🔭 astrophysics

The impact of strong feedback on galaxy group scaling relations

This paper demonstrates that highly ejective feedback models, which are calibrated to match recent low baryon fraction estimates, significantly under-predict the X-ray luminosity of local galaxy groups, suggesting that observable scaling relations are more reliable than baryon fractions for calibrating feedback mechanisms in cosmological simulations.

Original authors: D. Eckert, R. Seppi, J. Braspenning, A. Finoguenov, F. Gastaldello, L. Lovisari, E. O'Sullivan, S. Ettori, B. D. Oppenheimer, M. A. Bourne, D. -W. Kim, M. Sun, H. Khalil, G. Gozaliasl, Y. E. Bahar, V.

Published 2026-05-20

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: D. Eckert, R. Seppi, J. Braspenning, A. Finoguenov, F. Gastaldello, L. Lovisari, E. O'Sullivan, S. Ettori, B. D. Oppenheimer, M. A. Bourne, D. -W. Kim, M. Sun, H. Khalil, G. Gozaliasl, Y. E. Bahar, V. Ghirardini, W. Cui, K. Kolokythas, S. McGee

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 Picture: The Cosmic "Heater" and the Gas Clouds

Imagine the universe is filled with giant, invisible bubbles of hot gas. Inside these bubbles, galaxies (like our Milky Way) live and form stars. But there's a problem: these gas bubbles should be huge and full of matter. However, observations show that in smaller bubbles (called "galaxy groups"), a lot of the gas is missing.

Scientists think this is because of a cosmic "heater." At the center of many galaxies sits a supermassive black hole. When it gets active, it shoots out massive bursts of energy, like a giant blowtorch. This energy heats up the surrounding gas so much that the gas gets pushed out of the bubble entirely. This process is called feedback.

The Conflict: Two Different Stories

For a long time, scientists have been trying to figure out exactly how powerful this "blowtorch" is. They use supercomputer simulations (like the FLAMINGO project) to model the universe.

The Old Story: Recent measurements suggested the blowtorch is incredibly powerful. It's so strong that it blows almost all the gas out of the smaller bubbles. This would mean these galaxy groups are very "empty" of gas.
The New Story (This Paper): The authors of this paper looked at 44 real galaxy groups using the XMM-Newton space telescope. They wanted to see if the "super-powerful blowtorch" story was actually true.

The Experiment: Checking the Temperature and Brightness

To test the theory, the scientists looked at two things for each galaxy group:

How hot the gas is (Temperature).
How bright the gas glows in X-rays (Luminosity).

Think of it like checking a campfire. If you have a specific amount of wood (mass) and you know how hot the fire is, you can predict how bright the flames should be.

If the blowtorch is weak: The gas stays inside, the fire is bright, and the temperature is moderate.
If the blowtorch is super strong: The gas gets blown away. Even if the remaining gas is hot, there is so little of it left that the fire looks very dim.

The Results: The "Super-Blowtorch" is Too Strong

The scientists compared their real observations with the computer simulations.

The Simulation with the "Super-Blowtorch" (Strong Feedback): This model predicted that the galaxy groups would be very dim because so much gas had been blown away.
The Reality: The real galaxy groups were much brighter than the "super-blowtorch" model predicted.

In fact, the real groups were so bright that the "super-blowtorch" model was wrong by a huge margin. The authors calculated that the difference is statistically significant (5.7 sigma), which is like flipping a coin and getting heads 20 times in a row by pure luck—it's almost certainly not luck. The real universe has more gas in these groups than the "strongest feedback" models allow.

Why This Matters: The "Gas Fraction" Trap

The paper points out a tricky problem in how scientists usually measure this.

Often, scientists try to figure out how much gas is missing by estimating the total mass of the galaxy group first. But measuring the total mass of a galaxy group is like trying to guess the weight of a cloud by looking at its shadow—it's very hard and prone to errors.

The authors argue that instead of guessing the mass, we should look at directly observable things: how bright the group is and how hot it is. These are things we can measure directly without needing to guess the total weight of the system. When they did this, the "strong feedback" models failed the test.

The Conclusion

The paper concludes that while active black holes definitely push gas out of galaxy groups, the "strongest feedback" models (which assume the black holes are incredibly efficient at blowing gas away) are too extreme.

The real universe seems to have a "Goldilocks" feedback: strong enough to move some gas around, but not so strong that it empties the galaxy groups completely. The authors suggest that future computer simulations need to be tuned to match these real, observable brightness and temperature levels, rather than just trying to match complex, hard-to-measure gas fractions.

In short: The universe's "blowtorch" is powerful, but it's not as powerful as some recent computer models claimed. The galaxy groups still hold onto more of their gas than those models thought.

Technical Summary: The Impact of Strong Feedback on Galaxy Group Scaling Relations

Problem Statement
Active galactic nucleus (AGN) feedback is a critical component in galaxy evolution models, regulating star formation and shaping the stellar mass function. While simulations often calibrate feedback efficiency by matching the baryon fractions of galaxy groups, recent observational constraints from optically selected groups and the kinetic Sunyaev-Zel'dovich (kSZ) effect suggest lower baryon fractions than previously estimated. These new constraints imply that feedback may be significantly more "ejective" (expelling gas from halos) than standard models predict. Specifically, recent studies indicate that the FLAMINGO cosmological simulation suite, particularly the variant with the strongest feedback ( $f_{gas}-8\sigma$ ), aligns well with these lower gas fraction measurements. However, it remains unclear whether such highly ejective models are consistent with other direct observables, specifically the X-ray scaling relations of local galaxy groups.

Methodology
The authors analyze a sample of 44 nearby galaxy groups ( $10^{13} - 10^{14} M_\odot$ ) from the XMM-Newton Group AGN Project (X-GAP). This sample is a complete sub-sample of the AXES-SDSS survey, selected via cross-correlation of spectroscopic optical groups with ROSAT X-ray sources, ensuring a selection function that is not biased toward centrally peaked systems. The data were homogeneously reduced using XMM-Newton EPIC data, with temperatures and luminosities derived from spectral fitting in the 0.5–2 keV band.

The study focuses on two primary scaling relations:

Luminosity-Mass ( $L_X - M$ ): Compared against FLAMINGO simulations with varying feedback strengths (from $f_{gas}+2\sigma$ to $f_{gas}-8\sigma$ ).
Luminosity-Temperature ( $L_X - T$ ): Analyzed as a primary diagnostic because it is essentially insensitive to halo mass estimates, thereby avoiding biases associated with hydrostatic equilibrium (HSE) assumptions.

To address potential selection biases, the authors performed Monte-Carlo simulations drawing from the halo mass function and applying the specific X-GAP selection criteria (flux limits and angular size cuts) to the various FLAMINGO runs. This allowed them to predict the expected median temperature ( $\bar{T}$ ) and number of detected groups ( $N_{det}$ ) for each feedback model and compare these predictions with the observed X-GAP sample.

Key Results

Discrepancy in $L_X - T$ : The observed $L_X - T$ relation for the X-GAP sample agrees well with the fiducial FLAMINGO run ( $L1\_m9$ ). In contrast, the model with the strongest feedback ( $f_{gas}-8\sigma$ ) significantly under-predicts the X-ray luminosity of galaxy groups at a fixed temperature. At $T \approx 1$ keV ( $\sim 3 \times 10^{13} M_\odot$ ), the median luminosity of the observed groups exceeds the $f_{gas}-8\sigma$ prediction by approximately 0.5 dex.
Statistical Significance: The discrepancy is robust against selection effects. The Monte-Carlo analysis shows that the $f_{gas}-8\sigma$ model predicts a significantly lower number of detectable groups ( $N_{det} \approx 21$ ) and a higher median temperature ( $\bar{T} \approx 1.79$ keV) compared to the observed sample ( $N_{det} \approx 44$ , $\bar{T} \approx 1.13$ keV). Combining the deviations in both $N_{det}$ and $\bar{T}$ yields a statistical tension of 5.7 $\sigma$ between the X-GAP data and the strongest feedback model.
Mass Independence: The conclusion holds even when considering the $L_X - M$ relation, though mass estimates introduce additional uncertainties. The authors note that the $L_X - T$ discrepancy is driven by the integrated luminosity and temperature, which are directly measurable and minimally correlated, rather than derived quantities like gas fraction.
Systematic Checks: The authors argue that systematic errors in X-ray temperature and luminosity measurements (e.g., flux calibration, projection effects) are too small ( $\lesssim 0.1$ dex) to explain the observed 0.5 dex discrepancy. Conversely, they highlight that deriving gas fractions from stacked samples involves substantial systematics related to halo mass estimation and metallicity profiles, which can introduce factors of two in uncertainty.

Significance and Claims
The paper claims that the FLAMINGO models with highly ejective feedback (specifically the $f_{gas}-8\sigma$ run), while consistent with recent gas fraction and kSZ constraints, are in strong tension with the observed X-ray scaling relations of local galaxy groups. The authors argue that these models overly reduce the gas content in the inner regions ( $\lesssim 0.5 R_{500}$ ) of galaxy groups, leading to luminosities that are too low for the observed temperatures.

The study emphasizes that validating feedback models against observable scaling relations (like $L_X - T$ ) is necessary to ensure the physical validity of feedback implementations in cosmological simulations. The authors suggest that models must broaden the gas distribution without excessively suppressing X-ray luminosities, or that the FLAMINGO model may not perfectly reproduce true mass and redshift trends. They conclude that while the $f_{gas}-8\sigma$ model matches some indirect constraints, it fails to reproduce the direct observables of local galaxy groups, indicating a need for refined feedback prescriptions that satisfy both gas fraction constraints and X-ray scaling relations.