Galaxy Cluster Clash Points out Dark Matter
This image just released today may look like a galaxy cluster with some Photoshop airbrushing on it, but it’s really proof of an effect that had been observed once before but not completely proved. The Hubble Space Telescope and Chandra X-ray Observatory both studied the same area of the sky, looking at a collision of galaxy clusters into a giant supercluster, called MACS J0025.4-1222. The combined observations provide another clue to the existence and distribution of dark matter. This time the mechanism was provided by the cluster collision. When such objects collide, they pack a heck of a punch, and the energy of that punch separates dark matter from ordinary matter.
Here’s how the Chandra folks describe the mechanics of the collision: two galaxy clusters, each a quadrillion times the mass of the Sun, collided to form the system known as MACS J0025.4-1222. When they merged at speeds of millions of miles per hour, the hot gas in each cluster collided and slowed down, but the dark matter did not.
Optical images from Hubble were used to infer the distribution of the total mass — dark and ordinary matter — using a technique known as gravitational lensing (the blue area shows where light is “bent” as it passes by clumps and regions of dark matter and is influenced by the dark matter’s gravitational pull). Chandra data enabled astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (the pink regions.) The separation between the material shown in pink and blue provides direct evidence for dark matter. The fact that it could be separated from baryonic matter in such a powerful collision is another clue to the nature of dark matter.
Understanding this unseen material, particularly how much of it there is in the universe, is key to our understanding of so many other things about the universe. The expansion of the universe, the ordering of large-scale structure (into clusters of galaxies, superclusters of galaxies), and even such aspects of galaxies as their rotation and merger rates are all affected by this dark matter. It permeates the universe, yet it is extraordinarily difficult to detect using conventional observational techniques. It’s “easier” to infer its existence by observing its affect on light, for example. Which is what gravitational lensing does, and what makes it such a useful tool for astronomy. While we still don’t know the entire story of dark matter, discoveries like these are helping “fill in the puzzle pieces.”
This Fermi image shows gas and dust in the plane of the Milky Way glowing in gamma rays due to collisions with accelerated nuclei called cosmic rays. The famous Crab Nebula and Vela pulsars also shine brightly at these wavelengths. These are fast-spinning neutron stars, which form when massive stars die. The Crab and Vela pulsars were originally discovered by their radio emissions. The third pulsar shown here, named Geminga and located in Gemini, is not a radio source. It was discovered by an earlier gamma-ray satellite. Fermi is expected to discover many more radio-quiet pulsars, providing key information about how these exotic objects work.