LOFAR discovery of an ultra-steep radio halo and giant head–tail radio galaxy in Abell 1132

Abstract

LOFAR observations at 144 MHz have revealed large-scale radio sources in the unrelaxed galaxy cluster Abell 1132. The cluster hosts diffuse radio emission on scales of ∼ 650 kpc near the cluster center and a head-tail (HT) radio galaxy, extending up to 1 Mpc, south of the cluster center. The central diffuse radio emission is not seen in NVSS, FIRST, WENSS, nor in C & D array VLA observations at 1.4 GHz, but is detected in our follow-up GMRT observations at 325 MHz. Using LOFAR and GMRT data, we determine the spectral index of the central diffuse emission to be α = −1.75 ± 0.19 (S ∝ ν α ). We classify this emission as an ultra-steep spectrum radio halo and discuss the possible implications for the physical origin of radio halos. The HT radio galaxy shows narrow, collimated emission extending up to 1 Mpc and another 300 kpc of more diffuse, disturbed emission, giving a full projected linear size of 1.3 Mpc -classifying it as a giant radio galaxy (GRG) and making it the longest HT found to date. The head of the GRG coincides with an elliptical galaxy (SDSS J105851.01+564308.5) belonging to Abell 1132. In our LOFAR image, there appears to be a connection between the radio halo and the GRG. The turbulence that may have produced the halo may have also affected the tail of the GRG. In turn, the GRG may have provided seed electrons for the radio halo.

LOFAR Discovery of a Radio Halo in the High-redshift Galaxy Cluster PSZ2 G099.86+58.45

“…Massive, merging clusters should statistically have larger values of ν s making them detectable at relatively high frequencies (∼ GHz), a νs is defined as the frequency where the synchrotron spectrum is α = 1.9, with α calculated between νs/2.5 and νs (see Cassano et al 2010, for details) while less energetic mergers are expected to produce RHs with lower ν s which are only detectable at low frequencies (< few 100 MHz). The latter are referred to as ultrasteep spectrum RHs (USSRHs b ) and are predicted to be the dominant class of RHs in low-frequency radio surveys, such as LoTSS (Brunetti et al 2008;Cassano et al 2010;Wilber et al 2018). Theoretically, the two classes of RHs, the canonical and USSRHs, mark the extremes of a continuous distribution of properties of these radio sources.…”

“…deep, high-resolution, high-fidelity, low-frequency radio images, has opened up the possibility to study RHs at low frequencies with unprecedented detail and sensitivity (e.g., van Weeren et al 2016a;Shimwell et al 2016;Bonafede et al 2018;Botteon et al 2018Botteon et al , 2019Hoang et al 2018Hoang et al , 2019aWilber et al 2018Wilber et al , 2019.…”

A massive cluster at z = 0.288 caught in the process of formation: The case of Abell 959

“…; A1682, A2261, RXCJ1314.4-2515, ZwCL5247). Lastly, in the last two years, there has been a rapid increase in studies of individual or small samples of RH systems (see Bernardi et al 2016;Knowles et al 2016;Girardi et al 2016;Venturi et al 2017;Parekh et al 2017;Duchesne et al 2017;Hoang et al 2017;Wilber et al 2018;Hlavacek-Larrondo et al 2018;Cuciti et al 2018;Savini et al 2019). We have collected measurements from these studies to form a larger sample of RHs.…”

“…; O'Hara et al 2006;Mantz et al 2010;Giles et al 2017;Yuan et al 2015), or individual papers in some cases when available (e.g. ; A1132, ACT-CL J0256.5+0006, CIZA J2242.8+5301; Wilber et al 2018;Knowles et al 2016;Hoang et al 2017) 10 . Otherwise, we used the values from Piffaretti et al (2011) and even bolometric X-ray luminosity in some cases (e.g.…”

Ultra-steep spectrum emission in the merging galaxy cluster Abell 1914

“…The high sensitivity and abundance of short and long baselines makes LOFAR an ideal instrument to study low-surface brightness diffuse steep-spectrum objects in clusters (e.g. Shimwell et al 2016;van Weeren et al 2016;Hoang et al 2017;Wilber et al 2018;Botteon et al 2018b). This paper is structured as follows: We present the observations and data processing in Section 2.…”