🔭 astrophysics

Evidence for a thermal pressure deficit in galaxy groups from the tSZ effect and weak lensing

By combining thermal Sunyaev-Zel'dovich effect measurements with weak lensing data from the DESI Luminous Red Galaxy sample, this study reveals that current hydrodynamical simulations significantly overpredict the thermal pressure in galaxy groups, suggesting a deficit likely caused by missing non-thermal pressure support or departures from hydrostatic equilibrium.

Original authors: Jared Siegel, Alexandra Amon, Jenny E. Greene, Ian G. McCarthy, Eliot Quataert, William Coulton

Published 2026-08-04

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CC BY 4.0

Original authors: Jared Siegel, Alexandra Amon, Jenny E. Greene, Ian G. McCarthy, Eliot Quataert, William Coulton

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 ocean. While we can see the islands (galaxies) and the waves (light), the water itself—the hot gas that fills the space between galaxies—is mostly hidden from our eyes. This gas, called the intracluster medium, is incredibly hot and under immense pressure, like a giant pressure cooker sitting in the vacuum of space. Scientists have long tried to understand how this gas behaves. They know that massive galaxies and groups of galaxies act like heavy anchors, pulling this gas in with their gravity. But there's a twist: the galaxies themselves aren't just passive anchors; they are active chefs. Through violent explosions of stars and powerful jets from black holes, they heat up and push the gas around, changing its pressure and density.

To measure this invisible pressure, scientists use a clever trick involving the Cosmic Microwave Background (CMB), which is the faint afterglow of the Big Bang acting like a giant, cold backdrop. When the hot gas in galaxy groups pushes against the CMB photons, it gives them a tiny kick, changing their energy. This effect, known as the thermal Sunyaev-Zel'dovich (tSZ) effect, is like seeing the ripples in a pond caused by a hidden stone. By measuring these ripples, astronomers can calculate how much pressure the gas has. The big question is: does the gas behave exactly as our computer models predict, or is something missing from our understanding of how galaxies and their surrounding gas interact?

This paper sets out to solve a mystery in that invisible ocean. The authors, a team of astronomers, decided to take a fresh look at the pressure of gas around groups of massive galaxies, specifically those known as Luminous Red Galaxies (LRGs). They used data from the Atacama Cosmology Telescope (ACT) to measure the tSZ effect, but they didn't just look at the raw data. They realized that previous attempts to measure this pressure were like trying to hear a whisper in a noisy room; the signal was often drowned out by "noise" from dust and radio waves coming from the galaxies themselves.

To fix this, the team built a sophisticated model to separate the true pressure signal from the cosmic noise. They treated the data like a complex song with three instruments playing at once: the pressure signal (tSZ), the dust, and the radio waves. By carefully modeling the dust and radio components using the specific properties of the galaxies they were studying, they were able to isolate the pressure signal much more clearly than before. They then compared their new, cleaner measurements against the most advanced computer simulations of the universe available, specifically a massive simulation called FLAMINGO.

The results were surprising. The authors found that the computer simulations were predicting the gas pressure to be about twice as high as what they actually measured. It's as if the simulations were describing a pressure cooker that was set to "high," while the real universe was running on "medium." This discrepancy was consistent across different galaxy masses and distances, suggesting it wasn't just a fluke.

However, the story gets even more interesting. The team knew that the simulations had already been tweaked to match the amount of gas in these groups (a measurement called the kinetic SZ effect). In other words, the simulations were already correct about how much gas was there, but they were still wrong about how hot and pressurized it was. This means the problem isn't that the simulations have too much gas; it's that they think the gas is too hot.

The authors argue that this points to a missing ingredient in our understanding of the universe. The simulations assume the gas is in a state of balance, where gravity pulling in is perfectly matched by the gas pressure pushing out. But if the real gas is cooler than the simulations say, yet still holding its shape against gravity, there must be some other invisible force helping to hold it up. The paper suggests that this missing support could come from non-thermal sources, such as magnetic fields, cosmic rays, or turbulent motions that the current simulations aren't fully capturing.

In short, this paper doesn't just say "our models are a little off." It suggests that the standard recipe for how galaxy groups work is missing a key spice. The gas in these cosmic groups is cooler and less pressurized than the best computer models predict, implying that the universe has hidden ways of supporting its gas that we haven't fully figured out yet. It's a reminder that even in the vast, hot oceans between galaxies, there are still secrets waiting to be discovered.

Problem
Measurements of the thermal Sunyaev-Zel'dovich (tSZ) effect have historically failed to provide a consistent picture of the thermodynamic state of the intracluster medium (ICM). Interpretation is complicated by three primary roadblocks: (i) contamination of the tSZ signal by astrophysical foregrounds, specifically dust emission and radio point sources; (ii) the extreme sensitivity of tSZ theory predictions to halo mass and satellite fraction (scaling as $M_{halo}^{5/3}$ ); and (iii) the degeneracy between gas density and temperature when interpreting the tSZ signal. While recent studies suggest galaxy groups are more gas-depleted than simulations predict, the thermal state of the remaining gas remains unclear, with some studies reporting pressure deficits and others finding agreement with gravity-only expectations.

Methodology
The authors present new measurements of the tSZ effect around the Dark Energy Spectroscopic Instrument (DESI) Luminous Red Galaxy (LRG) sample, covering halo masses $M_{500} = 10^{13}–10^{14} M_\odot$ and redshifts $0.4 < z < 1$ . To address foreground contamination, the team utilizes Atacama Cosmology Telescope (ACT) DR6 single-channel temperature maps (90, 150, and 220 GHz). Instead of constructing Compton $y$ maps via multi-frequency component separation (which assumes unknown redshifts for contaminants), they measure the stacked spectral energy distribution (SED) of the target galaxies. They model this SED with three components: tSZ, dust emission, and radio point sources. By background-subtracting the photometry, they isolate emission originating from the targeted galaxies and their immediate surroundings, leveraging known redshifts and radio flux limits to constrain the dust and radio models.

To ensure a "like-with-like" comparison with theory, the authors employ the 1 Gpc $^3$ FLAMINGO hydrodynamical simulations. They calibrate the selection of simulated galaxies using new galaxy-galaxy lensing (GGL) measurements from the Hyper Suprime-Cam (HSC) Y3 shear catalog. This GGL calibration constrains the mean halo mass and satellite fraction of the simulated samples, mitigating biases introduced by uncertain stellar mass estimates. The analysis compares the observed stacked tSZ signal against forward-modeled predictions from two FLAMINGO variants: a fiducial simulation (L1 m9) and a strongest feedback simulation ( $f_{gas} -8\sigma$ ), which has been shown to reproduce the kinetic SZ (kSZ) signal (and thus gas density) of the same galaxy sample.

Key Contributions

Robust Signal Isolation: The paper introduces a method to isolate the tSZ signal by modeling the stacked SED of target galaxies in single-channel maps. This approach significantly reduces uncertainties from astrophysical foregrounds compared to map-level deprojection methods, particularly at small angular scales ( $\lesssim 3'$ ).
Like-with-Like Simulation Comparison: By using GGL measurements to calibrate the simulated galaxy selection, the authors decouple the tSZ signal interpretation from uncertainties in halo mass estimation and satellite fractions, which are known to masquerade as feedback signatures.
Breaking the Density-Temperature Degeneracy: By utilizing the fact that the strongest feedback FLAMINGO simulation already reproduces the observed gas density (via kSZ measurements), the authors isolate the discrepancy in the tSZ signal as a problem of gas temperature rather than density.

Results

Foreground Contamination: Dust emission is a dominant contaminant, contributing $>60\%$ of the signal at 150 GHz for low-mass groups ( $M_{500} = 10^{13} M_\odot$ ) within a $1.6'$ aperture, decreasing to $\approx 20\%$ for high-mass groups. Radio emission is subdominant but improves model goodness-of-fit. The authors' SED-derived Compton $y$ constraints are systematically higher than those derived from ACT+Planck deprojected maps, suggesting the latter harbor residual dust contamination.
Thermal Pressure Deficit: Across the full mass and redshift range, the fiducial FLAMINGO simulation overpredicts the observed tSZ signal by a factor of $\gtrsim 2$ at scales $\lesssim 3'$ (approximately $4 R_{500}$ at $z=0.7$ ).
Strongest Feedback Simulation: Even the strongest feedback simulation ( $f_{gas} -8\sigma$ ), which successfully reproduces the gas density inferred from kSZ measurements, overpredicts the thermal pressure (Compton $y$ ) by a factor of approximately two at $1.6'$ .
Robustness: The deficit persists across various simulation selection variants (e.g., centrals-only, different cluster masking) and SED modeling assumptions (e.g., varying dust spectral indices). The discrepancy is larger than the difference between the fiducial and strongest feedback simulations.

Significance
The paper claims that current hydrodynamical simulations overpredict the thermal pressure of galaxy groups by a factor of two. Because the strongest feedback models already reproduce the observed gas density, the remaining discrepancy cannot be explained by additional gas depletion alone. Instead, the results point toward missing non-thermal pressure support (e.g., cosmic rays, magnetic fields, or turbulent motions) or significant departures from hydrostatic equilibrium in the simulated gas. The authors suggest that resolving this deficit requires a re-examination of the physics governing gas in groups, particularly the role of non-thermal pressure sources and the thermodynamic state of the ICM, which may not be fully captured by current subgrid feedback models.

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