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A gas-rich cosmic web revealed by the partitioning of the missing baryons

Nature Astronomy volume 9, pages 1226–1239 (2025)Cite this article

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Abstract

Approximately half of the Universe’s dark matter resides in collapsed halos; significantly less than half of the baryonic matter (protons and neutrons) remains confined to halos. A small fraction of baryons are in stars and the interstellar medium within galaxies. The majority are diffuse (<10−3 cm−3) and ionized (neutral fraction <10−4), located in the intergalactic medium (IGM) and in the halos of galaxy clusters, groups and galaxies. This diffuse ionized gas is notoriously difficult to measure, but has wide implications for galaxy formation, astrophysical feedback and precision cosmology. Recently, the dispersion of extragalactic fast radio bursts (FRBs) has been used to measure the total content of cosmic baryons. Here we present a large cosmological sample of FRB sources localized to their host galaxies. We have robustly partitioned the missing baryons into the IGM, galaxy clusters and galaxies, providing a late-Universe measurement of the cosmic baryon abundance, \({\varOmega }_{{\mathrm{b}}}{h}_{70}=0.05{1}_{-0.006}^{+0.006}\), where Ωb is the baryon density parameter and h70 is the scaled Hubble constant. Our results indicate efficient feedback processes that can deplete galaxy halos and enrich the IGM (total baryon fraction in the IGM is \({f}_{{\rm{IGM}}}=0.7{6}_{-0.11}^{+0.10}\)), agreeing with the baryon-rich cosmic web scenario seen in cosmological simulations. Our results may reduce the ‘S8 tension’ in cosmology, as strong feedback leads to suppression of the matter power spectrum.

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Fig. 1: A full account and partition of the missing baryons.
Fig. 2: An MCMC fit of extragalactic gas parameters to a sample of localized FRBs.
Fig. 3: The extragalactic DM distribution of FRBs prefers a gas-rich IGM.
Fig. 4: A 10% measurement of the present-day baryon parameter.

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Data availability

The FRB data presented here are publicly available in a CSV file (Supplementary Data 1) and at the following link: https://github.com/liamconnor/frb_baryon_connor2024/blob/main/data/frbsample_connor0924.csv.

Code availability

We have created a reproduction package for our work that includes all code used for our data analysis and the production of each figure. We have placed this code on GitHub at https://github.com/liamconnor/frb_baryon_connor2024.

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Acknowledgements

We thank F. Walter, X. Prochaska and M. Oei for informative conversations. We also thank D. Nelson and C. Walker for their considerable help with IllustrisTNG.

Author information

Authors and Affiliations

  1. Center for Astrophysics ∣ Harvard & Smithsonian, Cambridge, MA, USA

    Liam Connor & Ralf M. Konietzka

  2. Cahill Center for Astronomy and Astrophysics, MC 249-17, California Institute of Technology, Pasadena, CA, USA

    Liam Connor, Vikram Ravi, Kritti Sharma, Stella Koch Ocker, Jakob Faber, Gregg Hallinan, Charlie Harnach, Greg Hellbourg, Rick Hobbs, David Hodge, Mark Hodges, Nikita Kosogorov, James Lamb, Casey Law, Paul Rasmussen, Myles Sherman, Jean Somalwar, Sander Weinreb & David Woody

  3. Owens Valley Radio Observatory, California Institute of Technology, Big Pine, CA, USA

    Vikram Ravi, Gregg Hallinan, Charlie Harnach, Greg Hellbourg, Rick Hobbs, David Hodge, Mark Hodges, Nikita Kosogorov, James Lamb, Casey Law, Paul Rasmussen & David Woody

  4. The Observatories of the Carnegie Institution for Science, Pasadena, CA, USA

    Stella Koch Ocker

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Contributions

V.R. and G. Hallinan led the development of the DSA-110. D.H., M.H., J.L., P.R., S.W. and D.W. contributed to the construction of the DSA-110. L.C. conceived of and performed the analysis techniques for studying the FRB sample, as well as the multiwavelength baryon analysis. L.C. led the writing of the paper, with assistance from all co-authors. K.S., V.R., L.C., C.L., J.S., J.F., N.K. and M.S. all conducted the optical/infrared follow-up observations presented in this work. K.S. and V.R. undertook the majority of the optical/infrared host-galaxy data analysis and interpretation. V.R., C.L., L.C., G. Hellbourg and R.H. developed the software pipeline for detecting FRBs on the DSA-110. R.M.K. led the investigation of ray tracing in the IllustrisTNG simulation.

Corresponding author

Correspondence to Liam Connor.

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The authors declare no competing interests.

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Nature Astronomy thanks Q. Wu and the other, anonymous, reviewers(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Optical/IR images of nine DSA-110 discovered FRB fields.

This subset of sources are the FRBs that did not make it into our companion paper’s host galaxy analysis, either because they were discovered after November 2023 or because their host did not meet the r ~ 23.5 mag criteria. Each image is centred on the host galaxy candidate preferred by PATH, identified by cyan cross-hairs6. The 90% confidence FRB localization regions are marked as red ellipses. All images were smoothed with a Gaussian kernel of σ = 0″. 2 – 0″. 4 to improve visibility.

Extended Data Fig. 2 The DM and total baryon distributions in IllustrisTNG.

Panels a-d show Macquart Relations for different components of diffuse cosmic gas from a mock FRB survey10. We plot \({{\rm{DM}}}_{{\rm{ex}}/{z}_{\rm s}}\) for intervening halos (green, panel a), the IGM (grey, b), and total DM (black, c). The solid curves are the mean extragalactic DM at that redshift. Panel d shows the total extragalactic DM distribution after including a log-normal host DM distribution, where the heatmap is the log probability and the redpoints are the actual samples. Panel e is a radial treemap showing the partition of baryons in TNG300-1.

Extended Data Fig. 3 Examples of cosmic DM distributions for different redshifts and cosmic baryon parameters.

The two upper rows show the 2D P(DMIGM, DMX) distribution, which is the relative probability in DMIGM and DMX for FRBs at zs = 0.5 (top) and zs = 1.0 (middle) for increasing values of the halo gas fraction, fX. We use a bivariate log-normal distribution that has been calibrated to mock FRB surveys in the Illustris TNG300-1 simulation. The distribution ensures that DMIGM and DMX are correlated, since sightlines that pass through halos are more likely to pass through overdensities in the cosmic web and have large DMIGM. In order to compute our final likelihood function, the 2D distribution shown in the top two rows must be integrated along curves of constant cosmic DM, \({{\rm{DM}}}_{\cos }={{\rm{DM}}}_{{\rm{IGM}}}+{{\rm{DM}}}_{{\rm{X}}}\). The resulting \(P({{\rm{DM}}}_{{\rm{ex}}}| {z}_{\rm s},\overrightarrow{\theta })\) are show in the bottom row, with μhost = 4.5 and σhost = 0.7. The bottom left and bottom right likelihoods are not supported by our data.

Extended Data Fig. 4 A direct measurement of the diffuse gas content.

The value of \({{\rm{DM}}}_{\cos }={{\rm{DM}}}_{{\rm{IGM}}}+{{\rm{DM}}}_{{\rm{X}}}\) averaged over a large number of sightlines is a direct measure of the cosmic density of diffuse, ionized baryonic matter, fd Ωb. We show the results from two different methods for estimating the ionized fraction of baryons, fd. The corner plot in panel a shows pairwise posteriors from an MCMC fit, which produces \({f}_{d}=0.9{3}_{-0.05}^{+0.04}\) after marginalizing over the host DM parameters. In panel b we plot the diffuse fraction of baryons as a function of redshift for our sample of localized FRBs with 1σ error bars (red markers). Over-plotted as black points are the zs > 0.2 sample we use to compute a weighted average of fd. The right side shows the probability P(fd) from kernel density estimation (KDE), where the red distribution is for the whole sample.

Extended Data Fig. 5 The matter budget in massive halos.

We plot the fraction of matter bound in massive groups and galaxy clusters above a given mass, defined here as M500. The left vertical axis shows the fraction of cosmic baryons within r500 of halos with mass greater than M500. The right vertical axis shows the total matter bound in halos. The divergence at low masses is due to the apparent scarcity of baryons in smaller halos.

Supplementary information

Supplementary Information (download PDF )

Supplementary Figs. 1 and 2, Discussion on ‘comparison with simulations’, and Table 1 with extra information on the optical FRB host galaxies.

Supplementary Data 1 (download CSV )

Properties of the FRBs used in this work.

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Connor, L., Ravi, V., Sharma, K. et al. A gas-rich cosmic web revealed by the partitioning of the missing baryons. Nat Astron 9, 1226–1239 (2025). https://doi.org/10.1038/s41550-025-02566-y

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