Bright galaxies put dark matter to the test

Over the past year and a half, the James Webb Space Telescope has provided astonishing images of distant galaxies formed shortly after the Big Bang, giving scientists their first glimpse of the nascent universe. Now a group of astrophysicists has upped the ante: find the smallest, brightest galaxies near the beginning of time, or scientists will have to totally rethink their theories about dark matter.

The team, led by UCLA astrophysicists, performed simulations that track the formation of small galaxies after the Big Bang and included, for the first time, previously overlooked interactions between gas and dark matter. They found that the galaxies created are very small, much brighter and form more quickly than in typical simulations that do not take these interactions into account, instead revealing much fainter galaxies.

Small galaxies, also called dwarf galaxies, are found throughout the universe and are often considered to represent the oldest type of galaxy. Small galaxies are therefore of particular interest to scientists studying the origins of the universe. But the small galaxies they find aren’t always what they think they should find. The planets closest to the Milky Way rotate faster or are not as dense as in the simulations, indicating that the models might have missed something, like these gas-dark matter interactions.

The new research, published in Letters from the astrophysical journal, improves the simulations by adding dark matter interactions with gas and finds that these faint galaxies may have been much brighter than expected early in the universe’s history, when they were just beginning to form. The authors suggest that scientists should try to find small galaxies that are much brighter than expected using telescopes like the Webb Telescope. If they only find weak ones, then some of their ideas about dark matter might be wrong.

Dark matter is a type of hypothetical matter that does not interact with electromagnetism or light. Thus, it is impossible to observe through optics, electricity or magnetism. But dark matter does interact with gravity, and its presence has been inferred from the gravitational effects it exerts on ordinary matter, the element that makes up the entire observable universe. Although 84% of the matter in the universe is thought to be dark matter, it has never been directly detected.

All galaxies are surrounded by a vast halo of dark matter, and scientists believe that dark matter was essential to their formation. The “standard cosmological model” used by astrophysicists to understand galaxy formation describes how clumps of dark matter in the very early universe attracted ordinary matter by gravity, causing stars to form and creating the galaxies we see Today. Since most dark matter particles – called cold dark matter – are thought to move much slower than the speed of light, this accumulation process would have occurred gradually.

But more than 13 billion years ago, before the formation of the first galaxies, ordinary matter, made up of hydrogen and helium gas from the Big Bang, and dark matter were moving relative to each other. ‘other. The gas flowed at supersonic speeds through dense thickets of slower-moving dark matter that should have attracted it to form galaxies.

“Indeed, in models that don’t take streaming into account, this is exactly what happens,” said Claire Williams, a doctoral student at UCLA and first author of the paper. “The gas is attracted by the gravitational pull of dark matter, forms clusters and knots so dense that hydrogen fusion can occur, and thus forms stars like our sun.”

But Williams and co-authors from the Supersonic Project team, a group of astrophysicists from the United States, Italy and Japan led by UCLA physics and astronomy professor Smadar Naoz, have discovered if they added the effect of scattering of different speeds between dark matter and ordinary matter. According to the simulations, the gas landed far from dark matter and could not form stars immediately.

When the accumulated gas fell back into the galaxy millions of years later, a massive explosion of star formation occurred all at once. Because these galaxies for a time had many more young, hot, luminous stars than ordinary small galaxies, they shone much brighter.

“While flux suppressed star formation in smaller galaxies, it also boosted star formation in dwarf galaxies, causing them to eclipse areas of the universe without flux,” Williams said.

“We predict that the Webb telescope will be able to find regions of the universe where galaxies will be brighter, accentuated by this speed. The fact that they are so bright could make it easier for the telescope to discover these small galaxies, which are generally extremely difficult to detect only 375 million years after the Big Bang.”

Because dark matter is impossible to study directly, searching for bright areas of galaxies in the early universe could offer an effective test for theories about dark matter, which have been unsuccessful until now.

“Discovering 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 speed between two types of matter can produce the type of galaxy we are looking for.” said Naoz, the Howard and Astrid Preston Professor of Astrophysics. “If dark matter does not behave like standard cold dark matter and the flow effect is not present, then these bright dwarf galaxies will not be found and we have to go back to the drawing board.”