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Astronomers have observed a distant and messy collision between galaxy clusters and discovered that dark matter, the most mysterious “stuff” in the universe, has sped through the wreckage like a cosmic ghost.
The dark matter was detected as it was speeding away from the conventional “normal” matter that makes up stars, planets, moons, and everything else we see around us in the colliding clusters. The galactic clusters “ghosted” in this study are part of a complex of thousands of galaxies collectively known as MACS J0018.5+1626, which is located about 5 billion light-years from Earth. Conglomerates like MACS J0018.5+1626 form the largest structures in the universe.
The individual galaxies of the colliding clusters emerged unscathed from this cosmic collision, thanks to the vast space between them. However, the dark matter in these galaxies was less affected by the incident.
To imagine what such a collision would look like, lead researcher Emily Silich, an astrophysicist at the California Institute of Technology (Caltech) in Pasadena, suggested picturing two dump trucks loaded with sand crashing into each other.
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“Dark matter is like sand, flying forward,” Silich said in a statement.
Scientists have previously observed dark matter rushing ahead of normal matter in similar collisions. This new work, using data collected by NASA’s Hubble and Chandra Space Telescopes, is the first time researchers have been able to directly study the “decoupling” of the speeds 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, Hawaii), the W. M. Keck Observatory, the Planck Observatory, the Atacama Submillimeter Telescope Experiment, and the now-retired Herschel Space Observatory collected data for the study.
The data not only come from a large number of instruments, but have been collected over decades. The analysis of the data itself took years.
Giving up the ghost. How did dark matter outshine ordinary matter?
The problem with dark matter is that it is frustratingly ‘antisocial’ when it comes to interacting with light, which makes it nearly invisible, and with ordinary matter.
It’s this lack of interaction (or the fact that the interactions are too weak to see) that leads scientists to believe that dark matter can’t be made of electrons, protons, and neutrons, the baryonic particles that make up 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 ghost, making you wonder how we even know it exists. Well, dark matter does. interact with gravity, and that influence can affect baryonic matter and light in ways we can observe.
For example, scientists know that galaxies are shrouded in enormous halos of dark matter, whose gravity prevents them from falling apart. They have also determined that dark matter, despite its seemingly immaterial nature, makes up 85% of the mass of 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, a pair of colliding galaxy clusters, also known as 1E 0657-56, located about 3.7 billion light-years away. In the Bullet Cluster, scientists saw dark matter streaking past hot gas as the two clusters slid past each other.
The lack of interaction with ordinary matter allows dark matter to avoid catastrophic collisions during its evolution.
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 is being viewed from a different angle, tilted at about 90 degrees relative to the Bullet Cluster. As a result, we are seeing MACS J0018.5+1626 in such a way that it appears as if one galaxy is flying away from Earth while the other is hurtling toward us.
This provides a vantage point from which scientists can measure the velocity of both the dark matter and baryonic matter involved in the collision. From there, they can then determine how the two types of matter decouple in an event like this.
“With the Bullet Cluster, it’s like we’re sitting in the stands watching a car race, and we can get these beautiful snapshots of the cars moving from left to right on the straightaway,” said Jack Sayers, the study’s principal investigator and a professor of physics at Caltech. “In our case, it’s more like we’re standing on the straightaway with a radar gun, standing in front of a car coming toward us, and we can determine its speed.”
The first light from the universe is a cosmic radar gun
The “radar gun” the team is using is a phenomenon called the Sunyaev-Zel’dovich (SZ) effect. It happens when photons that make up the first light in the universe, the cosmic microwave background (CMB), are scattered by electrons that are not bound to atoms in hot ionized gas as it travels toward Earth.
This causes the photons to undergo a Doppler shift, a change in the frequency and wavelength of a wave depending on whether it is traveling 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. This means that the SZ effect can be used to measure the rate at which hot gas is being produced, and thus the speed at which normal matter is moving, in MACS J0018.5+1626.
The team then used the Keck Observatory to measure the rate of mass concentration of galaxies in the clusters. Because most of this mass is dark matter, it and galaxies as a whole behave in similar ways during the collision, revealing to the researchers the speed at which the dark matter is moving.
This also showed the team something strange about MACS J0018.5+1626: the dark matter and the ordinary matter appear to be moving in opposite directions.
“We had this complete oddball with velocities going in opposite directions, and at first we thought it might 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 sorted it all out.”
Reconstruction of a cosmic accident
To solve the puzzle of the MACS J0018.5+1626 smashup, Silich turned to data from Chandra, which revealed the temperature of the hot gas from the merger and its location. This line of research also revealed how much this gas had been “shocked” by the collision process.
“These cluster collisions are the most energetic events since the Big Bang,” Silich said. “Chandra measures the extreme temperatures of the gas and tells us the age of the merger and how recently the clusters collided.”
The team then mapped the dark matter of MACS J0018.5+1626 by looking at the effect its mass has on the structure of space-time and thus on the passing light from background sources. This is called ‘gravitational lensing’.
From here, they were able to simulate the galaxy clusters colliding, a kind of cosmic accident reconstruction. They then combined this simulation with a large set of telescope data to determine the evolutionary stage of MACS J0018.5+1626 and the geometry of the cosmic collision. Such work showed that the galaxy clusters were hurtling toward each other at about 7 million mph (11 million km/h) just before they collided — about 1% of the speed of light!
Why do the dark matter and normal matter appear to travel in opposite directions? The team determined that this was due to the orientation of the collision and the two forms of matter separating from each other.
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“It’s taken a long time to put all the pieces of the puzzle together, but now we finally know what’s going on,” Sayers concluded. “We hope this will lead to a whole new way of studying dark matter in clusters.”
While these findings don’t provide much new information about dark matter, the team hopes that similar studies to follow could gradually shed more light on this mystery that has puzzled scientists for decades.
The team’s research was published last month in The Astrophysical Journal.