Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect

arXiv:2510.12553 · astro-ph.CO, astro-ph.GA · Submitted 2025-10-14 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Next we'll be talking about the paper "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect".

Jocelyn: The paper was written by the authors from Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, United Kingdom.

Vera: Stay tuned as we take you through the paper and discuss its implications.

The Summary of "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect": Vera: Now that we understand how they set up the comparison, let’s look at what they actually found in this study regarding the overall picture. The results are quite telling: there is clear evidence for a mass dependence of the halo baryon fraction.

Jocelyn: It’s a significant finding that low-mass groups—those roughly between ten twelve point five and ten fourteen solar masses —are noticeably depleted in their gas content compared to what we expect from the universe. This is a clear deviation from being fully stocked with gas.

Subrahmanyanyan: And they quantified this depletion, showing that for these lower mass systems, the gas fraction inside the virial radius is only about zero point three eight times the universal baryon fraction. This significant difference speaks directly to how energy processes affect smaller cosmic structures compared to larger ones.

Vera: It’s interesting that they found this low-mass deficit is consistent with an extended gas profile, which means that even if you look within the halo's measured boundary, you are missing a large amount of the baryonic mass. The gas is literally leaking out.

Jocelyn: That suggests the total baryon content reaches its full universal value only when we extend our view well beyond the virial radius, which is a crucial distinction for any researcher analyzing these systems. We can't just look at the immediate vicinity of a group and draw conclusions.

Subrahmanyanyan: This finding provides a concrete physical explanation for why low-mass halos are so different from their larger counterparts. It confirms that even though the overall budget might be consistent with the universe, the how they are distributed is not uniform across mass scales.

Vera: It’s compelling evidence that this depletion isn't just an error; it's a real physical outcome of how energetic processes, like feedback from galaxies, are working within these specific mass ranges. This ties directly into the cosmic web structure we observe.

Jocelyn: And it really points toward the idea that while the small systems are depleted, the large ones retain their gas, making them a much better reflection of the universal baryon fraction for any survey analysis.

Refining "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect": Vera: So, we've seen what they found; now let’s look closely at *how* they made those measurements so robust, which is a major part of the methodology. The authors suggest several ways to improve upon previous studies.

Jocelyn: One major improvement addresses the cancellation effect because a halo can be either moving toward us or away from us. Simply summing up all's signals would cancel out to zero, so they meticulously separate the approaching and receding groups to ensure we get a detectable kSZ signal.

Subrahmanyanyan: They also address potential inaccuracies in their velocity measurements, which are often skewed by the fact that photometric redshifts aren't perfect. They model these photo-z errors using distributions derived from previous research to clean up their reconstructed line-of-sight velocities.

Vera: This is a huge step because it’s not just relying on the raw signal; they are building a forward model of *all* the observational biases. They match the mass function of their simulated halos one-to-one with the observed groups, minimizing discrepancies before matching our data.

Jocelyn: The way they weight their observations is another key improvement; instead of giving every group equal importance, they use a momentum-weighted stacking approach, biasing the sample toward those high-velocity groups for a clearer picture. This makes the signal stronger.

Subrahmanyanyan: They also provide a robust method for estimating the gas profile using alpha, this scaling parameter that allows them to model how much of the gas is spread out. By allowing alpha to be a free parameter, they are making their findings less dependent on specific assumptions about the physics of distribution.

Vera: It’s a clever way to account for uncertainties; by demonstrating that even with imperfect weighting or miscentring effects, the measured f gas remains stable, they have significantly increased our confidence in the reliability of this entire study.

The Results of "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect": Vera: Now that we understand their robust approach, let’s look at *what* they found using the kSZ signal, focusing on specific results across different masses. The authors found a clear mass dependence in the gas fraction.

Jocelyn: They show that when you stack all groups—the full sample—the resulting average halo mass is quite high, around ten thirteen point seven M. This is because the stacking process favors those higher-mass systems.

Subrahmanyanyan: However, if we look at the gas fraction specifically for low-mass systems, say below ten fourteen M, they observe a significant deficit of gas within that small area. This points to active processes pushing baryons out of the central region.

Vera: It’s interesting to connect this depletion to their findings on the alpha parameter, which shows that the gas is not contained within the virial boundary. The gas profile is highly extended outwards, making it hard to capture all that mass just by looking locally.

Jocelyn: That implies a substantial amount of baryons are lurking in the outskirts. The total baryon content only reaches its universal value when we look way beyond the initial definition of the group’s boundary, which is vital information for our next steps.

Subrahmanyanyan: This finding provides a concrete physical example of how energy injection—feedback—is more effective in shallow potential wells, which is exactly what characterizes these low-mass halos. The kSZ signal acts as a direct probe for this effect.

Vera: It's compelling evidence that this depletion isn't just an observational artifact; it’s a real physical outcome of how energetic processes are working within these specific, less massive systems.

The Conclusion of "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect": Vera: We have seen how they modeled their data and what they found; now, let’s wrap up by discussing what this all means for the broader field. It's clear that "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect" has provided a powerful new lens.

Jocelyn: The key insight is that we can finally see how mass dictates whether a halo is able to hold onto its gas, which is something previous methods struggled to quantify so clearly. This gives us real data for our surveys.

Subrahmanyanyan: This work provides an essential observational anchor for our theoretical models of galaxy formation, giving us a concrete benchmark against the predictions of hydrodynamical simulations. We are now better equipped to test those theories.

Vera: It’s reassuring that the data confirms a depletion in low-mass systems, which is exactly what we expect from feedback processes like AGN and stellar winds. The results align with theory!

Jocelyn: The consensus across different weighting schemes and observational techniques validates this finding, making it a very robust conclusion for me as a researcher.

Subrahmanyanyan: It truly solidifies our ability to use the cosmic web as a laboratory to test these complex models of baryonic physics and energy transfer across vast distances.

Vera: So, as we conclude our deep dive into "Mass dependence of halo baryon fractions from the kinetic Sunyaev-Zeldovich effect," it’s clear this work provides an incredible roadmap for next-generation surveys.

Jocelyn: It leaves us with so many exciting avenues to explore regarding the gas dynamics in those outer, less detectable regions of the universe.

Subrahmanyanyan: We're now better equipped than ever to test the predictions of various simulations against this real-world data, and that’s a major milestone for this research.

Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, United Kingdom

astro-ph.CO, astro-ph.GA

Submitted: 2025-10-14

Updated: 2026-09-04

Comments: 16 pages, 7 figures. Published in MNRAS

DOI: 10.1093/mnras/stag1619

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 89/100

The gist: The study investigates the kinetic Sunyaev-Zeldovich (kSZ) effect to constrain the gas fraction (f gas) within dark matter haloes, providing a critical observational probe into the baryonic cycle of

Key concepts

Kinetic Sunyaev-Zeldovich effect
This effect is used as a probe in the study to measure properties of galaxy groups. It helps researchers investigate how energy processes affect cosmic structures by measuring the kinetic component of the Sunyaev-Zeldovich signal.
Halo baryon fraction
This refers to the ratio of baryonic mass (normal matter) to total mass within a dark matter halo. The study finds this fraction depends on the halo's mass, showing that low-mass systems have less gas than expected.
Feedback processes
These are energetic processes, such as feedback from galaxies like AGN and stellar winds, which push baryons out of the central regions of halos. The research suggests these processes are more effective in shallow potential wells found in lower-mass halos.

Terminology

Summary

The study investigates the kinetic Sunyaev-Zeldovich (kSZ) effect to constrain the gas fraction (f gas) within dark matter haloes, providing a critical observational probe into the baryonic cycle of the Universe. Understanding how baryons are distributed—specifically whether they are retained within a halo or expelled by feedback processes—is central to galaxy formation models and cosmology. This research is particularly important for addressing cosmological tensions, such as the S 8 tension, by providing empirical evidence regarding the mass dependence of gas retention in clusters and groups.

How it works

The researchers utilize a comprehensive forward modeling approach that integrates observational data with high-resolution simulations. The study employs the photometric DESI Legacy Survey group catalogue alongside cosmic microwave background (CMB) maps from the Atacama Cosmology Telescope (ACT). They generate mock galaxy group catalogues and synthetic kSZ maps based on the AbacusSummit cosmological simulations, creating a reconstructed peculiar velocity field that allows for photo-z errors, redshift-space distortions, and survey masks. This methodology allows them to compare a simulated signal against observed data to constrain the total baryon fraction within haloes.

Data Processing and Velocity Reconstruction

A major challenge in using the kSZ effect is that its signal depends on the bulk velocity of ionized gas integrated along the line of sight. To overcome cancellation effects (since a halo's radial velocity is equally likely to be positive or negative), the researchers employ a detailed velocity reconstruction process. This involves:

  1. Transforming haloes into comoving Cartesian coordinates and building a density field by convolving with a triangular-shaped cloud window function.

  2. Smoothing this field with a Gaussian kernel to mitigate the non-linear fingers-of-God redshift-space distortion (RSD).

3.Correcting for the effects of RSD and photometric redshift uncertainties by applying redshift perturbations to the mock data, ensuring that the observed kSZ signal is accurately modeled.

Key Findings: Mass Dependence and Depletion

The analysis reveals a significant mass dependence in how much gas is retained within a halo's virial radius. The findings indicate that low-mass groups exhibit substantial gas depletion compared to larger systems. Specifically, low-mass groups (10 12.5 M/h-1 M 10 14) are depleted to 0.38 plus or minus 0.11 times the universal baryon fraction. This depletion is not a sign that gas is entirely missing, but rather that the baryons are distributed differently than expected for a simple virialized system.

Interpretation of Gas Profiles

The observed low virial baryon fraction suggests an extended gas profile, meaning the total baryon content reaches its universal value well beyond the virial radius. This conclusion is consistent with previous analyses using X-ray, kSZ, and weak lensing data. The researchers conclude that this pattern reflects energetic feedback processes from the galaxies in these haloes, where mechanisms like stellar winds or AGN are particularly effective at expelling gas from the shallower potential wells of low-mass systems.

Conclusion on Robustness

The study demonstrates that while kSZ measurements cannot determine the total baryon content (due to baryons lying well beyond the detectable range), they can robustly estimate the baryonic mass within a fixed radius. The fact that f gas,vir is found to be consistent with this depletion at low masses, but consistent with a universal fraction when integrated over extended radii, provides strong evidence for the physical processes driving baryon redistribution in the Universe.

Improvements for AI systems

As a fastidious AI researcher, I have analyzed this manuscript. The methodology employed—the combination of complex forward modeling using AbacusSummit simulations, careful handling of observational biases (RSD, photo-z errors), and the statistical stacking procedure—presents several high-leverage opportunities for AI optimization and generalization.

The following improvements are structured to enhance an existing AI system (e.g., a deep learning framework or a Bayesian inference engine) by incorporating the specific physical constraints of this kSZ analysis.

The Improvement: Develop a dedicated Bias Correction Neural Network (B-CNN) that automates the complex, multi-step correction process currently performed manually in Sections 2, 3, and 4 (e.g., matching the AbacusSummit mass function to DESI-LS).

What the Improved AI System Can Do:

  • Input: Raw simulated data (AbacusSummit) and observed data (DESI-LS) as time series/spatial grids.

  • Mechanism: The B-CNN learns the mapping between the theoretical distribution of halo properties and the observed statistical degradation due to:

  1. Redshift-Space Distortions (RSD).

  2. Photometric Redshift (photo- z) errors, specifically using the empirically derived modified Lorentzian function from Hang et al. (2021).

  • Output: Provide a corrected synthetic kSZ template that is statistically indistinguishable from the observed data, removing the need for manual iterative correction (z corr) and significantly increasing computational throughput for future observational datasets.

Abstract

We detect the kinetic Sunyaev-Zeldovich imprint of peculiar motions of galaxy groups and clusters, using the photometric DESI Legacy Survey together with cosmic microwave background (CMB) maps from the Atacama Cosmology Telescope (ACT). We develop a comprehensive forward model based on the AbacusSummit cosmological simulations: mock galaxy group catalogues and synthetic kSZ maps are generated, together with a reconstructed peculiar velocity field that allows for photo- z errors, redshift-space distortions, and survey masks. We investigate possible contamination from the cosmic infrared background (CIB), finding that CIB effects are subdominant to the kSZ signal in the relevant ACT frequency channel. We then predict the kSZ signal expected when stacking CMB temperature maps around groups, taking account of their estimated radial velocity. Comparing the model with observations, we are able to constrain the total baryon fraction within haloes, as well as their internal gas profiles. We find evidence for mass dependence of the halo baryon fraction within the virial radius. The gas fraction in massive groups is consistent with the universal baryon fraction, but low-mass groups (10 12.5 M,/h-1 M 10 14) are depleted to 0.21 plus or minus 0.06 times the universal baryon fraction. We find this low virial baryon fraction to be consistent with an extended gas profile, for which the total baryon content reaches the universal value well beyond the virial radius. This conclusion is consistent with previous analyses using X-ray, kSZ, and weak lensing, and plausibly reflects energetic feedback processes from the galaxies in these haloes.

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