Let us now turn to the Reionization Era which is currently one of the frontiers of
observational cosmology and theoretical on as well.
A brief history of the universe is as follows.
The, there is a combination, one universe was about 380,000 years old and
microbackground is released. At that point, there were no more sources
of light in the universe. But the dark matter fluctuations keep
collapsing, and eventually, gas that would fall into those potential wells would get
sufficiently dense to make stars,, igniting star formation for proto-galactic
segments. Their UV radiation would then start
reionizing neutral hydrogen in the universe around them and eventually, those
bubbles of ionized gas would start to overlap at which point the universe
becomes transparent again to the UV radiation.
It's, of course, always transparent to the radio waves and infrared.
But, at wavelengths shorter than Lyman-alpha line of neutro hydrogen it was
completely opaque. That is what we call the reionization era,
and at that point, we can that galaxy evolution proper begins.
The universe was reionized by the first stars in flowing to massive black holes or
any black holes. Probably also played a role, but we think
that most of it was done by the first generations of stars.
It turns out that those first stars that formed just from hydrogen and helium,
without dust, as we see in Milky Way today or heavier elements like carbon, carbon
monoxide molecules and so on were different.
In the process of star formation, the protostar has to collapse.
That means it has to lose energy or cool just as we're talking about collapse of
galaxies requiring extra cooling. And, in star formation around us molecular
lines play substantial role in doing that. But, for the first stars only molecular
hydrogen was available, so they couldn't cool quite as efficiently.
And, that leads to formation of very massive generation of first stars.
Multiple theoretical studies have indicated the same thing, that the very
first stars probably had masses of the order of 100s or 1000s of solar masses.
Much more so than stars that are formed today, because the very massive stars are
also very hot and very luminous. They provided excellent sources of UV
radiation to reionize the universe. And indeed, if you look theoretical
Hertzsprung-Russell diagram, HR diagram the plot of luminosity versus temperature.
And if you compare where those stars without heavy elements, just hydrogen and
helium are. They kind of parallel the main sequence of
the young stars today, except they're much hotter.
And the reason they're hotter is that there were slightly smaller and also there
were no metals to absorb UV radiation in the spectra.
Which means they're even more efficient as sources of ionizing radiation [unknown]
galaxies. As those first stars evolved, they would
explode as Population [inaudible] Supernovae.
You may remember, a Population I of stars with a solar discs.
Population II where all stars are galactic halo and bulge.
And by the extension, Population [inaudible] where the very first
generation of stars, made up of hydrogen and helium alone.
Well, they do cook up some elements, some heavy elements and disperse them in some
supernova explosions, that creates the, some small abundance, from which the
Population II stars are formed. People have looked very hard to discover
stars with no metals, but there aren't any left.
The oldest stars, the most metal porous stars are maybe 100,000th of a solar
abundance, but they still have something, and so, that's what we think the first
Population II stars are from. One interesting thing is that many of
these massive stars probably exploded and created black holes, and in that process,
also created gamma-ray bursts. What's shown here is the mass of the
remaining remnant Newton star or black hole versus the initial mass, and for a
large range of masses black holes formed, and gamma-ray burst ensues.
Except for a small, [inaudible] mass, where star is completely destructed and
there is no remnant. So we can expect to see gamma-ray bursts
from the very first generation of super massive stars in the reionization era.
Indeed, here was the very high redshift gamma-ray burst discovered in 2005.
And, just as we can look for quasars or galaxies using the drop tech, drop
technique, here, we can also see it as a strong dropping continuum like that
Lyman-alpha line. And that is characteristic of the high
redshift objects and real spectrum obtained as shown here and that confirms
the photometric determination. Interestingly enough, in this particular
object, the host galaxy of it shows some metals.
So this was not from a really young galaxy.
This galaxy already evolved some. And here is one other redshift 8.2.
The redshift measurement is not 100% certain, because the spectrum was very
poor, but photometric dating indicate that was indeed the correct redshift.
And, this is probably the most distant object to which somewhat reliable redshift
estimate was made. What's shown here is the histogram of
gamma-ray bursts redshift that pretty much parallels the history of star formation in
the universe. And this one is obviously out there beyond
redshifts where other galaxies have been seen.
We think that these first stars started forming maybe around edshifts 20 or 30.
The first proto-galactic fragments, they probably reionized the universe by about
redshift 10 or 15. And that's indicated by micro background
measurements, as we'll see in the next module.
But then, conceivably there was a low in, in that primordial star formation, and in
the first supernovae from Population [inaudible] stars exploded, then they
started to make young Population II stars. And they may have reionized the universe
in a second wave of primordial star formation.
And by about redshift of six, the whole thing is pretty much finished.
So one possible scenario is that after the recombination in the Dark Ages when there
are no stars and all sources of light. You have the very first generation of
supermassive stars reionizing the universe, creating the metals, creating
now Population II stars. The low mass Population II stars are still
with us. They're in galactic halo and globular
clusters. But the very massive ones have, of course,
exploded early on. And then, they can do the second wave of
reionization, which may or may not be really distinct.
We can model this process using hydro simulations that include dark matter, gas
as well as radiation and supernova explosions.
And, here are snapshots from one of those. I can see that the ionization fronts
propagate through the primordial intergalactic medium, the original cosmic
web, in a way that's not simple and spherical, because some hydrogen shields
the material behind it. But eventually, it all does get ionized.
In the next module, we will talk about this in a little more detail.