We now turn to the study of galaxy formation for stars, galaxies and cosmic
reionization. This is probably the most active area of
observational cosmology today. First some preliminaries.
What do you mean by galaxy formation? Galaxies, kind of keep forming all the
time, there is no really sharp definition between formation and evolution.
And just as with galaxy evolution, there are two components to it.
How do you assemble mass and how do you turn gas into stars, and then their
effects on the remaining gas. Obviously galaxy formation as well as the
evolution have to be related to their large scale environment, because they're a
sort of low end extension of it. But unlike structure formation itself,
galaxy formation includes very dissipative processes, in particular star formation.
We now also know that processes of galaxy formation seem to be somehow closely
coupled to the formation of the central super massive black holes, and we'll
address that in a little more detail later.
But generally when people talk about galaxy formation they usually mean
assembly of large galaxies of today. Some were at large redshifts, certainly
greater than 3 or 4, maybe greater than 5 or 10.
The old dwarf galaxies may be just coming together today, and so we do see some
residual galaxy formation going on. So in the last decade or, or so of, we
discovered large populations of young galaxies, large redshifts young, simply
because universe was young then. And now we have a fairly good idea of how
the whole process works. And indeed, you can think of the frontier
as understanding the very first star formation and formation of first
protogalactic fragments and how they re-ionize the neutral universe, somewhere
around redshifts between 6 and maybe 15 or 20.
To recap the general picture of structure formation, we start with small initial
density perturbations that come out of the Big Bang.
They keep collapsing, driven by the dark matter, and that process begins well
before recombination when micro-background is released.
At that point there are no sources of light in the universe for awhile and
barions gas keeps falling into the potential oils delineated by the dark
matter. At some point the very first stars begin
to form. That probably happens around ratchets of
20 or 30 or so, but nobody really knows for sure.
Why, redshift of six or so, the young stars from, freshly formed proto-galaxies
or their fragments we ionize the universe, and it becomes transparent to UV radiation
again. All through this process and continued to
the present day, there is a hierarchical structure formation that galaxies merge,
or smaller fragments are absorbed into the large ones, so the galaxy growth
continues. As far as stellar populations are
concerned, massive stars will produce heavier chemical elements in their cores,
exploded super novi returning to interstellar medium from which new stars
are formed and so on. And in this way we have ever more
enriched, chemically enriched interstellar material as well as new generations of
stars. An important player in all this may be
energetics related to super massive black holes in galaxies, those responsible for
quasar-like activity, because they can dump substantial amounts of kinetic and
radiative energy in their hosts and modify their evolution as well.
So here is a nice schematic illustration of the early cosmic history.
This one is due to Avi Loeb. In the beginning of this particular
segment we have cosmic micro background recombination.
At which point universe enters what we now call dark ages because there are no
sources of light. Dark holes keep collapsing and eventually
you die substar formation and more and more of those happen.
When the ionized bubbles of gas around those first protogalaxies start to
overlap, this is what we call the reionization era, where the universe which
was filled with neutral hydrogen and helium ever since the microbackground was
released. Now becomes ionized again and that is
probably as good a boundary as any when we say, well this is roughly where galaxy
formation is really occurring. And after that obviously evolution of
galaxies continues. So the assembly of the mass is driven by
the dark matter and those potential wells keep us getting assembled through a
creation of More material. A way to characterize them is by their
mass function, how many there are in a given mass.
Here's a set of theoretical curves of how the mass function of dark halos evolves as
a function of time. There are many small ones, and there are a
few large ones. But if you look at curves, which are
labeled by the red shift, early on at high red shifts there are really very few very
massive hails and the growth keeps occurring.
There isn't so much growth at low mass end, but you start populating the high
mass end with evermore more massive halos, and that process continues until today.
So one of the important predictions of our general theoretical understanding of
structure formation is that there will relatively few massive potential wells,
massive galaxies early on. Well we discussed structure formation.
We talked about essential gravitational processes.
Galaxies follow that as well, but they're also much, much messier dissipative
processes, where gases are converted into stars, stars radiate energy, change to
gas, stars explode and so on and so forth. These processes are impossible to model
analytically, not because with their hard, they truly are impossible to model
analytically. And they can be only studied numerically.
So we do not have a clean-cut theory of star formation, although we do understand
it fairly well in some sense. And that then maps into our relatively
shaky understanding of how initial star formation proceeds.
Nevertheless, much progress was made, and we'll talk more about that later.
So, all of the things that I already mentioned are parts of the overall picture
of galaxy formation and evolution. But the basic paradigm is very simple.
You have potential wells formed by dark matter, which can keep merging, and they
form containers for the gas that will fall in, get sufficiently dense in the middle,
that will start making stars. So, generically we expect the dark halos
will be more extended than the stellar components of galaxies, which was exactly
what we've seen when we discussed global properties and structure of galaxies.
So for a while, protogalaxies, or really young galaxies, were something of a
mythical creature, and the question is how do you define one?
And there are many different ways in which you can define it, as some first X years
or fraction of the age of the galaxy's life, or when certain fraction of the mass
is assembled, or a certain amount of stars is assembled, and so on.
Or we can simply declare that beyond certain redshift everything should be
treated as a young galaxy. It doesn't really matter.
Essentially we've tried to map the whole process from the very first star formation
until galaxy evolution to the present day. But for a variety of reasons, we think
that in most young galaxies, there was very intense star formation.
And there should be very little luminous objects because of that.
Although, obviously, there will be a lot of small ones and very few really massive
ones early on. So it's the energy release from forming
galaxies that makes it possible for us to see them.
And there essentially two major mechanisms for young galaxy's to create energy.
First of all it's the collapse from large destruction.
You'll recall that, cooling plays a crucial role that galaxies are thousands
times denser than they should be if they're just extrapolation of logical
structure. That is, they collapse by an extra factor
of ten. And therefore binding energy change due to
the dissipative collapse overwhelms any changes in binding energy from simple
gravitational collapse. However it happened half the potential
energy today is equal to the kinetic energy in galaxies within a sign.
And therefore we can estimate kinetic energy of the galaxy today, and this much
energy was also released in process of galaxy, proto-galaxy collapse.
So we can take approximately mass of the cooling materials, that will be the
variants, and multiply it with a square of characteristic three D velocities in
galaxies and for typical numbers, that the relevant here for galaxy say like the
Milky Way, the relevant number is roughly n to the 59 ergs.
That's just for the variance part that cools.
There's 10 times that much in dark matter, but there is no visible signal from it.
It's simply a collapse and that sounds like a lot of edge.
It turns out actually that conversion of gas interstellar material to stars also
produces additional amount of energy which is approximately of the same order, so
just Putting things together will release of the order of ten to the 59 or few times
10 to the 59 at most of energy. But it's not the energy that matters but
luminosity or what time interval this is being done.
However, the second source of energy is actually more important.
And this is conversion of hydrogen into helium in massive stars.
The term nuclear fusion in stars, converting hydrogen into helium in the
heavy elements. Releases energy and approximately 0.1% of
the initial mass of hydrogen is converted into energy.
It turns out that subsequent fusion from helium into heavier elements is relatively
minor perturbation to this one. So it's really cooking up of the helium
stars that matters. Observing helium abundance, and
subtracting the primordial one, is actually very, very difficult, especially
for old stellar populations. Instead of that, we can model chemical
evolution of stellar populations and find that indeed there be correlation between
the amount of helium cooked up in stars, as opposed to primordial one.
And the amount of heavier elements which astronomers always call metals, that
includes things like oxygen and carbon and so on.
Now that is something that we can measure from spectroscopy, and so it turns out
that the typcial metallicities of stellar populations today are of the order of
solar, which is approximately two percent by mass or maybe one percent by mass if
you really account for all different stars, but so that order.
And for each gram of heavy elements that's made, approximately five grams of helium
that are made The actual numbers are a little iffy and depend on exact models of,
stellar population evolution, but they're of that order of magnitude.
And so we can estimate the net energy from star formation or, energy released by
stars, by taking the mass of all stars formed, times z squared.
Times the efficiency of thermonuclear reactions, which is 0.1% times the
fraction of the hydrogen that was converted into the helium and heavier
metals. And for typical numbers for galaxies
today, that turns out to be an order of magnitude more than what we got from just
release of the binding edge of the order 10 to the 60 ergs, or galaxy like the
Milky Way. So it's really burning of young stars that
will form the most important observe, observable signature of young galaxies,
and incidentally, active galactic nuclei that super massive black holes that they
create material and convert some fraction of it into energy can contribute
comparable amounts as does all of the star formation.
And right away that tells you that their energetics might be important component in
determining structure of galaxies and their star formation.
So those are the generic expectations and next we will take a look at the
observational evidence for galaxy formation.