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eROSITA cosmology with galaxy groups: hot gas budget out to the virial radius

Using eROSITA observations of 25 galaxy groups, this study maps hot gas distributions out to the virial radius to reveal sub-cosmic gas fractions and quantify baryonic feedback effects, finding that fiducial FLAMINGO and BAHAMAS simulations best match the data while strong feedback variants show significant tension.

Original authors: H. Khalil, A. Finoguenov, D. Eckert, R. Seppi, E. Tempel, L. Lovisari, F. Gastaldello

Published 2026-08-19

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CC BY 4.0

Original authors: H. Khalil, A. Finoguenov, D. Eckert, R. Seppi, E. Tempel, L. Lovisari, F. Gastaldello

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 universe is not merely a collection of stars and galaxies floating in empty space; it is filled with a vast, invisible ocean of hot gas that clings to these cosmic structures. In the spaces between galaxies, within the massive halos of dark matter that hold them together, this gas glows brightly in X-rays, acting as a tracer for the invisible forces at work. For decades, astronomers have known that the amount of this gas inside large galaxy clusters should match the average amount of matter in the entire universe. However, observations of smaller groups of galaxies have consistently shown a puzzling deficit: a significant portion of this gas is missing from the inner regions where it is expected to be. The leading theory is that energetic blasts from supermassive black holes, known as active galactic nuclei, act like powerful pumps, heating the gas and flinging it far beyond the visible edges of these galaxy groups. Understanding exactly how much gas is ejected and how far it travels is crucial, because this process reshapes the distribution of matter across the cosmos, influencing how the universe's largest structures form and evolve.

A team of astronomers has now taken a decisive step in solving this mystery by looking at the outskirts of galaxy groups with unprecedented clarity. Using data from the eROSITA telescope, which orbits Earth and scans the sky in X-rays, they studied a carefully selected sample of twenty-five galaxy groups. These groups were identified by cross-referencing the telescope's X-ray detections with a catalogue of galaxies mapped by the Two Micron Redshift Survey, ensuring the team was looking at a complete and representative set of systems rather than just the brightest or most obvious ones. The researchers focused on the region extending from the center of these groups out to their very edges, known as the virial radius, a boundary that marks the limit of the group's gravitational hold. By measuring the brightness of the X-ray glow at different distances from the center, they could map out the density of the hot gas and calculate exactly how much of it remained trapped versus how much had been pushed away.

The results reveal a distinct change in the behavior of the gas as one moves outward. Close to the center, the gas is distributed in a relatively flat, uniform way, but as the researchers looked beyond the halfway point of the group's radius, the gas density dropped off much more sharply than expected. This steepening of the profile suggests that the mechanisms pushing the gas out are most effective in the outer regions, effectively clearing the area just beyond the group's main boundary. At the median mass of their sample, the team found that the hot gas makes up only about 4.3 percent of the total mass within the inner half of the group's radius. Even when expanding their view to the very edge of the group, the gas fraction rises to only about 5.8 percent. This is far less than the cosmic average of roughly 15.7 percent, confirming that a substantial amount of baryonic matter—the normal matter that makes up stars, planets, and us—has been displaced from these systems entirely.

To understand the implications of this missing gas, the researchers compared their measurements against predictions from sophisticated computer simulations of the universe. These simulations attempt to model how gravity, gas physics, and the energy from black holes interact over billions of years. The team found that their observations align remarkably well with a specific set of simulations that use a moderate level of energy feedback from black holes. However, they explicitly ruled out models that rely on extremely powerful feedback mechanisms. Those models, which predict that black holes eject gas so violently that it leaves the outer regions of galaxy groups almost empty, were found to be inconsistent with the data, differing from the observations by a statistically significant margin. This suggests that while black holes are indeed powerful enough to reshape their environments, they do not do so with the extreme force previously hypothesized in some theoretical scenarios.

The study also quantified how this redistribution of matter affects the universe on the largest scales. By combining their measurements of the gas fraction with known amounts of stars in these groups, the researchers calculated how the presence of this displaced gas suppresses the clumping of matter across the cosmos. They determined that at specific scales, the matter distribution is reduced by between 10 and 15 percent compared to a universe containing only dark matter. This finding provides a concrete, observational anchor for cosmologists, helping to refine models of how the universe's structure grows. The work stands as a precise measurement of the baryon budget in galaxy groups, offering a clearer picture of the cosmic cycle of matter and the subtle, yet profound, influence of black holes on the architecture of the universe.

Technical Summary: eROSITA Cosmology with Galaxy Groups: Hot Gas Budget out to the Virial Radius

Problem Statement
Non-gravitational processes, specifically Active Galactic Nuclei (AGN) feedback, are known to expel hot gas beyond the virial regions of galaxy groups and clusters, reshaping the matter distribution on megaparsec scales. While the hot gas fraction ( $f_{gas}$ ) within $R_{500}$ (the radius enclosing 500 times the critical density) of galaxy groups is well-established as sub-cosmic, measurements at larger radii ( $R_{200}$ ) remain scarce and uncertain due to the low surface brightness of the intracluster medium (ICM) and elevated background levels. This uncertainty is critical because the outer regions are where the signatures of feedback—gas heated and expelled—should be most evident. Furthermore, discrepancies exist between observational constraints and hydrodynamical simulations (e.g., FLAMINGO, BAHAMAS, SIMBA) regarding the efficacy of feedback models in redistributing baryons. These discrepancies directly impact predictions for the suppression of the matter power spectrum (MPS), a key systematic for upcoming large-scale structure surveys.

Methodology
The authors utilize a complete sample of 25 galaxy groups selected from the first public release of the eROSITA-DE data (eRASS1) and cross-matched with the Two Micron Redshift Survey (2MRS) optical group catalogue. The sample is restricted to low redshifts ( $0.01 < z < 0.025$ ) to ensure spatial resolution of group outskirts and to minimize contamination from unresolved galactic emission.

Key methodological steps include:

Data Processing: The authors process eRASS1 event lists using the eSASS software system, filtering for the [0.6–2.3] keV energy band to avoid instrumental lines and "light leak" artifacts. They employ a multiscale wavelet decomposition for source detection and mask point sources.
Profile Extraction: Surface brightness ( $SB_x$ ) profiles are extracted from the background-subtracted images out to $R_{200}$ . The authors stack the profiles of the 25 groups to achieve sufficient signal-to-noise ratio for fitting in the outskirts.
Mass Derivation:
- Hot Gas Mass ( $M_{gas}$ ): Derived by converting $SB_x$ profiles to emission measure (EM) profiles using a constant temperature assumption based on the $L_X-T$ relation. A multiscale forward-fitting method deprojects the 2D EM profiles into 3D density profiles assuming spherical symmetry.
- Total Mass ( $M_{tot}$ ): Adopted from the 2MRS dynamical mass estimates, verified against velocity dispersion-mass relations.
- Gas Fraction ( $f_{gas}$ ): Calculated as the ratio of $M_{gas}$ to $M_{tot}$ within specific apertures ( $R_{500}$ and $R_{200}$ ).
Statistical Analysis: The authors perform a Bayesian hierarchical analysis to fit scaling relations ( $M_{gas}-M_{tot}$ , $L_X-M_{tot}$ , $L_X-M_{gas}$ ). Crucially, they implement a correction for aperture covariance, addressing the artificial flattening of relations caused by the shared dependence of integrated quantities on the integration radius.
Cosmological Inference: Using the measured baryon fractions and the SP(k) model (based on the ANTILLES simulation suite), the authors infer the suppression of the matter power spectrum relative to a dark matter-only universe.

Key Results

Surface Brightness Profiles: The stacked profile exhibits uniformly flat $\beta$ parameters within $R_{500}$ ( $\beta = 0.38 \pm 0.04$ ), rising to a steeper $\beta = 0.76 \pm 0.19$ beyond $R_{500}$ up to $R_{200}$ . This steepening suggests a weakening of baryon ejection in the outer regions, contrasting with strong feedback models that predict flatter outer profiles.
Hot Gas Fractions:
- At the median mass ( $M_{500} = 2.54 \times 10^{13} M_\odot$ ), the gas fraction is $f_{gas,500} = 4.32 \pm 0.42\%$ .
- At $R_{200}$ (median mass $M_{200} = 3.69 \times 10^{13} M_\odot$ ), the fraction increases to $f_{gas,200} = 5.78 \pm 0.69\%$ .
- Both values remain significantly below the cosmic baryon fraction ( $\sim 15.7\%$ ), confirming substantial baryon displacement.
Scaling Relations: The aperture-covariance corrected relations show:
- $M_{gas}-M_{tot}$ : A slope of $1.59^{+0.34}_{-0.28}$ at $R_{500}$ , slightly steeper than self-similar expectations.
- $L_X-M_{tot}$ : The observed luminosity-mass relation is best reproduced by the fiducial FLAMINGO (L1_m9) and BAHAMAS simulations. Strong feedback variants of FLAMINGO ( $f_{gas}-4\sigma$ , $f_{gas}-8\sigma$ ) underpredict luminosity by $6.5\sigma$ to $8.6\sigma$ .
Simulation Comparison:
- Fiducial Models: The fiducial FLAMINGO and BAHAMAS simulations provide the closest match to the observed $f_{gas}-M_{tot}$ and $L_X-M_{tot}$ relations.
- Strong Feedback Models: Strong feedback variants of FLAMINGO and the SIMBA simulation show significant tension with the data, particularly in the $L_X-M_{tot}$ relation and the steepening of density profiles beyond $R_{500}$ .
- Other Simulations: IllustrisTNG100, EAGLE, and FABLE tend to overpredict the gas fraction at the median mass.
Matter Power Spectrum: Using the SP(k) model, the authors infer a $10\%–15\%$ reduction in the matter power spectrum at $k = 5 h \text{ Mpc}^{-1}$ relative to a dark matter-only universe ( $0.85 \le P(k)/P_{DMO}(k) \le 0.90$ ). This result aligns with fiducial FLAMINGO and BAHAMAS but reveals growing tension with strong feedback variants on smaller scales.

Significance and Claims
The paper claims to present the first individual-system eROSITA constraints on the $M_{gas}-M_{tot}$ relation at $R_{200}$ for a complete sample of optically selected galaxy groups. By achieving twice the statistical precision of recent stacking studies, the work offers a robust test of AGN feedback implementations in cosmological hydrodynamical simulations.

The authors emphasize that their results:

Constrain Feedback Models: The observed steepening of density profiles beyond $R_{500}$ and the specific gas fractions challenge strong feedback models that preferentially suppress core densities while leaving outskirts close to fiducial values.
Validate Simulations: The agreement with fiducial FLAMINGO and BAHAMAS supports these specific feedback calibrations, while the tension with strong feedback variants suggests these models may over-eject gas or fail to capture the correct thermodynamic state of the outer ICM.
Impact Cosmology: The derived baryon fractions provide essential input for "baryonification" correction schemes, directly informing the expected suppression of the matter power spectrum for upcoming surveys like Euclid and the Nancy Grace Roman Space Telescope. The results suggest that while feedback suppresses power, the magnitude of this suppression is tightly constrained by the specific feedback physics implemented in the simulations.

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