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 galaxies and 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 - ee/universe/

Nearby region of the universe 1 billion light years

Universe

Tour of Universe – Sloan Survey 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 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