[Paper Review] Non-thermal emission and dynamical state of massive galaxy clusters from CLASH sample

M. Pandey-Pommier, Johan Richard+6 more|arXiv (Cornell University)|Dec 1, 2016

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This study analyzes 14 massive galaxy clusters from the CLASH survey using multi-wavelength data (GMRT, Chandra, HST) to link non-thermal radio emission with dynamical states inferred from X-ray and weak-lensing mass maps. It finds that ultra-steep spectrum radio haloes (α < -1.5) and offset radio-mass peaks indicate ongoing mergers, while aligned X-ray, radio, and mass peaks in cool-core clusters signal relaxed states, revealing a tight coupling between baryonic and dark matter in relaxed systems.

ABSTRACT

Massive galaxy clusters are the most violent large scale structures undergoing merger events in the Universe. Based upon their morphological properties in X-rays, they are classified as un-relaxed and relaxed clusters and often host (a fraction of them) different types of non-thermal radio emitting components, viz., haloes, mini-haloes, relics and phoenix within their Intra Cluster Medium (ICM). The radio haloes show steep (alpha = -1.2) and ultra steep (alpha &lt; -1.5) spectral properties at low radio frequencies, giving important insights on the merger (pre or post) state of the cluster. Ultra steep spectrum radio halo emissions are rare and expected to be the dominating population to be discovered via LOFAR and SKA in the future. Further, the distribution of matter (morphological information), alignment of hot X-ray emitting gas from the ICM with the total mass (dark + baryonic matter) and the bright cluster galaxy (BCG) is generally used to study the dynamical state of the cluster. We present here a multi wavelength study on 14 massive clusters from the CLASH survey and show the correlation between the state of their merger in X-ray and spectral properties (1.4 GHz - 150 MHz) at radio wavelengths. Using the optical data we also discuss about the gas-mass alignment, in order to understand the interplay between dark and baryonic matter in massive galaxy clusters.

Motivation & Objective

• To investigate the correlation between non-thermal radio emission and the dynamical state of massive galaxy clusters.
• To determine how merger activity influences spectral properties (spectral index) and morphology of diffuse radio emission in the intracluster medium (ICM).
• To examine the alignment between X-ray-emitting gas, total mass (dark + baryonic), and radio emission to infer the interplay between dark and baryonic matter during cluster mergers.
• To identify and characterize ultra-steep spectrum radio haloes (USSR) as indicators of post-merger or ongoing violent merger phases.
• To assess the role of cluster core state (cool-core vs. non-cool-core) in shaping the spatial distribution and spectral properties of diffuse radio emission.

Proposed method

• Utilized low-frequency GMRT observations down to 150 MHz and high-frequency VLA data at 1.4 GHz to derive spectral indices (α) between 150 MHz and 1.4 GHz.
• Combined X-ray data from Chandra to assess the morphological state (cool-core vs. non-cool-core) and centroid shift of cluster X-ray emission.
• Applied weak gravitational lensing analysis using HST and MUSE data to reconstruct total mass maps, revealing the distribution of dark matter.
• Overlayed radio contours (GMRT), X-ray emission (Chandra), and total mass maps (lensing) to study spatial alignment of radio, X-ray, and mass peaks.
• Classified clusters based on radio morphology (haloes, mini-haloes, relics, phoenix) and spectral index values to infer merger phase (pre/post-merger).
• Quantified cluster relaxation via the X-ray centroid shift (standard deviation of projected separation between X-ray peak and centroid within 500 kpc aperture).

Experimental results

Research questions

• RQ1 How do spectral indices of diffuse radio emission in massive clusters correlate with their dynamical state (relaxed vs. unrelaxed)?
• RQ2 What is the spatial relationship between radio emission, X-ray-emitting gas, and total mass (dark matter) in cool-core and non-cool-core clusters?
• RQ3 Do ultra-steep spectrum radio haloes (α < -1.5) preferentially occur in clusters undergoing or recovering from major mergers?
• RQ4 How does the alignment of X-ray, radio, and mass peaks reflect the degree of merger activity and the coupling between baryonic and dark matter?
• RQ5 What do offsets between radio and mass peaks indicate about the dynamical state and evolution of the cluster?

Key findings

• Ultra-steep spectrum radio haloes (USSR) were detected in 5 out of 14 clusters, with spectral indices α₁₄₀₀/₂₃₅ < -1.5, indicating they are likely post-merger or in late-stage merger phases.
• In 6 cool-core (CC) clusters (e.g., RX J1532+3021, MACS J0329-0211), the X-ray, radio (mini-halo), and total mass peaks are aligned, indicating relaxed dynamical states with minimal ongoing merger activity.
• In non-cool-core (NCC) clusters, such as MACS J0416.1-2403 and MACS J1149.5+2223, the radio halo and total mass peaks are offset, indicating ongoing or recent merger activity.
• Abell 209, a cool-core cluster hosting a giant radio halo, shows an offset between radio and mass peaks but alignment between X-ray and mass peaks, suggesting a transitional phase from NCC to CC after a violent merger.
• The X-ray centroid shift is smaller in CC clusters (mean shift < 100 kpc) compared to NCC clusters, confirming their more relaxed state, with higher X-ray concentration than total mass concentration.
• The total mass map (dominated by dark matter) shows lower central concentration than X-ray maps (baryonic matter), confirming the collisionless nature of dark matter and the collisional nature of baryonic matter during cluster mergers.