So, we saw how the primordial star formation, the very first stars of
Population II Stars, reionized the inverse as the first protogalactic fragments form.
Let's take a look at this in a little more detail.
Here, again, is the same schematic outline of how things happen that you start with
my career background, really, is an age of 300 or 400,000 years after the Big Bang.
Then, dark halos keep forming through what's called Dark Ages, when the universe
was about a few hundred million years old. Maybe about 500 million years old, the
very first stars formed. Then, through the next few hundred million
years until about age of about one billion years, the reionization commences and it's
complete by about the redshift of six. We can simulate the very first star
formation and this is from one of such simulations by Tom Abel and his
collaborators. And some of these first stars can ionize
the gas around them, as well as contribute metals through their explosions.
The way we can actually detect reionization here is through the
Gunn-Peterson Effect that we mentioned earlier when we spoke about intergalactic
absorption. This was developed already in the 1960s,
it was recognized when quasars were discovered that they can pro, provide
excellent probe of intergalactic medium with high redshifts.
And there is even small amount of neutral hydrogen, it will be very effective in
absorbing radiation blueward of the Lyman-alpha line.
It will manifest itself as a sudden drop influx blueward of the Lyman-alpha line
and that was finally seen circa 2000. Here is how it works.
If you're looking at the quasar, it's just inside the reionization era zone.
Then, the line of sight will go through a lot of these bubbles and the gaps between
them will be where neutral hydrogen is and they'll manifest themselves as continuum
between absorption lines. As you get closer and closer to the
reionization itself, the overlaps of the bubbles will be less and less and there
will be more neutral hydrogen that will just absorb large chunks of spectrum
blueward of the Lyman-alpha of the, of the quasar itself and the spectrum may look
like something like this. And indeed, this was observed.
In essentially all quasars at about redshift of 6 and above, the same
phenomenon was seen. There was a very sudden drop of flux at
Lyman-alpha line, and then at lower redshifts, the Lyman-alpha forest would
continue, but then you would run into the Lyman limit that corresponds to the
redshift of the quasar, and then flux will be gone again.
So, this was the most distant quasar for a while, and, shows beautiful example of the
Gunn-Peterson effect. So, the face value of it, we have found
reionization at about redshift of 6. If you look at transmission of Lyman-alpha
flux against redshift, the higher redshift you go, the less flux it goes through
because of Lyman-alpha forest. But then, by about red shift of 6, there
is a, just, dramatic increase in absorption, which was seen and manifested
itself as Gunn-Peterson effect, and this is why people think that that's pretty
much when reionization era ended. You can convert the observed absorption
into the opacity of neutral hydrogen or fraction of the neutral hydrogen, and the
sudden drop in transmitted flux maps into sudden increase in the fraction of neutral
hydrogen. But it's enough that just only a percent
or so of hydrogen is there to achieve the same effect.
There is also variaion in different lines of sight.
This shows the Lyman-alpha forest in the approach to reionization along four
different lines of sight to four different high redshift quasars.
We think we understand this because of the biasing, that there'll be substantial
cosmic variance, there'll be different large scale structures, there'll be
structures forming at the highest peaks. That means that there will be actually a
certain lumpiness in the reionization. It will not proceed everywhere at exactly
same pace. A new effect that was seen with WMAP
satellite that measured fluctuations in cosmic microwave background was a sudden
increase in electron opacity that corresponds to reionization, but this one
indicated that it happened around redshift of 10 or 20, which would be substantially
higher than what quasar data indicated. Now, the more modern analysis of the
microbackground data gives us optical depth due to free electrons, which come
from ionization of hydrogen and have a peak of reionization around redshift of 10
or so. It's consistent with an instant
reionization but that's physically not very likely and most likely, we're dealing
with an extended period from the formation of very first stars until the time where
all the ionization bubbles overlaped. And that may have taken form redshift that
are 20 or slightly higher, all the way down to redshift of 6.
In terms of actual age, that means from few hundred million years after the Big
Bang to close to about one billion years after the Big Bang.
And even though the quasar data by themselves cannot really guarantee that
there was a substantial amount of neutral hydrogen, remember, only a percent or even
a fraction of a percent is enough to cause the same effect.
We have other ways of looking at it. One is counting how many Lyman-alpha
galaxies are there in any given redshift interval.
And we expect that as we get into the region of neutral hydrogen, sufficiently
high redshifts, suddenly there'll be a drop in the numbers of Lyman-alpha
emitters, because light will be absorbed by the hydrogen.
And that's exactly what we've seen. There seems to be a, a rapid decline in
the number density of Lyman-alpha galaxies past redshift of 6.
Also, in the most distant gamma burst and quasars, we can fit the shape of the
absorption the cause is, is actually the, the damp wing of Lyman-alpha line and its
shape depends on the amount of neutral hydrogen.
So, that indicates that there was about 10% of neutral hydrogen by mass or by
volume at around redshift of 6. So, it's, the universe was not fully
neutral but it was pretty much at the end of that reionization phase then.
So, combining the data from Lyman-alpha galaxies, from quasars, and from microid
background yields a fairly consistent picture that we can see how star formation
density in the universe is dropping. This galaxy, as we go to high redshifts,
these galaxies are just forming back then that resulted in change of ionized or
neutral hydrogen fraction. It also, as galaxies were being built up,
you can infer how much mass was generated in stars nd you can also make predictions
of what should have been seen in microid background measurements and in, in fact
that matches the observations. So, we now seem to have a reasonably
consistent story about formation of first stars and galaxies in that era between
redshifts 20, 30 or so, when first stars form until about redshift of 6 when the
reorganization is complete. Going even deeper would require a
different kind of measurement. Neutral hydrogen, even before any stars
are formed, will still be detectable to 21 centimeter line.
That was used so effectively to map kinematics of gas and spiral galaxies.
And there are theoretical studies of this so that even before first stars are formed
that you can principal map assembly of baryons in potential wells of future
galaxies. And there are radio telescopes and
experiments being developed to try to detect precisely this.
So far it's too early to actually expect any detections, but within next several
years or 10 years everybody thinks that we will actually be able to observe these
protogalaxies before they even make stars using 21 centimeter line of neutral
hydrogen. And next, we will finally turn to active
galactic nuclei and their role in cosmology and galaxy formation.