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ICM-SHOX III. The case of MACS J0018.5+1626, a radio relic that looks like a radio halo?

This paper presents numerical modeling within the ICM-SHOX project demonstrating that the radio emission from MACS J0018.5+1626, which appears as a radio halo in LOFAR observations, is likely the result of two face-on merger-driven shocks produced by a binary cluster merger near pericenter passage.

Original authors: P. Domínguez-Fernández, J. A. ZuHone, E. M. Silich, E. Bellomi, J. Sayers, T. Mroczkowski, A. Botteon, R. J. van Weeren, L. Hernquist, G. Brunetti, J. Golec, S. Gupta

Published 2026-02-06

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: P. Domínguez-Fernández, J. A. ZuHone, E. M. Silich, E. Bellomi, J. Sayers, T. Mroczkowski, A. Botteon, R. J. van Weeren, L. Hernquist, G. Brunetti, J. Golec, S. Gupta

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: A Cosmic Collision

Imagine two massive galaxy clusters (groups of hundreds of galaxies held together by gravity) crashing into each other. This isn't a gentle bump; it's a high-speed collision that sends shockwaves rippling through the invisible gas filling the space between the galaxies.

Usually, when astronomers look at these collisions from the side, they see long, thin arcs of radio waves, like a cosmic "scar" or a "relic" of the crash. These are called radio relics. However, in this specific case, the cluster MACS J0018.5+1626 looks different. From Earth, it looks like a big, round, fuzzy blob of radio waves right in the center, which astronomers usually call a radio halo.

The big question the authors asked was: Is this actually a halo, or is it just two radio relics that we are looking at from a very strange angle?

The Detective Work: Putting the Clues Together

To solve this mystery, the team didn't just look at the radio waves. They acted like cosmic detectives, gathering clues from different parts of the electromagnetic spectrum:

X-rays: To see the hot gas.
Optical light: To see the galaxies.
Gravitational Lensing: To weigh the invisible dark matter.
The Sunyaev–Zel'dovich Effect: To measure how the gas is moving.

They used a sophisticated computer pipeline called ICM-SHOX to match all these clues against simulations. The result? The evidence strongly suggests that this cluster is in the middle of a crash, and we are looking at it almost head-on (like looking down the barrel of a gun), rather than from the side.

The Simulation: Replaying the Crash

The authors built a 3D computer model of this crash. Think of it like a high-end video game simulation where they can control the speed of the crash and the angle of the camera.

The Setup: They set up two galaxy clusters to crash into each other.
The Physics: They programmed the computer to track how the gas moves, how magnetic fields get stretched and amplified (like stretching a rubber band), and how particles get accelerated to near the speed of light.
The "Fokker-Planck" Solver: This is a fancy math tool they used to track the "energy" of the particles. Imagine a crowd of people in a stadium. When a shockwave hits, some people get pushed, some get energized, and some lose energy. This tool tracks exactly how that crowd's energy changes over time.

The "Aha!" Moment: Two Relics, One View

When they ran the simulation with the "head-on" angle suggested by the data, something amazing happened.

In a side view, you would see two distinct shockwaves crashing outward, creating two separate radio relics. But because they were looking face-on, those two shockwaves overlapped perfectly in the middle.

The Analogy: Imagine two people blowing bubbles. If you stand to the side, you see two separate bubbles. But if you stand directly in front of them, looking right at where the bubbles meet, it looks like one giant, round, glowing sphere.

The simulation showed that this "face-on" view of two merging shockwaves creates a round radio glow that looks exactly like the "radio halo" we see in the real telescope data.

The Catch: How Strong is the Engine?

To make this simulation match the real brightness of the radio waves, the authors had to tweak the "engine" of the crash.

The Engine: This is the efficiency of the shockwaves at accelerating particles.
The Problem: Standard physics suggests these shockwaves shouldn't be quite efficient enough to create such a bright glow.
The Solution: The authors found that if the shockwaves are slightly more efficient at accelerating particles (a bit more "optimistic" than the average) and if the magnetic fields are strong enough, the simulation matches the real data perfectly.

They also found that the timing matters. The crash has to be observed at a very specific moment—just after the two clusters have passed their closest point to each other (pericenter passage)—to get the right shape and brightness.

The Conclusion: A Trick of the Light

The paper concludes that MACS J0018.5+1626 is likely not a standard radio halo. Instead, it is a radio relic (or rather, two of them) that just happens to be viewed from a very specific, head-on angle.

This is a crucial distinction because:

It proves that projection effects (how we view an object from Earth) can completely change how we classify cosmic objects.
It shows that standard physics (Diffusive Shock Acceleration) can explain the radio emission, provided the viewing angle and magnetic fields are just right.
It highlights the power of combining many different types of data (X-ray, radio, optical) to solve cosmic puzzles that one type of data alone cannot solve.

In short, the universe is playing a game of "hide and seek" with our telescopes, and this paper shows how using multiple "eyes" (different telescopes) and a good computer model can reveal the true shape of the game.

Technical Summary: ICM-SHOX III. The case of MACS J0018.5+1626, a radio relic that looks like a radio halo?

Problem Statement
Diffuse radio emission in galaxy clusters, typically classified as radio halos (central, round morphology) or radio relics (peripheral, elongated, shock-driven), provides critical insights into the non-thermal energy budget and merger dynamics of the intracluster medium (ICM). However, the interpretation of such emission is often complicated by projection effects and the lack of multi-probe constraints on merger parameters (mass, impact parameter, viewing angle, merger stage). The galaxy cluster MACS J0018.5+1626 presents a specific challenge: LOFAR observations at 144 MHz reveal a diffuse, centrally located radio source with a morphology typical of a radio halo. Conversely, the Improved Constraints on Mergers with SZ, Hydrodynamical simulations, Optical, and X-ray (ICM-SHOX) pipeline suggests the cluster is undergoing a binary merger close to pericenter passage, viewed nearly along the merger axis. This configuration theoretically produces two merger-driven shocks that, if viewed face-on, could mimic a central radio halo. The central problem addressed is whether standard diffusive shock acceleration (DSA) acting on thermal electrons in such a "face-on" shock configuration can reproduce the observed radio power and morphology of MACS J0018.5+1626, or if alternative mechanisms (e.g., turbulent re-acceleration or fossil electrons) are required.

Methodology
The authors employ a hybrid numerical framework combining 3D magnetohydrodynamic (MHD) simulations with a Fokker–Planck solver to model particle acceleration and synchrotron emission.

MHD Simulations: Using the moving-mesh code AREPO, the authors simulate idealized binary mergers of galaxy clusters. The initial conditions are constrained by the ICM-SHOX pipeline results for MACS J0018.5+1626, specifically setting the redshift ( $z \approx 0.5456$ ), component masses, and gas profiles. The simulations explore variations in initial relative velocities ( $v_i = 2400$ and $3000$ km s $^{-1}$ ), impact parameters ( $b = 100$ and $250$ kpc), and initial plasma beta ( $\beta_p = 100$ and $200$).
Particle Tracking: Tracer particles are advected with the fluid flow to track the Lagrangian evolution of the gas. Only tracers encountering shocked cells (Mach number $M > 1.3$ ) are considered for particle acceleration.
Fokker–Planck Solver: A Fokker–Planck solver models the evolution of the cosmic-ray electron (CRe) momentum distribution. The solver accounts for diffusive shock acceleration (DSA), adiabatic expansion, and radiative losses (synchrotron, inverse Compton, Coulomb, Bremsstrahlung).
Shock Acceleration Models: The study tests two shock acceleration efficiency models: a constant efficiency ( $\eta_0$ ) and a Mach-number-dependent efficiency based on Kang et al. (2007). The electron injection efficiency is treated as a free parameter ( $\eta_e$ ) scaled by the proton efficiency, exploring values from $10^{-3}$ to $10^{-1}$ .
Synchrotron Emission: The solver computes synchrotron emissivity for tracers, generating 3D maps projected along various lines of sight (LOS). These maps are convolved with LOFAR beam sizes to compare directly with observations at 144 MHz.
Observational Comparison: Simulated radio and X-ray maps are compared against LOFAR (144 MHz) and Chandra (0.5–7 keV) data. Metrics include total radio power, radial surface brightness profiles, spectral indices, and point-to-point radio-X-ray correlation coefficients.

Key Results

Merger Configuration and Shock Properties: The simulations confirm that a merger viewed nearly along the axis (face-on) produces two shocks that overlap significantly at the cluster center. The resulting merger-driven shocks have typical average Mach numbers of $M_s \sim 2–3$ (with $\sigma_M \sim 0.5–1.5$ ).
Reproduction of Radio Power: Standard DSA can reproduce the observed LOFAR radio power ( $\sim 37$ $\sim 37$ mJy at 144 MHz) only under specific conditions:
- Efficiency: An electron shock acceleration efficiency of $\eta_e \sim 10^{-3}–10^{-2}$ is required. This corresponds to the optimistic end of theoretical estimates but is consistent with some Particle-in-Cell (PIC) simulation results for weak shocks.
- Magnetic Field: An initial plasma beta of $\beta_p = 100$ is sufficient to generate the necessary magnetic field amplification ( $\sim 0.8–1$ $\mu$ G in shock regions) to match observations. Models with $\beta_p = 200$ fail to reach the observed power.
- Epoch: The best match occurs shortly after pericenter passage (approx. 40–200 Myr post-pericenter), coinciding with the peak of shock energy dissipation.
Morphology and Viewing Angle: The face-on projection of the two overlapping shocks naturally produces a circular, centrally located radio morphology that closely resembles a radio halo. The study finds that viewing angles $\lesssim 30^\circ$ (for $v_i=2400$ km s $^{-1}$ ) or $\lesssim 36^\circ$ (for $v_i=3000$ km s $^{-1}$ ) provide the best match to the observed radial profiles and surface brightness distribution. Larger viewing angles result in non-overlapping, relic-like structures inconsistent with the observations.
Spectral and Correlation Properties:
- The integrated radio spectral index ( $\alpha_{int}$ ) ranges from $\sim -1.3$ to $-1.8$, consistent with typical values for radio halos and relics.
- The point-to-point correlation between radio and X-ray surface brightness is found to be variable. Depending on the merger stage, viewing angle, and resolution, the correlation can be superlinear, sublinear, or linear. This challenges the notion that sublinear correlations are a unique signature of radio halos (turbulent origin) versus relics (shock origin).

Significance and Claims
The paper claims that the apparent "radio halo" in MACS J0018.5+1626 is likely the result of the superposition of two merger-driven radio relics viewed nearly face-on, rather than a centrally located turbulent halo. This interpretation is supported by the multi-wavelength constraints from the ICM-SHOX pipeline, which favor a merger geometry that aligns with the face-on shock scenario.

The study demonstrates that standard DSA, when applied to a face-on shock configuration with optimistic but physically plausible acceleration efficiencies ( $\eta_e \sim 10^{-3}–10^{-2}$ ) and magnetic field strengths, can reproduce the radio power and morphology of the cluster without invoking turbulent re-acceleration or fossil electron populations. However, the authors note that the required efficiencies are on the optimistic side, suggesting that additional mechanisms (such as turbulent re-acceleration or the presence of fossil electrons) may still play a role.

The work highlights the critical importance of viewing angle and projection effects in classifying diffuse radio emission. It further establishes the ICM-SHOX pipeline as a powerful tool for linking thermal (X-ray, SZ) and non-thermal (radio) observables to constrain the physical state of merging clusters. The authors conclude that while their model successfully explains the observations under standard DSA, future high-frequency radio observations and polarization data are necessary to further distinguish between shock-driven and turbulent origins, and to better constrain the electron acceleration efficiency in the ICM.