Faint starlight in Hubble images reveals distribution of dark matter

Astronomers using data from the NASA/ESA Hubble Space Telescope have employed a revolutionary method to detect dark matter in galaxy clusters. The method allows astronomers to “see” the distribution of dark matter more accurately than any other method used to date and it could possibly be used to explore the ultimate nature of dark matter. The results were published in the journal Monthly Notices of the Royal Astronomical Society.

Two massive galaxy clusters — Abell S1063 (left) and MACS J0416.1-2403 (right) — display a soft blue haze, called
intracluster light, embedded among innumerable galaxies. The intracluster light is produced by orphan stars that no
 longer belong to any single galaxy, having been thrown loose during a violent galaxy interaction, and now drift freely
throughout the cluster of galaxies. Astronomers have found that intracluster light closely matches with a map of mass
distribution in the cluster’s overall gravitational field. This makes the blue “ghost light” a good indicator of how invisible
dark matter is distributed in the cluster. Dark matter is a key missing link in our understanding of the structure and evolution
of the universe. Abell S1063 and MACS J0416.1-2403 were the strongest examples of intracluster light providing a much
 better match to the cluster’s mass map than X-ray light, which has been used in the past to trace dark matter
[Credit: NASA, ESA, and M. Montes (University of New South Wales)]

In recent decades astronomers have tried to understand the true nature of the mysterious substance that makes up most of the matter in the Universe — dark matter — and to map its distribution in the Universe. Now two astronomers from Australia and Spain have used data from the Frontier Fields programme of the NASA/ESA Hubble Space Telescope to accurately study the distribution of dark matter.

“We have found a way to ‘see’ dark matter,” explains Mireia Montes (University of New South Wales, Australia), lead author of the study. “We have found that very faint light in galaxy clusters, the intracluster light, maps how dark matter is distributed.”

Intracluster light is a byproduct of interactions between galaxies. In the course of these interactions, individual stars are stripped from their galaxies and float freely within the cluster. Once free from their galaxies, they end up where the majority of the mass of the cluster, mostly dark matter, resides.

“These stars have an identical distribution to the dark matter, as far as our current technology allows us to study,” explained Montes. Both the dark matter and these isolated stars — which form the intracluster light — act as collisionless components. These follow the gravitational potential of the cluster itself. The study showed that the intracluster light is aligned with the dark matter, tracing its distribution more accurately than any other method relying on luminous tracers used so far.

This animation switches between an original image of the galaxy cluster MACS J0416.1–2403, as it was observed by the 

Frontier Field team, and a version, in which the intracluster light (in blue) is highlighted. Intracluster light is a byproduct 

of interactions between galaxies. It can be used to make the distribution of dark matter in galaxy clusters visible 

[Credit: ESA/Hubble, NASA, HST Frontier Fields team (STScI), and M. Montes & I. Trujillo]

This method is also more efficient than the more complex method of using gravitational lensing. While the latter requires both accurate lensing reconstruction and time-consuming spectroscopic campaigns, the method presented by Montes utilises only deep imaging. This means more clusters can be studied with the new method in the same amount of observation time.

The results of the study introduce the possibility of exploring the ultimate nature of dark matter. “If dark matter is self-interacting we could detect this as tiny departures in the dark matter distribution compared to this very faint stellar glow,” highlights Ignacio Trujillo (Instituto de Astrofísica de Canarias, Spain), co-author of the study. Currently, all that is known about dark matter is that it appears to interact with regular matter gravitationally, but not in any other way. To find that it self-interacts would place significant constraints on its identity.

For now, Montes and Trujillo plan to survey more of the original six clusters to see if their method remains accurate. Another important test of their method will be the observation and analysis of additional galaxy clusters by other research teams, to add to the data set and confirm their findings.

The team can also look forward to the application of the same techniques using future space-based telescopes like the NASA/ESA/CSA James Webb Space Telescope, which will have even more sensitive instruments able to resolve faint intracluster light in the distant Universe.

“There are exciting possibilities that we should be able to probe in the upcoming years by studying hundreds of galaxy clusters,” concludes Ignacio Trujillo.

Source: ESA/Hubble Information Centre [December 20, 2018]

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