image: This astounding view of galaxy cluster MACSJ0025 demonstrates how ordinary matter and mysterious dark matter interact. The blue cloud-shaped parts flanking the centre show the position of dark matter, mapped by the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope. The pink middle indicates ordinary matter, charted by NASA’s Chandra X-Ray Observatory. The position of the two matter types shown in the image are explained by MACSJ0025’s origin. It was formed when a pair of large galaxy clusters collided. Ordinary matter in the form of hot gas slowed down and pooled at the centre but ghostly dark matter passed straight through. Hubble used a technique known as gravitational lensing to obtain its data. The light observed was bent by the gravitationally massive galaxy cluster, resulting in an incredibly detailed image. This technique was originally predicted by Einstein. MACSJ0025 is located in the constellation Cetus, the Whale. view more
Credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara, USA), and S. Allen (Stanford University, USA).
New Hubble and Chandra observations of the cluster known as MACSJ0025.4-1222 indicate that a titanic collision has separated dark from ordinary matter. This provides independent confirmation of a similar effect detected previously in a target dubbed the Bullet Cluster, showing that the Bullet Cluster is not an anomalous case.
MACSJ0025 formed after an enormously energetic collision between two large clusters. Using visible-light images from Hubble, astronomers were able to infer the total mass distribution — dark and ordinary matter. Hubble was used to map the dark matter (coloured in blue) using a technique known as gravitational lensing. Chandra data enabled the astronomers to accurately map the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (shown in pink).
As the two clusters that formed MACSJ0025 (each almost a whopping quadrillion times the mass of our Sun) merged at speeds of millions of kilometres per hour, hot gas in the two clusters collided and slowed down, but the dark matter passed right through this smash-up. The separation between material shown in pink and blue therefore provides observational evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.
The international team of astronomers in this study was led by Maruša Bradač of the University of California, Santa Barbara, USA, and Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Stanford Linear Accelerator Center (SLAC), USA. Their results will appear in an upcoming issue of The Astrophysical Journal.