🔭 astrophysics

Precision Kinematic Sunyaev--Zel'dovich Measurements Across Halo Mass and Redshift with DESI DR2 and ACT DR6: Part I. Luminous Red Galaxies

This paper presents the most precise detection of the kinetic Sunyaev-Zel'dovich effect around luminous red galaxies to date by cross-correlating DESI DR2 and ACT DR6 data, revealing that gas profiles deviate from dark matter distributions and favor high-efficiency baryonic feedback models across a wide range of halo masses and redshifts.

Original authors: F. J. Qu, B. Ried Guachalla, E. Schaan, B. Hadzhiyska, S. Ferraro, J. Aguilar, S. Ahlen, A. Baleato Lizancos, D. Bianchi, D. Brooks, R. Canning, F. J. Castander, E. Chaussidon, T. Claybaugh, A. Cuceu

Published 2026-04-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: F. J. Qu, B. Ried Guachalla, E. Schaan, B. Hadzhiyska, S. Ferraro, J. Aguilar, S. Ahlen, A. Baleato Lizancos, D. Bianchi, D. Brooks, R. Canning, F. J. Castander, E. Chaussidon, T. Claybaugh, A. Cuceu, A. de la Macorra, B. Dey, P. Doel, A. Font-Ribera, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, H. K. Herrera-Alcantar, K. Honscheid, C. Howlett, D. Huterer, M. Ishak, R. Kehoe, T. Kisner, A. Kremin, O. Lahav, M. Landriau, L. Le Guillou, M. E. Levi, M. Manera, A. Meisner, R. Miquel, S. Nadathur, J. A. Newman, W. J. Percival, I. P'erez-R`afols, G. Rossi, L. Samushia, E. Sanchez, E. F. Schlafly, D. Schlegel, M. Schubnell, H. Seo, J. Silber, D. Sprayberry, G. Tarl'e, B. A. Weaver, R. Zhou

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. We can easily see the "islands" (galaxies) floating in it, but the water itself (the gas and dark matter) is mostly invisible to our eyes. For decades, cosmologists have been trying to map this invisible ocean to understand how the islands formed and how they interact with the water around them.

This paper is like a breakthrough in underwater sonar technology. The authors have used a new, ultra-precise method to "hear" the movement of this invisible gas around massive galaxies, revealing secrets about how the universe's "weather" (feedback from stars and black holes) shapes the cosmos.

Here is the story of their discovery, broken down into simple concepts:

1. The Invisible Clue: The "Cosmic Doppler Shift"

To find the invisible gas, the scientists didn't look at the gas directly. Instead, they looked at the Cosmic Microwave Background (CMB). Think of the CMB as the "static" or "hiss" left over from the Big Bang, a uniform glow that fills the entire sky.

When this light passes through a cloud of moving gas, it gets a tiny push.

The Analogy: Imagine you are standing on a train platform. A train (the gas cloud) is blowing a whistle (the CMB light) as it rushes past you.
- If the train is coming toward you, the pitch sounds higher (blue-shifted).
- If it's moving away, the pitch sounds lower (red-shifted).
The Science: This is the Kinematic Sunyaev-Zel'dovich (kSZ) effect. The gas around galaxies is moving, and as the CMB light hits it, the light gets slightly "warped" by that motion. By measuring this tiny warp, the scientists can map where the gas is and how fast it's moving, even though the gas itself is invisible.

2. The New Tools: A Massive Team-Up

To do this, the researchers combined two massive datasets, like merging two giant puzzle pieces:

DESI (The Galaxy Map): The Dark Energy Spectroscopic Instrument took a 3D map of 2.4 million specific galaxies (called Luminous Red Galaxies). These are like the "islands" in our ocean analogy.
ACT (The Sky Camera): The Atacama Cosmology Telescope provided a high-resolution map of the CMB "static" (the background light).

By cross-referencing the galaxy map with the sky camera, they could look at the space around each galaxy and ask: "Is the light here shifted because gas is moving?"

3. The Big Discovery: The Gas is "Leaking"

The most exciting finding is that the gas around these galaxies does not follow the rules of gravity alone.

The Expectation: If gravity were the only force, the gas would clump tightly around the galaxy, just like water swirling down a drain. It would follow the shape of the "dark matter" (the invisible skeleton holding the galaxy together).
The Reality: The scientists found that the gas is much more spread out and "puffed up" than gravity alone would predict.
The Metaphor: Imagine a campfire. Gravity wants the smoke to stay close to the fire. But the wind (energy from exploding stars and supermassive black holes) blows the smoke far away, creating a huge, diffuse cloud.
- The paper shows that this "wind" (called feedback) is much stronger than our current computer simulations predicted. The gas is being ejected from the galaxy's neighborhood much more efficiently than we thought.

4. The "Sonar" vs. The "Flashlight"

The authors used a clever new trick called harmonic-space analysis.

Old Way (Stacking): Imagine trying to hear a whisper in a noisy room by stacking 1,000 people's voices on top of each other. It's messy, and the noise gets mixed up.
New Way (Harmonic Space): Instead of stacking voices, they looked at the "frequencies" of the noise. It's like using a high-tech equalizer to isolate the specific sound of the whisper from the background noise. This allowed them to get a much clearer, sharper picture of the gas distribution, detecting the signal with 18 times the certainty of previous attempts (an 18-sigma detection!).

5. What This Means for the Future

This study is a "Part I" in a larger series. By mapping the gas around these specific galaxies, the team has created a new set of "targets" for supercomputer simulations.

The Lesson: Current simulations of the universe are like weather models that underestimate the wind. This paper tells scientists, "Hey, your models need to be more aggressive about how stars and black holes blow gas away."
The Goal: By understanding how this gas moves, we can better understand how galaxies grow, how they stop growing, and why the universe looks the way it does today.

In a Nutshell

The universe is full of invisible gas that we can't see, but we can "hear" it moving. Using a massive new map of galaxies and a super-sensitive camera of the early universe, scientists have proven that this gas is being blown around by energetic cosmic storms much more violently than we previously thought. It's a new chapter in understanding how the "weather" of the cosmos shapes the "landscapes" of galaxies.

1. Problem Statement

Understanding the distribution of baryons (ordinary matter) and the feedback processes that govern their evolution remains a critical challenge in modern cosmology. While baryons constitute over 15% of the universe's matter, their spatial distribution relative to dark matter is poorly constrained, particularly in the diffuse circumgalactic medium (CGM) and intracluster medium (ICM). This uncertainty hinders the precision of upcoming large-scale structure surveys (e.g., Rubin, Euclid, Roman) which require sub-percent modeling of baryonic effects.

The Kinematic Sunyaev-Zel'dovich (kSZ) effect offers a unique probe for these diffuse baryons. Unlike the thermal SZ effect (dependent on temperature) or X-ray observations (dependent on density squared), the kSZ signal depends solely on the electron momentum field (density $\times$ velocity). This makes it sensitive to the total ionized baryon distribution regardless of temperature or metallicity, allowing direct constraints on gas redistribution driven by feedback mechanisms (AGN and supernovae). However, previous measurements have been limited by sample size and statistical precision.

2. Methodology

The authors present a novel analysis combining 2.4 million spectroscopic Luminous Red Galaxies (LRGs) from the DESI Data Release 2 (DR2) with CMB temperature maps from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6).

Key Methodological Advances:

Harmonic-Space Cross-Correlation: Instead of traditional real-space stacking, the authors construct an explicit momentum-weighted kSZ template in harmonic space.
- They reconstruct the line-of-sight velocity field ( $v_{rec}$ ) for each galaxy using the continuity equation and the observed galaxy density field.
- This velocity field is projected onto the sky to create a template $\hat{\pi}(\theta)$ .
- The template is cross-correlated with the CMB temperature map ( $T_{CMB}$ ) using a pseudo- $C_\ell$ estimator (NaMaster) to measure the angular power spectrum $C_\ell^{\hat{\pi} \times T_{CMB}}$ .
- Advantage: This approach provides clearer scale-dependent information, better decoupling of angular scales, and facilitates joint analysis with other probes (e.g., galaxy-galaxy lensing) within a consistent covariance framework.
Halo Occupation Distribution (HOD) Forward Modeling: To convert measured galaxy-centered profiles into halo-centered profiles, the authors incorporate the HOD framework. They marginalize over HOD parameter uncertainties using a Gaussian Process (GP) emulator trained on $10^3$ evaluations of the class-sz code. This allows them to constrain gas thermodynamic parameters directly around dark matter halos rather than just galaxies.
Configuration Space Validation: As a cross-check, they perform a real-space stacking analysis using Compensated Aperture Photometry (CAP) filters to measure the stacked temperature profile around galaxies.
Null Tests: Extensive null tests were performed, including velocity shuffling (randomizing velocities to destroy the kSZ signal), comparing North/South Galactic Caps, and testing different CMB component separation maps (hILC vs. nILC) to rule out foreground contamination (tSZ, CIB) and systematics.

3. Key Contributions

Unprecedented Precision: The detection of the kSZ signal around LRGs at 18 $\sigma$ significance in both harmonic and configuration space, representing a $\sim$ 3 $\times$ improvement in sample size over previous DESI Year 1 results.
Harmonic-Space Template Construction: The development and application of a momentum-weighted template cross-correlation method in harmonic space, which yields nearly uncorrelated bandpowers and is consistent with large-scale structure analysis frameworks.
HOD-Integrated Gas Profiles: A rigorous framework that propagates HOD uncertainties to derive generalized Navarro-Frenk-White (GNFW) gas profiles centered on dark matter halos, providing empirical targets for hydrodynamical simulations.
Multi-Parameter Constraints: Simultaneous constraints on gas profile normalization, core radius, and outer slope across multiple redshift and stellar mass bins.

4. Key Results

A. Detection and Signal Characteristics

The kSZ signal is detected with high significance ( $SNR \approx 18$ ).
Redshift Evolution: Splitting the sample into four redshift bins ( $0.4 < z < 1.1$ ), the signal is detected in each bin ( $SNR \approx 5-10$ ). The amplitude decreases with redshift, consistent with the expected decline in mean halo mass for a fixed comoving number density.
Mass Dependence: Splitting by stellar mass, the kSZ amplitude scales approximately linearly with stellar mass. The highest mass bin shows a signal 3.6 $\times$ larger than the lowest bin, confirming that more massive galaxies host more extended gas halos.

B. Gas Profile Constraints (GNFW)

The authors fit the data to a Generalized NFW (GNFW) profile.
Deviation from Dark Matter: The measured gas profiles significantly deviate from dark matter-only (NFW) predictions, providing direct evidence that gas is redistributed by feedback processes.
Feedback Efficiency: The data strongly disfavor the Battaglia [79] profile (calibrated from hydrodynamical simulations). The measured signal is suppressed by a factor of $\approx 4$ relative to the Battaglia prediction ( $A_{Batt} = 0.248 \pm 0.015$ ).
Simulation Comparison:
- Illustris-1: Disfavored at $\sim 3\sigma$ .
- IllustrisTNG: Disfavored at $2.5\sigma$ (marginally acceptable only with extreme amplitude rescaling that implies physically implausible halo masses).
- Conclusion: Current state-of-the-art simulations tend to predict gas profiles that are too concentrated or have insufficient feedback efficiency to match the observed kSZ signal in group-scale halos.

C. Systematic Checks

Velocity Reconstruction Correlation ( $r$ ): The analysis relies on the correlation coefficient $r$ between reconstructed and true velocities. The authors adopt $r \approx 0.65$ based on light-cone simulations. They note that while $r$ introduces a systematic uncertainty in absolute amplitude normalization, it does not affect the shape of the profile or the detection significance.
Null Tests: All null tests (velocity shuffling, NGC/SGC split, frequency dependence) yielded results consistent with zero, confirming the signal is driven by coherent peculiar velocities and not foregrounds.

5. Significance and Implications

Baryonic Feedback Constraints: This work provides the most stringent constraints to date on baryonic feedback efficiency in group-scale halos ( $M \sim 10^{13}-10^{14} M_\odot$ ). The results suggest that feedback mechanisms in current simulations are either too weak or operate differently than modeled, leading to gas ejection that is more efficient than predicted by the Battaglia profile.
Benchmark for Simulations: The derived GNFW parameters and the specific suppression of the kSZ signal serve as critical empirical targets for tuning feedback parameters in hydrodynamical simulations (e.g., Flamingo, TNG, Illustris).
Future Cosmology: By demonstrating the power of spectroscopic kSZ measurements, this paper paves the way for using kSZ to map circumgalactic gas and constrain baryonic feedback across the cosmic web. When combined with the companion paper (Part II, covering BGS and ELGs), these results span $0.1 \lesssim z \lesssim 1.6$ and three orders of magnitude in stellar mass, offering a comprehensive view of baryon evolution.
Methodological Legacy: The harmonic-space cross-correlation technique and the HOD-marginalized forward modeling framework set a new standard for analyzing kSZ effects in large spectroscopic surveys, enabling future multi-probe constraints with next-generation CMB experiments (e.g., CMB-S4).

In summary, this paper delivers a high-precision measurement of the kSZ effect that not only confirms the presence of extended gas halos around LRGs but also reveals a tension with current hydrodynamical simulations, highlighting the need for refined feedback models to explain the observed baryon distribution.