Galaxies don't live in a perfect vacuum.
They're embedded in the intergalactic
medium, which is gas, which, some which
was never in galaxies, some which was
expelled from the galaxies.
And galaxies are a sort of ecological
equilibrium.
So briefly, intergalactic medium is
barriers between galaxies.
That gas tends to be very highly ionized,
mostly by radiation from our two galactic
nuclei.
And like any other test particles it
follows the evolution of the gravitational
potential Orbital large scale structure
forming voids and filaments and sheets.
In other words, just like those structures
we've seen in numerical simulations.
And because of that it's also sometimes
called a cosmic web.
So, unlike the gas in galaxies, which is
falling into deep potential wells.
Probably has dissipated quite a bit of
energy.
This gas is still in the linear region,
and because of that it offers a
possibility of being used to trace the
large-scale structure evolution in the
linear region that is not probed by
galaxies.
When the gas falls into the galaxy it
replenishes the fuel for star formation.
You may recall that the amount of neutral
hydrogen that is in spirals now would last
them only about a billion years so it had
to be replenished.
Likewise as we've seen from galactic
winds, some gas gets to be expelled back
into the intergalactic medium, and this is
where the metals come from.
So in some sense, the chemical evolution
of inter-galactic medium, which can be
probed in very effective ways, traces that
one/g of galaxies themselves.
This is now being simulated numerically at
the, in great detail, using[UNKNOWN]
simulations with gas affiliated with
large-scale structure.
So you can shoot lines of sight through
these theoretical representations of the
cosmic web and compute what would the
absorption spectrum look like if you had
light shining from the other end.
So this actually provides a complimentary
way to study galaxies and their evolution.
Typically, we would look for galaxies in
emission.
Visible light maybe re-radiated from
infrared from the dust or something like
that.
Here we don't look for galaxies in
emission.
It doesn't matter how bright they are or
if you can see them at all.
We're looking for galaxies in absorption.
There is a cloud of gas, which will absorb
certain wavelengths, and from that we can
learn about physics of the gas, maybe
fixed and evolutionary state.
So it's a very complementary way of
studying galaxies at high redshifts.
Traditionally, quasars provided those
sources of light.
That shine through the intergalactic gas.
They're very bright, seen far away, and
their spectra tend to be fairly simple.
They have broad emission lines but usually
it's a fairly smooth continuum.
So any absorption lines due to the clouds
will be easily distinguished against such
a continuum source.
More recently gamma ray bursts, which have
bright optical afterglows provided another
way of doing this.
And they probed somewhat different regime.
But maybe we'll get to that later.
I mentioned repeatedly during this class
how selection affects are something one
has to be aware of because we're looking
at very faint objects far away, there is
always flux limit, there is surface
brightness selection.
There's all kinds of selection effects.
When you look for sources in emission.
But when looking in absorption, none of
that matters.
It's a little bit like[INAUDIBLE] effect
where you're looking for clusters of
galaxies using their shadow, if you will,
in microwave background.
Well here we're looking some sense the
shadow produced by the gas associated with
galaxies or illogical/g structure.
There are a variety of different kinds of
absorption systems, usually called QSO
absorption lines because the QSOs, quasars
were used to find them.
Hydrogen being the most common element,
accounts for most of it.
And there, they're divided according to
their column density, by the amount of
hydrogen That you look through along that
line of sight.
The thinnest are so called Lyman alpha
forest clouds.
They're small subgalactic size clouds, and
they're heavily ionized.
But they're easily seen in absorption
lyman alpha line.
As you increase the column density you
find so called lyman limit systems and
then damp lyman alpha observers, and Talk
about that in more detail.
Usually there is some metallic line
absorption associated with high column
density hydrogen clouds.
This is probably because they are
associated with galaxies in someway or
another.
There also helium equivalents of all this
seen further in Ultraviolet but since
hydrogen is more common it's easier to
obseve usually people just worry about
hydrogen.
And finally they're metallic line
absorbers.
This is the gas that was processed inside
galaxies, expelled in supernovae winds.
And it can be studied in absorption as
parts of maybe extended galaxial halos, or
even clouds between.
So this is what the, spectrum of the quasa
looks like being ultraviolet rest in
ultraviolet.
The Quasa has sort of power like, power
like spectrum with very broad emission
lines superimposed on.
But now on top of that there are all these
absorption lines.
And you can see the bluer/g of the center
of the Lyman alpha line.
There is a whole lot of them.
This is not noise.
These are actually individual absorption
lines piled close together.
The reason why they're.
Occurring there is that Lyman Alpha is a
resonant line, a very strong absorbing
line, and the stuff that's between us and
the quasar is, of course, redshifted less.
So all those would be on the blue side of
the quasar's Lyman Alpha line.
To the red of the Lyman Alpha line you see
Some absorbers, not as many, they are
always due to some kind of metallic ionic
species, often as carbon or nitrogen or
magnesium or iron, things like that.
Because the Lyman Alpha Forest clouds are
so numerous and crowded together, they're,
they're often called the Lyman Alpha
Forest.
And here is what they might look like in
typical spectrum a quasar, not such a high
signal to noise as the one I'm showing
you.
The Lyman Alpha Forest lines are
unsaturated sharp lines.
But if you have enough column density of
hydrogen, you'll start making very broad
saturated line and those are Lyman limit
systems.
They're called so because in the limit of
Lyman series,[UNKNOWN], and 916 angstroms,
they're very effectively absorbing most of
the light.
So, you see the limit to it.
And then there are damped Lyman alpha
absorbers, which are really saturated
lines of high column inside, and they of
course will also have.
Strong limit systems.
So this is just a zoom in on[UNKNOWN]
spectrum I've shown you before.
To show you in little more detail, what's
going on.
Since we know the redshift of Lyman alpha
very precisely, then we know the redshift
of these absorbers with great precision.
And we can fit theoretical profiles
through those lines to estimate their
internal broadening or to the Doppler
effect and things like that.
This is a table of some of the most
commonly observed transitions in observing
clouds.
Not for you to remember, but it's a good
Able to have some where should you ever
need it to look up such things.
And can see they're all in ultraviolet
because that's where, where observing
quasars which at now at high redshift.
So how do we quantify that?
This was a very general discussion of
absorption in.
Transpiring plasmas, if you will.
And there is some atomic chorionic species
that absorbs at a particular wavelength.
The strength of the absorption, which can
be measured as the equivalent width, which
is defined as the width of a rectangular
line that would cover the same area.
[unknown] amount of absorption.
What] we want with is obviously going to
be proportional in some way to the numbers
of absorbers along the line of sight.
Since we're now looking at things
projected on the sky, it is the column
density.
It is a number of hydrogen atoms.
Projected in the sky per, say, centimeter
squared.
And in case of hydrogen clouds, the
relevant regime is in 10 to the 12 or so,
which is the weakest clouds we can detect,
to about 10 to the 21.
Hydrogen atoms per square centimeter
projected line, along the line of sight.
Incidentally, if you were to take the
Milky Way disk, and squish it, into a
plane, the surface density would be about
that much.
10 to the twentieth, or 10 to the 21,
atoms per square centimeter.
The, the physics of this is well
understood.
It is very well established atomic
physics.
And at first as we increase column density
you have proportional growth.
But then there is a little wiggle as you
start saturating.
And the line can't be any deeper than
complete absorption in, in the middle, so
zero transmitive flux.
Then it can only go wider, and so then
starts going up again.
The exact position of this regal, depends
on the doppler broadening its present in
the line.
And so by studying the equivalence with,
by calibrating this, we can then measure,
observe, properties, and the find out
both, what the column density is, and also
the Doppler broadening is.
Another thing that we can do, is we can
count how many clouds that we see, for
unit redshift, and then.
Therefore per unit area projected on sky,
and from that, we can infer what the
relative sizes of these absorbers are, and
find out as far a hydrogen concern, the
higher culumn densities are corresponding
to smaller sizes, their rare.
And that's easily understood.
Because the higher column density
presumably correspond to inner parts of
the galaxies.
Whereas, the outskirts will be much
thinner, and obviously bigger.
Likewise, different ionic speeches show
differences in absorption cross section, I
guess.
And that's partly due to their, spatial
extent and distribution.
Next we will talk in more detail about
hydrogen absorber of different kinds.