5.2.1
Candidate radio halo in Zwcl2120-1+2256
Zwcl2120-1+2256 (PSZ1 G072.78-18.70) is one of the less massive clusters in our sample, with M500 = 5.91 × 1014M (Planck Collaboration et al. 2014a). It is at
redshift z = 0.143 and the information available in the literature is rather sparse. According to the morphological analysis of the X-ray surface brightness distribution, we find that Zwcl2120-1+2256 (Z2120, hereafter) sits in the intermediate region be- tween merging and relaxed clusters. In fact, the X-ray emission of this cluster is quite interesting: it is fairly peaked at the center, however, there is a low-surface brightness “tail” extending to the South-West, which clearly indicates ongoing dy- namical activity, probably driven by a minor or off-axis merger event that did not completely disrupt the core (Fig. 5.10, right panel and Fig. 5.11, right panel).
The JVLA D array image of Z2120 is shown in Fig. 5.10 (left panel). In addition to the three compact sources (labelled A, B and C) in the cluster’ s central region, we detected some faint diffuse emission extending towards South-West. We subtracted the discrete sources (with the procedure outlined in Section 4.1) and we produced a low-resolution image tapering down the longest baselines. The residual emission is shown in Fig. 5.10 (right panel). It is elongated along the NE-SW axis and, interestingly, it follows the X-ray emission of the cluster, especially in the southern area. We note that, while the radio emission on top of the peak of the X-ray emission may be partly due to some residuals from the subtracted sources,
500 kpc
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Figure 5.10: Left : Z2120 JVLA D array image. The resolution is 3300× 3000 and the rms noise
is ∼ 60µJy/beam. Right : Z2120 low resolution (5500× 5400) JVLA contours after the compact
sources subtraction overlaid over the X-ray Chandra image. Contours are drawn at (3,6...)×σ, with σ = 0.1 mJy/beam. The first negative contour is dashed. The green crosses mark the position of the three sources at the cluster center, which have been subtracted.
the emission coincident with the low-surface brightness X-ray South-West tail is not affected by subtraction. The residual flux density measured inside the 3σ contours of Fig. 5.10 (right panel), considering also the emission in the central region of the cluster, is S1.5GHz ∼ 7.7 mJy. We classify this emission as a candidate radio halo.
A GMRT 330 MHz observation is available in the archive (P.I.:C. Jones). We reduced this dataset with the SPAM pipeline (Section 4.2.1) and then we imaged the processed data with CASA. Only some patches of diffuse emission are visible in the cluster central region on the “full resolution” image (∼ 1000, Fig. 5.11, left panel). In order to highlight the possible presence of diffuse emission, we subtracted the discrete sources and we tapered down the longest baselines to degrade the resolution of the image. A residual emission of ∼ 46 mJy is detected at low significance level (Fig. 5.11, right panel). Remarkably, this emission is spatially coincident with the one detected at higher frequency supporting the idea of a low surface brightness emission associated to the perturbed ICM.
The tentative combination of the two datasets suggests that the diffuse emission in Z2120 has a spectral index α ≈ −1.2. We scheduled Z2120 among the LOFAR SKP Tier 1 observations in cycle 9 to investigate the nature of this diffuse emission at low frequency.
5.2.2
Candidate radio halo in A3041
Abell 3041 (alternative names: RXC J0241.3-2839, PSZ1 G222.97-65.69) is a galaxy cluster at redshift z = 0.23. Its mass is M500 = 6.12×1014M (Planck Collaboration
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Figure 5.11: Left : DSS optical image if Z2120 with GMRT “full resolution” contours overlaid. Contours start at 0.1 mJy/beam and are spaced by a factor 2. The first negative contour is dashed. Right : Z2120 GMRT 330 MHz low resolution (32.700× 2800) contours after the subtraction
of compact sources superimposed on the Chandra X-ray image. Contours are (2,3,6...)×σ, with σ = 0.3 mJy/beam. The position of the discrete sources is marked with a green cross.
image (Fig. 5.12, right panel). There is no available redshift for the brightest one (∼ 94 mJy), while the fainter one (∼ 11.2 mJy) is associated with a member galaxy (Colless et al. 2003). On the Eastern side of the cluster there is a FRII radio galaxy extending over ∼ 5.8 arcmin. The nucleus is located ∼ 5.5 arcmin from the center of A3041, and no spectroscopic redshift is available for this source. The core is detected in the 2 MASS catalogue (Skrutskie et al. 2006), with a K magnitude of 15.62. Using the K − z relation by Willott et al. (2003) we estimated that the redshift of the FRII galaxy is z = 0.42 ± 0.16, suggesting that it may be a background radio galaxy.
We subtracted all the discrete sources from the dataset, except for the FRII galaxy. We paid special attention to the subtraction of the two central sources. In particular, we did not adopt the usual approach of subtracting the model component derived from the high resolution image produced cutting the shortest baselines (Section 4.1), but we subtracted a model made using the whole uv -range. In this way, we made sure that, if there is some faint extended radio emission associated with the central sources, it is subtracted from the data that we then use to produce the low-resolution image. Also, we double checked that all the flux density of these two sources was actually included in the model components. The contours of the low-resolution image, made after an accurate source subtraction, are shown in Fig. 5.12 (left panel). A residual emission of ∼4.1 mJy is present in correspondence of the cluster’ s X-ray peak, mostly detected at the 2σ level only. The spatial coincidence between the thermal and non-thermal emission indicates that these residuals may belong to a cluster radio diffuse source. However, the detection is marginal and we should note that even a small calibration error around the bright central source might have left some residuals showing up in the low-resolution image. We thus consider A3041 as a case of “suspect” diffuse emission. We asked and obtained uGMRT observations in the band 250-500 MHz to investigate the nature of this emission.
5.2.3
Candidate mini halo in A402
Abell 402 (alternative names: MACS J0257.6-2209, PSZ1 G210.08-60.96) is a mas- sive (M500 = 7.21 × 1014M , Planck Collaboration et al. 2014a) cluster of galaxies
42:00.0 50.0 40.0 30.0 20.0 10.0 2:41:00.0 40:50.0 44:00.0 Right ascension 42:00.0 50.0 40.0 30.0 20.0 10.0 2:41:00.0 40:50.0 44:00.0 Right ascension
Figure 5.12: Left : X.ray XMM-Newton image of A3041 (Credits: A. Botteon) with low-resolution (35.800× 25.500) JVLA C array contours overlaid. Contours are (±2, 3, 6...) × σ
rms with σrms =
80µ/Jy/beam. The discrete sources in the field are subtracted, except for the region of the FRII radio galaxy. The black cross marks the position of the central brightest radio source. Right : “Full-resolution” JVLA C array contours (red) of A3041 superimposed on the DSS optical image. Contours start at 0.1 mJy/beam and are spaced by a factor 2. The first negative contour is dashed.
at z = 0.32. Its temperature within R2500 measured on the [0.7–7] keV band ex-
cluding the central 70 kpc region is 8.0+1.1−0.9 keV (Cavagnolo et al. 2008; Giacintucci et al. 2017). Although the X-ray morphology of the cluster is fairly regular and peaked at the center, the central entropy of A402 is relatively high (K0 = 156 ± 25
keV/cm2, Cavagnolo et al. 2009; Giacintucci et al. 2017) suggesting that some sort
of dynamical activity may be taking place in this cluster, probably along the line of sight.
There are two archival GMRT 330 MHz observations available for this cluster (Obs No: 6153, P.I.: G. Macario and Obs No: 6837, P.I.:A. Bonafede). Both have been processed with the SPAM pipeline (Section 4.2.1) and both show evidence of diffuse emission at the cluster’ s center. Unfortunately they have been processed with two different versions of SPAM and this challenged the combination of the uv -datasets. From the combination of the two images, Giacintucci et al. (2017) classified the diffuse emission in A402 as a candidate radio halo. The images of A402 are shown in Fig. 5.13. On the left panel we show the GMRT 330 MHz “full resolution” contours from Obs No 6153 (Venturi et al. in prep.). There are several diffuse sources in the cluster’ s field. The most interesting one, for our purposes, is the central one, which seems to be confined in the core of the cluster, as it is typical for radio mini halos (Mazzotta & Giacintucci 2008; Giacintucci et al. 2014b). The flux density of this source, measured inside the 3σ contours shown in Fig. 5.13 (left panel) is ∼12 mJy and its LAS in the East-West direction is ∼ 8000 corresponding
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Figure 5.13: Left : A402 GMRT 330 MHz contours (Obs No: 6153, Venturi et al. in prep.) superimposed on the X-ray Chandra image of the cluster. Contours start at 0.35 mJy/beam and are spaced by a factor 2. The first negative contour is dashed. The beam is 13.400× 8.600 and the
rms noise of the radio image is ∼ 0.1 mJy/beam. Right : DSS optical image of the field of A402 with the same contours of te left panel (red) plus the low resolution GMRT 330 MHz contours (Obs No: 6837, credits: A. Bonafede). Contours start at 3 mJy/beam and are spaced by a factor 2. The rms noise of the image is ∼ 1 mJy/beam with beam=5700× 4200.
to ∼ 370 kpc.
On the right panel of Fig. 5.13 we also show the low resolution contours of the same portion of the sky derived from Obs No 6837 (credits: A. Bonafede), where some diffuse emission seems to be present on a larger scale. However, the image has a fairly low sensitivity (rms noise ∼ 1 mJy/beam with beam=5700 × 4200) and
it is not clear how much the discrete sources in the cluster’ s field (which have not been subtracted) contribute to the extended emission.
The diffuse radio source at the center of A402 deserves further investigation: higher resolution images would be useful to evaluate the contribution of compact sources and more sensitive low resolution images are needed in order to establish the extension and the nature of the diffuse emission.