🔭 astrophysics

Measurements of the Thermal Sunyaev-Zel'dovich Effect with ACT and DESI Luminous Red Galaxies

This study utilizes high-resolution CMB maps from the Atacama Cosmology Telescope and luminous red galaxy catalogs from DESI to detect the thermal Sunyaev-Zel'dovich effect with 19σ significance across redshifts 0.4 to 1.2, while addressing dust contamination and providing insights into halo thermodynamics and baryon distribution.

Original authors: R. Henry Liu, Simone Ferraro, Emmanuel Schaan, Rongpu Zhou, Jessica Nicole Aguilar, Steven Ahlen, Nicholas Battaglia, Davide Bianchi, David Brooks, Todd Claybaugh, Shaun Cole, William R. Coulton, Axel

Published 2026-02-20

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: R. Henry Liu, Simone Ferraro, Emmanuel Schaan, Rongpu Zhou, Jessica Nicole Aguilar, Steven Ahlen, Nicholas Battaglia, Davide Bianchi, David Brooks, Todd Claybaugh, Shaun Cole, William R. Coulton, Axel de la Macorra, Arjun Dey, Kevin Fanning, Jaime E. Forero-Romero, Enrique Gaztañaga, Yulin Gong, Satya Gontcho A Gontcho, Daniel Gruen, Gaston Gutierrez, Boryana Hadzhiyska, Klaus Honscheid, Cullan Howlett, Robert Kehoe, Theodore Kisner, Anthony Kremin, Aleksandra Kusiak, Andrew Lambert, Martin Landriau, Laurent Le Guillou, Michael Levi, Martine Lokken, Marc Manera, Paul Martini, Aaron Meisner, Ramon Miquel, Kavilan Moodley, Jeffrey A. Newman, Gustavo Niz, Nathalie Palanque-Delabrouille, Will Percival, Francisco Prada, Ignasi Pérez-Ràfols, Bernardita Ried Guachalla, Graziano Rossi, Eusebio Sanchez, David Schlegel, Michael Schubnell, Hee-Jong Seo, Cristóbal Sifón, David Sprayberry, Gregory Tarlé, Eve M. Vavagiakis, Benjamin Alan Weaver, Edward J. Wollack, Hu Zou

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: Finding the "Invisible" Gas

Imagine the universe is a giant, dark ocean. We know there are islands (galaxies) floating in it, but we suspect there is a massive amount of invisible water (gas) filling the space between them. Scientists call this the "missing baryons." We can't see this gas with regular telescopes because it's too hot and too thin to glow on its own.

So, how do we find it? The authors of this paper decided to use the Cosmic Microwave Background (CMB) as a giant, cosmic flashlight. The CMB is the oldest light in the universe, a faint glow that fills the entire sky.

When this ancient light passes through the hot gas surrounding galaxies, it bumps into electrons (tiny charged particles). This collision gives the light a little "kick," changing its energy. This phenomenon is called the Thermal Sunyaev-Zel'dovich (tSZ) effect.

Think of it like this: If you shine a flashlight through a foggy window, the light gets scattered and changes slightly. By measuring exactly how the "cosmic flashlight" changes as it passes through the neighborhood of a galaxy, we can figure out how much hot gas is there and how hot it is.

The Tools: A High-Res Camera and a Giant Catalog

To do this, the team used two massive tools:

ACT (Atacama Cosmology Telescope): A telescope in the Chilean desert that acts like a high-resolution camera for the CMB. It takes incredibly sharp pictures of that ancient light.
DESI (Dark Energy Spectroscopic Instrument): A robotic telescope that has taken pictures of millions of galaxies. Specifically, they focused on Luminous Red Galaxies (LRGs). You can think of these as the "elder statesmen" of the galaxy world—big, bright, red, and easy to spot.

The researchers took the DESI catalog of these red galaxies and used them as "pins" on a map. They then looked at the ACT telescope's CMB map right at those pin locations to see if the light had been "kicked" by gas.

The Challenge: The "Dust" Problem

Here is where it gets tricky. When looking at the CMB, there is a lot of "noise" or "static."

Imagine you are trying to hear a whisper (the gas signal) in a room where someone is playing a loud radio (the CMB) and someone else is blowing a horn (dust from the galaxies themselves).

The biggest problem in this study was dust. The galaxies they were studying contain dust, which emits its own infrared light. This dust light looks very similar to the signal they were trying to measure. It's like trying to measure the heat of a campfire, but the person holding the thermometer is also wearing a warm wool hat that confuses the reading.

If they didn't account for this dust, they would think there was more hot gas than there actually was.

The Solution: The "Noise-Canceling" Headphones

The team developed a clever way to filter out the dust, similar to how noise-canceling headphones work.

The "Stacking" Trick: Instead of looking at one galaxy (which is too faint to see clearly), they took millions of galaxies and stacked their data on top of each other. It's like taking a photo of a single firefly in the dark (you can't see it) versus taking a photo of a million fireflies all at once (suddenly, you see a bright glow).
The "Moment Deprojection": They realized that the dust signal behaves slightly differently than the gas signal depending on the "color" (frequency) of the light. By mathematically subtracting the specific "signature" of the dust, they could isolate the gas signal.
- Analogy: Imagine you have a soup that tastes like both salt and pepper. If you know exactly how much pepper is in the soup, you can mathematically "remove" the pepper taste to see how salty the soup really is. They did this with light frequencies.

What They Found

After doing all this complex math and filtering, they found some exciting things:

A Huge Detection: They successfully detected the hot gas around these galaxies with a confidence level of 19 sigma. In science, 5 sigma is usually enough to say "we found it." 19 sigma is like winning the lottery every single day for a year. It is an incredibly strong signal.
The Gas is "Expelled": When they compared their findings to computer simulations of how galaxies should form, they found a mismatch. The simulations predicted a lot of gas close to the center of the galaxy. The real data showed less gas near the center.
- Analogy: It's like a party where the simulation predicted everyone would stay in the living room. But the data showed that the "host" (the galaxy) was so rowdy (feedback from stars and black holes) that they kicked most of the guests (the gas) out into the backyard.
The Dust is Real: They confirmed that dust is a major contaminant. If you don't remove it, your measurements are wrong. They showed that their new method of removing dust works well, reducing the uncertainty significantly.

Why Does This Matter?

This paper helps us answer two big questions:

Where is the missing matter? It confirms that a lot of the universe's "missing" normal matter is hiding as hot gas around galaxies.
How do galaxies grow? By seeing that the gas is pushed away from the center, it tells us that galaxies are violent places. The energy from stars and black holes is strong enough to blow gas out of the galaxy, which stops the galaxy from growing too big too fast.

In a Nutshell

The authors used the universe's oldest light as a backlight to see the invisible hot gas surrounding millions of red galaxies. They had to be very careful to subtract the "static" caused by dust, using a new mathematical trick. They found that the gas is there, but it's been pushed away from the center of the galaxies, likely by the violent energy of star formation. This gives us a better map of the invisible universe and helps us understand how galaxies evolve.

1. Problem Statement

The baryonic content of the universe is a critical component of cosmological models, yet observations of galaxies in the late-time universe account for only ~10% of the expected baryon abundance. The majority of "missing baryons" are hypothesized to reside in the warm-hot intergalactic medium (WHIM) and the circumgalactic medium (CGM).

To locate and characterize these baryons, researchers utilize the Thermal Sunyaev-Zel'dovich (tSZ) effect, where Cosmic Microwave Background (CMB) photons scatter off hot free electrons in and around galaxies, gaining energy. While the tSZ effect is a powerful tracer of gas pressure and halo thermodynamics, previous measurements have been limited by:

Foreground Contamination: Specifically, thermal dust emission from the target galaxies and the Cosmic Infrared Background (CIB), which mimics or distorts the tSZ signal.
Resolution: Ground-based telescopes are needed to resolve gas on scales closer to galaxy centers compared to space-based missions like Planck.
Model Dependence: Previous attempts to remove dust contamination often relied on fixed spectral models that proved unstable.

This paper addresses these challenges by combining high-resolution CMB data with a large, spectroscopically calibrated galaxy sample to measure the tSZ signal with high significance while rigorously quantifying and mitigating dust contamination.

2. Methodology

Data Sets:

Galaxy Sample: The study uses Luminous Red Galaxies (LRGs) from the Dark Energy Spectroscopic Instrument (DESI). The sample is divided into four photometric redshift bins spanning $z \approx 0.4$ to $1.2$. The sample includes over 7 million galaxies (after masking) with a mean halo mass of $\log_{10} M_h \approx 13.4$ .
CMB Maps: High-resolution Compton- $y$ maps (tracing the tSZ effect) from the Atacama Cosmology Telescope (ACT) DR6, combined with Planck data. These maps were processed using a Needlet Internal Linear Combination (NILC) pipeline to separate the tSZ signal from other components.

Analysis Techniques:

Stacking: The authors stack the CMB temperature maps at the locations of the LRGs to extract the average tSZ signal.
Filtering (CAP): They employ Compensated Aperture Photometry (CAP) filters (disk-ring filters). This technique measures the integrated signal within a central disk and subtracts the signal from a surrounding ring of equal area. This cancels out large-scale CMB fluctuations and isolates the signal on specific angular scales (1–6 arcminutes, corresponding to ~0.5–4 virial radii).
CIB Mitigation Strategies:
- Simple Deprojection: Attempting to remove CIB using a fixed spectral energy distribution (SED) model.
- Moment Deprojection: A more robust method involving the deprojection of the CIB spectral template and its first derivative (with respect to the spectral index $\beta$ ). This removes the first-order sensitivity to the assumed spectral parameters, reducing modeling uncertainty.
Statistical Analysis: Covariance matrices were computed using bootstrap resampling (10,000 iterations) of the galaxy catalog to account for noise correlations between different aperture sizes.

3. Key Contributions

High-Significance Detection: The study achieves a combined detection significance of 19 $\sigma$ for the tSZ signal stacked on DESI LRGs across four redshift bins.
Quantification of Dust Contamination: The authors demonstrate that residual dust emission from the target galaxies is a dominant source of systematic error in standard tSZ stacks, particularly at small apertures (central regions).
Robust Mitigation Strategy: They establish that moment-deprojected maps (deprojecting both the CIB SED and its derivative) provide a stable signal. While simple deprojection is highly sensitive to assumed spectral parameters ( $\beta$ ), the moment-deprojection method reduces modeling uncertainty to ~10–20%.
Comparison with Simulations: The measured profiles are compared against the IllustrisTNG (TNG300) hydrodynamical simulation. The data reveals a significant deficit of gas at radii $R < 3$ arcmin compared to the simulation, suggesting that feedback mechanisms in real galaxies are stronger than those modeled in TNG300.

4. Key Results

Signal Detection: The stacked tSZ profiles show a clear positive signal consistent with hot gas in halos. The signal-to-noise ratio (SNR) ranges from ~19 to ~28 depending on the redshift bin and map type.
Dust Impact:
- In the fiducial (non-deprojected) maps, a "negative" signal is observed at the center of the stack (due to the CAP filter subtracting the positive dust emission at the center).
- Simple CIB Deprojection: Results vary wildly depending on the assumed spectral index $\beta$ (tested at 1.2, 1.4, 1.6), indicating high instability.
- Moment Deprojection: Yields a stable profile that converges with the fiducial map at large radii but corrects the central dust feature. The noise penalty for this method is manageable (~30% increase in noise).
Simulation Comparison:
- When matched to the same gas content at large radii, the observed tSZ signal is significantly lower at small radii than the TNG300 simulation predicts.
- This implies that galaxy feedback (e.g., AGN or stellar feedback) is more efficient at expelling gas from the inner halo than the simulation suggests.
- A direct amplitude fit yields a poor $\chi^2$ (983 for 8 dof), confirming a structural mismatch between the simulation and data independent of normalization.

5. Significance and Future Outlook

Baryon Census: This work provides one of the most precise measurements of the thermal pressure of gas around LRGs, contributing to the census of missing baryons in the low-redshift universe.
Feedback Constraints: The discrepancy with simulations offers direct observational constraints on the strength of feedback mechanisms in galaxy formation, suggesting current models may underestimate gas expulsion.
Methodological Advancement: The paper validates the moment-deprojection technique as a necessary tool for future high-precision tSZ studies, particularly as next-generation experiments (like CMB-S4) require sub-percent level systematics control.
Synergy: These tSZ measurements are designed to be combined with kinematic SZ (kSZ) and weak lensing measurements of the same DESI sample (in upcoming papers). This multi-probe approach will allow for model-independent constraints on gas density, temperature, and the full thermodynamic state of dark matter halos.

In conclusion, this paper successfully isolates the tSZ signal from DESI LRGs, rigorously addresses the challenge of dust contamination through advanced component separation, and provides evidence that current hydrodynamical simulations may need to be revised to account for stronger feedback processes in massive halos.