An Ancient Galaxy Collision Sheds Light on Dark Matter

Scientists think they might have explained the puzzle of galaxies free of dark matter.

Hubble picture of the system that accommodates DF2 and DF4. Image: Pieter van Dokkum et al./ Nature 2022


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Scientists assume they’ve reconstructed the epic origin story of a path of puzzling galaxies that would make clear the nice thriller of darkish matter, an unidentified substance that accounts for many matter within the universe, stories a brand new examine. 

It’s a story that features an historic cosmic crash, galaxies which can be bizarrely freed from darkish matter, and the medical tv drama House.

It all started a number of years in the past, when a workforce led by Pieter van Dokkum, who serves as Sol Goldman Professor of Astronomy at Yale University, found a distant dwarf galaxy, referred to as DF2, that seems to be completely bereft of darkish matter. Given that each one galaxies are thought to comprise darkish matter from their very origins, DF2 is a complete oddball that challenges conventional concepts about galactic evolution, whereas additionally providing a tantalizing check of present explanations for darkish matter. Flummoxed, van Dokkum and his colleagues reported the invention in a 2018 Nature paper that despatched ripples by way of the astronomical neighborhood.

“We decided to go ahead and publish it while saying and admitting that we don’t know how this happened, we don’t know where this thing came from, we don’t know how it formed, but here’s why we think this thing doesn’t have dark matter,” van Dokkum and his colleagues stated in a name. 

“At the same time, we made pretty grand claims about the implications of the objects—namely, that the fact that it didn’t have dark matter would imply that dark matter is real and is actually a thing that that galaxies can have or not,” he continued. “That, in turn, rules out a whole class of alternatives to dark matter.” 

The examine sparked spirited debate and hypothesis, which motivated van Dokkum and his colleagues to collect extra information with a view to attempt to clarify this vital cosmic puzzle. The workforce has now returned with an much more thrilling discovery: It seems that DF2 is a component of a complete path of dark-matter-free galaxies—together with one referred to as DF4, reported in 2019—which will have been fashioned in “a near head-on collision between two gas-rich galaxies” that occurred eight billion years in the past and straight-up shook the darkish matter out them, in accordance with a examine revealed on Wednesday in Nature

“We kept getting new data and kept checking what we had done,” stated van Dokkum, who additionally led the brand new examine. “So here we are: We have not two, but ten galaxies without dark matter,” which he referred to as a “super-exciting explanation.”  

Van Dokkum credit University of Oxford astronomer Joseph Silk for initially fleshing out this situation, named a “bullet-dwarf collision,” in a mannequin revealed in 2019. The identify derives from the Bullet Cluster, a pair of colliding galaxy clusters that’s thought to be extraordinarily vital for understanding darkish matter. 

This crash, which is situated almost 4 billion gentle years from Earth, has separated the enigmatic darkish matter within the clusters from the unusual matter that makes up extra acquainted objects, equivalent to stars and gasoline. This separation happens as a result of the gaseous parts of the cluster, that are made of standard matter, exert stress on one another as they collide, inflicting them to halt on the collision entrance, similar to a fender bender. 

Dark matter, in distinction, seems to solely work together gravitationally with common matter, which implies it retains proper on transferring throughout a high-speed collision. Voila: Dark matter and gasoline within the cluster appear to separate like oil and water. 

The Bullet Cluster was as soon as seen as an completely distinctive laboratory for finding out darkish matter in relative isolation. But now, van Dokkum and his colleagues assume they’ve witnessed a really comparable course of taking part in out on a a lot smaller scale on this path of galaxies that features DF2 and DF4, which is situated about 72 million gentle years from Earth. 

In addition to drawing on Silk’s mannequin and publicly accessible imaging information, Van Dokkum stated he was impressed by the tv present House, wherein Hugh Laurie’s physician character makes use of a method referred to as “differential diagnosis” to single out the illnesses of his sufferers. In a lot the identical manner, the workforce wrote all of their information to attempt to give you an overarching rationalization that match all of the items.

Van Dokkum initially started with the idea that it will be manner an excessive amount of of a coincidence for DF2 and DF4—two unprecedented dark-matter-free galaxies in the identical cosmic area—to be unrelated. The researchers then famous that the galaxies have been, in reality, spinning away from one another at excessive speeds. 

By rewinding their trajectories in time, the workforce realized that DF2 and DF4 might have been solid in an historic collision that will have spilled out a path of comparable dark-matter-free objects. This line of galaxies would, in flip, be bookended by the remnants of the galaxies that crashed eight billion years in the past, which since turned fully dominated by the darkish matter from the unique bullet-dwarf collision—and that’s precisely what the researchers assume photos of the system reveal.

“If there are two galaxies that are on this particular axis and still spinning away from each other, there must be other pieces as well, in particular the two galaxies at the edges of the trail,” van Dokkum defined. “Looking at that distribution of galaxies in the field, and then seeing this line jump out at you, was a huge moment. It’s like: ‘Oh wow, this is actually real.”

Indeed, van Dokkum’s workforce has pinpointed what is probably going the ultra-faint silhouette of one in all these dark-matter-dominated progenitor galaxies. They hope to seek for the opposite progenitor, and examine the entire system additional, with next-generation observatories, together with the not too long ago launched James Webb Space Telescope. This effort couldn’t solely verify this tentative rationalization for the bizarre galaxies, it might streamline the seek for hypothetical darkish matter particles, which is without doubt one of the most vital questions in science.

“If we can pinpoint where dark matter is, versus the stars in those galaxies at the edge, then we have really, really major results,” van Dokkum stated. “Then we know how the dark matter behaves and what kind of particle to look for.”   


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