and releasing energy.
It doesn't happen in the whole sun,
it only happens in the core
because that's where it's hot enough.
So in the cores of stars the temperatures reach
the necessary conditions for fusion to resume.
Now, in a star like our sun,
we can fuse hydrogen into helium.
Eventually when our sun evolves the core
will contract and become hotter,
and then we might be able to fuse some helium into carbon.
But every time we're making a heavier element,
so here's the kind of fusion chain of knocking together
some helium atoms to form beryllium,
adding another helium atom and forming a carbon atom,
every time we do that we're talking
about getting particles to collide
that have larger and larger positive charges.
And again, the more positive charge you have,
the more those things are gonna want to repel.
So we need higher temperatures once again.
So our sun isn't gonna reach temperatures
that are gonna allow us to fuse carbon
into, say, oxygen or silicon.
So the heavier elements are actually formed in stars
that are about 10 times more massive than our sun.
And they actually have an extremely interesting structure
were we to be able to carve away and look
at the inside of one of these massive stars.
Oh, I don't think it's going to let me use my pointer.
But we would see this kind of onion-skin like structure
where on the exterior we have
hydrogen fusing into helium.
Below that we have helium fusing into some nitrogen
and some carbon, below that carbon fusing into oxygen,
and so on all the way until we get to this iron core.
Now, iron is where the fun stops.
So all these other elements, when they undergo
this fusion process, let's see,
I think we had a good picture back here,
are releasing a lot of energy.
So in the process of combining to make a heavier element
some energy is liberated according to Einstein's equation
E equals mc squared.
So a little bit of that mass is being turned into energy.
And that energy is what's holding the star up.
So the energy is supporting that star
against gravitational collapse.
You're generating energy in the core,
and that's holding up those outer layers
which would otherwise want to collapse.
Because the sun is a plasma,
it's not a solid like the Earth's crust.
Okay, so we have that energy generation holding up the star,
we make iron.
Now, in order to make anything heavier than iron,
you actually have to add energy.
You don't release any energy in the process.
So when we fuse silicon into iron
we're releasing some energy,
but if we want to turn iron into gallium
or something heavier, we're gonna have to add energy.
And that doesn't work out so well for the star, right?
This is not what the star is all about.
The star wants to produce energy to hold itself up.
So the minute we have a fairly large iron core,
that core is inert.
It can't fuse in it anything else.
It's gonna get compressed, it's gonna get very hot,
but nothing else can happen to those atoms
to generate any energy.
So at a certain point the outer layers of the star
are gonna collapse and they're gonna hit that core,
which at that point is gonna be squeezed
into a ball of neutrons, and they're gonna rebound.
And that event is what we know as a supernova.
So essentially the star is gonna collapse on itself
and rebound, and that bounce back is
what we think of as a supernova.
So this is what's left over after a supernova event.
This is a beautiful, glowing nebula in our Milky Way
where at one point there was a supernova.
So it turns out that in the supernova itself,
of course, it's a fairly brief explosive event,
but in that supernova itself the conditions are right
to form some of those heavier elements.
We have a large flux of neutrons coming out
of the supernova, and the conditions are just right
to make some of those other things.
So here we have a Periodic Table where the color coding
reflects the origin of the elements.
So now we've essentially made the whole Periodic Table
and that hydrogen and helium, again, we make a little bit
of helium in stars, but almost all of it
really is made during the Big Bang.
And things like carbon and nitrogen and oxygen
can be fused in low mass stars similar to the sun.
But once we go beyond that, most of those elements
are made in those very massive stars,
stars more massive than about 10 times the sun's mass.
And then beyond iron, most of those elements
are made in the supernova explosion itself.
So Carl Sagan had the famous quote,
"We are star stuff," and that is completely true
and really profound when you think about it.
All of the elements that make up your body,
the carbon, the oxygen, all of that stuff was made
in a star somewhere.
And it's entirely possible that it was cycled
through a star more than once.
So we know that stars form out of gas.
The most massive stars are going to synthesize a lot
of these elements in their cores, and those are the ones
that are gonna explode as supernovas.
A lot of low mass stars will live for a really long time,
but the massive stars burn out quickly,
go supernova, and return all those newly-synthesized
chemical elements to the gas, where they get reincorporated
into the next generation of stars.
So it would not be at all surprising, in fact it's likely,
that some of the atoms that make up your body were
in one or more very massive stars.
Okay, so now we can switch gears just a little bit.
And so now we know where all
these elements come from, right?
They come from the Big Bang for the lightest elements,
and they come from stars and supernova
for the heavier elements.
We can think about how these elements
are distributed in the universe.
And hydrogen and helium is going
to be essentially everywhere in the universe.
But when we think about these heavy elements,
we usually associate them with galaxies
because stars form in galaxies.
So galaxies are basically just collections of dark matter,
gas, dust, and stars.
So we expect that's where we'd find most
of the heavy elements in the universe.
And in fact, if you can see in this picture
the little glowing, red regions, those are regions
of star birth where the surrounding hydrogen
is being excited by these very young, massive stars
that are producing a large amount of radiation.
So this is the whirlpool galaxy,
which is a relatively nearby galaxy.
Okay, so it we wanted to think about, okay,
we can buy the premise that maybe most of the metals
are inside of galaxies, that seems kind of reasonable.
But if we really want to think about
how they're distributed, we have to think
about how we might actually measure them.
And so on Earth, when we want to figure out
the chemical composition of something
we have a variety of ways that we can do it.
We can throw it into a mass spectrometer,
we can run some different experiments on it
and see what it reacts with.
But we don't really have that ability when we're looking
at an object out in space, so we have to think
of other ways to figure out what types
of chemical elements it contains.
And the key tool that astronomers use
to determine something's chemical composition
is spectroscopy.
And by spectroscopy I just mean this process
of splitting light up into its component colors
or component wavelengths, and we can do that with a prism.
Here you can see that being done for a beam of white light.
Astronomers like to slap our spectrographs on the back
of telescopes and take a look at some of that light
coming from stars and from that gas
that's being excited by the stars.
So if we want to measure the abundance of something,
say oxygen, it turns out that oxygen to hydrogen
is a really interesting quantity, because oxygen is made
in stars and hydrogen comes from the Big Bang,
so that kind of gives us a sense of how much chemical
enrichment has happened by looking
at the ratio of the two.
We can slap our spectrograph on the back of our telescope,
we can point it at one of those little regions
of glowing gas, and we might get a spectrum
like the one you see here.
So look at, first,
at the lines on the lower part of the slide.
And when we looked at the spectrum that was dispersed
by a prism, you could see there
was a continuous distribution of colors,
continuous distribution of wavelengths.
And here we see that light, it seems to be coming out
at very discrete wavelengths.
We see these bright bars.
And what you see on top there in red is just
if we made an intensity diagram
of the light coming out, so we're just saying how much light
is coming out as a function of wavelength,
and we can see that most of that light is coming out
in these two strong lines, the one labeled O three
and the one labeled hydrogen alpha.
So we have some of the lines that can tell us
this is gas that's being excited by stars
and it's emitting discreet radiation.
And we can use this to estimate the abundances of oxygen
and things like oxygen and hydrogen.
So again, if we want to see what the distribution
of metals are in a galaxy, we can take our spectrograph
and we can point it at all of these nice, reddish regions
where we think stars are forming, the gas is glowing,
and we can go in and use our spectrographs
and figure out what our ratio of oxygen to hydrogen is
by looking at those, what we call emission lines.
So if we did this, normally this would be
a pretty time-consuming process
because we would take these spectra one by one
in each position in the galaxy,
and we might need to expose for something like three hours,
even with a big telescope, to get a good spectrum.
Takes a lot longer to do spectra than imaging
'cause you're spreading that light out
into its component wavelengths.
So very painstakingly we could come up with a diagram
like the one you see on the bottom,
which shows that the galaxy has more oxygen to hydrogen
in its center than it does in its outskirts.
And that's kind of roughly what we'd expect
because more of the gas has been turned into stars
in the galaxy's center, so more of that chemical processing
has already taken place.
But we really want to understand, not just the distribution
at a few isolated points in the galaxy.
If we wanna really map the chemistry of the whole thing,
we need a different approach.
And in particular, we'd like to know how the metals
are distributed, not just in one galaxy or 10 galaxies
or 100 galaxies, we'd really like samples of 1000
or so, or 10,000.
- [Voiceover] Could you explain the ratio R on--
- Oh, sorry, so that's just radius.
And so it's just being ratioed relative
to what we call the half-light radius of the galaxy.
So where that ratio is one you are
as far out as where half of the galaxy's light
is contained within that radius on the bottom there.
Yeah, sorry.
And on the y-axis there, it's just the logarithm
of the oxygen to hydrogen ratio.
So if we want to actually understand how oxygen
is distributed in a large sample of galaxies,
several thousand galaxies, we need a different approach.
And that is one of the goals of this survey
that the University of Wisconsin is participating in,
and the survey is called MaNGA,
for mapping nearby galaxies at Apache Point Observatory.
The leader of our project is in Japan,
so "manga" is also a kind of Japanese comic book.
So that's partly the origin of the name.
So MaNGA is one of three surveys that are part
of this lone digital sky survey,
which is a kind of ongoing project at Apache Point,
New Mexico, that uses this telescope.
And this is actually not a very big telescope,
it's a 2 1/2-meter telescope, which the big telescopes
that people are excited about today are 10 meters.
We're busy trying to build a 30-meter telescope.
So this thing is dinky, right?
It's not very big.
But it's done an amazing amount of good science
because it has a very wide field of view
and it's able to look at a large number
of objects at once.
So we'll come back to that point in a minute.
So the MaNGA project, I won't dwell on this,
but it's just a very international collaboration.
It heavily involves the University of Wisconsin,
but we have people from China and Japan
and England and Canada and a whole host
of places that have made huge contributions.
Okay, so the original Sloan survey really made its mark
on galaxy evolution by going out and taking spectra
of a million galaxies.
And it could do this because it had a really wide field
of view, and it could take spectra of about 600 galaxies
at a single go.
And the way that it did this was by taking individual
fiber optic cables and placing those fiber optic cables
at the back of the telescope and focusing the light
from an individual galaxy on those fiber optic cables,
and then running those cables out to the spectrograph.
But the first Sloan survey really looked at galaxies
that were relatively distant, and it just put one fiber
down on a galaxy.
So you can see, here are some galaxies,
they don't look particularly exciting.
They're a little small and blurry.
And we're able to get a spectrum right
at that central region.
So that was useful for lots of things,
but if we want to think about how chemical elements
are distributed within a galaxy,
we need to do better.
We need to have spectra of the whole galaxy.
- [Voiceover] How long did it take
to get this much data?
- So the Sloan survey ran, I think the initial survey was
about five years to get a million galaxies and quasars.
So a long time, and it's running all the time,
any time it's clear.
So yeah, that's a good point.
So MaNGA's secret to looking at the whole galaxy
is to take the optical fibers and pack them together
into these little bundles,
these little hexagonally-packed bundles
which you can see here.
And these are called integral field units.
And then we're gonna plunk those on galaxies,
and everywhere you see that little circle,
that's a fiber optic cable that will take light
to the spectrograph so we can get a spectrum
of the galaxy position.
So we're pretty proud of these because they were prototyped
at the University of Wisconsin by Matt Bershaday,
and you can see him holding one there in his fingertips.
They're actually remarkably tiny.
When you look at these things, I can't remember,
the width of the fibers is really small.
It's not that much bigger than the width of a hair.
And we're packing a ton of these together,
and again you can see this relative to Matt's fingers here
holding this device.
Now, the amazing thing is that we're gonna point
that very precisely on the sky such
that it lands on a galaxy.
And you might wonder how we do that,
and it turns out the process is surprisingly manual.
So what we do is we take an image of the sky
and we decide what galaxies we'd like to target,
and then we take a three inch, or a meter diameter
aluminum plate and we drill it full of holes
where we can plug those fibers into.
And so that's what you're looking at here.
You can see these racks of aluminum plates.
Those get mounted at the focal plane of the telescope
and they have holes custom drilled,
very, very precisely drilled,
where these fibers will get plugged into.
And these are now bundles of fibers
that will take everything to the spectrograph.
So here it is sitting at the very back of the telescope.
If you can see that slightly curved piece,
that's a bit of one of those aluminum plates.
And then all of those fibers are fed into the spectrograph,
and because we want a nice, filled-in image,
not an image made up of little circles,
we actually take three exposures moving our fiber
bundle just a little bit.
And then we resample that data into something
that we call a data cube.
So basically we have a three-dimensional image
where two dimensions are positioned
and one dimension is wavelength.
So we can say, "Well, for a given position,
"let me take a look at that spectrum
"and I can maybe analyze what I see there
"and determine the chemical composition."
Or I could say, "Oh, let me take a slice
"at a particular wavelength."
I could take ionized hydrogen, H alpha,
I could take a slice at H alpha
and see what that galaxy looks like.
So there are various ways we can use these data cubes,
but it's a really immensely powerful tool
for analyzing an object.
So each of those little pixels there again corresponds
to a spectrum, and so a spectrum just being
the intensity of light as a function of its wavelength.
And here are some real spectra, just to show you
how rich and complicated they are.
And we're not gonna have to dwell on any
of the details here, but all of those features
you see labeled correspond to particular elemental
transitions, in some cases they're molecular transitions,
so you see some things like titanium oxide.
So the basic point is that there's a huge information
in each spectrum, so having a spectrum gives us a lot
more to work with than just having an image.
So MaNGA's goal is to obtain spatially resolved
spectroscopy of 10,000 nearby galaxies.
So this is gonna be a huge amount of data.
And again, this is a Hubble image of this particular galaxy,
and you can see a MaNGA image of it nearby.
And our resolution's lower, right?
We're not trying to be Hubble,
that's what Hubble does best.
But at each of those little boxes there
we can go in and extract a spectrum.
So again, that gives us this additional information.
So to date, MaNGA has observed about 1800 galaxies.
This survey is gonna take about six years to complete.
And so you can see these are some of the galaxies
we've already obtained data on,
and the little purple outlines there are
the fiber bundle kind of footprint.
So a few galaxies don't fit completely
in our fiber bundles, but most of them are chosen
such that they do.
And you can see there's a great diversity
in the galaxy population.
We can see some galaxies that are edge on,
some that have bars, some that are young and blue,
others that are old and red.
So we're trying to target everything
and see what it looks like.
Okay, so the particular mystery we want to solve
with regards to the abundance distribution is this.
So this is one of the things MaNGA would like to untangle.
So in spiral galaxies, we know that the ratio of gas
to stars is high in the center and low in the outskirts.
So in the center we formed a lot of stars,
in the outskirts not so many yet.
It's quite gas rich.
And so we can predict what we think the chemical
abundances should look like based on that, right?
Because every time we form stars we're going
to have some of them go supernova
and pollute the gas with all
these freshly-synthesized metals.
So the blue line there on my plot,
which is just schematic,
shows what we expect in terms of the trend
between the oxygen to hydrogen ratio
and radius in a galaxy.
And this expectation, again, is just based on looking
at the ratio of gas to stars.
But the mystery is that what we actually observe
is a lot shallower than that, and it looks like
we're just missing some metals overall relative
to what we would have expected.
So the question is, what's going on?
Why isn't the chemical evolution of a galaxy
a little bit more simple?
And I don't have an answer for you yet.
We're actively working on that,
but I'll tell you some of the possibilities.
So one possibility is that galaxies
don't evolve in isolation.
They often interact with their neighbors,
sometimes little neighbors, occasionally big ones.
So here's an example of two galaxies
that are starting to interact, and you can see
that that large galaxy is actually being quite distorted.
Most spirals are nice and symmetric,
and you can see it's already being distorted
by its interaction with the smaller galaxy.
So it might be that those interactions really make things
slosh around in the galaxy, and those interactions
might be a way of mixing up
the existing abundance gradients.
You might get a lot of gas moving about in the galaxy disc,
and that could flatten your abundance gradient.
It would just mix things up a little bit.
Another possibility is that there's a lot
of chemically poor gas, pristine gas that was made
in the Big Bang that's pouring into galaxies,
that's accreting onto galaxies.
And you might say, "Well, okay,
"why don't you know about this stuff?"
The answer is that this is an artist's rendition,
and we would love to be able to see something
that looks like this, but the gas that's coming in
is really diffuse and it's very, very difficult to see.
So we have a really difficult time.
We can't take an image and see gas accreting
because it essentially comes in almost invisible.
It's very diffuse and very difficult to detect.
So it may well be there, but we don't know.
Another solution, which this almost certainly happens,
the question is to what degree does it happen?
Is that metals are driven out of the centers of galaxies
by the collective effects of multiple supernova
explosions happening at once.
And so some of you may have attended John Chisholm's
talk last month.
He talked about this phenomena
which we call galactic winds.
And it's basically the idea that the supernova,
when a lot of supernova go off in close proximity,
they can essentially work together.
So those supernova can work together to build up
more pressure and drive some of this gas
out of the galaxy.
So many of you may know this galaxy's M82,
it's an edge on spiral, and what you're seeing in red
above and below the disc is gas
that's being driven out in one of these galactic winds.
So the real question is just, we know some stuff comes out,
the question is how often does it come out?
And how much is coming out?
Is the stuff that's coming out that raw supernova ejecta
that's super metal rich, or is it more ambient gas
that might not be as chemically processed?
So what we're looking at here in red is ionized hydrogen,
so it's H alpha emission.
So we do know that it's hydrogen gas,
but we would need to take a spectrum of it
to see how chemically enriched it is.
The complicating factor with galactic winds is
that we have gas at a wide range of temperatures.
So the stuff that we see in H alpha that
if we took a spectrum of it we could get
an abundance for it, might not be representative
of all the gas.
So we could also have some of that supernova ejecta
that was very, very hot, that we wouldn't be able
to see in the optical.
It would emit primarily in the X-rays.
So if we had an X-ray spectrograph,
we'd be able to see it and tell what's there.
But it's too hot to emit at optical wavelengths.
So galactic winds are really complicated just
because there is gas at so many different temperatures
that getting the full picture is tricky.
But that's a great question.
So one of the big surprises is that it turns out
that only about 25% of the metals
that we expect galaxies to produce
can be found in their discs.
And this was a huge surprise 'cause, again,
the stars produce the metals and you expect them
to stick kind of close to their sites of production.
But in fact, we're now starting to realize
that a huge reservoir of both gas and metals exists
around galaxies in something that we now call
the circumgalactic medium.
So it's fairly hot, diffuse gas that surrounds galaxies,
some of which was blown out in these supernova explosions,
some of it may be gas that accreted onto the galaxy later.
So that's kind of mind blowing,
only 25% of the total metals produced
actually live close to their sites of formation.
So our hope with MaNGA is to try to get a better idea
of how these different processes, all three of them
undoubtedly take place.
The real question is, which ones are most important
and which ones are most important when?
So we can do things like make maps of the galaxies,
the gas and the stars and how they're moving.
Those things can help us determine whether a galaxy
has recently experienced a merger.
We can look for signs of if we had a lot of accretion
coming in, we might see very low
metal abundances in particular parts of the galaxy.
So all the way on your right now,
so these are the MaNGA fiber bundles,
these are some galaxies that they were placed on,
and this is just an example of the kinds
of maps we can make, where I've just used false color
to code things.
So the one all the way over on your right
is an actual map of the metal abundance
distributions in galaxies.
Okay, so we should have more answers in a couple years.
Again, we're only a couple years into the survey.
We're just starting to get those big, statistical data sets
that we can chew on and really take a look
at how these chemical elements are distributed in galaxies.
But I wanted to spend the last few minutes commenting
on something that, kind of the global relevance
of all of this for us, and that is how are the prospects
for life in the galaxy influenced by chemical composition?
So we now know that on average
there's one planet, roughly one planet per star.
This is an artist's rendition,
just trying to give you a pictorial view
of planetary systems in galaxy.
And you can see that most of those objects
have one or more planets around them.
And the question is, which objects are gonna form planets,
and can we understand
which stars are likely to host planets?
Because as we start thinking about following them up
with big, expensive instruments, we wanna know
which stars we should be pointing at.
We now know that maybe one in five sun-like stars
will have an Earth-like planet in what we call
the habitable zone, so in the region
where we think water could be liquid.
These are great places to look for life.
But again, we want to find more systems,
we want to find the closest ones,
and we needed to know where we should look.
Which of those star systems are most likely to host planets?
And it turns out that planet formation
is pretty closely linked with the metal abundance.
This is a few years old, so we now know
of many more planets, but if we just look
at the number of planets as function
of their chemical abundance,
which here they're calling metallicity,
and they're measuring the metallicity
on the bottom axis here relative to the sun's metallicity.
And so we see that almost all of the planets
that have been found occur in systems
that are more than half the sun's chemical abundance.
So it turns out that things that have low chemical
abundances are bad at forming planets.
And that's kind of interesting,
and we can think about why that might be.
So the basic idea behind star and planet formation
is that we start out with a lot of gas.
That gas gets denser and it collapses
and it forms a rotating disc.
And within that rotating disc, we get the beginnings
of planetesimal formation, and then at some point
all of the gas and dust and ices get cleared away
and we're left with a planetary system.
But it turns out that a really, really key part of all this
is the early phases when those first little rocky
bits are starting to accrete into cores.
And so this is sometimes called
the core accretion hypothesis.
And so to make these cores you need lots of small rocky bits
to begin to stick together, rocky and icy bits.
And so the more metals you're gonna have,
the more chances you'll have of having this dusty...
Icy material that can eventually glom
together and form the seeds of planets.
So I think that's why we see such a strong link
with the chemical abundance of the host star.
The more metals you have, the easier it is
to form those first little bits
which will congeal together to form a planet.
Okay, so I just wanted to close with the thought
that all of this is really very connected, right?
We thought about hydrogen and helium forming
in the Big Bang, stars within galaxies creating
other heavy elements, some of those heavy elements
being blasted out of galaxies,
some of them sticking around and being incorporated
into the next generation of stars,
and as the galaxy becomes more and more metal rich,
so too does its ability to host planets,
and those planets are potentially places
where someday we may find life.
So the takeaway message here is remember,
we are all star stuff,
and really think about your origins.
It's a fascinating thought process.
So just to summarize, we think nearly all of that hydrogen
and helium was formed a few minutes
after the Big Bang.
These other elements come from the cores of stars.
Some of the very heavy ones are born
in the supernova explosions themselves.
Stars form out of gas, create new chemical elements,
return that to the gas, and we're starting
to understand how chemical elements
are distributed in galaxies thanks to this MaNGA
survey of 10,000 nearby galaxies.
And once that's complete, we'll have a very good picture
of the cycling of gas between a galaxy and its environment.
And this will have some implications for the ability
of stars to host planets.
So thank you very much, and I'll just leave you
with a reminder about Washburn Observatory.
(applause)