Dark matter is named because it does not interact with anything we can detect or see easily. It's basically invisible to us.
The only reason we believe that dark matter makes up 85% of the known universe is because of its observable gravitational effects. Now a new method developed by astronomers in Australia and Spain, using images captured by the NASA / ESA Hubble Space Telescope, can allow us to "see" dark matter using very faint light found in galaxy clusters.
A cluster of galaxies is a huge cluster of gravitationally connected galaxies. For example, our galaxy, the Milky Way, exists within a known as the Laniakea supercluster with hundreds of thousands of other galaxies. Within a cluster, galaxies interact, and sometimes stars are plucked from their home galaxy and sent freely out of the cluster. As these intergalactic wanderers travel freely through the cluster, they emit a dim light known as "intraclass light."
And that's the key.
"We found that very weak light in galaxy clusters, the intracluster light, maps how dark matter is distributed," said Mireia Montes of the University of Australia in New South Wales, lead author of the research (PDF) published in Monthly Notices of the Royal. Astronomical Society.
In the past, astronomers have used "gravitational lenses" to estimate the distribution of dark matter in galaxy clusters. Although the lens is powerful to reveal the structure of dark matter in clusters, it requires both intense observation and extensive time.
The method proposed by Montes and co-author of the study, Ignacio Trujillo of the Canarian Astrophysics Institute, requires only deep space images – such as those provided here by Hubble's Frontier Fields project – to accurately infer the properties of dark matter a given cluster.
In fact, gravitational lens studies were central to Montes and Trujillo in devising their own method. Previous research had already analyzed the same six clusters of galaxies, studying their dark matter profiles using gravitational lenses. Thus, Montes and Trujillo could compare the distribution of intracluster light with the previous analysis.
The researchers suggest that the intracluster light "follows the overall distribution of dark matter" in the agglomerates, and thus stars floating freely through the cluster have "a similar distribution to dark matter."
Future works will see researchers extend their findings to galaxy clusters larger than the six studied here. An extension of the Hubble Frontier Fields project, along with ultra-deep border fields and legacy observations (Buffalo), will allow observations on the edges of these galaxy clusters.
This will provide an opportunity to determine if this new way of seeing dark matter holds up.
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