[Paper Review] Diffuse radio sources in colliding galaxy clusters - Low frequency follow up of the GMRT Radio Halo Survey

S. Giacintucci|arXiv (Cornell University)|Feb 9, 2011

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This study presents new GMRT 325 MHz observations of four merging galaxy clusters—A2744, A1300, A1758N, and A781—to investigate the low-frequency spectral properties of diffuse radio halos and relics. The results reveal extended emission in the candidate relic of A781, with a spectral index of α ≈ 1.25, and no significant central diffuse emission, supporting its classification as a peripheral relic despite unusual edge-brightening.

ABSTRACT

The knowledge of the origin and statistical properties of diffuse radio emission in galaxy clusters has appreciably improved thanks to the GMRT Radio Halo Survey, a project based on 610 MHz observations of clusters belonging to a statistically complete sample. However, the spectral properties of cluster diffuse sources are still poorly known and uncertain. High sensitivity and multi-resolution observations at low frequency ($\le$0.3 GHz) are needed for accurate spectral studies. Here, GMRT images at 325 MHz are presented for the clusters A2744, A1300, A1758N and A781, all hosting cluster-scale diffuse emission in the form of a giant halo and/or relic. These observations are part of a new observational campaign to follow up with the GMRT at 150, 235 and 325 MHz all diffuse radio sources in the cluster sample of the GMRT Radio Halo Survey and obtain detailed information on their radio spectral properties.

Motivation & Objective

• To improve understanding of the spectral properties of diffuse radio halos and relics in galaxy clusters, which remain poorly constrained at low frequencies.
• To address the uncertainty in the origin of relativistic electrons in cluster-scale radio sources by obtaining high-sensitivity, multi-resolution low-frequency data.
• To test the hypothesis that radio relics trace merger shocks via diffusive Fermi acceleration, using multi-frequency observations.
• To determine whether candidate diffuse sources, such as in A781, are genuine relics or artifacts, through deeper low-frequency imaging.
• To provide a foundation for future multi-frequency studies of cluster radio emission by establishing a low-frequency follow-up campaign with the GMRT.

Proposed method

• Conducted deep 325 MHz GMRT observations with integration times of ~8 hours per cluster, using the spectral-line mode and a 32 MHz bandwidth.
• Employed multi-resolution imaging techniques to simultaneously resolve discrete radio sources and image diffuse emission, enabling accurate subtraction of compact sources.
• Produced both high-resolution (10–12") and low-resolution (35–40") images to optimize dynamic range and sensitivity to large-scale structures.
• Used a combination of u-v data editing and residual imaging to isolate diffuse emission after subtracting individual radio galaxies.
• Measured flux densities at 325 MHz and combined them with archival 1.4 GHz data (from NVSS and VLA) to derive spectral indices.
• Applied a consistent data reduction pipeline based on Giacintucci et al. (2008), achieving residual amplitude errors <5%.

Experimental results

Research questions

• RQ1 What is the spectral index of the candidate radio relic in A781 at low frequencies, and does it support its classification as a genuine relic?
• RQ2 Is there evidence of a central radio halo in A781 at 325 MHz, and how does its flux density compare to the peripheral relic?
• RQ3 How does the morphology and extent of diffuse radio emission in A2744, A1300, and A1758N change at 325 MHz compared to 610 MHz observations?
• RQ4 To what extent does the edge-brightening morphology of the A781 relic deviate from typical relic characteristics, and what does this imply about its origin?
• RQ5 What is the significance of the faint bridge connecting the halo and relic in A2744 at 325 MHz, and does it suggest a physical connection?

Key findings

• The candidate radio relic in A781 extends over ~700 kpc at 325 MHz, significantly more extended than at 610 MHz, confirming its large-scale nature.
• The spectral index of the A781 relic is α ≈ 1.25 (Sν ∝ ν−α), consistent with a steep spectrum typical of radio relics and supporting its identification as a genuine relic.
• No significant diffuse emission is detected at the cluster center of A781 at 325 MHz, with residual flux density at the 1σ level of 0.25–0.45 mJy/beam, indicating no central halo is present at this sensitivity.
• A 10 mJy extension of the relic is detected toward the cluster center, suggesting possible interaction with the cluster environment, though not a central halo.
• The total flux density of the A781 field, including the relic and discrete sources, is 526 mJy, with residual emission accounting for only ~5% of the total.
• In A2744, the halo and relic appear connected by a faint bridge at 325 MHz, consistent with previous 1.4 GHz observations and suggesting a possible physical link between the two structures.