A MeerKAT-meets-LOFAR study of MS 1455.0 + 2232: a 590 kiloparsec ‘mini’-halo in a sloshing cool-core cluster

Abstract

Radio mini-haloes are poorly-understood, moderately-extended diffuse radio sources that trace the presence of magnetic fields and relativistic electrons on scales of hundreds of kiloparsecs, predominantly in relaxed clusters. With relatively few confirmed detections to-date, many questions remain unanswered. This paper presents new radio observations of the galaxy cluster MS 1455.0+2232 performed with MeerKAT (covering the frequency range 872−1712 MHz) and LOFAR (covering 120−168 MHz), the first results from a homogeneously selected mini-halo census. We find that this mini-halo extends for ∼590 kpc at 1283 MHz, significantly larger than previously believed, and has a flatter spectral index (α = −0.97 ± 0.05) than typically expected. Our X-ray analysis clearly reveals a large-scale (254 kpc) sloshing spiral in the intracluster medium. We perform a point-to-point analysis, finding a tight single correlation between radio and X-ray surface brightness with a super-linear slope of $b_{\rm 1283\, MHz} = 1.16^{+0.06}_{-0.07}$ and $b_{\rm 145\, MHz} = 1.15^{+0.09}_{-0.08}$; this indicates a strong link between the thermal and non-thermal components of the intracluster medium. Conversely, in the spectral index/X-ray surface brightness plane, we find that regions inside and outside the sloshing spiral follow different correlations. We find compelling evidence for multiple sub-components in this mini-halo for the first time. While both the turbulent (re-)acceleration and hadronic scenarios are able to explain some observed properties of the mini-halo in MS 1455.0+2232, neither scenario is able to account for all the evidence presented by our analysis.

The Coma Cluster at LOFAR Frequencies. II. The Halo, Relic, and a New Accretion Relic

“…Outside the halo core, low surface brightness emission is detected, which is brighter toward the west. We call this emission the "outer halo" and discuss in Sections 4 and 6 whether the halo core and the outer halo show different properties, as recently proposed for some cool-core clusters (Savini et al 2018(Savini et al , 2019Biava et al 2021;Riseley et al 2022). The emission from the outer halo is only visible when baselines shorter than 80λ are included in the image, and hence it is filtered out in the LoTSS images.…”

“…They also have steep radio spectra, 16 with a spectral index α < −1 that makes them brighter at low radio frequencies. Hence, the advent of deep, low-frequency radio surveys, such as the LOFAR Two-meter Sky Survey (LoTSS; Shimwell et al 2017Shimwell et al , 2019, has both increased the number of new detections (see, e.g., Biava et al 2021;Riseley et al 2022;Botteon et al 2022;Hoang et al 2022) and allowed the study of known objects with unprecedented sensitivity and detail.…”

Radio fossils, relics, and haloes in Abell 3266: cluster archaeology with ASKAP-EMU and the ATCA

“…Historically, 'mini-haloes' have typically been found almost exclusively in relaxed, cool-core clusters (Giacintucci et al 2017;van Weeren et al 2019). However, next-generation low-frequency observations have blurred the lines significantly, with new detections of multi-component mini-haloes hosted by moderately-disturbed clusters or clusters that show signs of significant core sloshing (Venturi et al 2017;Kale et al 2019;Savini et al 2019;Biava et al 2021;Knowles et al 2022;Riseley et al 2022).…”

“…The linear size of source D3, 240 kpc, is typical of known mini-haloes, which are commonly around 100−300 kpc in size (van Weeren et al 2019), although next-generation observations are revealing that some mini-haloes are significantly larger, up to 0.5 Mpc (Savini et al 2019;Biava et al 2021;Riseley et al 2022). Mini-haloes frequently show an association with the radio BCG; from Figure 8 (panels a and b), we see that several cluster-member radio galaxies, including the BCG, appear embedded in the diffuse emission.…”

“…While tensions with limits from Fermi also exist for mini-haloes, recent work by Ignesti et al (2020a) showed that the limits do not completely exclude the hadronic scenario. Recent multi-frequency observations of mini-haloes have shown that our understanding of their underlying physics is far from complete, with multi-component mini-haloes being found in increasing numbers (Savini et al 2018(Savini et al , 2019Raja et al 2020;Biava et al 2021;Riseley et al 2022). In many of these cases, both the turbulent and hadronic mechanisms can explain some of the observational evidence, but neither scenario is able to explain all physical properties, and thus further work is required to understand the detailed and complex physical processes at work.…”