By measuring the sunshine of particular person stars between clusters of galaxies, astronomers can discover clues about darkish matter
Astronomers were able to measure extremely faint light in clusters of galaxies, and this measurement was surprising: it recorded the amount of invisible dark matter that scientists have been trying to determine for decades.
With the latest data release from the Dark Energy Survey, a massive scan of the sky designed to map the positions of galaxies across the universe, a team of astronomers instead focused on something else: the space between galaxies.
Each galaxy cluster in our universe is home to up to a thousand galaxies, and these galaxies are by far the greatest source of visible light from the clusters. But countless “rogue stars” that are not tied to a specific galaxy also contribute their own, much weaker light known as intracluster light (ICL).
The ICL is extremely difficult to see, as you have to be very far away from each individual galaxy to measure, but with so little light it burdens the sensitivity of the survey telescope. To make this possible, the team combined the data from over 800 galaxy clusters.
“Having a lot of data from the Dark Energy Survey allowed us to compensate for a lot of noise for this type of measurement. It’s statistical averaging, ”said Fermilab scientist Yuanyuan Zhang, who led the studies. The results were published in an article published in the Astrophysical Journal in April 2019. Another article appeared recently in Monthly Notices of the Royal Astronomical Society.
See in the dark
As a bonus, the team used a technique called weak gravitational lenses to map the amount of dark matter – an invisible form of matter that makes up most of the mass of the universe – within the same galaxy cluster. They found a simple relationship in which the amount of dark matter correlated well with the amount of ICL.
“We didn’t expect to find such a close connection between these radial distributions, but we did,” said Hillysson Sampaio-Santos, lead author of the latest paper.
On the left you can see a simulated image in which intracluster light is visible as a diffuse cloudiness between discrete brightness peaks – the galaxies. During observations, as can be seen on the right, this intracluster light component is largely drowned in the noise. Left picture: Jesse Golden-Marx; Simulation by The IllustrisTNG. Right picture: Dark Energy Survey and Yuanyuan Zhang
Oddly enough, computer simulations of galaxy clusters do not predict the same relationship.
“If the simulation didn’t get it right, it could mean that the simulated intracluster light is being generated at a slightly different time than when it was observed. The simulated stars did not have enough time to wander around and find dark matter, ”said Zhang.
Of course there is still a lot to be done.
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