The findings come from a new study published in The Astrophysical Journal Letters on 2026 August 18 by researchers Samuel McCarty, Liam Connor, and Ralf M. Konietzka from the Center for Astrophysics at Harvard and Smithsonian. By analyzing a sample of five local Universe fast radio bursts, the team successfully constrained the properties of the circumgalactic medium in host galaxies spanning a wide range of masses, from roughly 10^11 to 10^13 solar masses.
Probing the Circumgalactic Medium with Fast Radio Bursts
The circumgalactic medium acts as a critical interface between a galaxy and its cosmological environment. It plays a key role in the baryon cycle, controlling the growth of the galaxy through accretion from the intergalactic medium, feedback processes, and recycling. Understanding how cosmic baryons are distributed in this diffuse gas is a major limiting factor in modern cosmology and galaxy formation.
Traditional probes of halo gas, such as X-ray detection or the Sunyaev-Zeldovich effect, have significant drawbacks. They struggle to detect low- to intermediate-mass halos and rely on assumptions about gas temperature and metallicity. The kinetic Sunyaev-Zeldovich signal is intrinsically faint, meaning it has only been measured for galaxy group-scale and larger halos. Quasar absorption spectroscopy also depends on assumptions about the ionization state and structure of the gas.
Fast radio bursts, however, have emerged as a promising new method for studying this gas. These millisecond radio transients emit signals that are dispersed as they travel through cosmic plasma. The dispersion measure of these bursts provides a clean, direct probe of the electron column density along the line of sight, bypassing many of the assumptions required by older methods. The Milky Way circumgalactic medium is an obvious target for such studies, but recent works have only placed upper limits on its integrated dispersion measure. Utilizing larger samples of fast radio bursts will enable a more comprehensive characterization and may allow for the detection of anisotropies.
Evidence of Strong AGN Feedback
In the local Universe, the contribution to the dispersion measure from the intergalactic medium and intervening halos is subdominant. This allowed the researchers to study the circumgalactic medium of the host galaxies directly, provided the host interstellar medium could be sufficiently suppressed. The team selected five fast radio bursts with indicators that their host interstellar medium dispersion measure was negligible.
One of these sources, hosted by a massive elliptical galaxy, yielded striking results. The data strongly disfavors low feedback scenarios in simulations. The galaxy has been evacuated of its baryons, retaining only about 10 percent of the cosmological average within its virial radius. This massive elliptical galaxy shows clear evidence of a past episode of active galactic nucleus activity. The findings are consistent with the picture of strong AGN feedback in galaxy group-scale halos, where energy from a supermassive black hole expels gas from the galaxy, preventing it from cooling and forming new stars.
Implications for the Baryon Cycle and Galaxy Formation
The relative amount of baryons in different components of diffuse gas has been debated for decades, often referred to as the missing baryons problem. By using fast radio bursts to measure the gas mass in these halos, the researchers provided new insights into this long-standing mystery.
While the massive elliptical galaxy was largely evacuated of its baryons, the gas mass measurements for the other sources in the sample tentatively support more baryon retention in L* galaxies compared to group-scale halos. L* galaxies, which are typical spiral galaxies like the Milky Way, appear to hold onto a larger fraction of their cosmic gas. This distinction helps refine cosmological simulations, which must balance the accretion of new gas with the expulsion of existing gas through feedback mechanisms.
The suppression of the matter power spectrum due to baryonic effects on small scales remains a leading source of uncertainty in upcoming Stage IV weak lensing surveys by the Euclid satellite, the Vera C. Rubin Observatory, and the Nancy Roman Space Telescope. Better measurements of the circumgalactic medium directly improve our understanding of dark matter and dark energy, complicating the models used to interpret these massive cosmological surveys.
What Happens Next
The study demonstrates that a large sample of local Universe fast radio bursts will enable precision measurements of halo gas. As radio observatories continue to discover more of these transients, astronomers will be able to map the circumgalactic medium with unprecedented detail. Future research will likely focus on expanding the sample size of local Universe fast radio bursts to confirm the tentative findings regarding baryon retention in L* galaxies.
Additionally, targeting specific galaxy groups and clusters will help clarify the role of AGN feedback across different halo masses. Upcoming telescopes will be crucial in providing the volume of data needed to turn these preliminary measurements into a comprehensive map of the cosmic baryon cycle. By moving beyond upper limits and theoretical constraints, astronomers can use fast radio bursts to unlock the secrets of galaxy evolution.
— Priya Nair, science desk, AXO News