What is it about galaxies and dark matter? Most, if not all galaxies are surrounded by halos of this mysterious, unknown, but ubiquitous material. And, it also played a role in galaxy formation. The nature of that role is something astronomers are still figuring out. Today, they’re searching the infant Universe, looking for the tiniest, brightest galaxies. That’s because they could help tell the tale of dark matter’s role in galactic creation.
An international team of astronomers led by UCLA’s Smadar Naoz is doing simulations of early galaxy formation. Their computer programs track 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 neglected interactions between dark matter and the primordial “stuff” 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 didn’t include those motions. Now astronomers just need to find them, using JWST, in an effort to see if their theories of dark matter hold up.
How would interactions between baryonic matter and dark matter make a difference? Here’s one likely story: in the early Universe, clouds of gas moved at supersonic speeds past clumps of dark matter. It bounced off the dark matter. Eventually, after millions of years, the gaseous material fell back together to form stars in a blast of star birth. The team’s simulations track the formation of those galaxies right after the Big Bang.
A composite model of matter distribution in the Universe (with dark matter overlay) in a galaxy formation simulation made by the TNG Collaboration.
Naoz’s team thinks that the existence of those smaller, brighter, more distant galaxies could confirm the so-called “cold dark matter” model. It suggests that the Universe was in a hot dense state containing only gases after the Big Bang. Over time, it evolved to a lumpy distribution of galaxies (and eventually galaxy clusters). Along the way, stars and galaxies formed, but the earliest steps likely depend on gravitational interaction with dark matter. If the supersonic interactions that Naoz’s team modeled actually happened, then those little galaxies would be the result.
JWST has seen some pretty early galaxies during its time in operation. It hasn’t detected the very earliest ones—yet. However, the images it HAS provided are tantalizing hints at what might exist in earlier epochs and could provide insight into the role of dark matter. So, it makes sense that astronomers want to push its view 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 conception of starbursting galaxies in the early universe. Stars and galaxies are shown in the bright white points of light, while dark matter and gas are shown in purples and reds. Early gas clouds bounced past dark matter clumps, only to clump together again under dark matter’s gravity, sparking off 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 kinds of matter can produce the type of galaxy we’re looking for,” said Naoz. “If dark matter does not behave like standard cold dark matter and the streaming effect isn’t present, then these bright dwarf galaxies won’t be found and we need to go back to the drawing board.”
In a paper by the team member and first author Claire Williams (published in Astrophysical Journal Letters) the team suggests that scientists using JWST begin to look for galaxies that are much brighter than expected. If they exist, that will likely prove the interactions occurred early in cosmic time. If none can be found, then maybe scientists still might not understand dark matter interactions. The big question to answer is, if they exist, then how did they form so quickly and why are they so bright?
Let’s examine that 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, that caused stars to form, followed by galaxies. Dark matter is thought to move more slowly than light. So, astronomers predicted that the star- and galaxy-formation processes happened very gradually. At least, that’s what earlier simulations suggest.
But, what if something else was going on more than 13 billion years ago? How would that change things? It was a time before the first galaxies formed. But, it was a time when ordinary matter in the form of large overdensities of hydrogen and helium gas streamed through the expanding Universe. It bounced off slower-moving clumps of dark matter and outran its gravitational pull, at least for a time. Then, the baryonic matter massed together again, under the influence of dark matter. That’s when the star birth fireworks 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 that dark matter plays in early cosmic history.
“While the streaming suppressed star formation in the smallest galaxies, it also boosted star formation in dwarf galaxies, causing them to outshine the non-streaming patches of the universe,” Williams said. Essentially, the accumulated gas began to fall together after millions of years. That led to a huge burst of star formation. Lots of massive hot, young stars began to shine, out-brilliancing the stars in other small galaxies. Ultimately what this means is that since dark matter is impossible to “see”, those brightly shining patches of galaxies could be indirect evidence of its existence. And, they’d prove the role dark matter played in the creation of galaxies.
Nobody’s seen exactly what Naoz and the team are looking for—yet. Once they do, it will go a long way toward providing insight 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 kinds of matter can produce the type of galaxy we’re looking for,” said Naoz.
Of course, JWST is a perfect telescope to help see these galaxies. It should be able to peer into regions of the Universe where tiny infant galaxies are brighter than astronomers expect them to be. That extreme luminosity will help JWST spot them, showing them as they looked at a time when the Universe was only a few hundred million years old. Because dark matter is impossible to study directly, searching for those bright patches of baby galaxies in the early Universe could offer an effective test for theories about dark matter and its role in shaping the first galaxies.
Bright Galaxies Put Dark Matter to the Test
The Supersonic Project: Lighting Up the Faint End of the JWST UV Luminosity Function
