MWA and ASKAP observations of atypical radio-halo-hosting galaxy clusters: Abell 141 and Abell 3404

Abstract

We report on the detection of a giant radio halo in the cluster Abell 3404 as well as confirmation of the radio halo observed in Abell 141 (with linear extents $\sim\!770$ and $\sim\!850$ kpc, respectively). We use the Murchison Widefield Array, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to characterise the emission and intervening radio sources from $\sim100$ to 1 000 MHz; power law models are fit to the spectral energy distributions with spectral indices $\alpha_{88}^{1\,110} = -1.66 \pm 0.07$ and $\alpha_{88}^{943} = -1.06 \pm 0.09$ for the radio halos in Abell 3404 and Abell 141, respectively. We find strong correlation between radio and X-ray surface brightness for Abell 3404 but little correlation for Abell 141. We note that each cluster has an atypical morphology for a radio-halo-hosting cluster, with Abell 141 having been previously reported to be in a pre-merging state, and Abell 3404 is largely relaxed with only minor evidence for a disturbed morphology. We find that the radio halo powers are consistent with the current radio halo sample and $P_\nu$ –M scaling relations, but note that the radio halo in Abell 3404 is an ultra-steep–spectrum radio halo (USSRH) and, as with other USSRHs lies slightly below the best-fit $P_{1.4}$ –M relation. We find that an updated scaling relation is consistent with previous results and shifting the frequency to 150 MHz does not significantly alter the best-fit relations with a sample of 86 radio halos. We suggest that the USSRH halo in Abell 3404 represents the faint class of radio halos that will be found in clusters undergoing weak mergers.

The merging galaxy cluster Abell 3266 at low radio frequencies

“…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. Namely, individual 2-min snapshots are calibrated and imaged independently prior to linear co-addition/mosaicking.…”

“…To simulate the giant radio halo, we estimate the expected halo brightness based on the scaling relation derived by Duchesne et al (2021c), between cluster mass within 𝑟 500 13 (𝑀 500 ) and radio luminosity at 150 MHz (𝑃 150 MHz ) including the full literature sample of radio haloes. We note van Weeren et al ( 2021) originally derived the 150-MHz scaling relation from a sample of radio haloes detected at frequencies close to 150 MHz.…”

“…For consistency with scaling relations derived by Duchesne et al (2021c), we use the PSZ2 mass of ∼ 6.64 × 10 14 M . The expected power at 150 MHz is ∼ 3.5 × 10 25 W Hz −1 , corresponding to a flux 11 We choose the 216-MHz data due to the much longer integration time compared to the other bands combined with the highest nominal resolution resulting in less confusion.…”

“…13 The radius within which the mean density of the cluster is 500 times the critical density of the Universe. density of ∼ 4.2 Jy, or ∼ 2.5 Jy at 216 MHz, assuming a spectral index of −1.4 14 (as in Duchesne et al 2021c). The radial size of radio haloes has been found to somewhat correlate to their luminosity and host cluster properties (Feretti 2000;Cassano et al 2007) and following the relation obtained by Cassano et al (2007) we would expect an average radio halo radius of ∼ 450 kpc.…”

“…Johnston-Hollitt 2003;Bagchi et al 2006;van Weeren et al 2016;Duchesne et al 2021a), centrally located cluster merger and turbulence-driven giant, megaparsec-scale radio haloes (e.g. 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.…”