A KAT-7 view of a low-mass sample of galaxy clusters
Radio observations over the last two decades have provided evidence that diffuse synchrotron emission in the form of megaparsec-scale radio halos in galaxy clusters is likely tracing regions of the intracluster medium where relativistic particles are accelerated during cluster mergers. In this paper we present results of a survey of 14 galaxy clusters carried out with the 7-element Karoo Array Telescope at 1.86 GHz, aimed to extend the current studies of radio halo occurrence to systems with lower masses (M M). We found upper limits at the Watt Hz level for of the sample, confirming that bright radio halos in less massive galaxy clusters are statistically rare.
A KAT–7 view a low-mass sample of galaxy clusters
20–23 October 2015
(*) This conference has been organized with the support of the Ministry of Foreign Affairs and International Cooperation, Directorate General for the Country Promotion (Bilateral Grant Agreement ZA14GR02 - Mapping the Universe on the Pathway to SKA)
Giant Radio Halos (RHs) are Mpc-scale, diffuse radio sources with low surface brightness that are present in the central region of a certain number of galaxy clusters. Great progress has happened over the last decade on both the theoretical (see Brunetti & Jones 2014 for a recent review) and observational (see Feretti et al. 2012 for a recent review) study of the nature and formation of giant RHs, indicating that they are the product of turbulent re-acceleration of particles in the intracluster medium due to cluster mergers. This picture, recently supported by the discovery of a bimodal distribution of clusters in the P
In this paper we summarize the results of a pilot observational study aimed to populate the faint end of the P– correlation with the newly commissioned 7–element Karoo Array Telescope (KAT–7).
2 Observations and results
A sample of 18 nearby clusters was selected from the Planck SZ Cluster Catalogue (Planck Collaboration, 2014) with the following criteria:
M M mass range;
Radio observations were only available for four targets of the sample and we eventually carried out new observations for 14 clusters (see Table 1) with KAT–7. KAT–7 is an array of 7 12 m–diameter antennas, distributed in a randomized configuration that maximizes the coverage in 4 hours. Baselines span a range between 26 m to 185 m, giving an angular resolution of arcminutes at the central frequency of 1.86 GHz. Data reduction was carried out in a standard fashion and we refer the reader to Bernardi et al. (2016) for the details. The achieved sensitivity ranges between 0.3 and 1 mJy beam depending upon the target.
|Cluster name||z||RH power and upper limits||M|
Figure 1 presents an example of a cluster where no diffuse emission was found down to the sensitivity level of the KAT–7 observations. Out of the entire sample, upper limits to the presence of a giant RH could be set for seven clusters which either did not show any emission in the KAT–7 data or for which radio emission could be attributed to compact sources and reliably subtracted using higher angular resolution literature data. The targets shown in Figure 2 and 3 are of particular interest as they represent the clearest RH candidate in our sample. In the case of the Triangulum Australis, we found emission extending over arcmin, corresponding to kpc, broadly consistent with what reported by Scaife et al. (2015), although the comparison with the SUMSS data indicates that there may be some contribution from compact sources. Figure 3 shows the case of PSZ1G 018.75+23.57, where diffuse radio emission extends well beyond the contours of the two compact sources present in the NVSS image. Even after subtracting the NVSS sources, diffuse radio emission is still present over a 8-10 arcmin angular scale.
For six clusters of the sample we detected emission from compact radio sources and the KAT–7 limited angular resolution prevented a clear assessment of the presence of diffuse radio power potentially attributable to a RH.
Upper limits to the flux density of a possible giant RH were derived using the injection methods (Venturi et al. 2008, Kale et al. 2013) for the seven targets clearly void of emission. They are reported in Figure 4 that includes the most updated compilation of RH measurements and upper limits and the best fit to the P correlation (from Bernardi et al. 2016).
The KAT–7 data offer the first statistical information about cluster RHs in the M M range and confirm the lack of bright RHs in such systems, indicating that RHs more powerful than expected from the correlation must be rare which is in line with the predictions of the turbulent re-acceleration model (Cassano & Brunetti 2005). Under the assumption that the P holds for smaller systems too, our results indicate that this correlation has a steep slope, of the form P, with . Further observations at higher angular resolution and lower sensitivity will be key to probe the low end of the P correlation and provide more stringent tests of the RH formation models.
This work is based on research supported by the National Research Foundation under grant 92725. Any opinion, finding and conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard. This work was also partly supported by the Executive Programme of Scientific and Technological Co-operation between the Italian Republic and the Republic of South Africa 2014–2016. The KAT–7 is supported by SKA South Africa and by the National Science Foundation of South Africa. Participation to the Conference was supported by the Ministry of Foreign Affairs and International Cooperation, Directorate General for the Country Promotion (Bilateral Grant Agreement ZA14GR02 - Mapping the Universe on the Pathway to SKA).
- P is the radio halo power at 1.4 GHz and M is the total cluster mass within the radius , defined as the radius corresponding to a total density contrast , where is the critical density of the Universe at the cluster redshift.
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