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.