All galaxies (including our own) are thought to exist inside a giant cosmological structure called a dark matter halo — a cloud-like phenomenon made up of invisible matter that forms a scaffold to the visible universe.
These halos are key to the formation of stars, which are created as gas is pulled into the halos. Cool temperatures cause the gas to clump together. Over time, some of these clumps grow in size until gravity causes them to collapse, creating friction, which in turn creates heat and (eventually) results in the birth of a protostar.
It is not known whether any dark halos are totally devoid of stars. However, new research published in The Astrophysical Journal Letters suggests that there is a mass below which halos would be unable to make stars — and it is much lower than previously thought.
"Historically, our understanding of dark matter has been linked to its behavior in galaxies,” Ethan Nadler, a computational astrophysicist at UC San Diego and author of the paper, said in a press release.
“A detection of completely dark halos would open up a new window to study the universe," Nadler said in the release.
Read More: How Vera C. Rubin Revolutionized Dark Matter
How Small is too Small?
Previous estimates have varied. However, it is typically thought that stars are unable to form in dark halos weighing 100 million to 1 billion solar masses or less, a weight roughly equivalent to 100 million to 1 billion Suns. Yet, according to Nadler’s calculations, it is possible for stars to form in halos as small as 10 million solar masses.
Earlier estimates were based on the idea that the cooling of atomic hydrogen gas was a key part of the process driving star formation. However, Nadler has shown that formation can occur in much smaller galaxies thanks to molecular hydrogen cooling, which, as the name implies, involves hydrogen molecules (H2) rather than the hydrogen atoms (H1) on which these earlier calculations were based.
This, he writes, is exemplified by the abundance of smaller, satellite galaxies surrounding the Milky Way.
Their presence suggests that there are at least some ultrafaint dwarf galaxies — galaxies that have fewer stars and orbit larger galaxies like the Moon orbits the Earth — that are produced via the process of molecular hydrogen cooling.
Nadler’s study used a combination of models that use mathematical principles to predict the formation of galaxies and computer simulations that attempt to “recreate” the evolution of the cosmos.
Observing Dark Matter Halos Through the JWST
These findings suggest that stars can form in halos much smaller than previously thought, a prediction researchers hope to test further as additional data from the Rubin Observatory and the James Webb Space Telescope become available later this year.
Not only could we be one step closer to finding out if completely dark halos really do exist, but the research may also help us discover traces of some of the earliest stars in the universe.
“Looking forward, it is exciting to consider how this work informs searches for the remnants of the first stars,” writes Nadler. “Combining these predictions with stellar evolution models offers a promising avenue to probe the galaxy formation threshold through near-field cosmology, which we look forward to pursuing.”
Read More: The Heart of the Milky Way May Host a New Form of Dark Matter
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
The Astrophysical Journal Letters. The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold
Rosie is a freelance writer living in London. She has covered science and health topics for publications, including IFLScience, Newsweek, and Health.
