A new study by University of California, San Diego computational astrophysicist Ethan Nadler shows that star formation can occur in halos down to 10 million solar masses through molecular hydrogen cooling.
Nadler calculated the fraction of dark matter halos that exceeds a critical mass required for star formation. Image credit: Xiaodian Chen.
Every galaxy is thought to form at the center of a dark matter halo — a region of gravitationally bound matter that extends far beyond the visible boundaries of a galaxy.
Stars are formed when gravity within dark matter halos draws in gas, but astrophysicists don’t yet know whether star-free dark matter halos exist.
“What is the halo mass threshold for galaxy formation?” Dr. Nadler said.
“This question underlies key areas of research in galaxy formation and cosmology, including when and how the first galaxies formed, which galaxies drive cosmic reionization, and whether ‘dark’ (galaxy-free) dark matter halos exist.”
“Robust predictions for the galaxy formation threshold are critical given upcoming observations of faint galaxies and low-mass halos throughout cosmic history.”
In his new study, Dr. Nadler calculated the mass below which halos fail to form stars.
His work was done using analytic predictions from galaxy formation theory and cosmological simulations.
“Historically, our understanding of dark matter has been linked to its behavior in galaxies,” Dr. Nadler said.
“A detection of completely dark halos would open up a new window to study the Universe.”
Previously, this threshold for star formation was thought to be between 100 million to 1 billion solar masses due to cooling of atomic hydrogen gas.
The current study shows that star formation can occur in halos down to 10 million solar masses through molecular hydrogen cooling.
“With the Rubin Observatory coming online later this year and Webb already making unprecedented observations of our Universe, there will soon be new data to test these predictions and reveal whether completely dark halos exist,” Dr. Nadler said.
“This may have far-reaching consequences for cosmology and the nature of dark matter.”
The study appears in the Astrophysical Journal Letters.
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Ethan O. Nadler. 2025. The Impact of Molecular Hydrogen Cooling on the Galaxy Formation Threshold. ApJL 983, L23; doi: 10.3847/2041-8213/adbc6e
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