If galaxies are fun. Let's move one scale up in the hierarchy to clusters of
galaxies and let's see what we know about them.
so, our galaxy, like most galaxies, is part of a cluster or Local Group, as we
call our clusters, quite small, is dominated by the three giant spirals,
the Milky Way, slightly larger M31 and M33 in Triangulum, the Triangulum galaxy,
and the estimated mass of this entire collection of galaxies is 4 * 10^2 solar
masses, of which, at most 10% is barionic, which is the name we give to
normal matter, stuff where the mass is mostly protons.
And so, most of the mass of this cluster is dark matter and we get that by
estimating the masses of the galaxies based on star counts and gas counts and
our understanding of galaxies. And, comparing that to the kinematics,
the Newtonian calculation of the total mass that all of this is orbiting. And
based on the motion a merger of the Milky Way with Andromeda in about 4 billion
years is a distinct possibility. Of course, that won't be of too much
interest to denizens of the solar system, because in 4 billion years, life here
will have changed due to stellar evolution,
but it will be a very interesting experience for anybody still living on
some planet in the Milky Way. Note that it's not as remember, a
galactic collision is a stately affair. galaxies are mostly empty space,
stars only interact gravitationally. if these were objects full of gas, there
would be a lot of friction and some establishment of a combined angular
momentum and a combined disc. In this case the merger of these two
ellipticals may two, these two spirals may well end up as a elliptical galaxy,
because there's not enough friction to settle everything down into into a disk.
this is are local clusters, a very small cluster, a far more sizable cluster is
the well-known Virgo cluster in the constellation, you guessed it
that has 250 large galaxies and about 2,000 smaller ones scattered over a
distance of 16 million parsecs. And of these galaxies, 68% are spirals and only
about 20% are ellipticals, but that includes four huge, giant ellipticals
with a radius of between 1-300 kiloparsecs.
So three or eight times bigger than the solar system or 12 times bigger than the
Milky Way I mean. and this includes the center of the
cluster is dominated by three out of these four ellipticals, as we said,
ellipticals are often found at the centers of dense clusters.
And the galactic clusters, as I said, are
different from star clusters, in that, the intercluster medium of hot gas
contains on average about 8 times more mass than the sum total of all the mass
of all the galaxies. about 10% of the mass, by the way is
accounted for by intergalactic stars, which were suspected that only recently
directly. This involves detecting individual stars in a cluster so these
clusters are somewhat distant and recently that success has been achieved.
here is a beautiful image to the right, an optical image of a cluster Abell 2199.
You see the individual galaxies shining when you look at the x-ray image, you see
that the entire cluster is glowing in x-rays because there is a large density
of hot gas, and most of the mass, most of the baryonic mass of a cluster is in the
form of gas and this is quite important. Now as usual, I keep saying baryonic
mass, because as everywhere else in the universe, most of the mass is in the form
of this mysterious dark matter. And
how do we know? How do we find dark matter in a cluster far, far away? Well,
we can compute the total mass. we can also use gravitational lensing by
clusters to produce the mass distribution of the lense.
Remember, that the gravitiational lensing equation that I wrote said that the
deflection by which light of a angle of a beam of light passing a distance or a
distance of nearest approach r, and I think I called it b from an object
of mass M is given by this. You can imagine trying to compute what
distribution of masses would cause the, the lensing image we produce.
It's some kind of inverse, scattering problem.
We see here a rich cluster along with you can see here and there some lensed
images, and superposed this sort of blue smooth,
blue over coloring is the calculate, calculated distribution of dark matter or
of the total mass of the cluster, the lensing mass superposed on the image of
the galaxies. And we see that while the galaxies show
up at this distance as individual dots of light, the mass distribution is very
smooth. The dark matter does not clump nearly as
much as the galaxies do, and in this case, it's very interestingly
distributed. There's a sort of clump of dark matter at
the core of the cluster and then a ring of dark matter surrounding the cluster,
the dynamics of how this happened are far from clear, I should say.
And here is a map that gives, makes the same point.
This is a three-dimensional plot where the two axis are sort of along the plane
of the image we were just looking at, the vertical axis is the mass
distribution. We see the spikes, those are the galaxies, very dense regions of
mass, but we see that most of the mass is contained, this is a different cluster,
but still, most of the mass is contained in the big smooth distribution underlying
these individual peaks. Most of the mass of a cluster is not in the galaxies,
that would be true even absent in dark matter, but most of the mass of the
cluster is not even in the intracluster gas, which is itself eight times the
galactic masses. It's actually in the dark matter content
which is larger still. And, this brings us to one of the most
pretty observations of 2006 an object called the Bullet Cluster.
And, there is a lot going on in this image, so let's pay attention for a
second. what we see here is the process of two
galactic clusters in the process of colliding and essentially merging.
And the image has three pieces to it, there's an optimal image where we see two
concentrations of galaxies over here on the left, one concentration of galaxies,
and over here on the right, a second concentration of galaxies.
the collision occurred somewhere around here.
How do we see that? Well, the pink is the glow of the heated gas,
the infared light from gas that has been heated by the collision, and this is the
difference between two galaxies colliding and two clusters colliding.
When two galaxies collide they basically pass through each other, other than
gravitational dynamical friction, because galaxies are made of stars.
Clusters are mostly gas, so most of gas two clouds of gas that collide,
actually, interact strongly with each other, compress, and heat up.
So, when these two clusters pass through each other, essentially, the galaxies
went through, but the gas was trapped in the middle.
And so the entire gas content of the two galaxies in contain, is contained in
those two pink shock wavy, glowing collections of gas.
Okay, so the galaxies go through and the gas gets stuck.
Where does the mass go? Here is the question. So they, luckily behind these
colliding clusters are background galaxies whose lensing images we can
study. And we can use that, just as we did in
the previous picture to draw a mass distribution for this object. And, based
on everything I said, the mass distribution, since the gas in a cluster
is eight times more massive than the galaxies, you'd expect the mass
distribution to be centered in the middle where the gas is, if it was a gas. What
we see in blue is the mass distribution, and we see that the mass distribution,
like the galaxies, basically went right through.
Most of the mass of the cluster is not in the gas, it's in the even more massive
dark matter halo of the cluster, and dark matter being weakly interacting,
sailed right through. So in some sense, the fact that dark
matter follows the galaxies is paradoxically the, the fact that the mass
follows the galaxy is a validation of the fact that the mass is not in the forms of
matter we're familiar with. In terms of those, most of the masses in
the dust, we see the dust right there in the
middle, we see the galaxies and the dark matter following through, and this is a
very pretty image. some have called it a direct detection of
dark matter, and this is a not consistent with most modified Newtonian dynamic
theories, which suggested modifying gravity.
So this clusters it. The universe is made up of galaxies which fit into clusters.
Well, no, there is, follow Kant, there is a hierarchy, hierarchy of structures
clusters are organized in superclusters, indeed, our local group is on the edges
of the local superclusters. Supercluster of which Virgo is at 16
megaparsecs away from us, is the center, the size of the cluster is about 20
megaparsecs. at larger distances, we find more
superclusters and we can look at try to understand the local motions relative to
the Hubble flow. over and above the sort of expanding
Hubble flow. And motions of galaxy clusters in the
local neighborhood that we live in, suggests a great attractor in Centaurus,
some object with a mass of 10^16, I believe, solar masses.
There is nothing in that region that has sufficient mass, unless it's mostly dark
matter in ways we're not familiar. The Shapley Supercluster over here is the
most likely candidate for housing that great attractor, but it's not been
observed. and so, this is a map of all of the
superclusters this is a map of I think a 100 million or a 100 megaparsec around
the the Milky Way, and we see the con, the, the, the local region strewed with
super clusters of galaxies. And now when asked, are superclusters
clustered into superduper clusters, at what level does this stop? And that's a
very important question. The question is, is there a structure at
all scales? Is the universe lumpy all the way up to forever? At any size, there are
a super-duper-hyper clusters. And, to try to answer this at the largest
scales there is a famous survey that was done which found I believe hundreds of
thousands or couple hundreds of thousands of galaxies.
and, this was an attempt to look deep into the sky and in order to avoid
interference from the Milky Way the survey picked out two sort of slices of
the sky that avoid interference from the galactic plane and are, or oriented at
gaps in the galactic discs, so that we can see far away without galactic
interference. And they map out to z of 0.23, you can
compute the distance. And, the structure of these two sheets as found by this
galaxy redshift survey is here, the, sort of two slices are
these are two narrow angular slices that they surveyed.
And as a function of distance and angle within the slice, we see here the
structure of all the galaxies they found. And you see here redshift, on the one
hand, and distance in billions of light years, this was quite a deep sky survey
and we see lumpiness. We see these structures, these sort of
filaments. Remember this is a slice, so these are
presumably slices through some two-dimensional structures surrounding
large relatively empty bubbles or so called voids. And sort of
three-dimensionalish representation of the way space looks in our vicinity is
over on the right. And, again you see that there are these large gaps where
very, almost nothing is, so there is structure at these very large scales.
so is there, does, is the suggestion here that there is structure at all levels?
Well, not quite. You measure structure mathematically by
something called correlation, this roughly tells you the answer to the
question, if I find a galaxy or a cluster here how does this change the probability
of finding another cluster at some distance from there and at small
distances? Yeah, if we find a galaxy here, then, since all
galaxies are members of clusters, there is very likely to be one very close to
it. and this is a plot of the sort of correlation coefficient as a function
logarithmically of distance. And we see that correlations decline, and
the prediction is, from this graph, that a distance is well above 100 megaparsec,
correlations disappear. Correlations disappear, means that if you
find a collection of galaxies here at a distance of 100 megaparsecs, it tells you
nothing about the probably of finding more galaxies, so the universe at
distances of say a billion parsec should be quite smoother of 500 megaparsecs,
exactly where you'd draw the line is clear, is not clear, but at large
distances, the universe is homogeneous. You, all of the perturbations smooth out
at very large distances, there is not an infinite hierarchy of
super-duper-duper clusters, at large distances, we see homogeneity.
Now, even so, the size of these voids is interesting.
these are large regions of a galaxy of the universe where there are very few
galaxies. How did they form? Well, we don't yet
know. There are various theories of early
universe structure formation. We might talk about them next week, but what is
important is all of these voids have to be primordial.
We are looking at the way the universe look essentially at its creation, because
in the 13 billion years since the Big Bang, there has not been time for
galaxies, that might have populated, or matter that might have populated these
voids to have moved out, these voids are just too large.
Now, this statement, that the universe is homogeneous at scales above 100
megaparsecs or well above a 100 megaparsecs is something we will use
heavily in understanding cosmological considerations next week and is used
successfully by cosmologists, but nothing is an axiom.
recently, there's been reports of a collection of quasars of essentially
active galactic nuclei which are scattered in what appears to be a
coherent structure that is more than a billion parsecs across.
So these black dots are this sheet of what seems to be correlated structure,
some kind of coherent structure that is a billion parsecs across.
That is beginning to stretch the limits of our understanding of structure
formation. This is a brand new result.
We'll see how it either stands up to validation,
and if so, how it gets incorporated into models of structure formation.
As always in this class, science is a seem, succession of approximations to a
truth. This might be the beginning of an improvement in our understanding, or it
might just be a blip.