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.