🔭 astrophysics

The life of central radio galaxies in clusters: AGN-ICM studies of eRASS1 clusters in the ASKAP fields

This study analyzes 134 radio sources in 151 eRASS1 galaxy clusters to reveal that while AGN mechanical feedback shows a positive but scattered correlation with X-ray cooling luminosity in cool cores, it remains largely ineffective at offsetting radiative losses in high-luminosity clusters, contributing only 13%-22% of the required energy.

Original authors: Angie Veronica, Thomas H. Reiprich, Florian Pacaud, Marcus Brüggen, Bärbel Koribalski, Thomas Pasini, Tessa Vernstrom, Stefan W. Duchesne, Kathrin Böckmann, Jeremy S. Sanders, Y. Emre Bahar, Fabian Ba

Published 2026-01-28

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CC BY 4.0

Original authors: Angie Veronica, Thomas H. Reiprich, Florian Pacaud, Marcus Brüggen, Bärbel Koribalski, Thomas Pasini, Tessa Vernstrom, Stefan W. Duchesne, Kathrin Böckmann, Jeremy S. Sanders, Y. Emre Bahar, Fabian Balzer, Lachlan J. Barnes, Esra Bulbul, Nicolas Clerc, Jessica E. M. Craig, Johan Comparat, Simon Dannhauer, Jakob Dietl, Klaus Dolag, Vittorio Ghirardini, Sebastian Grandis, Duy Hoang, Andrew M. Hopkins, Zsofi Igo, Matthias Kluge, Ang Liu, Konstantinos Migkas, Vanessa A. Moss, Miriam E. Ramos-Ceja, Chris Riseley, Lawrence Rudnick, Mara Salvato, Stanislav Shabala, Riccardo Seppi, Jacco van Loon, Tayyaba Zafar, Xiaoyuan Zhang

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 is filled with massive, invisible "cities" made of hot gas called galaxy clusters. At the very center of these cities sits a supermassive black hole, acting like a giant, cosmic furnace. This paper investigates the relationship between that central furnace (the Active Galactic Nucleus, or AGN) and the hot gas surrounding it (the Intracluster Medium, or ICM).

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

The Problem: A Cooling City

Think of the hot gas in a galaxy cluster like a giant pot of soup on a stove. Normally, this soup is kept hot by the stove. But in these clusters, the stove is turned off, and the soup is radiating its heat away into space as X-rays. If nothing stopped it, this gas would cool down rapidly, lose its pressure, and collapse inward, potentially forming stars at a massive rate.

However, astronomers have noticed something strange: the gas isn't cooling down as fast as physics says it should. Something must be reheating the soup. The leading theory is that the central black hole acts as a "reheater." As it eats gas, it shoots out powerful jets of energy (like a cosmic blowtorch) that puff up bubbles in the gas, warming it back up.

The Experiment: Checking the Thermostat

The researchers wanted to test if this "black hole heater" is actually strong enough to balance the cooling. They looked at 151 galaxy clusters found in a new X-ray survey (eRASS1) and cross-referenced them with radio telescope data (ASKAP) to see the "blowtorches" (radio jets) in action.

They treated the clusters like a sample group to see if:

Bigger clusters (which have more gas to cool) have bigger heaters (more powerful radio jets).
The heat output from the jets is enough to stop the gas from cooling down.

The Findings

1. The Size of the Jet vs. The Distance from Home
The researchers found a curious link between how far the central galaxy is from the exact center of the cluster and how big its radio jets are.

The Analogy: Imagine a campfire. If the fire is right in the middle of a dense crowd (the cluster center), the people crowd around it, and the fire stays small and contained. If the fire is pushed to the edge of the crowd (a larger offset), it has more room to spread out and grow larger.
The Result: They found that galaxies slightly off-center tend to have larger, more spread-out radio jets. It seems the environment "pushes" the jets to expand when they aren't right in the thick of the center.

2. The "Bigger City, Bigger Heater" Trend
They found a weak but real connection: the more massive and luminous the cluster is, the more powerful the central radio galaxy tends to be.

The Analogy: It's like a city with a larger population (more gas) having a larger power plant (the black hole). The more "fuel" available in the cluster, the more the central engine revs up.

3. The Big Surprise: The Heater Isn't Strong Enough for the Biggest Cities
This is the most critical finding. The team calculated the energy coming out of the jets and compared it to the energy the gas is losing as it cools.

The Analogy: Imagine trying to keep a massive swimming pool warm in winter using a small space heater.
- In small, cooler pools (low-mass clusters), the heater works great. It balances the heat loss perfectly.
- In huge, deep pools (high-mass, high-luminosity clusters), the heater is woefully inadequate.
The Result: In the most massive clusters, the black hole's jets only provide about 13% to 22% of the energy needed to stop the gas from cooling. The "heater" is running, but it's not enough to keep the "soup" from getting cold in the biggest clusters.

Why This Matters

The paper suggests that while black holes are great at regulating small to medium-sized clusters, they might not be the sole solution for the universe's biggest structures. If the black hole isn't doing all the work, something else must be helping to keep the gas warm, or perhaps the cooling process in these massive clusters works differently than we thought.

Summary

What they did: Compared the "heat" (X-rays) and the "heaters" (radio jets) in 151 galaxy clusters.
What they found: Bigger clusters have bigger heaters, but the heaters in the biggest clusters are too weak to stop the gas from cooling down.
The takeaway: The central black hole is a key player, but in the largest cosmic cities, it's not strong enough to do the job alone.

Technical Summary: The Life of Central Radio Galaxies in Clusters: AGN-ICM Studies of eRASS1 Clusters in the ASKAP Fields

Problem Statement
The intracluster medium (ICM) in galaxy clusters radiates thermal energy primarily through X-ray emission, leading to rapid cooling in dense cores. Without a compensating heating mechanism, this would result in a "cooling flow" where gas loses pressure support and flows inward at rates up to $1,000 M_\odot \text{yr}^{-1}$ . However, observations reveal a deficit of cold gas and star formation rates far lower than predicted by cooling flow models. While Active Galactic Nucleus (AGN) feedback is widely regarded as the primary mechanism for heating the ICM and regulating cooling, the efficiency of this feedback, particularly in balancing radiative losses across different cluster masses and dynamical states, remains a subject of investigation. This study aims to quantify the relationship between AGN feedback power and ICM cooling by correlating the radio properties of Brightest Cluster Galaxies (BCGs) with the X-ray properties of their host clusters.

Methodology
The authors constructed a sample of 151 galaxy clusters from the first SRG/eROSITA All-Sky Survey (eRASS1) located within three specific fields observed by the Australian SKA Pathfinder (ASKAP): PS1, PS2, and SWAG-X.

Data Sources: X-ray properties (luminosity $L_X$ , mass $M_{500}$ , radius $R_{500}$ , and concentration parameter $c_{R500}$ ) were derived from the eRASS1 primary cluster catalogue. Radio data were obtained from ASKAP observations at 944 MHz (PS1, PS2) and 888 MHz (SWAG-X).
Sample Selection: The initial 260 eRASS1 clusters detected in these fields were filtered by an extent likelihood threshold ( $L_{EXT} \ge 6$ ) to ensure high purity (94%), resulting in a final sample of 151 clusters.
Radio Source Identification: The authors cross-matched BCG positions with ASKAP source catalogues (Böckmann et al. 2023 for PS1; PyBDSF for SWAG-X) and performed manual inspections for the remaining sources. They identified 134 radio sources associated with BCGs, treating non-detections as upper limits (3 $\sigma$ ).
Parameter Derivation:
- Radio Properties: Radio luminosity ( $L_{944 \text{ MHz}}$ ) and Largest Linear Size (LLS) were calculated. Non-detected sources were assigned beam sizes as their LLS.
- Dynamical State: Clusters were categorized into Cool Cores (CC) and Non-Cool Cores (NCC) using the concentration parameter $c_{R500}$ . A threshold of $c_{R500} > 0.26$ (the sample median) defined the CC subsample (75 clusters). BCG offsets from the X-ray center were measured to assess dynamical disturbance.
- Mechanical Power: The mechanical jet power ( $L_{\text{mech}}$ ) was estimated from radio luminosity using scaling relations from Heckman & Best (2014) and Shabala & Godfrey (2013).
- Cooling Radius: To better compare AGN heating with cooling, the integrated X-ray luminosity was scaled from the standard 300 kpc aperture to a cooling radius ( $R_{\text{cool}} \approx 0.08 R_{500}$ ), yielding $L_{X, r<R_{\text{cool}}}$ .
Statistical Analysis: Correlations were analyzed using linear regression (ASURV and LinMix packages) in log-log space. Robustness was tested via bootstrap resampling, partial correlation Kendall's $\tau$ tests (to account for redshift dependence), and flux-flux analyses.

Key Results

Sample Characteristics: The sample spans redshifts $0.033 \le z \le 1.126$ and X-ray luminosities $2.72 \times 10^{42} \le L_X \le 1.32 \times 10^{45} \text{ erg s}^{-1}$ . The radio detection rate for BCGs was 89%.
Morphology and Environment: A statistically significant positive trend was observed between the LLS of central radio sources and the BCG offset from the cluster center ( $p=0.0008$ ). This suggests that radio sources in more dynamically disturbed environments (larger offsets) tend to be more extended, potentially due to environmental influences on morphology.
Radio-X-ray Correlation: A weak but statistically significant positive correlation was found between the radio luminosity of BCGs and the X-ray luminosity of host clusters ( $L_R \propto L_X^{0.70 \pm 0.18}$ ). However, partial correlation tests suggest this relationship is heavily influenced by selection effects, particularly driven by high-flux clusters.
AGN Feedback Efficiency in Cool Cores:
- In the CC subsample, a positive but highly scattered correlation exists between mechanical jet power ( $L_{\text{mech}}$ ) and cooling X-ray luminosity ( $L_{X, r<R_{\text{cool}}}$ ).
- The linear fit indicates that AGN feedback becomes increasingly ineffective at offsetting radiative losses as cluster luminosity (and mass) increases.
- At a cooling luminosity of $L_{X, r<R_{\text{cool}}} \approx 5.50 \times 10^{43} \text{ erg s}^{-1}$ , AGN feedback contributes only 13% to 22% of the energy required to balance radiative losses.
- The best-fit line intersects the 1:1 line at a low luminosity ( $\approx 8.18 \times 10^{41} \text{ erg s}^{-1}$ ), suggesting that above this threshold, AGN feedback alone is insufficient to prevent cooling in these systems.

Significance and Claims
The paper claims to provide a statistical assessment of AGN-ICM coupling using a large, homogeneous sample of eRASS1 clusters with deep ASKAP radio data. The primary significance lies in the finding that AGN feedback is ineffective in high-luminosity (high-mass) clusters. While AGN jets may balance cooling in lower-luminosity systems, they fail to supply sufficient energy in massive clusters, contributing only a fraction (13–22%) of the required heating at high cooling rates. This supports the notion that additional heating mechanisms (e.g., turbulence, sloshing, or supernovae) or different feedback regimes may be necessary in massive systems. The authors note that their results are consistent with previous studies (e.g., Gupta et al. 2020; Igo & Merloni 2025) that found low ratios of jet kinetic energy to cooling energy in massive clusters. The study also highlights the importance of considering selection effects and the limitations of current survey depths in interpreting these correlations.

The Problem: A Cooling City

The Experiment: Checking the Thermostat

The Findings

Why This Matters

Summary

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