Now continuing our study of clusters of galaxies, let's consider their content.
Here's the X-ray gas. The origin of it is free-free emission
meaning, scattering of electrons on protons in fully ionized plasma, that is
it has temperatures of tens of millions of kelvin.
And in fact, x-ray emission from clusters is so conspicuous that now there are
surveys for clusters, that are based exclusively on their x-ray emission.
So whereas, we use approximation, vertical symmetry and such In reality we see
features in the X-ray gas that show us that clusters are not yet fully formed or
relaxed, to differ from cluster to cluster.
So for example, if you had as, a group of galaxies fall into a large cluster, there
would be a shock wave as the inter, intergalactic medium of both collides.
In the centers of clusters there is a possible phenomenon called the cooling
flow and it works like this. Gas emits radiation, cools.
Because of that it will fall towards the middle of the cluster, where it will heat
up again, because the x-ray emission is proportional to the square of the density
and so on. And so early estimates people thought that
their clusters are having extra gas flowing through the middle, not because of
gravity because of temperature differential.
At some rapid rate and is a real puzzle where this gas go, now we know that this
is only partly correct. And there are feedback processes that
reheat the gas. So that cooling flows are not quite as
important as people once thought . So where does the gas come from?
Some of it simply came from the intergalactic space.
And some of it was expelled from galaxies, intergalactic winds and driven by
supernova explosions. And the reason we know this is that the
gas is not pristine. It's not just hydrogen and helium but it
contains metals and roughly at about of 1 3rd solar.
So we know that some of that gas came through chemical evolution in stars in
galaxies. And the x-ray properties correlate very
well among themselves and also other with other physical properties of.
Clusters. Let's look at some of the X-ray pictures.
The cluster on the left here shows a cometary like thing which is a bow shock
from a clump of stuff falling through. The cluster on the right shows a strange
filament and we think that that's due to the jet from an active galactic nucleus.
So they're not Perfectly symmetric and homogeneous and so on but by and large
they can be and so our approximations are not bad.
And again, how do we determine masses? Simply applying the virial theorum that
kinetic energy has to be one half potential to within Plus or minus sign and
we can consider galaxies as test particles using their velocity dispersion.
Or we can use X-ray gas as test particles, measuring it's temperature.
Either way we measure kinetic energy precluding to mass.
And as you would recall, both of these methods yield the conclusion that the
amount of mass in clusters is much higher than can be accounted by stars or gas
alone. So therefore, there has to be some dark
matter. This is how strictly originally
discovered. People actually do a little more
sophisticated studies using x-ray gas and apply full blown hydronomical models and
so on. And that yields essentially the same
results. So, typically in a cluster 1 6th or so of
the matter would be variance, the rest would be dark matter.
And on the variance, gas and galaxies, meaning stars, would contribute to bulk
equal mass. Cluster collisions have recently led to
yet another piece of evidence for the existence of dark matter.
Here's one of those clusters that seems to have resulted from a merger as two
clusters of galaxies and what the picture here shows.
X-ray emission as well as galaxies. But also dark matter as inferred from
micro-lensing studies around this emerging cluster.
So the blue shading indicates where the mass should be, and the pink one is where
the gas is. You see that there is title shock but also
the two are displaced. Now this is what you'd expect if the dark
matter does not interact directly through electromagnetic radiation with the gas.
The two clusters which get, just going to pass through each other and eventually
settle together into a merger. But the gas cluster collides with one in
the other and does not participate in this boxing action.
So you see the gas is concentraded towards the middle, whereas the dark matter, halos
of clusters are now seperated apart.There are now several cases of these known and
They all yield the same results. That in the mass to light ratios of
clusters at several hundred times the solar mass to luminosity ratio, which
implies again there that must be a significant non-barionic component.
When we talked about numerical simulations of some of the modern hydro-dynamical
simulations, and these are now deployed to study evolution of clusters, including
their mergers the effects of octogalactic nuclei, star formation, stellar winds,
these are some snapshots of what those simulations showing various physical
properties. I mentioned the properties of clusters
correlated with each other and whats shown here is plot of mass infered from varial
arguments versus temperature. Now this trans look too good and the
reason for this is that we actually use temperature to for the mass.
But intuitively this is exactly what you'd expect.
The more massive clusters will have deeper potential levels, which will require a
higher velocity of particles which means a higher temperature.
And that's exactly heat we see. Likewise, X-Ray Luminosities itself is
proportional to temperature[UNKNOWN] power.
And X-Ray Luminosities also corelates with cluster mass.
Now this is reassuring to know, it's not terebly surprising.
It would be very surprising if they didn't correlate but it's good to actually know
measure it. So how do we study properties of clusters
as a population? Well we can form the luminosity function,
or mass function, usually in x-rays and see how it changes as a function of red
shift. And here is an example.
And you goes in the sense that at larger red shifts there are fewer more massive
clusters, which is exactly what you expect if the clusters are being built in time,
takes a while to build up the biggest ones.
This opens possibilities for use of clusters as chronological probes, because
in models with lower density and or positive cosmological constant.
There is more time, and so you expect to form more massive clusters earlier than
you would in high density zero cosmological constant models.
So simply by counting clusters, through some luminosity or mass function as a
function of red shift, you can consider it cosmolog.
And here are examples of cluster mass functions from x-ray measurements.
In two different regid shelves. The one on the left, shows the concordance
cosmology model, plotted against the data, as you can see it goes beautifully through
the data points, which is in a sense another vindication for the concordance
model. The one on the right has the same hotter
density, suppose we got that right, but suppose there was no dark energy, and you
can see that it fails. Completely in predicting the abundance of
clusters at larger ranges. So, just as we did before, combining
constraints from microarray background, from supernova, from verionica acoustical
oscillation and so on. Cluster measurements can be folded into
the same type of statistical analysis, and, beautiful enough, their air ellipsis
cross exactly where all other methods cross.
So they improve our knowledge of this logical parameters, but also because this
is a completely different way of measuring, they give us an extra
confidence that we've actually done a good job.
Now let's turn to galaxies and clusters. One thing that will happen is a galaxy
like the spiral galaxy plows through that X-ray gas.
It will get stripped away, of it's neutral hydrogen just because of the ram pressure
sweeping, and that's indeed what is observed.
The most gas deficient spirals are found in near a cores of clusters of galaxies,
where as those on the outskirts look like pretty much like spiral galaxies in the
field. The strength of this effect, the intensity
of stripping, if you will. It's proportional to cluster x-ray
luminosity, which is, again what you expect, because it's the x-ray gas that
strips away the neutral hydrogen in those galaxies around through it.
And if you look carefully, within individual spiral galaxies, the stripping
occurs primarily in their outer parts which are less likely bound to the galaxy
itself. So here is the plot of the hydrogen
deficiency versus X-ray luminosity. And deficiency goes from, no hydrogen
removed, to all hydrogen gone, and that correlates beautifully with x luminosity
of clusters which is measured about their mass, and simply the density of the gas.
So here is a very instructive map of vertical cluster galaxies in neutral
hydrogen produce by [unknown] collaborators.
These are [unknown] maps of the neutral hydrogen.
Density in galaxies and M87 is the big galaxy in the middle essentially the
center in argo cluster. And it's a obvious effect, that closer you
get to the cluster core, the smaller disks look in neutral hydrogen.
So this is exactly what you expect if they're being stripped by falling through
a cluster intergalactic meteor . So gas collisions will strip away gas.
What happens to stars? Gas will not effect them, but you may
remember in numerical simulations, say, these galaxies merge or just pass by each
other, their title features in some number of stars in these galaxies will get
unbound [unknown], but they're still bound to the cluster.
So they, they don't want, they no longer belong to a galaxy now.
They belong to a whole cluster. And they'll tend to accumulate in the
middle, but because galaxies interact everywhere.
You expect some defused light, and in fact this is what we see first in coma cluster
and here in the contours of very subtle, so low surface brightness distribution of
stars with galaxies and they're very high contrast break up.
And another one for Virgo cluster and the big holes are where many stars and
galaxies are, but you can see their features and in the slides which probably
corresponds to the title tails, streamers, and those galaxies and throughout.
So the whole picture holds together very well.
So let's recap what we learned about clusters of galaxies.
They're the most conspicuous point of the logical structure, and they represent
stage where structure becomes virialized. Gradually, typically a few mega parsecs
across, they have hundreds of thousands of galaxies in them, the mixture of luminous
and dark matter in them is exactly what is the overall mix from the cosmological
measurements, baryonic to non baryonic dark matter component.
Presence of the hot x-ray gas opens ways of finding them such as simply x-ray
emission or [unknown] effect, which is popular because, as you may remember, it
does not depend on Richert, the source is not the cluster, the source is the micro
array background and clusters just provide x-ray shadow for a background.
And those galaxies have by and large fully formed except for an occasional merger or
so. Clusters are very much still forming and
this is a process we now understand very well, thanks to the miracle of
simulations. Because of the dense environment both
proximity of many other galaxies to zip by or merge with and the dense intergalactic
medium of x-ray gas. Galaxy evolution is effected in clusters.
It proceeds a little differently from galaxy evolution in the general field,
because there are more galaxy encounters some of which result in merging.
There is certain stripping of gas which removes few of our star formation and so
on. So we do expect and we actually observe
differences in galaxy evolution between clusters and the general field.
So next, we will start talking about properties of galaxies themselves.