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.