Dark Matter Ring in CL0024
Extended interview with Aitana:
1. How was the ring discovered?
This Dark Matter ring was discovered unexpectedly
during what you could call a "routine" gravitational lensing analysis of the galaxy cluster CL0024.
The lead author on this paper, Myungkook James Jee,
was using this analysis to map out the Dark Matter halo, or cloud,
which surrounds and envelops this galaxy cluster.
And what he found was remarkable, an overdense ring of Dark Matter,
which is believed to have been created by the collision of two galaxy clusters along our line of sight.
Such a Dark Matter ring has never been observed before.
2. How did the ring form and why does it have the shape that it shows?
We believe the ring is the result of a head-on collision between two galaxy clusters with a mass ratio of about 2:1.
There have been many computer simulations of collisions like this,
both for individual galaxies and for galaxy clusters as we observe here.
And these simulations consistently show that after such a head-on collision,
some of the ejected material expands outward in a shell about each body.
When observed along the axis of the collision as in CL0024, we see these shells as an overdense ring.
It's all very similar as to when you throw a rock into a pond
and you see ripples which expand outward across the surface.
Well here we're observing a giant ripple of mass 2.6 million light-years across
which is expanding outward from CL0024.
3. Why do scientists refer to the ring as a ghost-ring? Why is dark matter so hard to detect?
Dark Matter is ghostly because it's everywhere in our universe,
it's all around us, but we can't see it and we can't feel it.
In fact right now there may be a billion Dark Matter particles passing through your body every second.
Luckily they rarely interact with the ordinary matter you and I are made of -- protons, electrons, and neutrons.
Nor do they interact with photons, which are particles of light.
Dark Matter doesn't produce light, nor does it reflect it.
So we can't actually see it in outer space;
we can only infer its existence indirectly by, for example, observing gravitational lensing.
Now experiments are currently ongoing to detect Dark Matter more directly in outer space.
The self-annihilations of Dark Matter particles with one another
may give off gamma rays and/or neutrinos which should be directly observable here on Earth.
In fact we observe a strong flux of gamma rays coming from the center of our galaxy,
and some of this flux may be the result of such Dark Matter self-annihilations.
But so far, we just can't say for sure.
There are also experiments trying to detect some of the Dark Matter particles within our galaxy.
These are the particles passing through each of us at the rate of a billion per second.
Every once in a while, these particles do interact with ordinary matter,
and it is these interactions which scientists hope to detect.
And particle physicists are also in a race to produce Dark Matter particles in the lab, in particle accelerators.
So this is an exciting time, as we hope to soon produce and directly detect Dark Matter.
As a result, we may finally learn just what Dark Matter is.
5. What analogy would you use to describe what the ring represents?
James has used the analogy that observing gravitational lensing
is like looking at pebbles on the bottom of a pond with ripples on the surface
distorting the shapes of the pebbles we see.
So in James's work, there is a galaxy cluster, CL0024, sitting on the surface of this pond, bending its surface.
And James has analyzed the distorted shapes of the galaxies behind it.
The collision of the two galaxy clusters sent ripples across the pond's surface,
further distorting the background galaxy shapes,
and allowing him to detect one of these ripples -- the Dark Matter ring.
6. What methods were employed to analyze/observe the ring? Did weak gravitational lensing play any role in the finding?
Yes, weak gravitational lensing analysis was the main tool used to detect the ring.
By measuring the shapes of many lensed background galaxies,
we can reconstruct the Dark Matter distribution of the lens.
In this case, a ring of Dark Matter is revealed.
There was also some strong lensing analysis involved.
Strong lensing is when you actually see multiple images of individual background galaxies.
And CL0024 hosts perhaps the most striking example of such strong lensing,
with an eerie blue ring-shaped galaxy repeating five times about the image.
This is very interesting, that this lensed galaxy exhibits the same ring morphology as the cluster,
suggesting that it too has undergone a recent collision.
Now this galaxy is located well behind CL0024, so it could not have interacted physically with the cluster.
But perhaps there is a long filament of Dark Matter connecting the two,
which helped induces these collisions along our line of sight.
7. Is it normal to find rings of this kind after collisions between galaxy clusters?
Actually, for about 30 years now, astronomers have been studying so-called radio relics and halos.
These are also rings, more or less complete, which are observed in radio images of some galaxy clusters.
As in CL0024, these rings are believed to be the result of the collision of galaxy clusters,
or of groups of galaxies within a cluster.
We have also observed many rings about individual galaxies, each resulting from the collision of two galaxies.
In these cases, the ring can be observed in visible light,
as the collision shock waves trigger star formation producing a bright ring.
Now if we lived for millions of years we could actually watch these rings expand outward.
But what is unique about James's paper is that such a ring is detected in the Dark Matter distribution.
This has never been done before.
Such accurate mapping of Dark Matter is difficult
as the presence of Dark Matter cannot be directly observed,
but only inferred using techniques such as gravitational lensing.
8. What does the ring tell about the nature of dark matter and our understanding of the Universe?
The Dark Matter ring has been offered as direct proof for the existence of Dark Matter.
While there is a wide range of observations which indicate the existence of a large quantity of Dark Matter
there is an alternative explanation for these observations.
It could be that our current theory of gravity simply breaks down on large scales.
Where we see a need for Dark Matter, it could be that gravity is simply stronger.
The law of gravity was last rewritten by Einstein 100 years ago.
Today there is no shortage of scientists who would love to be the next Einstein
and are currently working on new theories of gravity which attempt to explain away Dark Matter.
So today the question is which do you believe, a mysterious Dark Matter or a new theory of gravity?
Or perhaps both?
Most astronomers these days are persuaded that Dark Matter is the more likely explanation.
But there is little in the way of direct proof one way or the other.
Recently, you may have heard about the Bullet Cluster,
which provides strong direct proof for the existence of Dark Matter.
This is a collision of galaxy clusters, similar to CL0024,
but which the collision unfolding across the plane of our sky rather than along our line of sight.
In the Bullet Cluster, we see the galaxies and Dark Matter passed more or less straight through,
while the gas of these clusters was stripped off and left behind.
This separation of Dark Matter from gas is very difficult to explain away using alternative theories of gravity.
Now the Dark Matter ring in CL0024 is supposed to offer similar direct proof of the existence of Dark Matter.
But the result has been met with a bit more skepticism.
The bottom line is that the until we actually detect and produce Dark Matter particles in the lab,
it will be very difficult to rule out alternative theories.
As for our understanding of the universe,
I think we realize that there is a lot we don't understand
-- that we probably have yet to discover what over 80% of the universe is made of.
And this is widely recognized as the most urgent question facing both astronomers and particle physicists today --
What is the Dark Matter?
9. Anything else you'd like to add?
I'd like to point out that the images used in this study were taken by the
Advanced Camera for Surveys onboard the Hubble Space Telescope.
This camera is currently in need of servicing, and a mission to service the Hubble is planned for next August 2008.
Now you may remember, NASA originally cancelled this mission in 2004 after the Columbia disaster.
A lot of people, including most astronauts, were surprised by this decision,
and there was a huge outpouring of support from people like your listeners who wanted to save the Hubble.
This eventually helped change NASA's mind in favor of going ahead with the servicing mission.
So we're all really excited about that, and I thank all of you for your support,
not only for helping to save Hubble but also for your general interest in astronomy
which helps generate the salaries to pay astronomers like me and James to do something that I really love.
And I just hope that you all are enjoying the images that we're bringing back and the discoveries that we're making.
And thanks to you Aitana and to Earth and Sky for doing such a great job explaining these discoveries to your listeners.
Questions / comments? I'd love to hear them! Please e-mail me at coe(at)caltech.edu
--Dan Coe
Last modified: Tue Oct 16 11:50:39 PDT 2007