So now that we've seen all these measurements of evolving galaxy
populations, let's see if we can tie it together in the overall star formation
history of the universe. Here is a plot that's called a Madau
diagram after the first author of the paper that introduced him, Pierre Madau.
And it shows the inferred production of metals or density of star formation rate
as a function of red shift as inferred from Hubble deep field measurements.
The three sets of points and curves correspond to three different filters and
you can see that they're in reasonable agreement.
The actual curves are particular galaxy evolution models that are fit to the data.
And the interesting thing here, is that you can see that there is a peak.
There is a time in the history of universe, roughly around redshift of
unity, where it seems that comoving star formation rate, over all galaxies, was, at
its highest. And it declined since then as it
approached redshift of zero. It also seemed ot have declined at higher
redshifts which you might exepect would be the case as we first build up galaxies
they get brighter and brighter and then they fade off.
Turns out a lot of this apparent decrease of high redshifts is actaully due to the
obscuration. But let's have a look.
Now, we can measure reddenings of galaxies.
And we can correct for the apparent absorption by dust.
And if you do that, you plot the same diagram.
This would be diagram on the upper right here.
Due to stallion collaborators. You find out that.
True enough as you go from here, to the redshift of one, the comoving star
formation density over all galaxies increases rapidly, but then kind of stays
flat all the way out to redshift of four and maybe even higher.
Now plotting against the redshift is maybe slightly misleading because you really
want to plot as a function of the lookback time.
Time history of the universe. And when you do this, then this decline
after redshift of unity is actually much more gentle but nevertheless, there is a
decline. That was all for the unobscured star
formation, or just mildly obscured. Now, if we add component from.
Dusty, obscured sources like the SCUBA sources, then that boosts up the total
even higher. The curves shown here are models that
don't seem to fit terribly well, but they do imply that there will be substantial
component of hidden star formation contributing to the overall history.
Subsequently measurements have been obtained out redshift of six, where we
actually have spectroscopic measurements of galaxies and nowadays they push them to
redshift of the order of ten, but those are all based on photo metric redshifts.
And yes, there is a decline that sets in roughly past redshift of four or five.
And that's the picture that we expected. At first there is nothing, then you build
up galaxies, you get more and more star formation rate going on.
There is a very broad maximum of that around red shift two plus or minus factor
of two and then it declines since then. So we are now in the phase of history of
the universe where it's gradually fading away, that most of the action happened
when universe was few billion years old. We can convert these measurements into the
actual buildup of the present observed stellar mass in galaxies.
And, here is what it looks like. It starts at high redshifts as a very
small fraction. By about redshift of unity, most of the
stellar mass in galaxies is already assembled, and then stays roughly
constant. This is very much consistent with
everything else we've seen before. The more modern observations push this
further out to redshift of six now and even beyond and the trend continues.
So, we start with no galaxies whatsoever in our stars and build them up gradually.
The rate of the build up slows dramatically around redshift of unity.
And there is still some build-up because there is still some star formation, but we
now can actually see galaxies being assembled, stars being generated, and
galaxies over cosmic time. So this was the direct approach to looking
at galaxy evolution. We look at individual sources, measure
their distances, brightness, and so on. An alternative way is to observe sky and
integrate all of the nnergy that we get from it and actually obtain spectrum of
that overall integrated cosmic background. Now this is not the cosmic microwave
background. This is the radiation due to galaxies,
stars and galaxies and also maybe active nuclei.
This is a very difficult measurement to do, because it's hard to get zero
comparison. You're looking at the normal patch of
light and comparing it to what? So this is why it took so long to do it.
But nevertheless it was done in both optical and near-infrared...
And the upshot is that the total integrated density of optical infrared
backgrounds. Almost all of which is due to star
formation is about 100 nano-watts per meter square, per second if you have
telescope of certain collecting area, and divert some time, that, that will give you
the amount of power collected. This turns out to be only few percent of
the energy density of cosmic microwave background.
And of it only few percent is actually contributed to active galactic nuclei.
So here is the broadband picture. This is what happens when you integrate
the spectrum of the universe. In optical and infrared.
There're different measurements, there're upper limits, they come from ground, from
space, a variety of sources. If you recall the broadband spectrum of
Starburst Galaxy M82, it had two humps. There was Black bodyish radiation from
obscured stars, invisible light, and there was black bodyish radiation from heated
dust in far infrared. Well the same now applies to the overall
spectrum of the unverise, if you will, and what's plotted here is.
A quantity that's really proportional to the energy per unit logarithmic interval.
And the fact that a two peaks are roughly the same height means that approximately
equal amount of all star formation ever was in unobscured and obscured systems.
Let us now turn to the question of chemical evolution.
As stars are made, they explode, they release chemical elements and interstellar
medium, new stars are formed. Some of those are expelled in
intergalactic gas. Fresh gas comes in.
All of that contributes to the overall chemical evolution.
Of galaxies. How's their metallicity changing as a
function of time? Now here is a rough schematic diagram.
You begin, of course, with hydrogen helium and nothing else.
So star scoop up heavy elements. Some of those recycle into new stars.
Some are expelled out, fresh material comes in, and so on.
Often times these extremely complex processes are simplified.
For example, galaxies put all in a box, and there is no inflow or outflow.
Or, it's assumed that the moment stars explode, that material is immediately
recycled in new stars. Those are crude approximations, but they
give us at least some insight at what's going on.
Another schematic diagram which conveys more or less the same story is shown here.
But you may want to look at it and find it a little more informative as to all the
different connections between different processes and components are.
Now starbusts like that one in M82 can drive galactic winds that expel enriched
material just produced by supernovi. Thanks to a cotenancy of supernova
ejectile out in the intergalactic space. This can be modeled and also observed.
What happens to that enriched gas is that it contributes to the overall chemical
evolution of intergalactic medium. And at high redshifts it will be absorb,
observable in the form of. Metal absorption on clouds, which we'll
address in the next chapter. So as you make stars, so you make metals.
The star formation history of the universe and the chemical enrichment history of the
universe are tightly coupled and qualitatively should look the same.
So here is essentially a metal plot. It shows the production of metals as a
function of redshift, and with modern measurement we can look at dependence of
galaxy metallicity in mass. You'd expect that more massive galaxies
would achieve higher metallicities because they retain more of the supernova eject
and recycle them more effectively. And there is indeed a mass metallicity
relation. It's a very noisy one.
It's shown in this plot as the grey area. That's for essentially redshift to zero.
And now there is a set of measurements for distant galaxies.
Now it begins to make it more obvious. And we see that, that kind of a
relationship exists already early on, which is what you expect.
Regardless of the redshift, more massive galaxies will retain more of their
processed material. But the overall curve is shifted down.
Which is again what you expect. That you have lower metallicities and they
grow. They grow in galaxies of all different
masses and at every red shift there is this dependence that is generally
expected. Which will lead us into the next chapter.
Evolution and structure of the intergalactic medium.