Title: Cosmic Rays Masquerading as Cool Cores: An Inverse-Compton Origin for Cool Core Cluster Emission
Authors: Philip F. Hopkins, Eliot Quataert, Emily M. Silich, Jack Sayers, Sam B. Ponnada, Isabel S. Sands
First Author’s Institution: Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
Status: Submitted to the Open Journal of Astrophysics, available on arxiv
Galaxy clusters are the largest structures in the universe held together by gravity. Most of the space between galaxies in a cluster is filled with extremely hot, ionized gas. But in some clusters, known as “cool core” clusters, the gas at the center is noticeably cooler—about 30–50% lower in temperature than the outer regions. Astronomers have long believed that this hot gas gives off X-rays, and they’ve used X-ray observations to study the amount and properties of gas in clusters. However, there’s a puzzling mismatch in cool core clusters: the amount of gas estimated from X-ray line emissions (sharp peaks in the X-ray spectrum) is about one order of magnitude less than what is expected from the smooth background glow (the X-ray continuum), assuming it all comes from hot gas. To fix this problem, scientists have tried adjusting how energy from the central galaxy, especially from its active black hole, heats the surrounding gas. But these models usually either heat the core too much (making it no longer “cool”) or push too much gas away from the center, which then doesn’t match the observational profile we see in real clusters. In this paper, the authors suggest that the X-ray glow seen in the centers of galaxy clusters might not come from hot gas, as previously thought. Instead, these X-rays could come from low-energy cosmic microwave background (CMB) photons boosted by ancient cosmic ray (CR) electrons via inverse-Compton (IC) scattering.
CR electrons are high-energy particles launched into space by powerful events such as jets from active galactic nuclei (AGN). After being released, they gradually lose energy through two main processes: synchrotron radiation, as they spiral around magnetic fields, and IC scattering, as they collide with background photons and transfer energy to them. Because higher-energy electrons lose energy quickly, they fade rapidly, while lower-energy electrons, those below 1 GeV, can survive much longer. These long-lived “ancient” CR electrons are able to travel large distances, up to around 100 kiloparsecs, before finally losing their energy by scattering off CMB photons and converting them into X-ray light in the keV range. The authors show that if these electrons interact only with CMB photons, the resulting IC emission would closely mimic the kind of X-ray pattern expected from hot gas, which can be modeled using standard X-ray analysis tools that assume a mix of gas at different temperatures. This means that some of the X-rays observed in cool-core clusters, previously thought to come from hot gas, may actually be produced by CR electrons instead.
To estimate how much IC emission from CR electrons might influence what we observe in cluster cores, the authors explore an extreme case: they assume gas properties similar to a non-cool-core cluster, where the contribution from thermal gas is minimal. Using a typical rate at which these electrons are produced from active galaxies, they calculate the expected X-ray surface brightness, as well as the apparent gas density, temperature, and pressure of the resulting halo. Their results show that IC emission alone can reproduce much of the observed X-ray surface brightness within 100 kiloparsecs. From this brightness, one can infer gas properties that closely resemble those seen in cool-core clusters. However, these inferred properties tend to overestimate the gas density and pressure, while underestimating the true temperature of the thermal gas in the center. In short, the authors’ model suggests that long-lived CR electrons can produce X-rays that convincingly mimic the signatures of a cool-core cluster, even when the actual thermal gas plays only a minor role.
Figure 1. Surface brightness (upper left), gas density (upper right), temperature (lower left), and pressure (lower right) profiles, assuming non–cool-core gas conditions and a typical CR electron injection power of 10⁴⁴ erg/s. The surface brightness profile shows that IC emission from CR electrons can dominate the X-ray signal in the 0.1–1 keV range. This leads to an overestimation of the gas density and pressure, and an underestimation of the true thermal gas temperature when interpreting the X-rays as purely thermal in origin.
Based on estimates from their model, the authors propose an alternative view of how cool-core clusters evolve. In contrast to the traditional theory, which explains the observed X-ray brightness as a balance between energy injected from the AGN and gas cooling, the authors suggest a different story. Gas cooling can play a role as the cluster transitions from a non-cool-core to a cool-core state. Once the AGN becomes active and starts injecting CRs through its jets, the X-ray emission at keV energies becomes dominated by IC radiation from long-lived CR electrons. This radiation appears to have a similar magnitude and spectrum to thermal emission, even though it comes from CRs. This new explanation avoids the need for fine-tuning the energy released by AGN and naturally explains the observed connection between AGN activity, X-ray brightness, and radio emission—since the same CR electrons are responsible for both the radio and X-ray signals.
Since the radio and gamma-ray signals predicted by this model are far too faint to detect with current instruments, the authors suggest two alternative ways to test the CR-IC scenario. First, because CRs can provide extra pressure support, they can cause the thermal gas pressure to be overestimated when inferred from X-ray data. To reveal how much of the X-ray emission actually come from hot gas, one can compare this “apparent” pressure from X-rays with the “true” pressure estimated from the Sunyaev-Zeldovich (SZ) effect, a distortion of CMB caused by hot gas. Observations with high enough resolution already show that, in the central regions of cool-core clusters, the ratio of SZ-inferred pressure to X-ray-inferred pressure drops below 1, just as the CR-IC model predicts. Second, the model also affects metal emission lines. If most of the X-ray continuum comes from CR electrons instead of thermal gas, then the metal lines, produced by cooling gas, will appear weaker than the true value, since they’re measured against a non-thermal background. This could help explain why metallicities measured from X-rays appear sub-Solar, while optical and UV data suggest much higher, super-Solar values in the centers of nearby clusters. Together, these two observational signatures offer promising ways to test whether CRs are powering much of the X-ray glow in cluster cores.
Figure 2. A new interpretation of cool-core cluster formation proposed in this study. As a cluster transitions from a non–cool-core to a cool-core state, some gas cooling takes place. However, once the central AGN becomes active and begins accelerating CR electrons, IC emission dominates the X-ray output, producing a thermal-like signal as the actual thermal gas contribution is low.
This work presents a compelling alternative to the traditional view of cool-core clusters by highlighting the role of long-lived CR electrons. Instead of relying on finely tuned feedback from active galaxies to balance cooling, the CR-IC model offers a natural explanation for many observed features—including X-ray brightness, pressure profiles, and metallicity measurements. While direct detection of CRs remains challenging, indirect signatures like SZ-to-X-ray pressure comparisons and enhanced metal lines provide promising ways to test this idea. If proven right, this model could change the way we think about what’s really going on in the centers of galaxy clusters.
Astrobite edited by Lucie Rowland.
Featured image credit: Hopkins et al. (2025)
