Dark Energy Survey finds new evidence for emerging way to measure dark matter


“Observationally we confirmed that intracluster light is a pretty good radial tracer of dark matter. That means that where intracluster light is relatively bright, the dark matter is relatively dense,” said Zhang. 

“We did not expect to find such a tight connection between these radial distributions, but we did,” said scientist Hillysson Sampaio-Santos, the lead author of the new paper.

Comparing observations with simulations

To gain more insight, the team used a sophisticated computer simulation to study the relationship between intracluster light and dark matter. Curiously, they found that the radial profiles between the two phenomena in the simulation didn’t agree with the observational data. In the simulation, “the intracluster light radial profile was not the best component to trace dark matter,” said Sampaio-Santos, who is with the National Observatory in Rio de Janeiro, Brazil.

Zhang noted that it’s too soon to tell exactly what caused the conflict between observation and simulation.

“If the simulation didn’t get it right, it could mean that the simulated intracluster light is produced at a slightly different time than in observations. The simulated stars didn’t have enough time to wander around and start to trace dark matter,” she said.

Sampaio-Santos noted that further studies could yield insights into the dynamics occurring inside galaxy clusters, including interactions that gravitationally release some of their stars, allowing them to wander around.

“I’m planning to study the intracluster light and the effects of relaxation,” or spreading out, he said. For example, some clusters have merged together. These merged clusters should have different properties of intracluster light compared to clusters that are relaxed.

Enhancing signals in noisy data sets

The intracluster light that the team measured is about a hundred to a thousand times fainter than what Dark Energy Survey scientists normally attempt. That means the team had to deal with a lot of noise and contamination in the signal.

The technical aspect of the feat was challenging, Zhang said, “but because we had quite a bit of data from the Dark Energy Survey, we were able to cancel out a lot of noise to do this kind of measurement. It’s statistical averaging.”

To get the bigger picture and to beat down the noise, the Dark Energy Survey team statistically averaged about 300 galaxy clusters in the first study and more than 500 clusters in the second. All of them are a couple of billion light-years from Earth.

Teasing the signal from the noise of each cluster takes a lot of data. Luckily, that is exactly what the Dark Energy Survey collected. 


The intracluster light measurements probe clusters that are up to 3.3 billion light-years from Earth. In future studies, Zhang would like to study the redshift evolution of intracluster light—how it changes with cosmic time.

“My dream is to go all the way to redshift one—10 billion light-years,” Zhang said. “Studies say that’s when the intracluster light has just started to evolve.”

Going that far would enable scientists to see intracluster light building over time.

“But that’s really hard because it’s three times as far as the distance of our latest measurements, so everything is going to be extremely faint there,” she said.

The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.

Citation: “Is diffuse intracluster light a good tracer of the galaxy cluster matter distribution?” Sampaio-Santos et al, Monthly Notices of the Royal Astronomical Society, Nov. 26, 2020.

Funding for the Dark Energy Survey Projects: U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, Funding Authority for Studies and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science, Technology and Innovation, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.

Adapted from an article originally published by Fermilab written by Steve Koppes.

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