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Thermal Sunyaev-Zel'dovich Measurements of Locally Bright Galaxies with ACT DR6: Radio Source Contamination and Excess Compton-y Signal

Using high-resolution ACT DR6 data, this study confirms the stellar mass–tSZ signal scaling for Locally Bright Galaxies while revealing a previously unaccounted-for radio source contamination and a factor-of-two Compton- $y$ excess in galaxies hosting co-spatial radio sources, likely driven by AGN feedback or halo mass differences.

Original authors: Nicholas Battaglia, Jean-Baptiste Melin, J. Colin Hill, James Bartlett

Published 2026-07-10

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CC BY 4.0

Original authors: Nicholas Battaglia, Jean-Baptiste Melin, J. Colin Hill, James Bartlett

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 of hot gas surrounding every galaxy. This gas is so hot that it's invisible to our eyes, but it leaves a fingerprint on the oldest light in the universe—the Cosmic Microwave Background (CMB). When light from the Big Bang passes through this hot gas, it gets a little "kick" in energy, like a ping-pong ball hitting a moving paddle. Astronomers call this the thermal Sunyaev-Zel'dovich (tSZ) effect, and it's their way of weighing the invisible gas around galaxies.

For years, scientists using the Planck satellite found a neat pattern: the heavier a galaxy's stars were, the more "kick" the surrounding gas gave the light. It was a perfect power-law relationship, like a straight line on a graph. But now, a team of astronomers has taken a closer look using a sharper, more powerful telescope called the Atacama Cosmology Telescope (ACT). They didn't just confirm the pattern; they found a sneaky twist that changes the story.

The Sneaky Radio Ghost
When the team zoomed in on these "Locally Bright Galaxies" (LBGs), they noticed something weird. Some of these galaxies were hosting bright radio sources—think of them as cosmic radio towers, likely powered by hungry black holes at the galaxy's center. These radio sources were acting like a ghost in the machine. Because radio waves have a different "color" (frequency) than the gas signal, they were messing up the measurements, creating a small but noticeable error of a few percent. The team realized that previous studies hadn't fully accounted for this radio noise.

The Big Surprise: A Halo-Scale Excess
Here is the juicy part. The team split the galaxies into two groups: those with these co-spatial radio sources and those without. They kept the stellar mass the same for both groups to make a fair comparison. What they found was startling. The galaxies with the radio sources had a Compton-y signal (the measure of that hot gas "kick") that was roughly twice as strong as the galaxies without radio sources.

This wasn't just a tiny blip in the center of the galaxy. The extra signal stretched out to at least 6 arcminutes away from the center. To put that in perspective, that's a huge distance in the sky, suggesting the effect isn't just a local radio tower interfering with the sensor; it's a massive, halo-scale phenomenon affecting the entire cloud of gas surrounding the galaxy.

What Caused the Extra Heat?
The authors are careful not to claim they have solved the mystery. Instead, they suggest two main possibilities, and they can't yet tell which one is the culprit:

The Heavy Halo Theory: Maybe the galaxies with radio sources are actually sitting inside much heavier "halos" of dark matter than the others. If the halo is about 50% more massive, it would naturally hold more hot gas, explaining the stronger signal. Previous studies hinted at this, but the current data isn't strong enough to prove it definitively.
The AGN Feedback Theory: Or, perhaps the radio sources (powered by Active Galactic Nuclei) are actively pumping extra heat into the surrounding gas. The team calculated that the energy output from these black holes, if they are inefficiently eating matter (a state called Advection-Dominated Accretion Flows), is just enough to account for the extra thermal energy they measured. However, they also noted that this energy is likely not quite enough to blow the gas away completely; it's more like a warm bath than a hurricane.

What This Means
The paper doesn't declare a victory or a final answer. Instead, it sounds a warning bell for future research. It shows that if you want to measure the gas around galaxies, you have to be very careful about which galaxies you pick. If you include galaxies with radio sources without checking, your data will be skewed.

The authors suggest that future telescopes, like the upcoming DSA-2000 radio survey or the Simons Observatory, will help clear up the dust (literally and figuratively) by better identifying these radio sources. Until then, the universe is still keeping a secret: are those extra-hot galaxies just sitting in heavier boxes, or are their black holes really cooking the gas? The answer is still out there, waiting for sharper eyes to find it.

Technical Summary: Thermal Sunyaev-Zel'dovich Measurements of Locally Bright Galaxies with ACT DR6

Problem Statement
The connection between a galaxy's stellar mass and the thermodynamic properties of its circumgalactic medium (CGM) is fundamental to understanding galaxy evolution. Previous work by the Planck collaboration (Planck Collaboration et al. 2013) identified a remarkably tight power-law relationship between stellar mass and the thermal Sunyaev-Zel'dovich (tSZ) signal for the Locally Bright Galaxy (LBG) sample. However, Planck's large beam size limits the ability to spatially resolve the pressure profile on CGM scales and disentangle the tSZ signal from contaminants. Furthermore, potential contamination from radio sources in component-separated maps had not been rigorously characterized in this context. This paper re-examines the LBG tSZ signal using higher-resolution data to assess residual contamination and investigate the physical origins of the tSZ signal in galaxies hosting radio sources.

Methodology
The authors utilize the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) component-separated Compton- $y$ maps, which offer lower noise and higher angular resolution (1.6 arcmin FWHM) compared to Planck. The analysis focuses on the LBG sample (Planck Collaboration et al. 2013), restricted to galaxies overlapping the ACT footprint with stellar masses $11.1 \le \log_{10}(M_*/M_\odot) \le 12.0$ .

Key methodological steps include:

Component Separation: The study employs three types of Internal Linear Combination (ILC) maps: a nominal ILC, a CIB-deprojected ILC, and a moment-based CIB-deprojected ILC. The moment-based approach is used as the fiducial result to robustly suppress Cosmic Infrared Background (CIB) residuals without requiring precise prior knowledge of the CIB spectral energy distribution (SED).
Stacking and Flux Extraction: The authors use compensated aperture photometry (CAP) with a 3-arcminute aperture to extract the cylindrical Compton- $y$ signal ( $Y_{3'}^{\text{cyl}}$ ) from stacked sub-maps centered on LBGs. This method avoids assumptions about the pressure profile shape, unlike the matched filters used in the original Planck analysis.
Radio Source Subselection: The LBG sample is cross-matched with the NRAO VLA Sky Survey (NVSS) at 1.4 GHz. The sample is divided into two subsamples: galaxies with co-spatial radio sources (17% of the total) and those without ("Radio Cut").
Systematic Checks: The authors perform robustness tests against CIB contamination by varying SED parameters ( $\beta$ and $T_{\text{CIB}}^{\text{eff}}$ ) and verify that the observed signals are not artifacts of the component separation method.

Key Contributions and Results

Power-Law Scaling: The study recovers a consistent power-law scaling between the cylindrical Compton- $y$ signal and stellar mass, validating the original Planck findings over a slightly smaller mass range due to the smaller ACT footprint and the use of assumption-free flux extraction.
Radio Source Contamination: The analysis identifies a negative decrement in the Compton- $y$ maps at the center of stacked galaxies containing co-spatial radio sources. This is attributed to low-frequency synchrotron emission that persists in the component-separated maps, a contamination source previously unquantified in this context.
Excess Compton- $y$ Signal: A primary finding is a systematic excess in the Compton- $y$ $y$ signal for LBGs hosting co-spatial radio sources compared to those without, at fixed stellar mass. This excess is approximately a factor of two in $Y_{3'}^{\text{cyl}}$ $Y_{3^{'}}^{cyl}$ .
- This excess persists to radii of at least 6 arcminutes, suggesting it is a halo-scale effect rather than a central point-source artifact.
- The result is robust across different ILC methods and CIB deprojection parameters.
- The same excess is recovered when applying the radio source subselection to the original Planck MMF results.
Energetics: The authors calculate the thermal energy ( $E_{\text{th}}$ ) associated with the excess signal. The additional energy in radio-hosting galaxies is comparable to the total thermal energy of non-radio hosts. They estimate that energy injection from Active Galactic Nuclei (AGN) via Advection-Dominated Accretion Flows (ADAFs) is sufficient to account for this excess thermal energy, though it is likely insufficient to unbind the halo gas.

Significance and Claims
The paper claims that the identification of a factor-of-two excess tSZ signal in radio-hosting galaxies is a critical finding with direct implications for tSZ cross-correlation measurements. The authors assert that:

Radio Source Fraction Matters: Future analyses of tSZ signals in galaxy samples must carefully characterize the fraction of galaxies hosting radio sources, as this introduces a systematic bias in the measured thermal energy.
Physical Interpretation: The excess signal is intrinsic to the galaxies and likely reflects either a systematic difference in halo mass at fixed stellar mass (consistent with Mandelbaum et al. 2009) or thermal energy injected into the CGM by AGN feedback. The authors note they cannot currently distinguish between these two explanations with the available data.
Methodological Impact: The study highlights the necessity of low-frequency radio surveys (e.g., DSA-2000) to identify and mitigate radio contamination in tSZ measurements. It also suggests that modeling the specific selection effects of the LBG sample (including radio source fractions) is essential for accurate comparisons with cosmological simulations.

The authors remain modest regarding the distinction between halo mass differences and AGN feedback, stating that while both explanations are well-motivated and consistent with the data, current CMB lensing measurements cannot statistically distinguish between the halo masses of the two subsamples. They propose that future galaxy-galaxy lensing measurements from surveys like LSST and Euclid will be required to resolve this question.

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