Let's now talk about the most common absorbers which are those due to hydrogen.
We can divide them according to the projected column density, as I mentioned
earlier. So, Lyman alpha Forest, are clouds which
have unsaturated lines, and they correspond to column densities ranging up
to ten to the 16 per square centimeter, or thereabouts.
They have very little, if any metals, and their, their sizes are bigger than those
of galaxies, they're not yet collapsed into galaxies.
Next are the Lymon limit systems. Those have column densities ranging from
10 to the 16 through maybe 10 to the 18 or so square centimeters.
They're called so because there is enough absorption going on that you can see the
break of the alignment series at rest frame by length of the 912 angstroms.
The Lyman alpha lines themselves are already saturated.
And so it's the amplitude of the break that's actually interesting quantity here.
Going to the higher densities, we see so-called damped Lyman alpha systems.
They're completely saturated. They're called damped, because what you
see is the absorption by so-called damping rings of absorption line.
And if you take in any stellar atmospheres, or.
Maybe atomic physics that deals with this, then you know what it means.
We believe that these high-cone density systems are almost certainly parts of this
galaxy with high redshift, or at least their progenitors.The things that we can
measure, therefore, are the equivalent widths and the broadening.
And there are many Codes that can be used for this.
Now this is a well understood and well developed art.
And so you're in highly complex spectrum, like the one I show you here.
Can be fitted very well with a collection of modeled lines, that go well through the
data points. And this is what a damped Lyman alpha
system looks like. It looks like there this hole in the
spectrum with kind of curved wings to it and the, the amount of absorption
determines how wide that system is going to be.
So how many of which they there? It turns out that the probability
distribution of column densities is a parallel.
As shown here. And it's a power wall with a slope of
minus 1.7, which is interestingly close to the slope of the two point correlation
function and that's not entirely an accident.
But it goes from the lowest column densities that we can see to the highest
ones. [inaudible] So now let's take a look at
the evolution of these absorbers. They're comoving number density, their
comoving cubic megaparsec changes in time. It's lower here and it's much higher at
high redshifts and here is a good illustration of this.
There are two quasars, very different redshifts, one low, one high, and they've
been plotted in the rest frame wavelength, so see that.
For the lower redshift one there hardly any Lyman alpha clouds to be seen.
At high red shift there is ubiquity of them.
And so that's probably because these clouds eventually get absorbed into
galaxies. Using simple Freedman models you can
derive formulae that will correspond to number.
Per unit ratchet interval, as a function of ratchet, as a function of cosmological
parameters, and here, I'm giving you a formula for models with now, with
non-cosmological constant, although the, those can be computed as well.
And it turns out that also high ratchets. The number increases as parallel of the
stretch factor 1 plus z to the 1.8 power. But, does this not apply to all redshifts.
And one of the interesting results from Hubble space telescope was.
When we can finally observe quasar spectra outside of Earth's atmosphere so that
ultraviolet can be observed. Note that the atmospheric transmission
window starts around 3,000 angstroms. [unknown] of that it's all absorbed by
the, by the oxygen or ozone, but outside the atmosphere we don't have that
limitation. So we could now look at Lymon alpha forest
at lower redshifts then we could do from the ground.
Say from the ground it can do from about redshift 1.6, 1.8.
Up. And, from space, you can look at arbitrary
low red shifts. And they turn out that there is a
substantial change in the slope. There is a change in population.
This may or may not having something to do with the peak of the cosmic star formation
history that happens at around the same red shift.
You may think that. Clouds have been absorbed into galaxies up
until then and then things kind of slow down so the column density doesn't change
very much. And, indeed, as we go to higher redshifts
you get thicker and thicker forests but then what happens is that lines start to
overlapping. The trees are lined up so dense that you
don't see gaps between the trees and this is illustrated here as a set of windows in
redshift space and lines of sight towards some high redshift quasars.
At around redshift of four you see a very thick Lymon alpha forest and then it
superficially looks at, like it's thinning out, but actually it's just overlaps of
the lines. And what looks like little emission spikes
are just gaps between the lines. As you go to ever higher redshifts, it all
fills up. And so the inter-galactic medium is
effectively opaque to the Lyman alpha line.
This leads into the so-called reionization that we will talk about in more detail.
It is quantified by so-called Gunn-Peterson effect.
Named after the two astronomers who came up with it.
And they noticed that if you increase the net column density of neutral hydrogen, at
some point it is so saturated that you're just going to completely lose all the flux
below of to the center of[UNKNOWN]. It doesn't take a lot.
It takes maybe one part in 10,000 of the neutral hydrogen, in the IGM at that
redshift, to do this. So, recall the history of the universe, in
some sense, is that there was first ionized plasma, it recombines, and this is
when cosmic microwave background is relased.
Age little shy of 400 thousand years. Then there are no sources of light in the
universe, and the universe is filled with neutral hydrogen, still some, and neutral
helium, as well as dark matter, and so on. And that neutral hydrogen would
effectively absorb all photons blueward at 1216 axtroms in space.
Radio waves of course can still go through.
Then sources of light may start turning on.
They reionize the intergalactic medium. And thus they make it, transparent again,
to ultraviolet light, except where there are little clouds of hydrogen.
And those are the Lyman alpha forest clouds.
So transition from purely opaque more or less continues not very ionized
intergalactic medium with very high redshifts to the one that's ionized by
stars and quasars. Is called the reionization era and until
recently this was one of the major goals of cosmology to find where it happens and
now we know. The first examples of this we're seeing
circa 2000, 2001 the[UNKNOWN] quaser is discovered by Sloan digital sky survey.
And now there is a number of those several tens and around redshift of six those
effects start to come in. You see sudden drop, essentially complete
absorption[UNKNOWN] the Lyman alpha line until you get to sufficiently low
redshifts so then things can start coming up again and then you run into the limit.
This is the end of[UNKNOWN] era. The beginning starts maybe oh, redshift 20
or. 30, that's still subject of some research,
but we've seen this signature at least. And we'll talk more about this in the next
chapter when we talk about galaxy formation and reionization.
So here is a plot of the transmitted flux at Lyman alpha wavelength as a function of
red shift from a large number of different quasars.
And as you can see, as you go to every higher red shifts, more and more is
absorbed, less and less gets through, and then there is essentially waterfalls,
steep cut at around red shift of six. Which is essentially the effected that I
just described to you. Next we will talk about connection between
galaxies and intergalactic absorbers.