The only reason we believe that dark matter accounts for 85 percent of the known universe is in the observable effects of gravity. A new method, developed by astronomers in Australia and Spain and using images taken with the NASA / ESA Hubble Space Telescope, may enable us to "see" dark matter with very weak light in galaxy clusters.
A galaxy cluster is a huge gravitational accumulation of galaxies. For example, our home galaxy, the Milky Way, exists within a Laniakea supercluster with hundreds of thousands of other galaxies. In a cluster, galaxies interact, and sometimes stars are torn out of their home galaxy and thrown freely through the cluster. As these intergalactic vagrants move freely through the cluster, they emit a faint light called the "accessory light."
And that's the key.
"We have found that very weak light in galaxy clusters, the intracluster light, maps the distribution of dark matter," said Mireia Montes, Australian University of New South Wales, lead author of the Monthly Notices research (PDF) of the Royal Astronomical Society.
In the past, astronomers have used "gravitational lenses" to estimate the distribution of dark matter in galaxy clusters. While lensing makes the structure of dark matter in clusters clear, it requires intense observation and a long time.
The study proposed by Montes and the co-author Ignacio Trujillo of the Spanish Institute of Astrofisica de Canarias require only a depth image – as presented by Hubble's Frontier Fields project here – to accurately determine the properties of the dark matter within the project of a given cluster.
In fact, studies on the gravitational lens for Montes and Trujillo were central to the development of their own method. In earlier research, the same six galaxy clusters were investigated and their profiles of dark matter were investigated with gravitational lenses. Therefore, Montes and Trujillo were able to compare the distribution of intracluster light with the previous analysis.
The researchers suggest that intracluster light "exquisitely follows the global distribution of dark matter" within clusters, and therefore the stars freely floating through the cluster have "an identical distribution to dark matter."
Future work will see researchers expand their findings to galaxy clusters that are larger than the six studied here. An extension of the Hubble Frontier Fields project, Beyond Ultra-Deep Frontier Fields and legacy observations (Buffalo), allows observations at the edges of these galaxy clusters.
This will provide an opportunity to see if this new view of dark matter persists.
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