LOFAR observations of galaxy clusters in HETDEX

Abstract

Diffuse cluster radio sources, in the form of radio halos and relics, reveal the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). These cosmic rays are thought to be (re)accelerated through the ICM turbulence and shock waves generated by cluster merger events. Here we characterize the presence of diffuse radio emission in known galaxy clusters in the HETDEX Spring Field, covering 424 deg2. For this, we developed a method to extract individual targets from LOFAR observations processed with the LoTSS DDF-pipeline software. This procedure enables improved calibration as well as the joint imaging and deconvolution of multiple pointings of selected targets. The calibration strategy can also be used for LOFAR low-band antenna and international-baseline observations. The fraction of Planck PSZ2 catalog clusters with any diffuse radio emission apparently associated with the ICM is 73 ± 17%. We detect a total of ten radio halos and twelve candidate halos in the HETDEX Spring Field. Of these ten radio halos, four are new discoveries, two of which are located in PSZ2 clusters. Five clusters host radio relics, two of which are new discoveries. The fraction of radio halos in Planck PSZ2 clusters is 31 ± 11%, or 62 ± 15% when including the candidate radio halos. Based on these numbers, we expect that there will be at least 183 ± 65 radio halos found in the LoTSS survey in PSZ2 clusters, in agreement with past predictions. The integrated flux densities for the radio halos were computed by fitting exponential models to the radio images. From these flux densities, we determine the cluster mass (M500) and Compton Y parameter (Y500) 150 MHz radio power (P150 MHz) scaling relations for Planck PSZ2-detected radio halos. Using bivariate correlated errors and intrinsic scatter orthogonal regression, we find slopes of 6.13 ± 1.11 and 3.32 ± 0.65 for the M500–P150 MHz and M500–P150 MHz relations, respectively. These values are consistent with the results of previous works.

Spectral study of the diffuse synchrotron source in the galaxy cluster Abell 523

“…In the latter case the radio largest linear size of the source would be about 570 kpc, its radio power 8 at 144 MHz ∼ 1.7 × 10 26 W/Hz and at 1.410 GHz ∼ 1.8 × 10 24 W/Hz, still consistent with typical radio powers observed for radio halos (see Giovannini et al 2009, Feretti et al 2012and van Weeren et al 2019. Assuming the scaling relation 𝑃 150 MHz -𝑀 SZ,500 , 𝑃 1.4 GHz -𝑀 X,500 , and 𝑃 1.4 GHz -𝐿 X,0.1−2.4 keV (van Weeren et al 2021;Yuan et al 2015), we predict a mass of the system 𝑀 500 in the range 8 − 11 × 10 14 𝑀 and an Xray luminosity 𝐿 X,0.1−2.4 keV = 4 − 18 × 10 44 𝑀 . These values are consistent with those observed for galaxy clusters at similar redshift (e.g., Schneider 2015).…”

“…No SZ mass estimate is available for A523 and the X-ray derived mass is 𝑀 X,500 = 2.2 − 3.6 × 10 14 𝑀 . Considering our flux measurements at 1.410 GHz and 144 MHz, taking into account the different cosmology, the frequency scaling when appropriate, and k-correction 10 , the value predicted according to the 𝑃 1.4 GHz − 𝑀 X,500 scaling relation in Yuan et al (2015) is 𝑀 500 ≈ 6 − 10 × 10 14 𝑀 , while 𝑀 500 ≈ 6 − 9 × 10 14 𝑀 from the best fit relations 𝑃 150 GHz − 𝑀 SZ,500 by van Weeren et al (2021).…”

“…We performed a further refining of the data in the region where the cluster is located through amplitude and phase selfcalibration. Flux contributions from sources outside a square box with sides of length 0.4 • centered at approximately the location of the X-ray peak of the cluster (RA 04h:59m:07s and Dec 08 • :45 :00 ) have been subtracted in the uv plane using the direction-dependent calibrations and final ddf-pipeline model, a phase-shift has been applied to locate the target in the phase center, and finally a correction for the LOFAR station beam towards this direction has been applied (this procedure is detailed in van Weeren et al 2021). We reimage the data that are calibrated in the direction of our target using wsclean-2.10 ( Offringa et al 2014).…”

The LOFAR Two-metre Sky Survey

“…The effects of non-deconvolution of diffuse sources at the full resolution of LoTSS-DR2 can be significantly mitigated by making use of the low-resolution (20 ) images that are also provided as a standard DR2 data product. Alternatively, the released uv-data can be reimaged with tailored masking and deconvolution or even further processed to optimise the image fidelity and deconvolution parameters for a particular target (see van Weeren et al 2021). Finally, we note that the analysis presented in this subsection has focused on the recovery of faint diffuse emission in the images and not on the accuracy of the PYBDSF characterisation of such objects.…”

The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223

“…Data were processed by means of the LOFAR Surveys Key Science Project reduction pipeline v2.2 3 (Shimwell et al 2019;Tasse et al 2020), which performs both direction-independent and direction-dependent calibration using PREFACTOR (Offringa et al 2013;van Weeren et al 2016;Williams et al 2016;de Gasperin et al 2019) and KillMS (Tasse 2014a,b;Smirnov & Tasse 2015) and delivers images at 6 and 20 resolution of the full LOFAR field of view using DDFacet . In order to improve the image quality towards MACS J1149, we used the models produced by the pipeline to subtract the sources 3 https://github.com/mhardcastle/ddf-pipeline outside a region of sizes 27 × 27 centered on the target from the uv-data.…”