Free Astronomy Magazine July-August 2015

SPACE CHRONICLES pare the distribution of the dark matter with the positions of the lu- minous galaxies. Although dark matter cannot be seen, the team could deduce its lo- cation using a technique called grav- itational lensing. The collision hap- pened to take place directly in front of a much more distant, unrelated source. The mass of dark matter around the colliding galaxies severe- ly distorted spacetime, deviating the path of light rays coming from the distant background galaxy — and distorting its image into characteris- tic arc shapes. Our current understanding is that all galaxies exist inside clumps of dark matter. Without the constraining ef- fect of dark matter’s gravity, gala- xies like the Milky Way would fling themselves apart as they rotate. In order to prevent this, 85 percent of the Universe’s mass must exist as dark matter, and yet its true nature remains a mystery. In this study, the researchers ob- served the four colliding galaxies and found that one dark matter clump appeared to be lagging be- hind the galaxy it surrounds. The dark matter is currently 5000 light- years (50 000 million million kilome- tres) behind the galaxy — it would take NASA’s Voyager spacecraft 90 million years to travel that far. A lag between dark matter and its asso- ciated galaxy is predicted during col- lisions if dark matter interacts with Dark matter may not be completely dark after all by ESO T his image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more dis- tant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the gal- axies is not moving with the galaxy itself, possibly implying dark matter- dark matter interactions of an unknown nature are occuring. [NASA] Using the MUSE instrument on ESO’s VLT in Chile, along with im- ages from Hubble in orbit, a team of astronomers studied the simulta- neous collision of four galaxies in the galaxy cluster Abell 3827. The team could trace out where the mass lies within the system and com- F or the first time dark matter may have been observed inter- acting with other dark matter in a way other than through the force of gravity.