[Paper Review] Dissecting nonthermal emission in the complex multiple-merger galaxy cluster Abell 2744: Radio and X-ray analysis

K. Rajpurohit, F. Vazza+15 more|arXiv (Cornell University)|Jan 1, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy138 references63 citations

TL;DR

This study presents deep low-frequency radio observations of the multiple-merger galaxy cluster Abell 2744 using the upgraded GMRT, combined with VLA and Chandra data, revealing complex nonthermal emission. It demonstrates that the central radio halo exhibits spectral index fluctuations and a bimodal correlation with X-ray brightness, indicating two distinct components with different evolutionary histories, while four radio relics show power-law spectra and high polarization, consistent with strong shocks (Mach 3.0–4.5).

ABSTRACT

We present the first deep low frequency radio observations of the massive and highly disturbed galaxy cluster Abell 2744 using the upgraded Giant Metrewave Radio Telescope (uGMRT). The cluster is experiencing a very complex multiple merger and hosts a giant halo and four radio relics. The uGMRT observations, together with existing VLA (1-4 GHz) and Chandra observations, allow us to study the complexity of the physical mechanisms active in this system. Our new images reveal that the central halo emission is more extended toward low frequencies. We find that the integrated spectrum of the halo follows a power law between 150 MHz and 3 GHz, while its subregions show significantly different spectra, also featuring high frequency spectral steepening. The halo also shows local regions in which the spectral index is significantly different from the average value. Our results highlight that an overall power-law spectrum, as observed in many radio halos, may also arise from the superposition of different subcomponents. The comparison of the radio surface brightness and spectral index with the X-ray brightness and temperature reveals for the first time different trends, indicating that the halo consists of two main components with distinct evolutionary signatures. All four relics in this system follow a power-law radio spectrum, compatible with shocks with Mach numbers in the range 3.0†-†4.5. All relics are also highly polarized from 1-4 GHz and show low Faraday dispersion measures, suggesting that they are located in the outermost regions of the cluster. The complexity in the distribution and properties of nonthermal components in Abell 2744 supports a multiple merger scenario, as also highlighted by previous X-ray and lensing studies. Our unique results demonstrate the importance of sensitive and high-resolution, multifrequency radio observations for understanding the interplay between the thermal and nonthermal components of the ICM.

Motivation & Objective

• To investigate the origin and spectral properties of diffuse nonthermal emission in Abell 2744, a complex multiple-merger galaxy cluster.
• To resolve the spatially resolved radio spectrum of the halo and relics to test re-acceleration and shock models.
• To examine the correlation between radio and X-ray properties to infer the physical conditions of the intracluster medium.
• To determine the magnetic field structure and Faraday rotation in relics to constrain their location and environment.
• To assess whether the observed integrated power-law spectrum of the halo arises from superposition of distinct components with different spectral indices.

Proposed method

• Conducted deep low-frequency (300–950 MHz) observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) to image diffuse radio emission.
• Combined uGMRT data with existing VLA (1–4 GHz) and Chandra X-ray observations for multi-wavelength analysis.
• Produced high-resolution, multi-frequency radio images and spectral index maps using self-calibration and CLEAN-based imaging techniques.
• Performed point-to-point correlation analysis between radio spectral index, surface brightness, and X-ray brightness/temperature.
• Used the integrated radio spectrum of relics to infer shock Mach numbers via the DSA model and compared with X-ray shock measurements.
• Analyzed Faraday rotation measures (RMs) and Faraday dispersion to constrain the magnetic field structure and line-of-sight environment of the relics.

Experimental results

Research questions

• RQ1 What causes the complex spectral structure of the radio halo in Abell 2744, and does it arise from a single population or multiple components?
• RQ2 How do the spatially resolved spectral index and radio-X-ray correlations vary across the halo, and what do they reveal about particle acceleration mechanisms?
• RQ3 Are the four radio relics in Abell 2744 consistent with diffusive shock acceleration, and what do their spectral indices and polarization imply about shock strength and magnetic fields?
• RQ4 How do the Faraday rotation and dispersion properties of the relics constrain their location within the cluster and the magnetic field structure?
• RQ5 To what extent do the observed spectral and morphological features support a multiple-merger scenario for Abell 2744?

Key findings

• The central radio halo exhibits a complex, spatially resolved spectral index distribution with significant fluctuations, including regions of steepening and deviations from the average spectral index of −1.15.
• The halo's integrated spectrum follows a power-law from 150 MHz to 3 GHz, but subregions show distinct spectral behaviors, indicating that the overall power-law may result from superposition of components with different evolutionary histories.
• Point-to-point comparisons reveal two distinct trends: a strong anticorrelation between spectral index and X-ray brightness in the northern halo, and a positive correlation in the southern halo, indicating at least two physically distinct components.
• The spectral index also shows a moderate positive correlation with ICM temperature in the northern halo, further supporting the presence of two components with different physical conditions.
• All four radio relics (R1–R4) exhibit power-law radio spectra with integrated spectral indices between −1.10 and −1.19, consistent with shocks of Mach number 3.0–4.5, and show high polarization (1–4 GHz) and low Faraday dispersion (<10 rad m⁻²), indicating they are located in low-density, magnetized outer regions.
• The main relic R1 has a spectral index of −1.17 ± 0.04 and a shock Mach number of 3.6, in excellent agreement with the X-ray shock Mach number (3.7 ± 0.4) derived from the temperature jump, supporting the standard DSA model with low acceleration efficiency (<1%) and magnetic fields ≥ few µG.