🔭 astrophysics

The suppression of the matter power spectrum: strong feedback from X-ray gas mass fractions, kSZ effect profiles, and galaxy-galaxy lensing

By jointly analyzing X-ray gas mass fractions and kinetic Sunyaev-Zel'dovich effect profiles within a baryonification framework, this study reveals that baryon feedback suppresses the matter power spectrum by $10 \pm 2\%$ at small scales—a stronger effect than predicted by current hydrodynamical simulations—and demonstrates a multi-wavelength approach to calibrating feedback models for future cosmic shear surveys.

Original authors: Jared Siegel, Leah Bigwood, Alexandra Amon, Jamie McCullough, Masaya Yamamoto, Ian G. McCarthy, Matthieu Schaller, Aurel Schneider, Joop Schaye

Published 2026-06-24

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CC BY 4.0

Original authors: Jared Siegel, Leah Bigwood, Alexandra Amon, Jamie McCullough, Masaya Yamamoto, Ian G. McCarthy, Matthieu Schaller, Aurel Schneider, Joop Schaye

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

Imagine the universe as a giant, invisible sponge made of dark matter. For a long time, scientists thought this sponge was the only thing shaping the structure of the cosmos. But there's another ingredient: "baryons," which are the normal gas and stars we can actually see.

This paper is about how the "normal stuff" (gas and stars) pushes back against the "invisible stuff" (dark matter) and changes the shape of the universe's sponge. Specifically, the authors are trying to figure out how much the normal gas gets kicked out of the cosmic sponge by powerful energy blasts from black holes, and how this kicking changes the way the universe clumps together.

Here is a breakdown of their findings using simple analogies:

1. The Problem: The "Cosmic Blender"

Think of a galaxy cluster as a giant blender. Inside, there is dark matter (the heavy, invisible base) and hot gas (the liquid). When a supermassive black hole at the center of a galaxy wakes up, it acts like a high-powered blender blade, shooting out massive jets of energy.

The Effect: These jets blow the hot gas out of the center of the blender and push it far away.
The Consequence: Because the gas is pushed away, the total "clumpiness" of the universe changes. On small scales (like inside a galaxy cluster), the universe becomes less dense than scientists previously thought. This is called "suppressing the matter power spectrum."

2. The Mystery: "How hard is the blender spinning?"

Scientists have been arguing about how powerful these "blender blades" (black hole feedback) actually are.

The Old Guess: Computer simulations (like FLAMINGO and BAHAMAS) tried to predict this. They guessed the gas gets pushed out a little bit, but not too much.
The Reality Check: The authors wanted to know the real answer by looking at the actual universe, not just computer guesses.

3. The Detective Work: Three Different Magnifying Glasses

To solve this, the team didn't just look at one thing; they combined three different ways of seeing the gas, like using three different types of flashlights to find a lost cat in the dark:

The "Wind" Detector (kSZ Effect): They looked at how the gas moves. As gas rushes away from a galaxy, it leaves a tiny "wind" imprint on the Cosmic Microwave Background (the afterglow of the Big Bang). This tells them how far the gas has been pushed out.
The "Heat" Detector (X-ray Gas): They looked at how much hot gas is left inside the galaxy clusters. If the "blender" is strong, there should be less gas left inside the cluster.
The "Gravity" Detector (Galaxy-Galaxy Lensing): They used the gravity of the clusters to weigh them accurately. This was crucial because you can't know how much gas was blown away unless you know exactly how heavy the cluster was to begin with.

4. The Big Discovery: The Blender is Stronger Than We Thought

When the team combined the data from the "Wind" and "Heat" detectors, they found something surprising:

The Gas is Further Out: The gas has been pushed much further away from the center of galaxy clusters than the computer simulations predicted.
The "Sponge" is Flatter: Because the gas is so spread out, the universe is "smoother" on small scales than we thought.
The Numbers: They calculated that this effect reduces the clumpiness of the universe by about 10% on small scales.

The Conflict:
There was a slight disagreement between their data and a specific older dataset (HSC-XXL).

The kSZ and eROSITA (new X-ray) data said: "The blender is very strong; gas is pushed far away (10% suppression)."
The HSC-XXL (older X-ray) data said: "The blender is moderate; gas isn't pushed as far (5% suppression)."
The Verdict: The authors trust the new, combined data (kSZ + eROSITA) more because it covers a wider range of galaxy sizes and distances. They believe the "strong feedback" (10% suppression) is the correct answer.

5. Why This Matters for the Future

Imagine you are trying to measure the shape of the universe by looking at how light bends around galaxies (a technique called "cosmic shear").

The Problem: If you don't know exactly how the gas is being pushed around, your measurements get blurry. It's like trying to take a sharp photo through a foggy window.
The Solution: This paper provides a "cleaning cloth." By using the new data to calibrate exactly how the gas behaves, future telescopes (like the upcoming LSST) can look at smaller, fuzzier parts of the universe without getting confused.

In short: The authors found that the universe's "gas" is being blown out of galaxy clusters much more aggressively than our computer models predicted. By measuring this directly, they have created a better map for future astronomers to use, allowing them to see the universe's structure with much sharper clarity.

Technical Summary: The Suppression of the Matter Power Spectrum by Baryon Feedback

Problem Statement
Baryon feedback processes, particularly energy injection from active galactic nuclei (AGN), redistribute gas relative to the underlying dark matter distribution. This redistribution suppresses the matter power spectrum on small, non-linear scales by tens of percent compared to dark matter-only predictions. While this effect is a known source of systematic uncertainty for cosmic shear (weak gravitational lensing) analyses, the precise amplitude and scale dependence of the suppression remain uncertain. Current hydrodynamical simulations lack the resolution to treat feedback from first principles, relying instead on "sub-grid" prescriptions that yield markedly different predictions for power suppression. Consequently, weak lensing analyses often adopt conservative scale cuts, discarding valuable small-scale signal, or utilize flexible models that, without external calibration, effectively downweight small scales and degrade cosmological precision.

Methodology
This work constrains the impact of baryon feedback on the matter power spectrum by jointly analyzing multi-wavelength gas observables and galaxy-galaxy lensing (GGL) data. The study employs the baryonification framework, a method that perturbatively displaces particles in dark matter-only N-body simulations according to observationally motivated, spherically symmetric analytic density profiles to mimic feedback effects.

The analysis integrates three primary datasets spanning group and cluster masses ( $13 < \log_{10} M_{500}/M_\odot h^{-1} < 14$ ) at $0 < z < 1$ :

Kinetic Sunyaev–Zel'dovich (kSZ) Effect Profiles: Stacked measurements from SDSS/DESI+ACT (Schaan et al. 2021; Ried Guachalla et al. 2025), which probe the outskirts of halos (beyond several $R_{500}$ ) at higher redshifts.
X-ray Gas Mass Fractions: Measurements from the first eROSITA All-Sky Survey (eRASS1) and the HSC–XXL sample, which probe the inner regions ( $< R_{500}$ ) of low-redshift clusters.
Halo Mass Constraints: Mean halo masses derived from GGL (Siegel et al. 2025; Umetsu et al. 2020) are used to resolve the degeneracy between gas expulsion magnitude and halo mass.

The authors utilize the BCemu emulator (trained on 2,700 baryonified simulations) to map between the baryonification model parameters (governing gas density profiles) and the suppression of the matter power spectrum. The model is validated through injection-recovery tests using the FLAMINGO hydrodynamical simulation suite, demonstrating the framework's ability to accurately predict power suppression from gas observables and vice versa.

Key Results

Joint Constraints: The joint analysis of kSZ profiles and eROSITA X-ray gas mass fractions yields precise constraints on the matter power spectrum suppression: $10 \pm 2\%$ at $k = 1 \, h \text{Mpc}^{-1}$ . The inferred gas profiles are more extended, and the power suppression is stronger than predicted by the fiducial models of recent hydrodynamical simulation suites (including FLAMINGO and BAHAMAS).
Dataset Tension: The HSC–XXL gas mass fractions, which the fiducial simulations were calibrated to reproduce, prefer a more moderate power suppression of $5 \pm 4\%$ at $k = 1 \, h \text{Mpc}^{-1}$ . The joint fit of kSZ and HSC–XXL data results in a poor goodness-of-fit ( $\chi^2_{\text{red}} = 2.5$ ), suggesting potential systematics in X-ray calibration, selection effects, or kSZ velocity reconstruction.
Simulation Comparison: Only simulation variants with particularly strong feedback (e.g., BAHAMAS with $\log_{10} \Delta T_{\text{AGN}} = 8.0$ and FLAMINGO variants calibrated to gas fractions shifted down by $8\sigma$ ) are consistent with the joint kSZ and eROSITA constraints.
Cosmological Impact: Using a simulated LSST Year 1 weak lensing analysis, the authors demonstrate that applying their observationally calibrated feedback priors recovers half the cosmological precision otherwise lost when using flexible, uncalibrated baryon models. Without a baryon model, the inferred cosmology is significantly biased; with uncalibrated models, constraining power degrades by a factor of two.

Significance and Claims
The paper claims to provide the most comprehensive multi-wavelength constraints to date on how baryonic feedback suppresses the non-linear matter power spectrum. The primary significance lies in transforming feedback modeling from a source of systematic uncertainty into an observationally calibrated component of cosmological inference.

The authors argue that:

Strong Feedback is Evident: Independent kSZ and eROSITA data consistently favor strong feedback, implying a suppression level that exceeds current state-of-the-art simulation predictions.
Calibration is Essential: External constraints on gas distribution (via X-ray and kSZ) are indispensable for next-generation surveys (e.g., LSST, Euclid, Roman). These constraints allow weak lensing analyses to safely extend to smaller angular scales, recovering statistical power that is typically discarded via conservative scale cuts.
Discrepancy Requires Investigation: The tension between the HSC–XXL data and the kSZ/eROSITA results highlights the need for further cross-instrument calibration and forward modeling to understand residual systematics in X-ray selection functions and kSZ velocity reconstructions.

The work concludes that while baryon feedback is a limiting systematic for cosmic shear, observationally calibrated models offer a viable path to unlocking the small-scale statistical power of future cosmic shear surveys.

1. The Problem: The "Cosmic Blender"

2. The Mystery: "How hard is the blender spinning?"

3. The Detective Work: Three Different Magnifying Glasses

4. The Big Discovery: The Blender is Stronger Than We Thought

5. Why This Matters for the Future

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