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Goal: To understand clusters of
galaxies including the one we are
located in called the Local Group
Objectives:
1) To learn about the Sizes of
Clusters
2) To explore some Nearby clusters.
3) To understand the Distribution of
Clusters
4) To understand the Properties of
clusters
Galaxy clusters
Pisces-Perseus supercluster
Picture – 100 degrees
So, what are the sizes?
• Some, like our local group, can be a few
million light years across and contain a
few trillion solar masses.
• Some can be tens of millions of light years
across and contain hundreds of trillions of
solar masses.
Coma Cluster
• Here is one example of a cluster.
• It has thousands of galaxies which span
millions of light years.
Virgo Cluster
• Closest cluster to ours.
• We are actually falling towards this one.
Classifications
• Group – contains < 50 galaxies in a span
of 3-6 million light years.
• We live in a group called the Local Group
which contains about 40 galaxies.
• Cluster – contains 50-1000 galaxies and
10-100 trillion solar masses.
• Diameters are 7-32 million light years.
• Super clusters – clusters of galaxy
clusters!
• Super cluster complex – clusters of Super
clusters
Local group http://www.anzwers.org/fr
ee/universe/
Nearby
region of
the
universe
1 billion
light
years
Universe
Tour of Universe – Sloan
Survey
• http://www.youtube.com/watch?feature=pl
ayer_embedded&v=08LBltePDZw
Properties of clusters
• Clusters of galaxies are gravitationally
bound.
• They orbit around one another.
• There are a lot of galactic collisions and
mergers as a result.
• So, you get a lot of elliptical galaxies.
• But you also get…
Gas
• When 2 galaxies collide and each has gas
the gas collides with the gas.
• What happens to the gas?
• A) forms stars
• B) heats up
• C) gets thrown around the cluster
• D) goes into orbit around the cluster
Fast gas
• The gas gets heated to millions of
degrees.
• As a result it moves around very quickly
(temperature is a measure of the random
speeds of the gas particles).
• What limits how hot the gas can get?
Fast gas 2
• The limit is set by the mass of the cluster.
• If it gets too hot it can escape from the
cluster.
• So, if we measure the temperature of the
gas and measure its speed – and see it is
NOT escaping – then we know the
minimum mass of the galaxy.
• Oh, and if it is in orbit then we also know
the total mass of the galaxy.
Dark matter
• Yet again the masses we find from
temperature and from orbital motions do
NOT match up with the mass we see.
• For individual galaxies the discrepancy
tends to be a FACTOR of 5-10.
• For clusters this factor is 50!
• So, 98% of all mass of a cluster is mass
we cannot see – or dark matter!
What is dark matter?
• Short answer:
• We don’t know!
• Long answer:
• Start of answer… Dark matter is some
stuff that we cannot see.
• We cannot see it because either it does
not emit light (and does not interact with
stuff other than through gravity) or is just
too dim for us to see.
What is dark matter - continued
• So, dark matter can be ordinary stuff or it could
be weird stuff we know nothing about.
• But, which is it?
• If ordinary stuff.
• Dust/gas – only accounts for 10% of the mass of
most galaxies at most – so probably not it.
• Dim stars/black holes – possible but not likely
(reasons to come) - these are all dubbed
MACHOs (massive compact halo objects)
Not ordinary
• WIMPs – weakly interactive massive
particles.
• These are atomic particles much heavier
than a proton that would not emit much
light.
• Possible, but probably ruled out.
• Then there is the other possibility – does
dark matter really exist?
How do we know it does?
•
•
•
•
How can we observe dark matter?
Directly: we can’t!
Indirectly – through gravity.
But what if gravity does not work as we
think it is supposed to at long distances (it
probably does, but suppose it does not).
• Then it is possible – if unlikely – that dark
matter does not exist at all.
Lets assume it does.
• Is there any way to get ANY idea of what
the dark matter could possibly be?
• Well yes there is – microlensing.
• But what is lensing (gravitational lensing)?
Gravitational lensing
• Gravity affects light!
• When light moves close to an object with a
lot of gravity (either really close to a
moderately massive object or some
distance away from a really massive
object) the light gets bent
– just like light gets bent
by a lens.
wikipedia
microlensing
• Microlensing is just a smaller – but not
less impressive version of gravitational
lensing.
• Microlensing involves smaller masses for
which light gets much closer.
• For this you usually match a moving object
vs a more stationary one.
• As the two pass each other, the closer one
will lens the further one.
More microlensing
• For a short period of time the closer object
brightens by a factor of 1.1 to 5 for a few
seconds (the time it takes for the two to
pass).
How useful?
• How can we use this to try to find out what the
dark matter is?
• Imagine the dark matter where in our galaxy and
we observed stars further away hoping the dark
matter would lens them.
• If a star passes directly behind a bit of dark
matter, it will briefly brighten.
• The amount of the brightening will be a reflection
of the mass and distance of the dark matter
(max lensing when the dark matter is halfway
between us and the star).
Great lets do it!
• One problem, if we use stars in our galaxy,
even though we have a lot, it takes a
LONG time for them to line up right.
• Takes many, many years to get ONE
lensing event!
• Ooops.
• So, um what is the solution?
LMC to the rescue!
• Chris Stubbs (at the time a professor at U.
Washington, but now at Harvard) realized
that if you used a nearby galaxy (such as
the LMC) that there are a LOT of stars –
hundreds of millions – all in one part of the
sky to do lensing events (potentially a
hundred a year).
• So, he did that.
And he found
• Not much.
• He discovered that the dark matter definitely
could not be in the size range of the mass of the
sun to ten million times less massive than the
sun.
• Objects ranging from the mass of the sun to 30
times the mass of the sun could not account for
more than 40% of the unseen mass – so it is
also ruled out.
• Therefore the dark matter has to be very small
or very big.
• But this is just for our galaxy. Could clusters be
different?
Bottom line
• We may know what dark matter is not, but have
no idea what it is.
• The only way we have to observe the dark
matter is through gravity.
• This large discrepancy poses a big problem –
called the – you guessed it – Dark Matter
Problem.
• Turns out most of our universe is made of stuff
we cannot see… Only 20% of the mass of the
universe is observable mass (baryons we can
see).
• The other 80% or so is Dark Matter.
As for the overall
• As for the overall distribution of clusters you will
have noticed that they fall onto a spider web.
• That is because all of the mass (read here the
dark mass) tends to pull all the mass to
centralized locations.
• When two threads connect you get a cluster of
galaxies (or even a super cluster).
• And this forms the hierarchy of the masses in
our galaxy.
Conclusion
• Galaxy groups come in all sizes and forms from
Groups to Super Clusters.
• Most of the mass of these clusters are in the
form of dark matter.
• While we have some idea of what dark matter
isn’t, we have NO clue what it is.
• The distribution of galaxies affects the galaxies
in the group and the evolution of the galaxies.
• Also, this creates large regions that the galaxies
move away from – regions called VOIDS