Galaxies and groups of galaxies exist in dark-matter haloes filled with diffuse gas. The diffuse gas represents up to 80% of the mass in baryonic matter within the haloes1,2, but is difficult to detect because of its low density (particle number densities of ≲10−4 cm−3) and high temperature (mostly >106 K). Here we analyse the impact of diffuse gas associated with nearby galaxies using the dispersion measures (DMs) of extragalactic fast radio bursts (FRBs). FRB DMs provide direct measurements of the total ionized-gas contents along their sightlines. Out of a sample of 474 distant FRBs from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB Catalog 1 (ref. 3), we identify a subset of events that probably intersect the dark-matter haloes of galaxies in the local Universe (<40 Mpc). The mean DM of the galaxy-intersecting FRBs is larger than that of the non-intersecting FRBs with a probability >0.99 and the excess DM is >90 pc cm−3 with >95% confidence. The excess is larger than expected for the diffuse gas surrounding isolated galaxies, but may be explained by additional contributions from gas surrounding galaxy groups, including from the Local Group. This result demonstrates the predicted ability of FRBs to be used as sensitive, model-independent measures of the diffuse gas contents of dark-matter haloes4,5,6,7.
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All the data used in these analyses are publicly available. CHIME/FRB Catalog 1 can be found at https://www.chime-frb.ca/catalog. The GWGC can be downloaded at http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=GWGC.
This research made use of the open-source hmf, NFW, CHIME/FRB’s cfod package and frb (https://github.com/FRBs/FRB/tree/main/frb) python packages. All custom code used in our analysis is available from the corresponding author upon request.
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We thank C. Hummels, W. Lu, J. M. Shull and the Caltech FRB group for helpful discussions. We also thank C. Leung, K. Masui and M. Bhardwaj for valuable comments on the manuscript. This research was partially supported by the National Science Foundation under grant number AST-1836018.
The authors declare no competing interests.
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Extended Data Fig. 1 A simulation of CHIME/FRB DMs to compare the statistical power and appropriateness of three tests: Student’s one-sided t-test, our jackknife test, and the Kolmogorov-Smirnov (KS) test.
We simulate 474 FRB DMs with a similar distribution to the CHIME/FRB sources. In the top row, we have not added any excess DM to the galaxy-intersecting sources. In the bottom row, excess DM has been added to the 25 simulated FRBs that intersect a foreground galaxy, with a normal distribution of mean 150 pc cm−3 and standard deviation 50 pc cm−3. From the top row, we see that none of the tests produces spurious low p-values and their p-values are uniform as expected. The bottom right panel demonstrates that the KS-test is less sensitive to DM offsets than the t-test and the non-parametric jackknife test. The t-test and jackknife tests are one-sided, in that they explicitly look for a positive mean DM difference, whereas the KS test measures if the two samples were drawn from different distributions and does not make that distinction.
Extended Data Fig. 2 Predicted DM excesses for CHIME/FRB galaxy intersections assuming isolated galaxy halos.
The histograms show the relative binned counts of DMs accrued by the fiducial sample of 26 FRBs with b⊥ < 200 kpc. Two models for the radial density distribution of the CGM are shown: a ‘beta’ model with β = 0.5, and a modified NFW model. Results are shown in blue and orange respectively. The mean DMs of the data shown in each histogram are noted in the figure legend.
Top: We use a ‘beta’ model with β = 0.5 and a modified NFW model for the radial density distribution. Results are shown in blue and orange respectively. For each halo mass, 1σ error ranges are shown based on 100 simulations of samples of 26 FRB intersections within the virial radii. The FRB positions relative to the halo centres are simulated using the offsets and CHIME/FRB Catalog 1 position uncertainties of the 26 FRBs in Table 1. The grey shaded area indicates the likely (95% confidence) DM excess. fgas = 1 is assumed. Bottom: Same as top, but with values of fgas specific to each halo mass derived from four cosmological simulations (see text for details). A modified NFW radial-density model is assumed.
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Connor, L., Ravi, V. The observed impact of galaxy halo gas on fast radio bursts. Nat Astron 6, 1035–1042 (2022). https://doi.org/10.1038/s41550-022-01719-7