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Astrophysicist Maxim Markevitch on Dark Matter

CXC: How did the idea to study 1E 0657-56 come about? Why did you go about mounting an observation campaign using so many different telescopes?

MM: Initially, my CfA colleagues and I were interested in the X-ray aspects of this object. Pre-Chandra X-ray observations showed that it is among the hottest and most X-ray luminous known clusters, so a snapshot Chandra observation was obtained in 2000. It has revealed a gas "bullet" and a spectacular shock front, a textbook example of shocks and the first one ever seen in a cluster (see our paper 2002 ApJ Lett., 567, L27). We then overlaid the X-ray image on an optical image, noticed the offset between the galaxies and the gas, and realized that this cluster offers a unique experimental setup for dark matter studies -- we only needed to map its dark matter distribution. This is where our optical colleagues joined in (Doug Clowe, Anthony Gonzalez, Marusa Bradac), wrote observing proposals for various telescopes and over the years assembled this rich optical dataset. In the meantime, we obtained a very long Chandra observation.

CXC: What's your interest in dark matter? Why is it important to understand the behavior of dark matter?

MM: The nature of dark matter is one of the most important topics in astronomy, so everybody is interested. Little is known about it -- all that the numerous searches for dark matter particles have done is ruling out various hypotheses, but they never got any "positive" results. So any new piece of evidence is valuable.

CXC: What did you think when you realized you'd found direct evidence of dark matter?

MM: It was Doug Clowe's realization. Initially, we were planning to use the lensing maps to answer a more technical question: whether the dark matter particles are perfectly collisionless, which is the current assumption, or they can collide with each other (see our paper 2004 ApJ 606, 819). But Doug looked at the mass/X-ray overlay and realized that it's also a direct proof of the dark matter existence.

CXC: How important or big is this discovery to astrophysics and science? What does this discovery tell us about the general nature of the universe?

MM: Most people already believe that dark matter (DM) exists, because it fits various indirect observations so well. However, it is uncomfortable for a scientist to have to invoke something invisible and undetectable to account for 90% of the matter in the universe. Furthermore, some observations are difficult to explain with the current DM models. This is why people also explored alternatives to DM, for example, modifying the gravity laws on intergalactic scales in such a way that visible matter would be sufficient to explain all those effects that we normally ascribe to DM. This idea has now been disproved -- although we did not prove that gravity laws are correct, we did show unambiguously that there is dark matter on cluster scales, not just the visible matter.

Since the DM concept is so well-established, it means little in practical terms. For example, searches of dark matter particles have been going on before and will continue. However, such proof is important for our general understanding of the universe, because it gives us confidence in our basic assumptions. We all know of numerous examples in the history of science when a well-established theory turned out to be wrong.

CXC: How might dark matter relate to Earth and the solar system?

MM: It most probably doesn't have any detectable effects. On such astronomically small linear scales, the normal matter dominates. The gravity in the solar system is well-measured and fits perfectly what's expected from the sun and planets. As for interactions of DM particles with things on Earth, the most sensitive experiments have tried to detect them and failed, so it should have no practical effects.

Return to 1E 0657-56 (21 Aug 06)