Tonight speaker is Professor John Weiss
here on faculty here at Georgia Tech.

In part of our center for
relativistic astrophysics.

He also has the distinction
I believe of being the only

physics professor who was also
an undergrad here at Georgia Tech.

I think you're the only one I think so.

That's right.
And

he graduated from Georgia Tech not so
long ago.

So two thousand and one and after
graduating from Georgia Tech keep entered

a Ph D. program at Penn State
where he worked under Tom Abel and

he finished his Ph D. with Tom at Stanford
University in two thousand and seven and.

From there he did he was
a NASA post-doctoral fellow at

the Goddard Space Institute
under John Mather and

a few others before going on to Princeton
where he was a Hubble fellow for

two years before we were lucky enough
to nab him as one of our own his

research is primarily in
the primordial star and

galaxy formation with the specialization
in photo realistic visualization.

What that translates into is that
you'll get to see some very amazing

images in animations backed
up by real science tonight.

Not just Hollywood stuff.

So please give us a peek around
of applause welcoming him and

afterwards we'll have a question period
in your welcome to come down and

speak talk to the speaker.

Thank you thank you.

AD and everyone hear me OK.

Yes OK So I hope tonight I'll
bring you want to journey back in

time on how our Milky Way actually
formed over billions of years but will

mainly be focusing on the first billion
years of its evolution and formation.

And we'll be talking about
these baby galaxies East.

Tiny galaxies which are the building
blocks of all the galaxies that we see

today and I just want you to
imagine yourself somewhere

that's totally desolate No no lights and
no lights no light pollution.

That's blocking your
view of the night sky and

if you look up lay on your
back on the ground and

look up you'll see something like
this you'll see the Milky Way.

And this is our own galaxy in which we
live in and you'll see a multitude of

stars but along the some plain
some line of sight in the sky.

You'll see dust.

You'll see some some some over density of
stars here and this is our the Milky Way.

This is our own galaxy that we live in and

this is this is a photograph from
the southern hemisphere in Australia.

You can see that there's
these two smudges here.

And those are actually galaxies
these are the smaller galaxies which

are abundant around these larger galaxies
like your own Milky Way and you'll be able

to see these only in some really dark
place and that's some southern hemisphere.

But just imagine yourself
laying on your back

on the ground looking up at the heavens
and you can just if you had enough time no

distractions this pondering how did
the Milky Way actually form you can ask.

Several different
questions simple questions.

How did the our own
Milky Way actually form.

Where did the stars form and how did
they form and when did our own Milky Way

form and also what actually were in
the presented years of our own Milky Way.

So this is I mean science in action.

We want to question everything.

You want to say take your observations.

I mean what your neck and I can look
out at the sky at the Milky Way and

just ponder how did we become how did
our own galaxy come into existence.

So I mean unfortunately we're
in our own Milky Way we can

actually look at how it looks like from.

From the outside but luckily our
own Milky Way It's it is unique but

there are other galaxies that are similar
properties to our own Milky Way.

So is a spiral galaxy it actually has
some a bar component in the middle.

But this is a good representation of what
the Milky Way actually looks like it's

a spiral galaxy and
we're about halfway around.

Where around twenty thousand light
years from the center of the Milky Way.

And here you can see it in
this one particular galaxy.

There's some smaller galaxy that's
a satellite Galaxy next to it but

also if you have a keen eye you can
have there are all these little specks

all these background galaxies.

These are galaxies far in the background
and we're actually looking at these.

At some earlier state and their
evolution and we'll go over some of this

some of these details why we're actually
looking back in time and this just to

keep this number in your head that our own
Milky Way is on the order of a trillion.

Solar masses or
a trillion times the mass of our sun and

will look be looking at galaxies
that are much smaller than these.

So and going out to even larger
distances just to give you a sense of

our own place in the universe we are pore
part of what's called the Local Group.

It's local group has three major galaxies
our own Milky Way and drum and a galaxy

which is actually visible to the neck and
I in a dark sky and I'm thirty three.

So these the three main galaxies
in their own local group and

it's populated also with the smaller
galaxies the satellite galaxies these

galaxies that are orbiting
around our own Milky Way and

also in galaxy and we're separated by just
around two and a half million light years.

So this is just to give.

Going farther and bigger and

bigger in space to actually show you
what is our place in the universe.

Now when the Hubble is lost when
the Hubble Space Telescope was launched.

Is actually.

So it peered into the sky and
just open the shutters over many orbits

start peering into some of the one
the darkest portions of sky and

and what it saw was ten thousand
galaxies in a tiny field of view.

So the field of view that we're actually
looking at the first largest picture.

If you imagine you hold a dime.

At arm's length and
you actually look at F.D.R.

as I that's the actual field of
view in this in the cemetery here.

So you can see there.

There are many objects in this.

So who can tell me how many stars

exist in this in this picture
that exist in their Milky Way.

And then once they say one

hundred ten billion in
our own Milky Way So

there's you can actually see stars
by their diffraction pattern.

These are these radial arms
coming out of the star so

here's one star and
the second star is up here.

Everything else is a galaxy.

And that's mind blowing
looking at a tiny part and

this in our night sky just
the small field of view.

So just imagine there are ten thousand
galaxies in this field of view.

But there are ten million of these
field of views in the whole entire sky.

So if you do the math.

There are one hundred billion galaxies
that are be visible to the to the Hubble

to to the Hubble if they could
expose three weeks at a time.

Each time in field but
that's practically impossible

because there are ten million of these
field of views and they'll take.

Most a million years to actually
observe the whole night sky.

At this detail but

I'm hopeful I'm optimistic that someday in
the future decades probably that we can

actually have exquisite detail on these
galaxies at the same sensitivity.

So that just gives you a sense of we're
not really special in the universe that

we're one in significant galaxy and
one hundred billion galaxies and

orbiting around a normal star and
in our own Milky Way has

one hundred billion stars but
I mean you can also look at the sense that

most stars now we're finding more and
more of these exit points these planets.

It's very common to actually find planets
orbiting around typical stars and

can think about how many stars there
are in the in the entire universe and

how many planets there are so

it's just mind blowing to think about
the quantities involved in the universe.

So there are some things that we can
actually learn from this from this.

From this image we can actually.

If we look at the universe.

So the idea that the universe is
expanding this been known for for

almost one hundred years.

But it's expanding and but we're actually
looking at cars mile cosmologically

distant objects and actually takes
a finite time for the light to travel from

this galaxy to our own telescopes and just
to give you a sense of some time scales.

It takes around a third of a second for
like to travel from the moon to us

takes about eight minutes for
like to travel from the sun to us and

it takes two and a half million years
to travel from the Andromeda galaxy or

our close this spiral
galaxy neighbor to us.

But we can actually use
this idea as a time machine

back into space to actually look at
galaxies how they looked early on.

So this is a great animation that
came from from Japan and just imagine

that you're looking at some cosmological
baby some baby out in the on the.

Farthest reaches regions of space and
we want to observe this baby so.

So there's light is emitted propagates at
the speed of light and this these photons

that come out from this baby travel
at some finite at the speed of light.

But the baby this girl is actually aging
as a as image is traveling toward us.

So we're actually looking
at how the galaxies were.

We're we're looking at.

You know if you can read Japanese or

you can get some sense if you
know you know Chinese can write.

I don't know what this means but.

But you can get the sense of we're
actually looking at galaxies

at a at their national states if you
look farther and farther back and

in and distance we're actually looking
farther and farther back and in time.

So we can utilize this this the speed
of light to actually look back

in time and remember that the image
of the Hubble Ultra Deep Field

how there are ten thousand
galaxies we can actually measure

how far that away they
are by how far how so.

The farther these galaxies are there
is something called redshift so

that as the as a light propagates
from this galaxy to us.

It's actually expanding with the universe.

So this wavelengths are stretching with
as it travels from the galaxy to us and

this called rushed and we can actually
use this as a distance measure.

And we can measure the red shifts
of these galaxies to actually

pinpoint what is their distance
to each individual galaxy and

you can actually turn
this into a three map.

So for each galaxy.

We know their distance and

now we're flying through this whole
alter our Hubble Ultra Deep Field.

So you can see now we get a good sense
of where the galaxies actually live.

And as we progress through this volume of.

Case you'll see that we progressed
from the spiral galaxies here

will see some larger galaxies
redder galaxies that you

actually read in dead
galaxies that look to go.

Most massive galaxies in the universe.

But as we progress farther and

farther into space right now where around
five billion light years away from Earth.

So we see these galaxies.

Now we're seeing more and

more irregular galaxies like
this because galaxy formation.

It's the most intense.

Just when the universe was
around a few billion years old.

And and what we want to know
what my research actually

focuses on is the very
beginnings of galaxy formation.

What were the first galaxies.

What were the first stars in the universe
really pushing the limits of

of our theoretical knowledge and making
predictions of what Hubble successor

successor will actually see and that is
called the James Webb Space Telescope and

it is due to launch in twenty
eighteen in about five years.

So we can actually put
this in a different way.

What are we actually looking at.

So this is a another animation
from from the from Japanese T.V.

and H.K. but it gives you a good sense of
what we're actually looking back into so

here we're at the present day we have our
local universe all these galaxies here and

you can just imagine flying through
the universe in this column.

So this isn't this is one billion
years five billion years backwards

into time and there's this big tower of
galaxies that we're actually looking at.

And here's the big bang.

This is thirteen point seven
billion years in the past and

we actually look up the tower in the
universe and we're all the way at the top

and you can just imagine how many
galaxies we can actually observe.

So here is the luminous galaxies that
we're observing that we can actually

get a good sense good statistics
of what their properties are.

And how numerous there are there are but
I mean we want to see.

What these first galaxies
were what were the origins

of all the galaxies that
we see around us today and

to put this in the different sense this
is another timeline of the universe.

So starting up here the big bang.

This is where everything all matter is
created and space and time is created and

eventually we were starting out with
some very hot plasma where photons and

and gas are our one and eventually
the universe expands it cools off

until it reaches a cold enough temperature
three thousand Calvin it's still hot.

I mean compared to that's.

That's around what seven hundred
Celsius and that's as cold enough so

that electrons and protons can actually
combine together to form neutral hydrogen.

So this is what's called recombination and
this occurs around four hundred

thousand light four hundred
thousand years after the Big Bang.

But after this.

The universe is completely dark.

I mean there is there is
some infrared infrared

radiation emanating throughout
the universe but there are no stars.

No galaxies.

This is what's called the Dark Ages.

Where we have no luminous
structure whatsoever.

But as as time goes on we build larger and
larger structures we can actually

get structures that can actually form some
of the first stars and first galaxies

in the universe and what's depicted
depicted here are heated bubbles heated in

ionized bubbles that are surrounding
the first stars in first galaxies.

But you can see this this goes on.

You can see more and
more of these bubbles appear and

they coalesce and
they become larger and larger.

So it really goes.

Undergoes some transition
from neutral to ionized.

And we go farther in time and the universe
goes under what's called if we can

think about this as some sort of phase
transition and the whole universe going

from just like how water goes from ice
to water you can think about the whole

universe undergoing the same type of phase
transition from neutral to ionized and

eventually So this is the first billion
years which will be focusing on in the in

the last later in this talk and
then afterwards the universe becomes

becomes transparent to most radiation or
most light that emanates from stars and

galaxies which we observe today and
there on Earth.

So there are a couple problems that
we need to solve cosmologically So

at the very early times we can see this
what's called the cosmic microwave

background or C. and B. and at these early
times only four hundred thousand light

four hundred thousand
years after the Big Bang.

We can actually observe this.

This surface of last scattering this is
this is when the universe recombines and

you can see the photons
that that are emitted

from this from the surface only four
hundred thousand years after the Big Bang.

But what's shown here are fluctuations
in temperature and you can or

directly relate this to fluctuations
in density and the contrast of

tremendously by order of of
ten to the hundred thousand.

So you're seeing fluctuations on the order
of one part and one hundred thousand.

So there are very small differences
between the blue and red but

how do we go from this almost homogeneous.

Density homogeneous medium that
emanates through the whole galaxy

to something like this.

It takes billions of years
of galactic evolution

to actually build up
these galaxies over time.

This is maybe one this is
a nearby spiral galaxy is.

Ursa Major if you're if you're interested
actually look you don't need that powerful

of a telescope to actually
see this this galaxy.

But how do we actually go from something
like this that that's almost homogeneous

at only four hundred thousand four hundred
thousand years after the Big Bang.

To something very
non-linear like a galaxy.

And there's something else that's
one problem how do we actually that.

How do we actually go from some
homogeneous matter to a galaxy.

So this is been thought about Blott and

what actually happens is what's
called higher structure formation.

So what we're depicted depicting
here is just dark matter.

We're looking at how structure actually
forms through mergers of smaller objects

and merges together to form even larger
objects and this happens time and

time over again.

Eventually you build galaxies that
are like your own Milky Way you have

a huge range of different
types of of galaxies.

So I said that we're only
depicting dark matter here.

Dark matter actually makes up
as around six times as abundant

as regular gas as regular matter
that we see today baryons.

But all the dark matter halos all these
dark matter objects that we see here.

So this is a very large scale right
one hundred million light years and

at the center.

Each one of these dark matter
objects exist the galaxy.

So this can give you some sense have how
galaxies actually form through mergers and

slow the creation of gas and
stars eventually building up to

the galaxies that we see today and
what we're seeing looking at right here is

a galaxy cluster one of the most massive
gravitationally bound objects in

the universe today it's they have around
on the order of a thousand galaxies.

Just gravitationally bound orbiting around

each other in some
gravitationally bound object.

So this is it's.

Around thousand times more massive
than your own Milky Way and

the nearest galaxy cluster is the Virgo
Cluster and that hosts many of these

massive galaxies that we can actually
observe with with amateur telescopes.

So we can go on to this other
question that they alluded to.

There's another problem where from the
Big Bang three minutes after the Big Bang

undergo Big Bang nucleus
Sensis where we we have it so

it's a well known process where
you have this reaction network.

It's not that complicated of a reaction
network we have twelve reactions if we had

many more elements or
be much much more complicated but

in the end what we're left
is with mainly hydrogen and

helium we have some
trace amounts of lithium

Briley M is is unstable it doesn't really
have a very low abundance of that but

mostly you can take away that we have
seventy five percent of the universe

from the Big Bang isn't hydrogen twenty
five percent of that is in helium.

But if you look at our own sun
we see this abundance pattern.

You see all of these elements not even
the sun on our own Earth that we see

a typical one of these we see
all these heavier elements.

So a stronger speak for

metals is just anything heavier
than helium and it's totally wrong.

In other disciplines but this is how we
speak just so anything heavier than all

of these are metals but how are these
actually produced and where and

how did all these metals come from
where did they all come from.

So we have you know all the way
from lithium to uranium.

How do these how were these produced and
also you can look not only our sun we can

look at many different stars in our own in
our own Milky Way So this is a spectrum.

These are for spectra or
went for spectrum of a.

Of Stars.

These are calcium lines of absorption
lines that come from the atmosphere of

the stars but you can see as we go
down this ladder these get weaker and

weaker and this is just a high resolution
specter of the four same stars.

So this just shows that there's a
decreasing amount of calcium in the stars.

We're getting or we can find stars
throughout our own Milky Way that have

varying medalist these varying
fractions of metals and.

So you can still ask we
can ask again how and

where did all these metals form
because you can look we haven't

found a star today that has
a primordial abundance pattern and

we have hydrogen and helium everything
all the stars that we found today.

Have some trace amounts of metal and so

you can just progress do with some
sort of thought of experiment.

On just stepping farther and
farther back in time.

Because these metals I haven't mentioned
that these metals they're primarily

produced in the in the stellar interiors
of stars as they undergo a fusion and

through supernovae.

But you can just think of
what was the first star.

What were the characteristics
of these first stars and

I mean were they any
different from today's stars.

If we were to expect.

Why don't we see any of these stars
that only have hydrogen helium.

Why do we see these stars that have
you can find some star that has very

trace amounts of metals.

But why don't we see any today.

So this spring.

This brings us to the first stars.

A rendition of one of my simulations of
and ionizing heated region around one of

these first stars you can see
everything's dark red is cold blue and

what this blue white is hot.

So I can see it's surrounded by
some cold medium and in this space.

Lighting up its surrounding regions.

This is around three thousand light
years across this this heated region.

OK so

we can just step back and think about some
simple questions about the first stars.

First we can ask yourselves
what I was just pounding on.

Why do all stars contain metals and
if you are thinking to think about

a metal Freestar I will get
into the details later but

without any metals because
metals they have a much more

complicated atomic structure you
have many more channels to cool and

you have many more channels to cool the
gas through through atomic transitions.

And this is basically the gateway to
cooling gas and forming stars but

without these metals.

How do you actually cool the gas and
condense to form the stars and finally.

Where and when do the first our sport.

So I like to I like history.

I like looking back at historical
paper seminal papers and

this is one paper that back and
this is what

sixty years ago that two famous
astronomers short shield and

Spitzer they actually were thinking
about what were the first stars.

And they looked at the data
that they had on hand and

they they saw three things they saw that
there's a lot of abundance of metals and

these and these early stars
population three stars or

Population two stars that
means metal pore stars.

We saw they saw high frequency of
white dwarfs which is the remnant

of a star like your own sun and

they saw these distant massive galaxies
elliptical galaxies that are red.

So red meaning they were
just left over with old and

massive stars but because they saw
on no metal for the stars that

led them to believe that the first
generation of stars were very massive.

And you can just think
about that if we go.

If we can think about a star
the more massive A Star is

the faster it burns us feel and
the shorter its lifetime.

Our own son lives for ten or
ten billion years but

if we're just to look at some star that's
around eight times this mass of our sun

only live for twenty million years.

That's a huge.

That's a huge range of of going from
twenty million years to ten billion years.

So if you can just imagine that that
these first stars were relatively massive

they would die off before the before
we before they reach the present day.

So if they did form if they did form and
they were very massive.

They won't be around today if the they
form that these very early times.

And also.

You can ask what I was talking about how
does this gas cool without any metals

without this complex Tomic
structure that that metal.

Metallic atoms actually have so
if we look at nearby star forming regions.

Molecular hydrogen and actually the key
coolant to actually cool the gas and

form stars but they actually form
on dust grains dust grains like

that are composed of graphite and
silicate.

But those are or are built of metals but
without these dust grains.

How does that actually cool.

It's actually warms in the gas phase
through this this two stage process.

And neutral hydrogen and combines with
the free electron to form some negatively.

Chart hydrogen ion H. minus.

And this goes on to form H two but
this very slow process.

Compared to something that forms and
dust grains.

And in what was done about five
years later they took this

result and actually took this result
is realized that this was important in

the early universe and just focus on
this graph here so this is how the gas.

Behaves as it collapses.

So as it collapses.

It starts out with some low density it
heats up because it's being compressed to

three eighty Batek A and then it reaches
a point at some density it can actually

cool from this molecular hydrogen and
then it cools down to around a few hundred

Kelvin around room temperature and
this just the building

blocks of our understanding our current
understanding of the first stars.

And second or lastly.

When and where do the first stars form.

So in the eighty's it was realised
that the first stars actually form in

very early in the universe around one
hundred to one hundred fifty million years

after the Big Bang and they form in these
in these dark matter objects that have

to have a mass of around a million
solar masses this is much much smaller

than their own Milky Way remember that our
Milky Way lives in a tenth of the twelve

or a trillion solar mass
trillion solar mass.

Dark matter halo so

it's a it's a bill million times
smaller than their own Milky Way.

And I like this graph a little
complicated but it shows you how much.

Molecular hydrogen you can form.

In the age of the universe.

So this is how much you can form in
the age of the universe when the universe

was fifteen million years fifteen million
years old fifty million years old and

one hundred forty million years old.

And this is what you need
to actually collapse and

where these two lines actually
intersect dictates what kind of dark.

What kind of temperature and that's what
kind of dark matter halo you need to

actually form one of these first stars.

So this is approximately.

This is how as it was realized what kind
of dark matter halo you needed to actually

collapsing need a dark matter halo
of around a million solar masses or

a million times smaller than our own Milky
Way and the other thing to realize is.

Is what actually comes
from these first stars.

Depending on the masses.

So this this is a just focus on this
on the slower half you can get a mix

of both supernovae the British and the so
I could fit in this little explosion here

supernovae black holes more supernovae and
more black holes so depending on the mass.

This is goes for

eight solar masses all massive stars eight
forty one hundred forty two hundred sixty.

So depending on the mass you get a nice
mix of both metal production and

supernovae and in these black holes.

So how does the how do these
first stars actually look like.

So this is an animation that we did from
two simulations with my collaborators.

So this is focusing on the formation of
when these first stars were zooming out

zooming out from around a solar radio

to cosmic blanks around ten thousand
light years across so you can see it's

it takes hundreds of millions of years to
actually condense and cool the dense core

that goes on to form one of these first
stars see is turbulent in nature.

What we're looking at is
cells of constant density

zooming out over orders of
magnitude in length and

here's a dark matter halo that exists here
is a filament that that was that you saw

in this this large scale structure
permission and once a star forms.

It's a very massive star it's on the order
of tens of times the mass of our own sun

so it's very luminous and
what happens is when it when it's born and

it heats the surrounding regions.

So this blue here is in heat heated and
and

at the end it produces a supernovae
you can see it just obliterates

its host gas cloud and
just blows out all this this metal and

gas all the elements heavier
than than than helium.

So these happen.

These supernovae these first stars.

Warman these first supernovae happen
in many different places in the middle

in the first billion years of the universe
and you can just think about.

Just do the thought experiment where how
many how much of these metals are actually

exist on earth today and you can you
can think about there's there's around

the teaspoon of these metals that come
from the first star and our own bodies

because we're primarily made up of water
and we have a bunch of oxygen carbon but

I mean that small fraction actually
came from these first supernovae.

So it's just amazing to see that our own
personal connection with the very first

episode the universe and

you can think about this on an even larger
scale these first stars have exploding

in the Uniting many different times and
something to realize is these first

stars they form first they form before
galaxies they form in isolation and

then only afterwards you form Star So
here's density a projection of density

This is one of our own simulations
density and temperature but

you can see how these first stars actually
light up the surrounding regions.

How it's actually light heating up.

You can see there.

If you saw in the very first parts of it.

They only live for a few million years
they are very short lived because they

are very massive and it's basically
like cosmic fireworks going off and

eventually we form a big enough dark
matter halo enough gas to actually

form one of the first galaxies and these
are just the building blocks of the way

there are much smaller then
than your own Milky Way there.

Around the thousand to ten thousand times
smaller than their own Milky Way and

this only showed the first quarter
billion years after the Big Bang.

They're still thirteen point thirteen and
a half billion years or

more galactic evolution to go after this.

And these these stars.

They're supernova actually.

Produce the first metals
in in the universe.

The first the first instance is
of nitrogen carbon silicate or

silicon magnesium and so on.

Iron and what happens is the supernova
they actually blast out these these

these metals out to very large
distances so this is this is around.

This is around five hundred
thousand light years and

co-moving means if we because the universe
is expanding this just takes out

the expansion of the universe this is what
the volume would be at the present day.

So for it to actually follow
this to the present day.

This would be around one hundred
thousand light years across or

basically the diameter
over a very milky way.

So these first stars are important because
they provide the first battles and

the building blocks that this
this baseline of metals for

starts to form in more normal galaxies and
now that brings us to

the first galaxies now that we've
talked about the first stars.

Now we can move on to the next stage where
the galaxies form and this is a rendition

of one of my simulations of when the first
galaxies that form here blue is hot red is

cold again you can see these filaments
here here's a population three star that's

actually blowing out most of
this gas in the filament and

it's a very I mean it's a very competent.

Intense process to actually run these
simulations so I mean this was run.

Maybe three years ago it took
around six weeks of runtime on

five hundred compute course.

So it's a very it's a very
complication expensive process but

aren't the knowledge that you gain
from these computational experiments.

And it's it's tremendous.

What you can actually learn from all
of the data that are produced from

these simulations and
here is another galaxy that's forming

You can see it's heated heating
up the surrounding gas.

And how do these first galaxies form.

So remember back to when I was taught.

About higher structure mation how
smaller objects merge together to form

larger objects.

So here we have the first stars forming
around two hundred million light

years million years after the Big Bang
a few of these these are dark matter halos

that merged together to form a larger
dark matter halo that's around.

So this is a million solar masses and
the first galaxies they primarily form and

dark matter halo as they are on
one hundred times as large and

this is where the bulk of
these first galaxy form.

Roughly around a half a billion
years after the Big Bang.

And you can ask yourself what's so
special.

Why is it ten of the solar masses.

It's primarily what does
it take to cool the gas

because you need to cool the gas and
in order to for

this diffuse gas to actually reach high
enough densities so stars can form.

So I'm showing here a cooling function.

So how fast that gas can actually
cool as a function of time so

you just focus on this on the solid
line here this is the cooling curve.

Cooling function for a primary with
yes you can see this steep rise

at ten thousand kelvin And what's so
special about ten thousand kelvin.

This is where I mean how does
the gas cool here this undergo this

this happens because you have what's
called Collision all excitation you have.

Just imagine you have two hydrogen
atoms two neutral hydrogen atoms.

Here they are moving around at some speed.

According to their temperature they
collide together this kinetic energy

actually can excite one of the electrons
in the hydrogen atom to some excited state

that decays to the ground state and
what's emits a photon.

So what you're doing here is
converting kinetic energy or

thermal energy to radiation
which going to into space.

So you're basically cooling the gas.

So at temperatures around ten to the four
Calvin This is when the losses of

the hydrogen atoms are.

Great enough to actually excite hydrogen
and thus you can cool the gas and

this this bump is from hydrogen and
this bump is from helium here but

you can convert this temperature and
to Halo masses so

this is right around one
hundred million solar masses.

So this is this is like a very critical

mass scale where the first galaxies
actually form at these early times

it's a shift in the Certainly just
to just to demonstrate this so

what we have here is this pump here this
is this this collision where we convert

the kinetic energy into some excited
state and this excited state decays and

back into the ground state emitting some
photon converting this this kinetic energy

into radiation and just cooling
the gas but we can we can do this

in look at all these first stars forming
and and how the first galaxies formed.

So here is I showed density here.

This is you can see all
this this cosmic web.

This is a projection of temperature and
these are our projections of metals

that are produced in supernovae but
you can see if we progress through time.

You can see these cosmic fireworks going
off these first stars just lighting up.

Boom boom boom boom boom just going off
lighting up the surrounding universe.

And this sesa stage for the first galaxies
to actually form in the supernovae from

the first stars they provide the space
level of metals in which stars form in

the first galaxies and these metals here
this projection here actually shows.

What kind of metals come
from the first galaxies.

And this is just a more.

More pretty.

Rendition of the same simulation
blue is hot red it's cold again.

So here's the most massive
galaxy that's forming here and

it's around a billion solar masses or
a thousand times smaller than their

own Milky Way you can see just
progressively the these stars as.

Only visualize only
the gases visualize here.

You know the stars but
embedded in all of these gas.

Clouds exist these first galaxies and

first stars they can see over time and
becomes more and more heated and

you can see these filaments here you can
see all these these clumps but over time.

Just remember back to this cosmic timeline
how these bubbles are growing with time

and in coalescing together to
form some neutral universe

and if we actually look at the galaxy
that's doing all of this work here

you can see all these mergers happening.

What is actually look like
if we look at the stars.

If we rendered this realistically.

So this is looks.

Sorry I was thinking ahead.

So one one last thing is we can actually
zoom in to these into these and

to to return to these
galaxies to these galaxies

you have a whole multitude of different
properties of the first galaxies.

These are the building blocks of our own
Milky Way that we're just looking at

the first billion years
of Galaxy evolution.

So we can see some turbulent
some irregular galaxy like this.

That's very turbulent we.

Here's the temperature here at the metals
and you can see that it's much different

than the other galaxy here the smaller
galaxy that that form some small disk.

This has around a total mass of

attended the solar masses this critical
mass that was talking about before and

this galaxy has is around ten times
more massive and houses most massive

Suman to this most massive galaxy forming
You can see all of these halos forming

here first stars being formed in these
small halos all these mergers going on.

So you can see how explosive and dynamic.

The gas dynamics are just because
of the the energy that's input from

from the stars.

Groups area.

So you can see all of the gas you can see
they just disappear all the gas just.

Pierce but in the process.

These stars actually light up and heat.

The surrounding gas here.

So this is what that galaxy actually
looks like if we if we zoom in and

realistically render
what the stars look like

in some similarly telescope if if we
have some magical telescope that has

infinite angular resolution we could
look at one of these first galaxies.

This is what we would see what's rendered
here is blue and red are the gas again.

The intensity of the gas is directly
proportional to the density and the stars

are colored so that there is what your
neck and I would look like if you're

at the same if you're just some number
of light years away from this galaxy.

But remember that because these
galaxies are forming at the very

farthest regions of the of the universe.

They'll be redshifted So most of the most
of this radiation that's coming from these

stars are coming out in the ultraviolet
in optical but when we look at these

we need infrared detectors because
they're being redshifted they're they're

wavelength they're being stretchered
stretched by on the order of around ten.

So if you can imagine some light coming
out at at at around four hundred meters.

I mean that's in the in the very loose
regions of what we can actually see

now and redshift its wavelength
would actually stretch to around for

microns into the infrared and for that
we need the James Webb Space Telescope.

This is being launched
in twenty eighteen and

this is a full scale model of
Bell's actually at Goddard Space.

Space Flight Center in
just outside of D.C.

You can see how big it is say it's
around this six meter telescope.

Telescope and
will be orbiting at that little ground.

To point to point.

That's not going to be in
lower earth orbit and and

so it really needs to be has to be

perfect when we when it's actually being
launched because we can't service it.

But the hope is this will
have enough sensitivity.

If I was on the order of two metres.

It's it's it's much smaller than
the James Webb Space Telescope.

But the hope is that this telescope.

Will actually observe these first galaxies
and you can actually do the calculation

that you'll see abundant number
of these first galaxies and so

on in five years it will be
a tremendous time for for

a strong American astrophysicist
to actually pore through

the data that are coming from this new
space telescope in five years time.

So at the heart of all of this
is how these first stars and

first galaxies actually
contribute to rain on ization or

the space transition from
neutral to ionized So here you

can see these bubbles growing this just
some dynamical process a cold simulation.

That's it's a cube and spinning so

we're doing some periodic
boundary conditions here and

at the heart of all of these these ionized
and he he did bubbles are galaxies.

So this is a very large box
here this is two hundred

two hundred million year
light years across.

So there's some large scale evolution
of the eye nice fraction the universe

in these yellow dots
are the galaxies that are actually

there are actually providing the photons
the radiation to do this heating.

But there's a lot of action going on below
what we can actually see in the animation

is what's actually happening to these
first galaxies how dynamic are they so

this is to move a movie
from one of my simulations.

This is density and the radiation field.

So it's very an isotropic it's very
it's rapidly evolving so this you

can see how the angular dependence of the
radiation feels actually Xscape ing into

into the into the surrounding regions and
how the supernova in.

The in the stellar feedback that
the radiation from the stars

are actually affecting
the gas in this galaxy.

You can see it's undergoing
some breathing mode it's

you forming stars you're pulling
stuff out is falling back in.

It's just pulsation will move this some
sort of regulation of star formation.

But this leads to this time
varying radiation that's going out

and isotropic pattern because
if we go back to to here.

One too many to here you can see it's and
what's being missed is is where is

this radiation actually coming from where
these first stars and first galaxies.

How does the radiation actually skate from
the first from these it's its own self and

in only a part of that radiation
actually scapes from the galaxy and

leads to these bubbles forming.

So it's very important to
actually do the same elations

to actually calculate what type of
stars form in these first galaxies and

how much radiation actually
scapes from these first galaxies.

And.

And you can also look at
what kind of stars actually

are produced in these first galaxies.

So this is some star formation history.

So as a function of time.

What is this metallicity and you can see
at some steadily increasing function of

time and you can actually condense this
you can collapse this distribution here

this history on the time axis and
get some distribution of the medalists and

you can actually observe this and
in in local dark galaxies.

So you can ask yourself are these first
galaxies are they similar to local

galaxies and this is some way that
we test our own simulations because

we're simulating objects
that we can observe yet

we want to make predictions for
the James Webb Space Telescope.

So we this is one way that we test whether
simulations are actually producing

realistic galaxies and and today.

This is how we actually calibrate what's.

Models that we actually
include in our simulations and

to produce some realistic galaxy you
can look at other properties as well

like their sizes there
their distributions of.

Stars there.

How how spread out these stars are and
so on and

what happens to these first galaxies
because this is only the first billion

years if you can think about this
in in the sense of a human being.

The human being.

This is only the first billion
years out of thirteen point eight

billion years in the evolution
of the universe.

Take a life expense.

Expectancy of eighty years.

This is only the first six
years of of humans lifetime.

So this is only the first and
this is going from infant toddler to

just kind of garden but there's the whole
lifetime of the galaxy ahead of itself.

So most of these galaxies they merge
together to form larger galaxies

like this elliptical galaxy here and
maybe seven groups.

I was there.

So this is maybe seven.

This is some large elliptical
galaxy what you're seeing here this

buzz is just stars.

There's no cold gas
whatsoever any if you have

a good eye you can see the jet coming
out of the central regions there's

actually a super massive black
hole in the center of this galaxy.

But a small fraction around ten percent
actually survive into the present day

in this is a dark Galaxy some isolated or
galaxy.

That's around two million or
two million light years away and

what you can see is this
is just forming stars now.

So it underwent some burst of star
formation the first billion years that was

shut off by by by some prophecies and then
it had some rebirth of star formation.

But this is on the order of ten to
around a billion solar masses and

total mass and some small fraction of
the first galaxies actually survive until

the present day and this is what we want
to actually compare with our simulations

to these local dwarf galaxies.

What were the pretenders.

Years of these both these local Dorf
galaxies and our own Milky Way So

as I said the only but the beginning
this is only the first billion years in

the universe and there's a lot of
evolution to evolution that goes on after

the space billion years there's they're
still twelve and half billion years to go.

So here's a nice movie
from some of my work

actually shows what goes on after
the formation of these first galaxies.

So this is a movie showing only stars

of how Star how galaxies actually merge
together and form a galaxy like your own.

So here we are zooming in there
are all these small dark galaxies.

Here's one cluster here.

There's another cluster here.

If we just focus on this one.

The closer one to the camera
as you can see it's.

You have all these mergers and
when you see some pink burst of emission

that's actually indicating
new stars being formed.

So you can see as these smaller galaxies
collide with this larger galaxy.

It's being torn apart in
in being drawn together and

building this this one spiral galaxy.

You see these dust lanes that
are forming in this galaxy and

eventually these two galaxies
merge together and form.

You have some spic starburst
here in these total tails and

they coalesced together
to form one galaxy.

So the simulation.

We ran it until the universe was
around five around four or five

billion years old but if we look we know
our own Milky Way hasn't gone you're gone.

One of these made.

What's called major merger some
equal mass murder of two galaxies.

Cents since around.

Eight to nine eight to
nine billion years ago.

So what would happen is you had this
major merger that produces Milky Way and

then evolves passively it forms it reforms
its disk and forms a galaxy like around.

And this brings us back to
what we actually observe

what is our own Milky Way.

Now we've gone through the whole
evolution of our own Milky Way And

this is what's left today and this is what
we observe with our own that could die.

And if you're in some dark region
in that summer hemisphere.

But you can you can use
your imagination and

think about what's going to happen
because this is not stopping.

There is going to be if
mergers don't stop and

we're in this gravitationally
bound local group.

We have our own Milky Way and
drawn a galaxy which is two and

a half million light years apart.

So this.

So these two our own galaxy and and

then drama the galaxy will merge in on the
order of five billion years in the future.

So this is some I thought was
a beautiful rendition of what

a hypothetical human on earth.

I mean the Earth one exe one probably
I mean sun's going to die before

before before this actually happened.

So if you're off to Mystic that
we haven't killed each other.

In five billion years for
on some other planet and then in

the Milky Way this what you'd actually
observe here is the Andromeda Galaxy.

You can actually see it with your neck and
eye and here's a Milky Way as time

progresses it gets closer and closer until
it almost fills one corner of the sky and

then these two galaxies merge
together crane this big star burst.

In this in this merging galaxy.

You see the starburst going on.

You see some total destruction of
the of the of the morphology of these

spiral galaxies and then you go on to form
this a little galaxy that's devoid of

cold gas and
it's not really forming stars anymore.

And then you'll be left with some the star
that the humans are on in five billion

years or many stars and many galaxy or
whatever you want to use or imagine a.

About side by but this is what you would
actually see this elliptical galaxy.

Instead of a nice spiral galaxy like so.

So just to summarize That's all I have
to say about the about baby galaxies and

how our Milky Way actually formed.

And and

so I just want you to take take away these
these few take away the take home points.

So the first stars they produce these
first metals that pretty rich the first

galaxies they produce all of the help
that the heavy elements that go on to

form in the first galaxies and
these first galaxies what we're

seeing is they are having a very
similar cellar properties tear own

to the local Dorf galaxies and
third that these first galaxies these

small galaxies they're actually producing
most of the ionizing protons that lead to

this phase transition from neutral to
ionized in the space galaxies small Dorf

galaxies they're actually the building
blocks of all galaxies we see today and

roughly around a thousand of these dwarf
galaxies actually merge together in our

own Milky Way but I just want to point out
that understanding these first galaxies.

It's a single important
piece of the puzzle.

Gallus from ation and and

how do we actually understand the cosmic
structures around ourselves and

I want to thank you for your attention and
I'll take any questions.

So if you have a question just raise your
hand and I'll pass you the microphone and

if you could please ask your
question in the microphone so

that everyone can hear not just
the speaker was there a question on front.

How do you how do you gauge
how large the energy.

Right around particular galaxies are.

So dark energy is constant
throughout space.

So we can actually measure this from
the cosmic microwave background

we can measure their the power spectrum of
all those fluctuations that you sell and

that's very sensitive to
how much dark energy.

We have and how much dark matter we have
and how much gas we have in the universe

and this is how we initialize
are our simulations.

How

do you explain the appearance of
the spiral arms and the in general.

Why are these things.

Disk shaped I mean I'm going to understand
why the solar system might be so

if you can imagine for
discounts to actually form it

can undergo one of these major mergers
undergo one of these mergers so

it's some isolated system and
it has some angular momentum.

So if you just imagine some
rotating sphere as it collapses

conserving a momentum and it's going to
flatten So that's why we see disks and

the reason why we see spiral arms.

It's actually a outstanding research
problem I mean it's not fully understood

why there are several theories out there
but you can think of about density waves

that are propagating they
are actually in the disk itself and

he has some convergent flows
where we have over and

over dancing and you trigger
star formation in these arms and

these density waves in and
spiral galaxies.

So I mean still this is
an outstanding question is there.

People have primary two or

three different areas of spiral arms
actually form and why are they stable.

I've read in the last year or so

an interesting result actually
quite disturbing result for me and

that is that ninety five percent of all
stars it ever will form have formed.

Which leads me to the question since these
are simulation mathematical simulations.

Do you run them forward and
see what I know many people do.

But yeah.

Because we want to we want to look at
we want to explain what we're actually

observing but there have been a few
people I thought that the images that I

showed of the collision in
between the Andromeda and

Milky Way galaxy is actually taken from
some simulation of two merging galaxies.

I can think of a cup.

All groups in the world actually
looking into the future

because what's going to happen is dark
energy is again going to become more and

more dominant over time and.

We've actually passed far past
the speak peak of star formation.

What's happened around three
billion years after the Big Bang.

So it's I mean that is I didn't.

I don't know that number but
it sounds about right ninety five percent.

So you mentioned the Webb telescope
will be going up in five years.

I think he said five years.

So if the data comes in.

The computational models are correct
in their career correctly predicting

what astronomers are seen and
what would be the next step from there.

Once you sort of had that verification
of your your computational models.

Right.

So then you can actually look
at where these first galaxies

where they where they exist today.

If we can see where the stars were
where they propagate into and

where they exist and in our own Milky Way
or what were the building blocks of that

now that we have confirmation that
our own simulations are are there.

We very constrained our simulations
to some point back to make concrete

predictions of these these first galaxies
or you can then you can just wait for

more powerful telescope because
the first stars they will be observed by

by that Webb telescope and they
supernova might be but individual stars.

There's no way they're
going to be observed.

So you can do more predictions on what
the next generation of telescope would

actually observe and
twenty thirty or whatever.

This one is working.

So as a number forty two ever appeared.

Only on my program in.

So I have a more general question
about the actual simulations.

So what technologies are going
in your actual like to predict

like what kind of physics or
what kind of computational.

Commutations stuff like yeah yeah.

So I don't resent using rank.

So the simulation code is not.

It's not unique but

we use that very competition heavy
routines are written in Fortran.

Because it just fast because it doesn't
need to do anything but math and loops and

memory management we see plus plus and
for analysis we use Python and

it's paralyzed with M.P.I.
message passing interface and

also it's paralyzed
further on using open M.P.

within a single computer so
as a hybrid parallel code.

It also has you can also leverage G.P.
use as well.

So it's a multi parallel program.

And it's scalable up right now.

At scale up to around ten thousand course.

So it's very hard to actually
get up to one hundred thousand

there are a million course but
I mean we actually have

a couple of computer scientists
working on that on that because

I'm not trained as in computer science I
do the best I can without formal training.

I love it but there's only only so
much I can do.

The last question and
then I would think the speaker and

if you if you have questions or would
like to speak with Professor wise you're

welcome to come down after
that after the end here.

How do you find
the competition in the stars.

Sorry I didn't hear how do you find
the composition of metals in stars.

So you look at their spectrum so.

So you can actually look
at the light in and

spread it along wavelength and
actually look at

whatever there is the option lines because
you can you can look at the photosphere.

The surface of the star and

that material on the surface of the star
will actually absorb some light and

you can look at the strengths of these
individual absorption lines iron calcium

oxygen and so on to actually measure

what is abundance in each star and you
can compare that to what we observe in.

The sun and you can get some sense of
what is the what was the origin of

that star I mean what types of supernovae
especially when you get to the lowest

metallicity stars because only
a few of supernova actually enrich

that stellar that star forming
cloud that form the stars.

So you can get some idea of what
where the pretenders that enrich this

gas that goes on to
form that is the Stars.

You Thanks.