So galaxies and intergalactic medium have a sort of ecological interplay.
The gas comes into galaxies, gets expelled from the galaxies.
This is where metallic line absorbents have to come from.
And I already mentioned that very high-column density ones probably are
associated with galactic disks or progenitors thereof.
As the line of sight for some high redshift quasar goes through space, it can
encounter more than one of these. And here's a good example of a quasar, and
along the line of sight to this quasar there is a whole number of separate
metallic line absorbers. Each of which we believe now is affiliated
with some sort of halo of some small galaxy or another.
And the way to find out is to first take deep images of fields around those
background quasars, identify galaxies around them, get their redshifts, and see
if any of those actually correspond in redshift to one of the absorption line
systems. This was now done for a large number of
these, and in fact. Essentially every time there is an
association of a galaxy with a given metallic line absorber.
Even so, some of those absorbers can be a very large apparent radius from the
galaxy. And we, we know that this is the case
because we can absorb. Multi-baryonic species.
Not just one absorption line, but whole number of them.
Which then correspond to the same redshift.
And they're matched against the galaxy's own redshift.
So here is an example of whole number of those in one of the deep fields.
And those galaxies tend to have pretty much properties of field galaxies at same
redshifts, which is reassuring. Now what about damp Lyman alpha systems?
Which we always thought were counterparts of high redshift disks.
For some number of them we can also detect Lyman alpha in emission that's offset from
the absorption line of sight. And so here is an example of a quasar with
a damped Lyman-alpha object present. Then in a plot on the right you can see a
spectrum from blue to red and the orthogonal direction is the longest slit.
There's a gap that corresponds to the damped absorber and just offset from it is
a blob that corresponds to emission from the same absorber.
From the strengths of those emission lines, from their separation, from, other
properties, it was concluded that, indeed, they're perfectly consistent with being
disks or youngest, or progenitors of young disks, at high redshifts.
So if metallic line absorbers are associated with galaxies, they should also
cluster like galaxies, and in fact they do.
Here is a comparison of such a coloration function for the metallic line absorbers,
adjusting radio coordinate that gets the only one we can measure and also for Lyman
alpha clouds. Lyman alpha clouds are also correlated but
much, much less. So, metallic absorbers do correspond to
high redshift galaxies whereas Lyman alpha clouds are still material that's in linear
regime that belongs to large scale structure, not to galaxies themselves.
And then we can compare how the absorbers of different types involving red-shift.
As we look at hydrogen absorbers, their numbers increase with redshift, as we
already noted before. For the metallic line absorbers, we see
first an, well, looking from high redshifts, towards us, meaning in the
direction of time. We see an increase as you build up
galaxies and star formation, presumably, and then things roll over and flatten out.
So this is exactly what we would expect from this picture, in which.
Galaxies are built up slowly, then they eject gas out.
And so the metal rich intergalactic medium is also being accumulated in time.
So, I mentioned that damp Lyman alpha absorbers are likely to be progenitors of
present day disks. And one way to look at this is to look at
their chemical abundances. The column density is so high that in
addition to hydrogen we can measure various metallic species and from that we
can infer metallicity just as we would for stars.
Here the histograms comparing metallicity distributions in Damped Ly-alpha
absorbers, the solid blue. With metallicities of stars in the thick
disks of the Milky Way on the left, the red one.
And the thin disk of the Milky Way, the green one, on the right.
As you can see the, they overlap. But Lymanova absorbers are much closer in
their metallicity to the thick disk stars, the old disk population, than to the thin
disk stars, which are just relatively recently made.
Chemically evolved systems. And, just as we looked at the history of
chemical enrichment in galaxies we can do the same thing now for absorption clouds
and here is the plot of the metal abundance of damped Lyman alpha systems.
It's a function of redshift. The individual red dots are individual
objects and then they're being, those are the big air process.
So you can see there's a gradual increase. You start with the broad distribution
tending to belong with the list of high redshift and you gradually climb towards
almost solar abundance low redshifts. Now one has to beware that different types
of absorbers may be evolving in a different fashion.
In fact, they correspond to different parts of galaxies so damped Lyman alpha
absorbers are kiloparsec scales, disc scales whereas Lyman alpha forest clouds
may be hundreds of kiloparsec scales. On the other hand you can look at things
like quasars whose metallicity you can also measure from their spectra and they
can be up to 10 times solar and they probably correspond to regions in the very
cores of their host galaxies or near the black hole.
So the chemical evolution will proceed a different pace in different environments.
It will grow faster in the dense environments, we get more of recycling of
stars and so on but we can outgrow that over both range of scale.
So we can add up all this hydrogen that is to be used for star formation in galaxies
and if we have proper census of absorbers per unit redshift and so on we can
integrate that as briefly shown here. So from that we can then deduce.
The evolution of the, say, omega in hydrogen gas, that belongs to these clouds
as a function of redshift. And so here it is.
Compared to the measurement of the local stellar dentsity.
That's the red point on the left, upper left, as well, some of the theoretical
models. And you can see that there is
approximately one part in thousand of omega in hydrogen gas that belongs to this
class. You may remember that omega variance, all
variance, is approximately 0.45. Which is, so 45 times this.
But the omega of stars that we see in galaxies is only something like, 5 times
10 to the minus 3. So this is, a fraction, but sizable
fraction, of all material that we see in stars.
So to recap, the intergalactic medium is forming cosmic web.
It's associated with large scale structure that's still collapsing linear regime, but
metal rich parts in the highest density hydrogen clouds are associated with
galaxies. Pieces that have already fallen together
and made stars. The intergalactic medium is almost
entirely ionized from here out to redshift about six, at which point you start to get
more neutral hydrogen because stars and quasars simply didn't have time yet to
re-ionize it fully before that. The metals themselves come from galactic
winds, products of stellar evolution that have been expelled from galaxies and mixed
in with the rest of intergalactic medium. And these studies using absorption
systems, using quasars or gamma ray bursts as background lights A compliment other
ways of studying galaxy evolution. In the next chapter, we will finally look
at galaxy formation itself and cosmic reionization in more detail.