March 1, 2024

Webb can directly test a theory for dark matter

What’s with galaxies and dark matter? Most, if not all, galaxies are surrounded by halos of this mysterious, unknown, but omnipresent material. And it also played a role in the formation of galaxies. The nature of this role is something astronomers are still discovering. Today, they are searching the infant Universe, looking for the smallest, brightest galaxies. That’s because they could help tell the story of dark matter’s role in galactic creation.

An international team of astronomers led by Smadar Naoz of UCLA is running simulations of early galaxy formation. Their computer programs record the circumstances of galactic births not long after the Big Bang. These “hot off the press” computer models include some new wrinkles. They take into account previously overlooked interactions between dark matter and the primordial “matter” of the Universe. That would be hydrogen and helium gas. The result of the simulations: tiny, bright galaxies that formed more quickly than in computer models that did not include these movements. Now astronomers just need to find them, using JWST, in an effort to see if their theories about dark matter hold up.

Dark matter interactions with supersonic baryonic matter

How would interactions between baryonic matter and dark matter make a difference? Here’s a likely story: In the early Universe, clouds of gas moved at supersonic speeds, passing through clumps of dark matter. It bounced off the dark matter. Eventually, after millions of years, the gaseous material came back together to form stars in an explosion of star birth. The team’s simulations trace the formation of these galaxies shortly after the Big Bang.

A composite model of matter distribution in the Universe (with dark matter overlay) in a galaxy formation simulation carried out by the TNG Collaboration.

Naoz’s team believes that the existence of these smaller, brighter and more distant galaxies could confirm the so-called “cold dark matter” model. This suggests that the Universe was in a hot, dense state containing only gases after the Big Bang. Over time, it evolved into an irregular distribution of galaxies (and eventually galaxy clusters). Along the way, stars and galaxies were formed, but the first steps likely depend on gravitational interaction with dark matter. If the supersonic interactions modeled by Naoz’s team actually happened, then the result would be these small galaxies.

Simulating galaxy formation and the influence of dark matter

JWST has seen some very old galaxies during its time of operation. He hasn’t detected the first ones – yet. However, the images provided are tantalizing clues about what might have existed in earlier times and could provide insights into the role of dark matter. So it makes sense that astronomers would want to go back in time as far as they can. And that means looking for bright patches of light that existed a few hundred million years after the Big Bang.

Artist's conception of star-forming galaxies in the early universe.  Stars and galaxies are shown in bright white points of light, while more diffuse dark matter and gas are shown in purples and reds.  The first gas clouds passed through clumps of dark matter, only to clump together again under the dark matter's gravity - triggering star formation.  Credit: Aaron M. Geller/Northwestern/CIERA + IT-RCDS
Artist’s conception of star-forming galaxies in the early universe. Stars and galaxies are shown in bright white points of light, while dark matter and gas are shown in purples and reds. The first gas clouds passed through clumps of dark matter, only to clump together again under the dark matter’s gravity, triggering star formation. Credit: Aaron M. Geller/Northwestern/CIERA + IT-RCDS

“The discovery of patches of small, bright galaxies in the early Universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two types of matter can produce the type of galaxy we are looking for,” said Naoz. “If dark matter does not behave like standard cold dark matter and the outflow effect is not present, then these bright dwarf galaxies will not be found and we will have to go back to the drawing board.”

In a paper by team member and first author Claire Williams (published in Astrophysical journal letters), the team suggests that scientists using JWST start looking for galaxies that are much brighter than expected. If they exist, this would likely prove that the interactions occurred at the beginning of cosmic time. If nothing is found, scientists may not yet understand the interactions of dark matter. The big question to be answered is: if they exist, how did they form so quickly and why are they so bright?

Streaming through dark matter corridors

Let’s examine this by looking at the role of dark matter. The standard cosmological model says that the gravitational pull of clumps of dark matter in the early Universe attracted ordinary matter. Eventually, this caused stars to form, followed by galaxies. Dark matter is believed to move more slowly than light. Thus, astronomers predicted that the processes of star and galaxy formation occurred very gradually. At least that’s what previous simulations suggest.

But what if something else was happening more than 13 billion years ago? How would this change things? It was a time before the first galaxies formed. But it was a time when ordinary matter, in the form of large excess densities of hydrogen and helium gas, flowed through the expanding Universe. It bounced off slower-moving clumps of dark matter and overtook their gravitational pull, at least for a while. Then, the baryonic matter came together again, under the influence of dark matter. It was then that the fireworks of star birth began.

This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen from, among other things, excited hydrogen atoms — Lyman-alpha emission.  Scientists look to this and other young galaxies to understand the role dark matter plays in early cosmic history.
This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen from, among other things, excited hydrogen atoms — Lyman-alpha emission. Scientists look to this and other young galaxies to understand the role dark matter plays in early cosmic history.

“Although the flow suppressed star formation in the smallest galaxies, it also boosted star formation in the dwarf galaxies, causing them to outshine the flowless areas of the Universe,” Williams said. Essentially, the accumulated gas began to fall together after millions of years. This led to a huge burst of star formation. Many young, massive stars began to shine, outshining stars in other small galaxies. Ultimately, what this means is that since dark matter is impossible to “see,” these bright patches of galaxies could be indirect evidence of its existence. And they would prove the role that dark matter played in the creation of galaxies.

Finding these bright spots

No one has seen exactly what Naoz and his team are looking for – yet. Once they do, it will be a big step toward providing insights into the role of cold, dark matter. “The discovery of patches of small, bright galaxies in the early Universe would confirm that we are on the right track with the cold dark matter model because only the velocity between two types of matter can produce the type of galaxy we are looking for,” said Naoz.

Of course, the JWST is a perfect telescope to help observe these galaxies. It should be able to observe regions of the Universe where small infant galaxies are brighter than astronomers expect them to be. This extreme luminosity will help JWST locate them, showing them as they were at a time when the Universe was just a few hundred million years old. Since dark matter is impossible to study directly, searching for these bright patches of baby galaxies in the early Universe could offer an effective test for theories about dark matter and its role in the formation of the first galaxies.

For more informations

Bright galaxies put dark matter to the test
The Supersonic Project: Lighting the Weak End of JWST’s UV Brightness Function

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