Bullets and Musket Balls: Examining Dark Matter with Galaxy Interactions

Dark matter is important stuff. It constitutes about 23% of the mass-energy in the universe. Yet understanding its nature and characteristics is a daunting task.

Once again, we can rely on the creativity of the names that astronomers and physicists have come up with to help gain a basic understanding of dark matter: the “dark” stems from dark matter particles not interacting with light. Simply, this outlines the main problem in probing the particles that likely make up dark matter. Since they do not interact with light, we cannot simply collect the light that they emit or reflect, the way most astronomical objects are observed. The other half of the name, however, the “matter” portion, states that these particles have mass. We can rely on this fact to ensure that they interact gravitationally, and this can be used advantageously. It turns out that this is really the only way to examine dark matter. Dark matter particles do not interact with normal, baryonic matter, except through gravity, so astronomers and physicists have to exploit their gravitational interactions in order to study them in detail.

When galaxy clusters collide

A relatively recent method to study the nature of dark matter has been through the interactions between galaxy clusters. Galaxy clusters are large and compacts groups of galaxies. When these collide with each other, the gas between the individual galaxies starts to run into each other, causing it to slow down, and heat up and emit radiation in X-ray. Meanwhile, the dark matter particles present in both galaxy clusters are thought to pass right through, not interacting with all the matter during the collision nor itself.

There is observational evidence of this taking place, as predicted, in a few cluster collisions, namely the Bullet Cluster.

In the image above, the optical image of the cluster has been overlaid with pink and blue areas. The pink indicates the hot gas, imaged by the Chandra X-ray Observatory. The blue, on the other hand, represents the areas of the cluster with the most mass. This information was deduced by gravitational lensing¹. By statistically measuring how much background galaxies were warped, astronomers could map out how much mass was in the cluster at which locations. The most obvious fact that results from this image is that the massive areas appear to have passed right through the center of the collision! Much of the mass in galaxy clusters is predicted to be composed of dark matter, so it appears that the dark matter has sailed right through the collision, as had been predicted.

Just a few weeks ago, another example of the same effect was observed in a different cluster. This cluster was named the Musket Ball Cluster (since it appears to be an older version of the Bullet Cluster²)

The gas is shown by the pink regions, and the regions of high mass are in blue. The same effect seems to be taking place in the Musket Ball Cluster as well. Since it is similar but older than the Bullet Cluster, though, we can see how such a system evolves over time. Although clear results have not yet been announced, the discovery of a system like the the Musket Ball Cluster can help us learn how a collision between galaxy clusters affects the stars making up the constituent galaxies. Collisions could cause large groups of gas to collapse and perhaps trigger increased amounts star formation in the galaxies as the gas coalesces together.

Curveball

Sometimes the universe isn’t completely nice towards theories, and the evidence doesn’t always fit in squarely with predictions. That is what’s happening in Abell 520. The concentrations of dark matter mass are shown by the blue color, while the gas is shown in green and the light from stars is in orange.

In Abell 520, the dark matter is concentrated exactly in the center of the galaxy cluster, unlike most other examples that seem to confirm our understanding of the behavior of dark matter. Instead of sailing right through the group of galaxies, as is seen in the Bullet Cluster and the Musket Ball Cluster, it appears that there is something that could be holding the dark matter in the middle.

This evidence may initially suggest that unlike the other cases, dark matter may be interacting with itself, causing it to be held back in the middle of the cluster. This wouldn’t fit extremely well with previous observations and our current best understandings of dark matter. Most theories claim that dark matter particles do not readily interact with each other. The Bullet and Musket Ball Clusters support this claim since the highest concentrations of dark matter are located in two different peaks and not bunched up together in the center. Astronomers have been exploring possible explanations that line up with current theories, and one particularly attractive route may be that the collision of these particular galaxy clusters is happening along our line of sight. So even though the dark matter of the two clusters may have gone past the collisions into two peaks, we are only seeing a single peak of mass from our vantage point.

Ultimately, the importance of Abell 520 is that it could lead us to slightly amend our picture of dark matter. Our understanding of the universe is never quite complete, and new pieces of data always carry the ability to prove that our simple understanding could in fact be wrong.

Sources

• Clowe, D., Bradač, M., et al. 2004, ApJ, 648, L109.
• Dawson, W., Wittman, D., et al. 2012, ApJ, 747, L42.
• “Discovery of the Musket Ball Cluster”, 2012.
• Jee, M. J., Mahdavi, A., et al. 2012, ApJ, 747, 96.
• Plait, Phil. “Dark matter, apparently, is midichlorians”, 2012.
• Plait, Phil. “Desktop Project Part 24: A deep, dark mystery”, 2012.

Image Credits

• Bullet Cluster: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
• Musket Ball Cluster: X-ray: NASA/CXC/UCDavis/W. Dawson et al; Optical: NASA/STScI/UCDavis/W. Dawson et al.
• Abell 520: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)

The Moon and Me

For the past few weeks, I’ve been working with MyMoon as part of their Street Team. MyMoon is a website designed to promote the importance of the Moon, from art to science to human exploration. I’m incredibly excited to be a part of the team and am looking forward to celebrating and discussing the Moon on the website.

So far, I have a couple of blog posts up on MyMoon (here and here). There are also a great number of other interesting posts on the blog. Besides the blog, MyMoon also has occasional video webcasts, such as one by Dr. Brent Garry on Lunar Volcanism on April 18. Overall, the website is not just a source of content, but a community. You can contribute and discuss with others about Lunar science and exploration, and the role of the Moon in society in general.

The Moon occupies a special place in my life. Growing up, I would dream of working on space missions that would eventually reach the Moon. I voraciously read through everything I could find about the Apollo missions and the astronaut’s first hand accounts of the glorious landscape there. When my family got our first telescope, the Moon was the easiest and first target. The Moon essentially represented everything that existed beyond, the unknown waiting to be explored.

Most importantly, though, it taught me to look up more, to care about the rest of the universe beyond the Earth. Once I started to care about the Moon, the rest of the Solar System began to become an easier target to focus on. I wanted to know about Mars, or strange and giant planets like Jupiter and Uranus. Other planets also had moons, I found out, stranger than our own, each with their own unique characteristics. Our moon was just one of the many fantastic places beyond our planet, and as I slowly began to comprehend the vastness of the universe, I realized that the Earth was just one of those places, a tiny portion of the entire, grander cosmos.

Now, as I study bigger and much further away things, like pulsars and galaxies, I still remember my first stepping stone into the universe. It is still every bit as glorious and even more interesting than in my own first explorations. There is a lot that the Moon can teach us about our own planet and the Solar System. And as my story relates, the Moon can serve as a huge source of inspiration and discovery. I hope by contributing to MyMoon, I can help spread my same wonder and excitement about the universe beyond the planet.