Galaxies eject matter more violently than expected
A new analysis reveals that supermassive black holes in galaxies fling out baryonic matter far more aggressively than previously thought, explaining the long-missing cosmic gas. Researchers at the University of California, Berkeley combined observations of the cosmic microwave background to map how this ordinary matter diverges from dark matter. The findings highlight the powerful role of black holes in shaping the universe's mass distribution.
For years, cosmologists have puzzled over the whereabouts of baryonic matter, the ordinary stuff made of particles like protons and neutrons that forms stars, planets, and gas. While dark matter dominates the universe, baryons seemed partially absent, with only a few percent locked in stars and the rest as diffuse, hard-to-detect gas.
Boryana Hadzhiyska at the University of California, Berkeley, and her team addressed this by analyzing the cosmic microwave background—the afterglow of the big bang. They examined how baryon matter casts shadows on this radiation and how gravitational fields from massive objects distort it. This allowed them to pinpoint where baryonic matter clings to dark matter halos and where it separates, both inside galaxies and across intergalactic space.
The results show baryonic matter is more spread out than dark matter, suggesting supermassive black holes at galaxy centers eject it with unexpected violence. "Matter consists of dark matter, which is the predominant component, and baryonic matter or, essentially, gas. For that gas, only about a few percent is in the form of stars, and the rest of it is in the form of diffuse gas," Hadzhiyska explained.
Experts praise the work for clarifying black hole influences. "Understanding exactly how this process happens and how strong it is, so how much of the matter can actually get ejected from a given galaxy has [so far] remained extremely uncertain," said Colin Hill at Columbia University. "It gives us a complementary probe to understand the role of supermassive black holes in moving gas around galaxies," added Alex Krolewski at the University of Waterloo.
These insights could resolve debates on the universe's clumpiness, where gravity bunches matter. The team plans to incorporate more data, like fast radio bursts passing through the gas. "An even better 'baryon census' with fewer uncertainties is still necessary," noted Michael Shull at the University of Colorado Boulder. Hadzhiyska hopes it might reveal deviations from standard cosmology, particularly in dark matter behavior. The study appears in Physical Review D (DOI: 10.1103/kclp-x5j1) and arXiv (DOI: 10.48550/arXiv.2507.14136).