Low-frequency integrated radio spectra of diffuse, steep-spectrum sources in galaxy clusters: palaeontology with the MWA and ASKAP

Abstract

Galaxy clusters have been found to host a range of diffuse, non-thermal emission components, generally with steep, power law spectra. In this work we report on the detection and follow-up of radio halos, relics, remnant radio galaxies, and other fossil radio plasmas in Southern Sky galaxy clusters using the Murchison Widefield Array and the Australian Square Kilometre Array Pathfinder. We make use of the frequency coverage between the two radio interferometers—from 88 to $\sim\!900$ MHz—to characterise the integrated spectra of these sources within this frequency range. Highlights from the sample include the detection of a double relic system in Abell 3186, a mini-halo in RXC J0137.2–0912, a candidate halo and relic in Abell 3399, and a complex multi-episodic head-tail radio galaxy in Abell 3164. We compare this selection of sources and candidates to the literature sample, finding sources consistent with established radio power–cluster mass scaling relations. Finally, we use the low-frequency integrated spectral index, $\alpha$ ( $S_v \propto v^\alpha$ ), of the detected sample of cluster remnants and fossil sources to compare with samples of known halos, relics, remnants and fossils to investigate a possible link between their electron populations. We find the distributions of $\alpha$ to be consistent with relic and halo emission generated by seed electrons that originated in fossil or remnant sources. However, the present sample sizes are insufficient to rule out other scenarios.

The merging galaxy cluster Abell 3266 at low radio frequencies

“…However, at the lower frequencies we become more limited by confusion rather than sensitivity within and near the cluster. The MWA-2 fields '7' and '10' presented in Duchesne et al (2021d) along with supplementary data from miscellaneous projects in the region are processed and co-added. Data processing follows the procedure outlined by Duchesne et al (2020Duchesne et al ( , 2021c using the MWA calibration and imaging pipeline, 5 , and associated software.…”

“…Calibrated RACS visibilities from two adjacent observations covering the cluster are retrieved from the CSIRO 7 ASKAP Science Data Archive (CASDA; Chapman et al 2017;Huynh et al 2020) and imaged with . We use a 'Briggs' robust +0.25 image weighting following Duchesne et al (2021d), resulting in a 25 × 25 resolution image with a local rms noise of 0.28 mJy beam −1 near the cluster. The brightness scale of this robust +0.25 image is consistent within a few per cent of the available RACS survey image for the region of interest.…”

“…following the process described in Section 2.4 of Duchesne et al (2021d). Uncertainties for flux density measurements, 𝜎 𝑆 𝜈 , are…”

“…Relevant discrete sources detected in the RACS image are subtracted by extrapolating their 887-MHz flux density to other frequencies assuming a power law model with spectral index 𝛼 = −0.7. Additionally, 𝜎 discrete in Equation 1 incorporates both the measurement uncertainty of the discrete sources and the uncertainty in this assumed 𝛼 by adding in quadrature the difference between the extrapolated flux densities when using a range of 𝛼 (between −0.5 and −1.0; for further detail refer to Section 2.4 of Duchesne et al 2021d). We use the robust +2.0 MWA-2 images for measuring the flux densities of all sources except the western fossil, which becomes too blended with the nearby wide-angle tail (WAT) source, PKS 0429−61.…”

“…Feretti et al 2001;Wilber et al 2020;van Weeren et al 2021;Duchesne et al 2021c), mini-haloes (e.g. Gitti et al 2002;Giacintucci et al 2014;Duchesne et al 2021d), and a variety of other, generally smaller-scale re-accelerated/-energised radio plasmas (e.g. Slee et al 2001;van Weeren et al 2017;Mandal et al 2020;Hodgson et al Robertson & Roach 1990;Quintana et al 1996;Cypriano et al 2001;Finoguenov et al 2006).…”