Maunakea, Hawaiʻi – Astronomers have untangled a messy collision between two massive clusters of galaxies in which the clusters’ vast clouds of dark matter have decoupled from the so-called normal matter. The two clusters each contain thousands of galaxies and are located billions of light-years away from Earth. As they plowed through each other, the dark matter—an invisible substance that feels the force of gravity but emits no light—sped ahead of the normal matter. The new observations are the first to directly probe the decoupling of the dark and normal matter velocities.
The discovery was made using data from space- and ground-based telescopes, including two Maunakea Observatories on Hawaiʻi Island: W. M. Keck Observatory and the Caltech Submillimeter Observatory, or CSO (which was recently removed from its site on Maunakea and will be relocated to Chile). Some of the observations were made decades ago, while the full analysis using all the datasets took place over the past couple of years. The findings are detailed in a new study published in The Astrophysical Journal.
Galaxy clusters are among the largest structures in the universe, glued together by the force of gravity. Only 15 percent of the mass in such clusters is normal matter, the same matter that makes up planets, people, and everything you see around you. Of this normal matter, the vast majority is hot gas, while the rest is stars and planets. The remaining 85 percent of the cluster mass is dark matter.
During the tussle that took place between the clusters, known collectivity as MACS J0018.5+1626, the individual galaxies themselves largely went unscathed because so much space exists between them. But when the enormous stores of gas between the galaxies (the normal matter) collided, the gas became turbulent and superheated. While all matter, including both normal matter and dark matter, interacts via gravity, the normal matter also interacts via electromagnetism, which slows it down during a collision. So, while the normal matter became bogged down, the pools of dark matter within each cluster sailed on through.
Think of a massive collision between multiple dump trucks carrying sand, suggests Emily Silich, lead author of the new study. “The dark matter is like the sand and flies ahead.” Silich is a graduate student working with Jack Sayers, research professor of physics at Caltech and principal investigator of the study.
Such decoupling of dark and normal matter has been seen before, most famously in the Bullet Cluster. In that collision, the hot gas can be seen clearly lagging behind the dark matter after the two galaxy clusters shot through each other.
The situation that took place in MACS J0018.5+1626 (referred to subsequently as MACS J0018.5) is similar, but the orientation of the merger is rotated, roughly 90 degrees relative to that of the Bullet Cluster. In other words, one of the massive clusters in MACS J0018.5 is flying nearly straight toward Earth while the other one is rushing away. That orientation gave researchers a unique vantage point from which to measure the speed at which the hot gas was traveling.
“With the Bullet Cluster, it’s like we are sitting in a grandstand watching a car race and are able to capture beautiful snapshots of the cars moving from left to right on the straightway,” says Jack Sayers, a research professor at Caltech and principal investigator of the study. “In our case, it’s more like we are on the straightway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed.”
METHODOLOGY
The team used Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS) to learn the speed of the galaxies in the cluster, which told them by proxy the speed of the dark matter (because the dark matter and galaxies behave similarly during the collision). To measure the speed of the normal matter, or gas, in the cluster, researchers used CSO to perform an observational method known as the kinetic Sunyaev-Zel’dovich (SZ) effect.
“The Sunyaev-Zeldovich effects were still a very new observational tool when Jack and I first turned a new camera at the CSO on galaxy clusters in 2006, and we had no idea there would be discoveries like this,” says Sunil Golwala, professor of physics and Silich’s faculty PhD advisor. “We look forward to a slew of new surprises when we put next-generation instruments on the telescope at its new home in Chile.”
The team also gathered data from the European Space Agency’s now-retired Herschel Space Observatory and Planck observatory, as well as the Atacama Submillimeter Telescope Experiment in Chile.
MACS J0018.5 showed signs of something strange going on—the hot gas, or normal matter, was traveling in the opposite direction to the dark matter.
“We had this complete oddball with velocities in opposite directions, and at first we thought it could be a problem with our data. Even our colleagues who simulate galaxy clusters didn’t know what was going on,” Sayers says. “And then Emily got involved and untangled everything.”
For part of her PhD thesis, Silich turned to data from NASA’s Chandra X-ray Observatory to reveal the temperature and location of the gas in the clusters as well as the degree to which the gas was being shocked.
“These cluster collisions are the most energetic phenomena since the Big Bang,” Silich says. “Chandra measures the extreme temperatures of the gas and tells us about the age of the merger and how recently the clusters collided.”
The team also worked with Adi Zitrin of the Ben-Gurion University of the Negev in Israel to use NASA’s Hubble Space Telescope to map the dark matter using a method known as gravitational lensing.
Additionally, John ZuHone of the Center for Astrophysics at Harvard & Smithsonian helped the team simulate the cluster smashup. The scientists found that, prior to colliding, the clusters were moving toward each other at approximately 3000 kilometers/second, equal to roughly one percent of the speed of light.
With a more complete picture of what was going on, the researchers were able to figure out why the dark matter and normal matter appeared to be traveling in opposite directions. The orientation of the collision, coupled with the fact that dark matter and normal matter had separated from each other, explains the oddball velocity measurements.
NEXT STEPS
In the future, the researchers hope that more studies like this one will lead to new clues about the mysterious nature of dark matter. “This study is a starting point to more detailed studies into the nature of dark matter,” Silich says. “We have a new type of direct probe that shows how dark matter behaves differently from normal matter.”
Sayers, who recalls first collecting the CSO data on this object almost 20 years ago, says, “It took us a long time to put all the puzzle pieces together, but now we finally know what’s going on. We hope this leads to a whole new way to study dark matter in clusters.”
Learn more:
- “Dark Matter Flies Ahead of Normal Matter in Mega Galaxy Collision” – Caltech Press Release, July 24
ABOUT DEIMOS
The DEep Imaging and Multi-Object Spectrograph (DEIMOS) boasts the largest field of view (16.7arcmin by 5 arcmin) of any of the Keck Observatory instruments, and the largest number of pixels (64 Mpix). It is used primarily in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently probing the most distant corners of the universe with high sensitivity.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.