Free Astronomy Magazine March-April 2019

34 MARCH-APRIL 2019 SPACE CHRONICLES by NASA/ESA U tilizing Hubble’s past observa- tions of six massive galaxy clusters in the Frontier Fields program, astronomers demon- strated that intracluster light — the diffuse glow between galaxies in a cluster — traces the path of dark matter, illuminating its distribution more accurately than existing meth- ods that observe X-ray light. Intracluster light is the byproduct of interactions between galaxies that disrupt their structures; in the chaos, individual stars are thrown free of their gravitational moorings in their home galaxy to realign themselves with the gravity map of the overall cluster. This is also where the vast majority of dark matter resides. X- ray light indicates where groups of galaxies are colliding, but not the underlying structure of the cluster. This makes it a less precise tracer of dark matter. “The reason that intracluster light is such an excellent tracer of dark mat- ter in a galaxy cluster is that both the dark matter and these stars forming the intracluster light are free-floating on the gravitational potential of the cluster itself — so they are following exactly the same gravity,” said Mireia Montes of the University of New South Wales in Sydney, Australia, who is co-author of the study. “We have found a new way to see the location where the dark matter should be, because you are tracing exactly the same gravita- tional potential. We can illuminate, with a very faint glow, the position of dark matter.” Montes also highlights that not only is the method accurate, but it is more efficient in that it utilizes only deep imaging, rather than the more complex, time-intensive techniques of spectroscopy. This means more clusters and objects in space can be studied in less time — meaning more potential evidence of what dark matter consists of and how it behaves. “This method puts us in the position to characterize, in a statisti- cal way, the ultimate nature of dark matter,” Montes said. Faint glow within galaxy clusters illuminates dark matter “The idea for the study was sparked while looking at the pristine Hubble Frontier Field images, ” said study co-author Ignacio Trujillo of the Ca- nary Islands Institute of Astronomy in Tenerife, Spain, who along with Montes had studied intracluster light for years. “The Hubble Frontier Fields showed intracluster light in unprece- dented clarity. The images were in-