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Cross-correlating galaxies and cosmic dispersion measures: Constraints on the gas-to-halo mass relation from 2MASS galaxies and 133 localized fast radio bursts

By cross-correlating 2MASS galaxies with 133 localized fast radio bursts and finding a null signal inconsistent with IllustrisTNG-300 predictions, this study constrains the hot-gas mass fraction in $10^{12-13}\, M_\odot$ halos to be below $\sim 10\%$ of the global baryon fraction, thereby demonstrating the power of galaxy-FRB correlations as a direct probe of feedback processes in galaxy formation.

Original authors: Masato Shirasaki, Ryuichi Takahashi, Ken Osato, Kunihito Ioka

Published 2026-06-24

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CC BY 4.0

Original authors: Masato Shirasaki, Ryuichi Takahashi, Ken Osato, Kunihito Ioka

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Listening to the Universe's "Static"

Imagine the universe is filled with a giant, invisible fog made of free-floating electrons. This fog is everywhere, even in the empty space between galaxies.

Fast Radio Bursts (FRBs) are like cosmic lighthouses. They are incredibly bright, short flashes of radio waves coming from deep space. As these flashes travel toward Earth, they have to pass through that invisible electron fog. The fog slows down the radio waves slightly, and the lower the frequency of the wave, the more it gets slowed down. This creates a "delay" or a "stretch" in the signal, similar to how a heavy backpack makes a runner slow down more than a light one.

Astronomers call this delay the Dispersion Measure (DM). By measuring how much the signal is stretched, scientists can calculate how much electron fog the signal passed through.

The Experiment: Connecting the Dots

The scientists in this paper wanted to answer a specific question: Where is this electron fog hiding?

They knew that galaxies are like islands in a sea of dark matter. They suspected that the electron fog might be trapped inside the "halos" (the gravitational bubbles) surrounding these galaxies. To test this, they performed a massive cross-reference check:

The Map: They used a catalog of 43,000 nearby galaxies (from the 2MASS survey) as their "islands."
The Messengers: They used 133 localized Fast Radio Bursts (FRBs) as their "messengers" flying past these islands.
The Method: They looked at the FRBs that were behind the galaxies. They asked: "Does the FRB signal get more stretched (more fog) when it flies close to a galaxy compared to when it flies far away?"

Think of it like standing in a field with a friend holding a flashlight. If you stand close to a tree, you might see more dust motes dancing in the light beam than if you stand far away from the tree. The scientists were checking if the "dust" (electrons) was clustering around the "trees" (galaxies).

The Surprise: The Simulation Was Wrong

The scientists compared their real-world data against a super-computer simulation called IllustrisTNG-300. This simulation is like a very detailed video game of the universe that tries to predict how galaxies and gas form.

The Result:

The Prediction: The computer simulation said, "If you look close to a galaxy, you should see a lot of electron fog."
The Reality: The actual data showed nothing. The signal was flat. There was no extra fog detected near the galaxies.

It's as if the video game predicted that every tree in the forest was covered in thick moss, but when the scientists went to the forest, the trees were bare.

What This Means: The "Feedback" Problem

Why was the simulation wrong? The paper suggests that the simulation has too much "hot gas" (the electron fog) trapped inside the halos of medium-sized galaxies (those with masses between $10^{12}$ and $10^{13}$ times the mass of our Sun).

In the real universe, something must be kicking that gas out.

The Analogy: Imagine a party in a house (the galaxy). The simulation thinks the party is so wild that the guests (gas) are stuck inside the house. But in reality, the "hosts" (supernovae explosions or black holes) are so loud and energetic that they are blowing the guests out the door and into the street.
The Conclusion: The "feedback" (the force pushing gas out) in these galaxies is much stronger than the computer simulation thought. The scientists estimate that the hot gas inside these galaxy halos makes up less than 10% of the total normal matter (baryons) available. The simulation predicted it should be much higher.

Why This Matters

This study is a new way to weigh the invisible gas in the universe.

Old ways: Scientists usually try to measure this gas using X-rays (like taking an X-ray of a bone) or by looking at how the gas distorts light (the Sunyaev-Zel'dovich effect). These methods often rely on guessing the temperature of the gas.
This new way: This method counts the electrons directly by how they delay radio waves. It doesn't need to know the temperature; it just counts the "stuff."

Summary

The paper is a detective story where astronomers used Fast Radio Bursts to check the "weight" of gas around galaxies. They found that the gas is much lighter than the best computer simulations predicted. This tells us that nature has a more powerful "wind" (feedback) that blows gas out of galaxies than we previously thought, and our computer models need to be updated to reflect this stronger force.

Technical Summary: Cross-correlating Galaxies and Cosmic Dispersion Measures

Problem and Motivation
Understanding the distribution of baryonic matter, particularly hot gas within dark matter halos, remains a critical challenge in cosmology. While hydrodynamical simulations (e.g., IllustrisTNG) predict specific gas-to-halo mass relations, these models rely on subgrid recipes for feedback processes that must be validated against observations. Current constraints often rely on X-ray emission or Sunyaev-Zel'dovich (SZ) effects, which can be limited by assumptions regarding electron temperature or contamination from point sources. Fast Radio Bursts (FRBs) offer a unique probe of free electrons via their dispersion measures (DM). However, utilizing FRBs to constrain the gas content of specific halo mass ranges requires precise localization and robust separation of the cosmic DM component from contributions by the Milky Way and the FRB host galaxy. This paper addresses the need for a direct, temperature-independent constraint on the hot-gas mass fraction in halos hosting low-redshift galaxies by cross-correlating a galaxy catalog with localized FRBs.

Methodology
The authors perform a cross-correlation analysis between the Two Micron All Sky Survey (2MASS) galaxy catalog and a sample of 133 localized FRBs (compiled in Takahashi et al. 2025).

Datasets: The foreground sample consists of 43,533 2MASS galaxies with spectroscopic redshifts ( $z \sim 0.03$ ), weighted by redshift completeness. The background sample comprises 133 FRBs with precise host galaxy identifications.
Estimator: A novel real-space cross-correlation estimator, $\hat{\xi}_{gd}(R)$ $\hat{ξ}_{g d} (R)$ , is introduced to measure the correlation between galaxy overdensity and FRB dispersion measures as a function of comoving separation $R$ $R$ .
- Background Selection: To isolate the intergalactic medium (IGM) and circumgalactic medium (CGM) at the redshift of the foreground galaxies, the authors select FRBs with redshifts $z_s \geq z_g + \Delta z$ (where $\Delta z = 0.05$ ).
- Residual DM: The estimator uses a residual dispersion measure $\Delta DM$ , subtracting the Milky Way contribution and the mean cosmic background, but intentionally leaving the host-galaxy component un-subtracted to avoid model dependence.
- Systematics Control: A "boost factor" is calculated to verify that the background selection is not compromised by scattering effects or misidentification of host galaxies. The analysis employs a jackknife method (40 subregions) to estimate the covariance matrix and statistical uncertainties.
Theoretical Modeling: Predictions are generated using a halo-model framework (Cooray & Sheth 2002).
- Galaxy Distribution: The Halo Occupation Distribution (HOD) of 2MASS galaxies is constrained by Ando et al. (2018).
- Electron Density: The free-electron density profiles are initially drawn from the IllustrisTNG-300 (TNG300) hydrodynamical simulation.
- Phenomenological Modification: To test the robustness of the TNG300 prediction, a phenomenological cutoff function ( $\tanh(M_{500}/M_{cut})$ ) is applied to the gas mass in the model to reduce contributions from lower-mass halos.

Key Results

Null Detection: The cross-correlation signal $\xi_{gd}(R)$ is measured over the range $0.01 < R [h^{-1}\text{Mpc}] < 1$ . The measurements are consistent with a null detection (signal-to-noise ratio $\sim 2\sigma$ ) across this range.
TNG300 Inconsistency: The TNG300-based prediction significantly overestimates the cross-correlation signal at small scales ( $R \sim 0.01 h^{-1}\text{Mpc}$ ). This discrepancy indicates that the simulation predicts too much hot gas in halos with masses of $10^{12}–10^{13} M_\odot$ .
Constraints on Gas Fraction:
- By introducing a phenomenological cutoff, the authors derive a lower limit on the cutoff mass $M_{cut} > 1 \times 10^{12} M_\odot$ at a 95% confidence level.
- This implies that the hot-gas mass fraction in halos of $10^{12}–10^{13} M_\odot$ must be below $\sim 10\%$ of the global baryon fraction (or $f_{gas,hot} \lesssim 0.03$ ).
- This constraint is consistent with limits derived from X-ray stacking (Popesso et al. 2024) and thermal SZ measurements (Lim et al. 2021), but it provides a direct estimate that does not rely on electron-temperature assumptions.
Model Sensitivity: The analysis demonstrates that the signal at $R \sim 0.01 h^{-1}\text{Mpc}$ is primarily sensitive to the total gas mass fraction in group-sized halos rather than the detailed radial distribution of the electron density.

Significance and Claims
The paper claims that galaxy-FRB cross-correlations offer a powerful, independent probe of baryonic feedback processes in galaxy formation. By utilizing precise redshift information to select background sources, the method effectively isolates the free-electron density around foreground galaxies. The results suggest that current hydrodynamical simulations (specifically TNG300) may require stronger feedback mechanisms to reduce the hot-gas content in halos of $10^{12}–10^{13} M_\odot$ to match observational data. The authors emphasize that while the current sample size ( $\sim 100$ FRBs) limits the statistical precision, the methodology is robust against systematic errors related to host-galaxy contamination and scattering. They conclude that future surveys with larger samples of localized FRBs will enable more precise mapping of the hot gas content in the universe and tighter constraints on the evolution of baryonic feedback.