PSZ1 G139.61+2420 (PSZG139, hereafter), discovered by Planck, has a mass of M500 = 7.09 × 1014M and it is at redshift z = 0.27. It has a global temperature
of 7.5 keV and a low-entropy (K0 = 10 ± 10) cool core (Giacintucci et al. 2017).
The high value of the concentration parameter that we derived for PSZG139 is consistent with the presence of the cool core, however, the centroid shift parameter lies at the boundary separating merging and non-merging clusters (Section 7.1). In fact, the X-ray morphology of the cluster is slightly elongated in the SE-NW direction, indicating some trace of dynamical disturbance. A deeper analysis of the X-ray surface brightness profile reveals the presence of a discontinuity around the cool core, in the NW direction. Specifically, this edge is a cold front (the
Figure 5.6: Left : A1443 spectrum. Flux densities are measured at 330, 610 and 1500 MHz on images made with the same uv -range, uniform weighting scheme and same resolution.
temperature in the upstream region is higher, Savini et al. submitted), suggesting that the cold gas in the core may be sloshing. Deeper X-ray data are necessary to investigate the presence of large scale discontinuities in this cluster.
There are two GMRT observations at 610 MHz of PSZG139: Obs. No 7380, P.I. R. Cassano and Obs No 8039, P.I. S. Giacintucci. We analysed Obs. No 7380 with the procedure described in Section 4.2 and then we combined our image with the image produced by S. Giacintucci from Obs No 8039. The combined “full resolution” image is shown in Fig. 5.7 (left panel), superimposed on the optical DSS image of the cluster field. A weak source (labelled S1) of ∼ 0.4 mJy appears associated with the central galaxy. North of source S1, we detected an extended source (S2) without a clear optical counterpart and a faint point like source (S3) probably associated with a member galaxy (the redshift of the optical counterpart is not available). The flux density of S2 is ∼ 2.4 mJy, while S3 is ∼ 0.24 mJy. In the image with a slightly lower resolution (Fig. 5.7, right panel) the three sources are blended with a diffuse component that seems confined within the cluster core. We classify this diffuse source as a mini halo. The total extension of the diffuse emission in the East-West direction is ∼ 100 kpc. We measured the total flux density inside the 3σ contours shown in Fig. 5.7 (right panel) and we subtracted the contribution of the discrete sources mentioned above, obtaining a flux for the mini halo S610M Hz ∼ 1.2 mJy. The position of the X-ray cold front is roughly
coincident with the edge of the radio diffuse emission, as found for other mini halos in the literature (Mazzotta & Giacintucci 2008; Giacintucci et al. 2014b,a).
In Fig. 5.8 we present the JVLA 1.5 GHz C array “full resolution” image of the central region of PSZG139 compared to the low resolution GMRT 610 MHz contours. The sources S1, S2 and S3 are detected also at 1.5 GHz, with flux densities of ∼ 0.28 mJy, ∼ 0.76 mJy and ∼ 0.18 mJy respectively. At a similar resolution, the mini halo appears less extended towards the West at high frequency.
20.0 10.0 6:22:00.0 50.0 40.0 30.0 21:20.0 41 74 :4 0: 00 .0 Right ascension 20.0 10.0 6:22:00.0 50.0 40.0 30.0 21:20.0 41 30 .0 Right ascension
Figure 5.7: Left : GMRT 610 MHz “full resolution” contours of PSZG139 overlaid over the optical DSS image. Contours are (±3, 6, 9, 12...) × σrms with σrms = 30 µJy/beam and beam=500× 600.
The first negative contour is dashed. Right : GMRT 610 MHz low-resolution contours of PSZG139 overlaid over the X-ray Chandra image of the cluster. Contours are (±3, 6, 9, 12...) × σrms with
σrms= 30 µJy/beam and beam=700× 700. The first negative contour is dashed. The red dashed
line represents the location of the cold front (Savini et al. submitted).
We estimated the flux density of the mini halo in the same region used for the GMRT image (3σ contours shown in Fig. 5.8) and we subtracted the contribution of the discrete sources. We obtained a flux density of the mini halo S1.5GHz ∼ 0.36
mJy, which would give a spectral index of the mini halo α330M Hz
1500M Hz ≈ −1.3.
PSZG139 has been also observed with LOFAR at 144 MHz as part of a project aimed at studying the low frequency radio diffuse emission in dynamically relaxed or intermediate clusters (P.I. F. Savini). In the LOFAR image the mini halo ap- pears much more extended, especially towards South-East (Fig. 5.9, Savini et al. submitted) and has an integrated flux density S144M Hz = 38 ± 6 mJy. Savini et
al. analysed the brightness radial profile of the emission, fitting it with an ex- ponential form I(r) = I0e−r/re (Murgia et al. 2009), and found best fit values
I0 = 13.4 ± 0.6 µJy/arcsec2 and re = 91.6 ± 6.5 kpc; this value of the radius places
PSZG139 in between the typical values of giant and mini-radio halos. In order to place an upper limit to the extended emission that is detected with LOFAR but not with the GMRT at 610 MHz, we injected a simulated radio halo in the GMRT uv -dataset (Section 4.3). We modelled the fake radio halo with the parameters I0
and rederived from the brightness radial profile and we created a set of fake sources
with decreasing flux densities, i.e. with decreasing spectral indices. Starting from α = −1, we lowered the value until the radio halo could not be detected any more. This analysis suggests that the diffuse source has a spectral index α = −1.7; a similar constraint can be derived using the data at 1.5 GHz.
10.0 6:22:00.0 50.0 40.0 21:30.0 74:42:00.0 30.0 41:00.0 40:30.0 Right ascension Declination
Figure 5.8: JVLA C array 10.3 × 5.1 resolution image of the center of PSZG139 with GMRT 610 MHz contours from Fig. 5.7 (right panel) overlaid.
emission are rare, still PSZG139 is not the first case (Bonafede et al. 2014b; Sommer et al. 2017; Venturi et al. 2017). In particular, some similarities can be found with the cluster A2142, where a slightly flatter spectrum diffuse emission, surrounded by cluster scale emission has been detected (Venturi et al. 2017). However, A2142 is not a cool-core cluster (Giacintucci et al. 2017) and the size, the spectrum and the radio power are those typical of giant radio halos in clusters of that mass. PSZG139 is particularly interesting because it is the first case of a cool-core cluster where a mini-halo (the diffuse component detected at higher frequencies) coexists with a diffuse emission on larger scales detected at low frequency (Savini et al. submitted). These properties may be expected in a situation where a massive cool-core cluster undergoes a minor merger. In this case the cool core would not be disrupted and the cluster may appear slightly disturbed (elongated, with relatively high values of the centroid shift) in the X-rays. At the same time, the energy dissipated on large scales may not be sufficient to generate a classical giant radio halo and the emission would appear very steep and glow up only at lower frequencies. Since minor mergers are common in cool-core clusters with evidence of gas sloshing we may expect that PSZ139 is only the first of many cases of very steep spectrum halos surrounding mini-halos that will be detected by LOFAR observations.
Figure 5.9: Left : PSZG139 optical Pan-STARRS g,r,i mosaic with low resolution (3400× 2600)
144 MHz LOFAR contours overlaid. Contours level are at (±3, 6, 12...) × σrms with σrms =
330µJy/beam.Right : X-ray Chandra image of PSZG139 with 144 MHz LOFAR high resolution (blue) and low resolution (red) contours superimposed. Low resolution contours are the same as in the left panel. High resolution (1100× 800) contours are (±3, 6, 12...) × σ
rms with σrms =
150µJy/beam. From Savini et al. (submitted).