So, I didn't get a chance to give, a lot of detail about elliptic galaxies or the
patterns of, mass versus rotation and spiral galaxies, we'll get to some of
that maybe later. But, I want to tell you what we know
about galactic evolution, because it'll give us a framework for thinking about
this in the same way that the history of the solar system gave us a framework
thinking of the solar system. galaxies, it turns out, evolve in a very
different way than stars, stars essentially unless they're in a very
close binary, evolve pretty much independently.
A star, evolves, on it's own evolutionary track.
Galaxies are far more social creatures remember that most galaxies are found in
clusters, many stars are found in clusters, but in a cluster of galaxies
the fraction of the volume taken up by the galaxies is much larger.
Remember that if you count the halos Andromeda and the Milky Way essentially
are touching. Even if you don't count the galaxies
compute the ratio of size to distance between say, Andromeda and the Milky Way
and size to distance between say the sun and Proxima Centauri, and you will get
the idea that while interstellar space is vast and empty, intergalactic space is
far less vast especially in the context of a cluster.
And so interactions between galaxies, therefore are going to be far more
important in the evolution of a single galaxy than is the case in stars.
Now, I talked about spiral galaxies we find as part of our hints to
understanding how galaxies evolve That elliptical galaxies are far more common
in the interior center of the core of the densest clusters in the areas where they
are densest. And this is the region where you expect
galactic interactions to occur more than anywhere else, because the galaxies are
packed tight near the center. And galactic interactions, it turns out,
can destabilize the disc structure. This is sort of a reasonable thing.
imagine remember that the disc maintains itself because of conservation of angular
momentum. So you have a bunch of stars orbiting in
one particular direction, but then they collide with another bunch of stars with
a different direction of angular momentum.
Those collisions will lead to a combinium momentum, but the flatten disk structure
will probably not survive. And the result of collisions is likely to
be since you have collisions with I mean collisions of stars are gravitational
collisions. They're scattering events, but the result
will be that stars will go off in with varying orbital parameters and the
net result is sort of this elliptical galaxy where stars orbit with all kinds
of inclinations. And so if you believe that collisions and
mergers happen in a cluster you would expect ellipticals to survive in clusters
and that is indeed what, in the centers of clusters, that is indeed what we find.
There's another difference between galaxy clusters and star clusters or galaxies as
large clusters of stars, which is that in between stars, the interstellar medium is
there but the interstellar medium, remember gas and dust per whole
interstellar medium in the Milky Way comprises the tiny fraction of the mass
of the Milky Way unless you count the extended halo in which case may be it
counts for about half the mass of the Milky Way excluding dark matter.
In the case of galaxies we find this is very different.
The typical galaxy cluster, galactic cluster is full of hot gas whereby hot,
again I mean million kelvin gasses and the mass of this hot gas usually exceeds
the sum of the masses of all of the galaxies.
So, a majority of the, again excluding dark matter, a majority of the mass of a
galactic cluster is actually spread out in between the galaxies in the form of
intergalactic medium and again we think of interactions as the reason for this in
inter, intergalactic interactions, tidal forces can strip a small fraction of a
galaxy's gas away and ejected out into the interstellar medium.
And then, if we have recurring interactions, then over time some
significant quantity of gas will be transferred to the intsteller medium.
The net result is the observation that in galactic clusters, most of the mass is in
between the galaxies and not in the galaxies.
So what do interactions between galaxies look like? Well, galaxies colliding is
certainly not like cars colliding because galaxies are sort of for all that they
are massive they are extended objects. And the main interaction between two
galaxies is gravitational. So, the typical interaction at slow
relative velocities is a very stately thing, it's a gravitational interaction.
and a typical case to study is sort of the motion of a dwarf galaxy through the
plain of the Milky Way or a globular cluster on it's orbit through the plain
of the Milky Way or of a globular cluster from one galaxy through the plain of
another galaxy, and, it's a little bit tricky to make the calculation, but you
can sort of imagine but if you have a bunch of stars scattered around, and
along comes a globular cluster, pushing through moving this way, then in its
wake, what is going to happen is that the stars are going to gravitationally
attracted to this visitor and so there will be a motion of stars towards and
after it takes a while for these stars to move and after the cluster of pass, has
pass, there will be a wake. A region of increased mass density from
where it is attracted star.
So, an object moving through a field of stars always is dragging behind it
increased mass density which is decelerating it.
And it in turn is of course accelerating these stars so momentum is being
transferred from the moving object to the galaxy relative to collection of stars
relative to which it is moving. This is called dynamical friction and
because it's second order of gravitational effect, it's not surprising
that it increases like GM^2. Why GM^2 because the first GM is the
magnitude of the wake, that the star creates and the second GM is the
magnitude of the force that that wake exercises on the star.
So, 2 GM row is the density of the medium of course the denser the medium exerts
more friction and then this 1 / v^2 can be argued either from dimensional grounds
v being the speed with which this thing is moving relative to the stars around it
or because the, the basic idea is, the faster you're moving through the less
time there is for this wake to establish itself and exert a force.
The net result is that a visitor will be slowed down so a globular cluster, as it
passes through the disk, through the disk population, will be slowed down and
slowly spiral into a smaller and smaller orbit and eventually its interaction with
the discs will become so frequent. That tidal and forces will then break it
up and the globular cluster will not live forever.
It will eventually merge with a disc. And, this will happen faster for massive
clusters. so if you started globular clusters with
all kinds of masses, the smallest one would survive the longest.
Another, sort of, observation to file away this is what a slow collison between
galaxies look like. there are faster collisions where the
relative speeds are bigger and, essentially, the 2 galaxies pass through
each other without time for wakes to get organized.
Without the stars having time to react, v is much larger.
The stars still rearrange each other but they rearrange with a delay, this
liberates gravitational energy that was converted while the galaxies were
accelerating towards each other. Some of that is converted into kinetic
energies of the stars, the result can be the explosion.
A large amount, of kinetic energy of the stars in one galaxy and in dramatic
cases, like the the, something cartwheel nebula.
[LAUGH] Like the cartwheel galaxy, this has caused the emission of a lot of gas
in a circular ring around the galaxy, and the stars that we see here, these blue
stars are of course nothing to do With a. They were not ejected from the galaxy.
The gas was ejected. The gas is plowing into the intergalactic
medium. That's creating a high density shock wave
and that's generating star creation so this beautifully decorative ring around
the galaxy is marking the place where the shock wave is progressing through the
intergalactic medium and this may well have resulted From a collision with one
of these two objects here over on the right.
another case is this elliptical galaxy, which has what is called a polar ring,
likely the result similarly of a collision.
Perhaps the collider was destroyed and merged with the elliptical but the
resulting energy ejected this ring. Now, these rings are very useful because
they do not orbit in the plane of a galaxy, they are influenced by the
gravity of whatever is around there. This gives us an opportunity to measure
by studying their orbit or parameters, the dark matter distribution
perpendicular to the plane. And somebody might have argued based on
the orbit of the sun and the stars and the clouds and the Milky Way that dark
matter could be confined to the disk of the Milky Way.
the reason we are absolutely certain it's a spherical distribution is the, that,
the orbits of these objects, which orbit way outside the plane of the galaxy agree
with the predictions of the dark matter, cold dark matter model.
when the collisions are more delicate or in any collision between galaxies,
remember galaxies are in free fall. The gravitational attraction or the
gravitational interaction of one with the other, is a tidal interaction and this
can lead to these beautiful objects. On the left, of course M51 the whirlpool
galaxy interacting with not quite a collision, but obviously an interaction
with the smaller galaxy in the upper right,
We see that one of the spiral arms has grown and is extending towrads this
galaxy and slightly outside the frame the other spiral arm extends very far to the
left. These are both tidal effects gas or
material and stars are being attracted on the one hand and attracted away from the
galaxy on the other hand in the same way that we get high tide on two sides of the
Earth. These are the tidal effects of the
passing galaxy. And an even more brilliant example of
these sort of tidal tails, are these antennae galaxies, as they are called
here. And then a pair of galaxies in collision.
And we see the effects of tidal forces in these large tendrils of gas that have
been emitted by both of the galaxies. in another recent observation of the same
effect, NGC68672 has recently been, been measured to be the largest spiral galaxy
known. the scale here is 100,000 light years so
this thing is about 2 or 3 times the size of the Milky Way, but that's kind of
cheating. One of the reasons these spiral arms are
so extended is because it seems that this has undergone a collision, perhaps over
here, in the upper left, and so it has got these two tightly extended spiral
arms coming out on two sides. And the most exciting thing here is that
there is some structure in this upper left its tidal tail and it seems as
though some of the tail has become sufficiently cold and dense to collapse
and there's the region that is in circle there is a region where astronomers
suspect that a actual globular cluster or a dwarf galaxy might be forming.
So this is the weird case where rather than destroying galaxies and merging
them, here a collision is actually creating a new galaxy, a new wrinkle on
our theories of galaxy interactions, something modelers will indubitably
incorporate as time goes on, if indeed this thing is verified.
Now an interesting question you might ask, is, well.
So, if what you are telling me is that very often, two galaxies, will in fact,
merge, because they will be slowed down, ripped apart by each other's tidal
effects, and in fact, end up as one merged object.
Well that's very nice for the stars, they can orbit whatever they want and the gas.
Each of these galaxies would have had a super massive black hole in its center.
What do those do? Well that's an interesting question.
And what you would guess, of course, is that the two black holes, being so
massive, would very rapidly find themselves in the core, of the combined
object. So you could find yourself, in the
interesting situation, like NGC6240 which is a galaxy whose shape clearly suggest,
a past in which some interaction happened and a x-ray image of the center, shows 2,
x-ray sources, two black holes. This has a binary super massive black
hole and they are seperated by, this doesn't say, but what it meant to say, is
that these things are separated by an order of 3000 light years or a kiloparsec
an eighth of the distance from the sun to the center of the galaxy is the distance
between these two super massive black holes.
Those are some very interesting highly relativistic, orbital dynamics, and over
time of course friction with the dynamical friction, with the medium
as well as gravitational radiation will cause these things to eventually over
hundreds of millions of years or billions of years to move closer to each other,
eventually merge with each other, and there you will get a blaze of
gravitational radiation, that perhaps someone will be able to detect at that,
to detect at that point. So, based on all these observations of
galactic interactions we can try to understand, a model of galaxy formation.
And the original model of galaxy formation was basically the solar nebula
at large. So, remember, the solar nebula formed
from a cloud that collapsed, a proto star, that story.
Imagine the same thing on a galactic scale so this is the this is the cartoon
version of the ELS model of galaxy formation.
You start with a protogalactic cloud. The protogalactic cloud collapses in the
center where the density gets highest. stars starts forming long before the
densities of the outskirts are high enough.
Eventually of course angular momentum will flatten the cloud just as it did the
protosteller disc but, in the center perhaps enough stars formed in clusters
managed to collapse so there was fragmentation and smaller sub clouds
became critical and collapsed before the flattening so then you'd find the old
globular cluster stars orbiting in orbits that are not necessarily in the plane of
the disk, and then eventually the disk flattens out and you get the issue of
the, the, the shape of the galaxy as we know it.
This is a very reasonable model, and it probably is part of the truth.
There are some issues with it. One is, it does, it explains why you
find, halo stars and halo clusters orbiting in inclined orbits, but not why
so many of them are in retrograde orbits. Orbits considering that as the cloud
collapsed angle of momentum reservation would have sped up whatever rotation it
has. It also turns out that this collapse
occurs very rapidly and it does not have, it does not leave for a cloud the size of
The Milky Way this is the Kelvin Helmoltz gravitational collapse time scale.
It's a rapid timescale and even in the case of a cloud the size of the Milky Way
you don't have 2 billion years in which to create halo stars.
Remember the ages of globular clusters were between 11 and 13 billion years.
This is 1 of the key pieces of evidence that this model is no sufficient.
In addition there's this correlation between distance, orbital radius and
modelocity in globular clusters that says that the distant clusters are
younger. This does not match, in this theory you'd
expect the clusters to be formed in the center to be the.
The oldest ones and, and also the fact that the disk components, the thin and
thick disk differ in age, in this case, in this model everything happens fast and
expect stellar creation to start in the disk as soon as densities got high
enough. And there is not, again a billion,
billions of years of separation that can explain all of the phenomena that we see.
So this is not a sufficient model. Another sort of opposing model is what's
called the heirarchical merger model, in this case a galaxy forms more the way we
said planets formed. Form, so, there is a collapse and what
collapses are basically globular clusters or dwarf galaxies, protogalactic
fragments and they can a mass from a million solar masses to maybe bigger than
a galaxy. But in the usual way that fragmentation
works we know that the most common will be the smallest ones.
So you'll have a lot of, globular cluster size.
Protogalactic fragments, with a mass of about a million solar masses.
And then, as these orbit each other, and interact with each other, dynamical
friction slows them down, and, eventually, they are brought into this,
big spherical, mass distribution. And, there, density becomes high enough
that they start forming stars and globular clusters in the center, and
because these things have formed stars individually then in the center of each
cloud you start star formation then this explains why you get different chemical
histories because different clusters independently initiated different
fragments independently initiated star formation and then these things are now
beginning star formation at the same time that they're interacting with each other.
Collisions and tidal forces will break some of them.
Which ones will break most likely? Well the ones with, which experience the most
dynamical friction namely the mass, most massive one.
So the more massive ones are more likely to be slowed down and disrupted because
of dynamical friction those that have been disrupted will produce the halo
field stars and, some of the others the stars and the gas will have been blown
away, but the globular clusters as more massive, will have survived.
So we'll have bare globular clusters and we'll still have some dwarf galaxies.
Now, in this model about 90% of all the globular clusters formed, especially the
massive ones, as I've said, are going to be destroyed.
The globular clusters we see in the halo today will be the 10% that survived and
with an emphasis on the less massive ones which is why we typically find globular
clusters with masses up to a million solar masses.
And then very early in the dense center as this these things are starting to
merge. You form a bulge, so in this case star
formation proceeds from the inside out, in this hierarchical merger model and so
you'd expect very old stars in the bulge. Explaining why there's new stars in the
bulge, is a detail of the model I won't really have time to get into.
Of course in principal both models probably have a role to play.
We do not have I should say a satisfactory theory of galaxy formation,
this is an area of active research. But I think the, the consensus is that,
the Hierarchical Merger Model in a slightly more elaborate version than what
I've described is the more likely, it's more likely to be
closer to what it is that really goes on. Can we see the evidence of this? So for
stars, we used clusters to test our theories of stellar evolution.
How do you test your theories of galactic evolution? we do not have a long main
sequence understanding so we don't, can't time galaxies in the way we timed stars,
but we can look for very young galaxies.Where would you find very young
galaxies? You'd find very young galaxies by looking very, very far so this is the
very famous Hubble extreme dark field.
basically the Hubble Telescope was aimed at a region, the constellation Formax.
This shows just a region 3 by 3 arc minutes in the sky which as far as anyone
knew when they aimed at it, was completely empty of anything.
There was no known objects in that region and then they took a very long exposure
and in this field they found about 10,000 formerly unknown galaxies.
They looked at a region where there was nothing in order to estimate, in order to
be able to focus on very dim and therefore very distant objects.
These galaxies are billions of light years away.
Aha, billions of light years away tells us that we are seeing them billions of
years ago, we'll get into that soon and this means that the galaxies we see here
are likely to be younger than the ones we see a million light years away like
Andromeda or the Milky way which we see right now.
And so, in this Hubble Deep Field indeed, we see that the galaxies seem to be bluer
than the galaxies we see around us, and their shapes are far more irregular.
It is a possibility that's under study that what we're seeing here is, in some
of these cases the proto-galactic fragment that will eventually form the
galaxies that we see today. As I said, galaxy formation is still a
very active field of study and the understanding we have of the dynamics of
the qualitative level we can handle, I think is all we're going to be able to
achieve today.