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So now that we've seen all these
measurements of evolving galaxy

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populations, let's see if we can tie it
together in the overall star formation

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history of the universe.
Here is a plot that's called a Madau

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diagram after the first author of the
paper that introduced him, Pierre Madau.

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And it shows the inferred production of
metals or density of star formation rate

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as a function of red shift as inferred
from Hubble deep field measurements.

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The three sets of points and curves
correspond to three different filters and

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you can see that they're in reasonable
agreement.

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The actual curves are particular galaxy
evolution models that are fit to the data.

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And the interesting thing here, is that
you can see that there is a peak.

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There is a time in the history of
universe, roughly around redshift of

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unity, where it seems that comoving star
formation rate, over all galaxies, was, at

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its highest.
And it declined since then as it

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approached redshift of zero.
It also seemed ot have declined at higher

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redshifts which you might exepect would be
the case as we first build up galaxies

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they get brighter and brighter and then
they fade off.

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Turns out a lot of this apparent decrease
of high redshifts is actaully due to the

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obscuration.
But let's have a look.

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Now, we can measure reddenings of
galaxies.

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And we can correct for the apparent
absorption by dust.

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And if you do that, you plot the same
diagram.

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This would be diagram on the upper right
here.

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Due to stallion collaborators.
You find out that.

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True enough as you go from here, to the
redshift of one, the comoving star

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formation density over all galaxies
increases rapidly, but then kind of stays

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flat all the way out to redshift of four
and maybe even higher.

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Now plotting against the redshift is maybe
slightly misleading because you really

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want to plot as a function of the lookback
time.

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Time history of the universe.
And when you do this, then this decline

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after redshift of unity is actually much
more gentle but nevertheless, there is a

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decline.
That was all for the unobscured star

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formation, or just mildly obscured.
Now, if we add component from.

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Dusty, obscured sources like the SCUBA
sources, then that boosts up the total

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even higher.
The curves shown here are models that

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don't seem to fit terribly well, but they
do imply that there will be substantial

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component of hidden star formation
contributing to the overall history.

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Subsequently measurements have been
obtained out redshift of six, where we

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actually have spectroscopic measurements
of galaxies and nowadays they push them to

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redshift of the order of ten, but those
are all based on photo metric redshifts.

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And yes, there is a decline that sets in
roughly past redshift of four or five.

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And that's the picture that we expected.
At first there is nothing, then you build

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up galaxies, you get more and more star
formation rate going on.

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There is a very broad maximum of that
around red shift two plus or minus factor

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of two and then it declines since then.
So we are now in the phase of history of

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the universe where it's gradually fading
away, that most of the action happened

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when universe was few billion years old.
We can convert these measurements into the

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actual buildup of the present observed
stellar mass in galaxies.

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And, here is what it looks like.
It starts at high redshifts as a very

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small fraction.
By about redshift of unity, most of the

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stellar mass in galaxies is already
assembled, and then stays roughly

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constant.
This is very much consistent with

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everything else we've seen before.
The more modern observations push this

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further out to redshift of six now and
even beyond and the trend continues.

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So, we start with no galaxies whatsoever
in our stars and build them up gradually.

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The rate of the build up slows
dramatically around redshift of unity.

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And there is still some build-up because
there is still some star formation, but we

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now can actually see galaxies being
assembled, stars being generated, and

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galaxies over cosmic time.
So this was the direct approach to looking

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at galaxy evolution.
We look at individual sources, measure

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their distances, brightness, and so on.
An alternative way is to observe sky and

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integrate all of the nnergy that we get
from it and actually obtain spectrum of

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that overall integrated cosmic background.
Now this is not the cosmic microwave

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background.
This is the radiation due to galaxies,

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stars and galaxies and also maybe active
nuclei.

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This is a very difficult measurement to
do, because it's hard to get zero

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comparison.
You're looking at the normal patch of

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light and comparing it to what?
So this is why it took so long to do it.

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But nevertheless it was done in both
optical and near-infrared...

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And the upshot is that the total
integrated density of optical infrared

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backgrounds.
Almost all of which is due to star

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formation is about 100 nano-watts per
meter square, per second if you have

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telescope of certain collecting area, and
divert some time, that, that will give you

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the amount of power collected.
This turns out to be only few percent of

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the energy density of cosmic microwave
background.

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And of it only few percent is actually
contributed to active galactic nuclei.

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So here is the broadband picture.
This is what happens when you integrate

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the spectrum of the universe.
In optical and infrared.

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There're different measurements, there're
upper limits, they come from ground, from

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space, a variety of sources.
If you recall the broadband spectrum of

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Starburst Galaxy M82, it had two humps.
There was Black bodyish radiation from

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obscured stars, invisible light, and there
was black bodyish radiation from heated

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dust in far infrared.
Well the same now applies to the overall

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spectrum of the unverise, if you will, and
what's plotted here is.

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A quantity that's really proportional to
the energy per unit logarithmic interval.

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And the fact that a two peaks are roughly
the same height means that approximately

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equal amount of all star formation ever
was in unobscured and obscured systems.

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Let us now turn to the question of
chemical evolution.

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As stars are made, they explode, they
release chemical elements and interstellar

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medium, new stars are formed.
Some of those are expelled in

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intergalactic gas.
Fresh gas comes in.

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All of that contributes to the overall
chemical evolution.

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Of galaxies.
How's their metallicity changing as a

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function of time?
Now here is a rough schematic diagram.

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You begin, of course, with hydrogen helium
and nothing else.

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So star scoop up heavy elements.
Some of those recycle into new stars.

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Some are expelled out, fresh material
comes in, and so on.

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Often times these extremely complex
processes are simplified.

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For example, galaxies put all in a box,
and there is no inflow or outflow.

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Or, it's assumed that the moment stars
explode, that material is immediately

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recycled in new stars.
Those are crude approximations, but they

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give us at least some insight at what's
going on.

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Another schematic diagram which conveys
more or less the same story is shown here.

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But you may want to look at it and find it
a little more informative as to all the

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different connections between different
processes and components are.

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Now starbusts like that one in M82 can
drive galactic winds that expel enriched

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material just produced by supernovi.
Thanks to a cotenancy of supernova

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ejectile out in the intergalactic space.
This can be modeled and also observed.

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What happens to that enriched gas is that
it contributes to the overall chemical

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evolution of intergalactic medium.
And at high redshifts it will be absorb,

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observable in the form of.
Metal absorption on clouds, which we'll

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address in the next chapter.
So as you make stars, so you make metals.

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The star formation history of the universe
and the chemical enrichment history of the

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universe are tightly coupled and
qualitatively should look the same.

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So here is essentially a metal plot.
It shows the production of metals as a

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function of redshift, and with modern
measurement we can look at dependence of

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galaxy metallicity in mass.
You'd expect that more massive galaxies

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would achieve higher metallicities because
they retain more of the supernova eject

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and recycle them more effectively.
And there is indeed a mass metallicity

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relation.
It's a very noisy one.

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It's shown in this plot as the grey area.
That's for essentially redshift to zero.

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And now there is a set of measurements for
distant galaxies.

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Now it begins to make it more obvious.
And we see that, that kind of a

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relationship exists already early on,
which is what you expect.

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Regardless of the redshift, more massive
galaxies will retain more of their

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processed material.
But the overall curve is shifted down.

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Which is again what you expect.
That you have lower metallicities and they

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grow.
They grow in galaxies of all different

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masses and at every red shift there is
this dependence that is generally

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expected.
Which will lead us into the next chapter.

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Evolution and structure of the
intergalactic medium.