Dark matter ghosts its way through powerful (and messy) collision of galaxy clusters

Observing a distant and messy collision between galaxy clusters, astronomers have discovered that dark matter, the most mysterious "stuff" in the universe, passed through the wreckage like a cosmic phantom.

The dark matter was detected racing away from the conventional "normal" matter that comprises stars, planets, moons and everything we see around us in the colliding clusters. The galactic clusters "getting ghosted" in this research are part of a complex of thousands of galaxies that are collectively known as MACS J0018.5+1626, which is located around 5 billion light-years from Earth. Conglomerations like MACS J0018.5+1626 constitute the largest structures in the universe.

The individual galaxies of the colliding clusters escaped unscathed from this cosmic pile-up because of the vast space between them, but the dark matter content of those galaxies was even more untroubled by the incident.

To imagine what this collision looked like, study lead author Emily Silich, an astrophysicist at the California Institute of Technology (Caltech) in Pasadena, suggested picturing two dump trucks carrying sand smashing together.

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"The dark matter is like the sand and flies ahead," Silich said in a statement.

Scientists have detected dark matter racing ahead of normal matter in similar collisions before, but this new research, which used data collected by NASA's Hubble and Chandra space telescopes, represents the first time that researchers have been able to directly study the "decoupling" of the velocity of dark matter and "normal" matter.

Silich and colleagues used a plethora of telescopes to observe the collision of MACS J0018.5+1626. In addition to data from Hubble and Chandra, the Caltech Submillimeter Observatory (until recently located on Maunakea in Hawai‘i), the W. M. Keck Observatory, the Planck Observatory, the Atacama Submillimeter Telescope Experiment, and the now-retired Herschel Space Observatory gathered data for the study.

The data doesn't just come from a vast array of instruments; it was also collected over the course of decades, with the analysis of the data itself taking years.

Giving up the ghost. How did dark matter give ordinary matter the slip?

The problem of dark matter hinges on the fact that it is is frustratingly "antisocial" when it comes to interacting with light, something that makes it nigh-invisible, and with ordinary matter.

It is this lack of interaction (or the fact that the interactions are too weak to see) that makes scientists think dark matter can't be made up of electrons , protons and neutrons, the baryonic particles that comprise the atoms that make up stars, planets, moons and everything else we see around us. That's because these baryons do interact with each other and with light.

This might make dark matter sound like a cosmic phantom, leaving you to wonder how we can know it exists at all. Well, dark matter does interact with gravity, and that influence can impact baryonic matter and light in ways we can detect.

That's how scientists know that galaxies are shrouded in vast haloes of dark matter, the gravitational influence of which prevents them from splitting apart. It is also how they have determined that, despite its seeming insubstantial nature, dark matter makes up 85% of the stuff with mass in the universe.

Related: What is dark matter?

One of the best pieces of evidence we have for the existence of dark matter is the Bullet Cluster , two colliding clusters of galaxies also known as 1E 0657-56 and located about 3.7 billion light years away. In the Bullet Cluster, scientists observed dark matter shooting past hot gas as the two clusters slipped past one another.

It's the lack of interaction with ordinary matter that allows dark matter to escape the cataclysmic collisions as it is progressing.

The collision that forms the basis of MACS J0018.5+1626 is similar to that of the Bullet Cluster. What sets it apart is the fact that it's seen at a different angle, tilted at about 90 degrees relative to the Bullet Cluster. As a result, we see MACS J0018.5+1626 in such a way that it appears one galaxy is racing away from Earth as the other rockets our way.

This results in a vantage point that allows scientists to measure the velocity of both the dark matter and the baryonic matter involved in the collision. From there, they can then determine how the two types of matter decouple from each other in an event like this.

"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 straightaway," said study principal investigator Jack Sayers, a Caltech professor of physics. "In our case, it's more like we are on the straightaway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed."

The universe's first light is a cosmic radar gun

The "radar gun" used by the team is a phenomenon called the "Sunyaev-Zel'dovich (SZ) effect." This occurs when photons that comprise the first light in the universe, the cosmic microwave background (CMB) , scatter from electrons that are not bound to atoms in hot ionized gas as this gas travels toward Earth.

This causes the photons to experience a Doppler shift , a change in the frequency and wavelength of a wave depending on whether it is heading toward or away from an observer. This results in a change in the brightness of the CMB light that is proportional to the speed at which the scattering electrons are moving. That means the SZ effect can be used to measure the speed at which hot gas is produced, and thus the speed that normal matter moves, in MACS J0018.5+1626.

The team then used the Keck Observatory to measure the speed of the mass concentration of galaxies in the clusters. Because most of this mass is accounted for by dark matter, it and galaxies as a whole behave similarly during the collision. Thus, this revealed to the researchers by proxy the speed at which the dark matter is moving.

This also demonstrated something odd to the team about MACS J0018.5+1626: The dark matter and ordinary matter seem to be moving in opposite directions.

"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 explained. "And then Emily got involved and untangled everything."

Cosmic accident reconstruction

Aiming to solve the puzzle of the MACS J0018.5+1626 smashup, Silich turned to data from Chandra, which revealed the temperature of the merger's hot gas and its location. This line of inquiry also revealed how much this gas had been "shocked" by the collision process.

"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 then mapped MACS J0018.5+1626's dark matter using an effect its mass has on the fabric of space-time and, through this, on passing light from background sources, called " gravitational lensing ."

From here, they were able to simulate the collision of the galaxy clusters, a type of cosmic accident reconstruction. They then combined this simulation with a vast array of telescope data to determine the evolutionary stage of MACS J0018.5+1626 and the geometry of the cosmic collision. Such work showed that just before they collided, the galaxy clusters were racing together at around 7 million mph (11 million kph) — about 1% of the speed of light!

Why do the dark matter and normal matter appear to be traveling in opposite directions? The team determined this was because of the orientation of the collision and due to the two forms of matter separating from each other.

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"It took us a long time to put all the puzzle pieces together, but now we finally know what's going on," Sayers concluded. "We hope this leads to a whole new way to study dark matter in clusters."

Though these findings don't reveal much new information about dark matter , the team hopes similar studies that may follow could gradually help shed some light on this mystery that has confounded scientists for decades.

The team's study was published last month in The Astrophysical Journal.