respectively. We therefore have the following global spectral indices for PKS 2324-02: α1400610 = 0.86andα61074 = 0.63. A multiwavelength study of this source will form part of a future paper on the GMRT cluster sample.
cluster sample can be be explained if the clusters are relaxed systems. This indeed appears to be the case based on the dynamical studies of the ACT-E sample from optical redshift information (Sif´on et al., 2015). The only cluster in our observed sub-sample with an indication of merging activity from dynamical information is ACT-CL J0256.5+0006, which is also the only target to host observable diffuse emission. As low-resolution SZ cluster detections, such as those provided by ACT, SPT, and Planck, provide no dynamical information on the cluster state, this means that blind SZ-selected radio halo or relic observing samples may have a high fraction of non- detections.
TheP1.4GHzupper limits for our non-detections are higher than the upper limits quoted in the literature, for all but our lowest redshift cluster (§3.3.1.1) which is also the only cluster in our sample below a redshift ofz = 0.48with a non-detection. The upper limits from the literature are for clusters below a redshift ofz = 0.4. We note, however, that our upper limit fluxes are fairly consistent with those from the literature (see e.g. Kale et al., 2013). Since the angular size of a 1 Mpc halo decreases with increasing redshift, the radio power relating to a given flux will increase with increasing redshift. For example, an upper limit 610 MHz flux of 8 mJy gives a 1.4 GHz log radio power (in W Hz−1) of 23.9 atz = 0.3, but produces a value of 24.5 at a higher redshift of z = 0.5. After compensating for varying radio halo sizes over the range of masses in our cluster sample, most of the high-redshift upper limits are below the radio power/mass correlation. Only our two highest redshift clusters, ACT-CL J0022.2−0036 and ACT-CL J0059.1−0049, are still slightly above this scaling relation. However, the upper limits are still above the P1.4GHz–Y500 correlation, although well within the scatter. We performed a survival analysis for both relations and found that the upper limits could belong to the population of detected radio halos without significantly changing the correlation found using only the true detections.
High-redshift clusters would need to be observed for much longer than their closer coun- terparts in order to achieve upper limit powers of the same order of magnitude. However, the rms noise of an observation, and thus the upper limit flux, is limited by the telescope properties.
The theoretical noise level for an image cannot be improved on by increased integration time, but only by using a more sensitive instrument. We therefore note that our cluster upper limits which lie above or on the radio power correlation don’t exclude the existence of radio halos in
these clusters. Longer integration times with better sensitivity may yet reveal diffuse emission in these clusters. With that in mind, studies of diffuse emission in high redshift cluster samples will become more feasible with the new generation instruments such as the SKA and its precursor telescopes.
Finally, as the masses used for the pilot project sample (§3.1.1) were preliminary values, and the mass selection for the high-redshift sample was based on the UPP-profile masses from Has- selfield et al. (2013) (§3.1.2), we do not have a complete mass-selected sample when considering the published B12 scaling relation masses, which were the final SZ-inferred mass values at the time of this work. Using these published B12 cluster masses, there are an additional 16 clusters which satisfy theM500 >5×1014Mcriterion over a redshift range of 0.15−0.7. Observations of these remaining clusters are necessary in order to infer any reliable diffuse radio emission statistics on a SZ-selected ACT cluster sample.
A GIANT RADIO HALO IN LOW-MASS SZ-SELECTED CLUSTER: ACT-CL
J0256.5+0006
4.1 Introduction
Multiwavelength observations of galaxy clusters provide a wealth of information about the physics of the intracluster medium (ICM) and its relationship with cluster galaxies. The optical and X-ray bands have historically been used to identify merger activity via optical substructure (Carter and Metcalfe, 1980; Geller and Beers, 1982; Rhee and Katgert, 1987; Dressler and Shectman, 1988;
Rhee et al., 1991; Wen and Han, 2013) and morphological parameters determined from X-ray images (Mohr et al., 1993; Jeltema et al., 2005; O’Hara et al., 2006; Santos et al., 2008). In the last decade, a link has been found between a cluster’s merger status and the presence of large- scale diffuse synchrotron emission (see Brunetti and Jones, 2014, and references therein). This cluster-scale radio emission, dubbed a giant radio halo (GRH) if∼Mpc in size, exhibits a steep spectrum and has no obvious link to the individual cluster galaxies (Buote, 2001; Feretti and
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Giovannini, 2008; Ferrari et al., 2008; Feretti et al., 2012). Radio halos appear to trace the non- thermal ICM and typically have spectral indices ofα∼1.1–1.5. However, ultra-steep spectrum radio halos (USSRHs,α ∼1.6–1.9), presumably associated with more pronounced synchrotron ageing, have also been detected within the population (Brunetti et al., 2008; Dallacasa et al., 2009; Venturi et al., 2013).
The existence of USSRHs is predicted by one of the current leading theories for the origin of GRHs (Brunetti et al., 2008), namely the turbulent re-accelerationmodel in which the syn- chrotron emission is powered by turbulence generated during cluster mergers (Schlickeiser et al., 1987; Ensslin et al., 1998; Brunetti and Lazarian, 2011; Beresnyak et al., 2013). In this model one expects an USSRH to be seen when the turbulent energy in the cluster has decreased sufficiently for it to be less efficient in accelerating high energy electrons in the cluster. This scenario can also explain the observed bimodality in scaling relations between the 1.4 GHz GRH power and ther- mal cluster properties, in which clusters are either radio loud or radio quiet. This dichotomy has been observed in cluster samples selected via X-ray luminosity (Brunetti et al., 2007; Cassano et al., 2008) and the Sunyaev-Zel’dovich (SZ) effect (Sunyaev and Zel’dovich, 1972), although it is less pronounced in the latter case (Sommer and Basu, 2014). In practice, one anticipates a population of clusters in transition between these two states that will have intermediate radio power.
The observed bimodality could be due to selection effects in the cluster sample (Sommer and Basu, 2014) or a physical effect related to the cluster evolutionary state. Magnetohydrodynamic (MHD) simulations by Donnert et al. (2013) show that a GRH is a transient phenomenon that exhibits a rise and fall in radio halo emission over the course of a merger. This evolutionary model suggests that for a merging cluster, the observable diffuse radio emission depends strongly on the phase of the merger in which the cluster is being observed, which likely contributes to the scatter in the observedP1.4GHz scaling relations with thermal cluster properties.
Moreover, one would expect to find two separate types of systems that populate the inter- mediate region of radio power: late-stage mergers with old GRHs that are in the process of switching off, and early-stage mergers in which the radio halo emission has recently switched on but not yet reached its maximum radio power. The former scenario is indeed the case for the
USSRHs, which are starting to fill in the intermediate region of GRH power. Clusters that are in the early stages of merging would also be interesting systems to identify and study as they would complete the evolutionary picture.
Cassano et al. (2010) find that the observed dichotomy is strongly related to cluster dy- namical state, with morphologically disturbed systems hosting GRHs. However, several GRH non-detections in merging clusters are seemingly incongruent with this trend (A141, A2631, MACSJ2228: Cassano et al. 2010; A119: Giovannini and Feretti 2000; and A2146: Russell et al. 2011). In the case of A2146, Russell et al. postulate that the lack of a GRH in this strongly- merging system is due to the relatively low mass of the cluster. Low-mass systems are expected to generate less turbulent energy during their mergers, yielding weaker synchrotron emission, and hence GRHs that are too faint to observe with current telescopes. The era of LOFAR (Vermeulen, 2012), SKA precursors such as MeerKAT (Booth and Jonas, 2012) and ASKAP (DeBoer et al., 2009), and the SKA itself (Taylor, 2013) will bring with it highly sensitive observations of these systems, and should reveal the underlying GRH emission.
In this chapter we present the detection of a GRH in a low-mass system that we argue is in the early stages of merging. As discussed, such early-stage merging systems are interesting because they allow us to probe the full evolutionary cycle of GRHs and are expected to fill in the intermediate region in radio halo power.
The structure of the chapter is as follows. We present existing multiwavelength data on ACT- CL J0256.5+0006 in §4.2, and we describe the radio observations and data reduction process in§4.3, with the radio results presented in§4.4. X-ray and optical morphological analyses are discussed in sections 4.5.1 and 4.5.2, respectively. We construct a model for the merger geometry in§4.6 and infer merger time-scales from this model in§4.7. We conclude with a discussion in
§4.8. We adopt aΛCDM flat cosmology withH0 = 70km s−1Mpc−1,Ωm= 0.27 andΩΛ= 0.73.
In this cosmology, at the redshift of our cluster (z=0.363), one arcminute corresponds to 305.8 kpc. We assumeSν ∝ ν−α throughout, whereSν is the flux density at frequencyν andαis the spectral index.
Table 4.1: Published properties of J0256. a R.A. and Dec. (J2000) of the SZ peak of the cluster, with an astrometric accuracy of 5-1000. b Cluster redshift as per Menanteau et al. (2013).
c Integrated 0.1–2.4 keV X-ray luminosity and X-ray mass from Majerowicz et al. (2004), corrected for the cosmology in this paper. This band luminosity is obtained from integrating the spectrum obtained by M04. d Integrated Compton y-parameter and B12 SZ mass from Hasselfield et al. (2013).
R.A. (hh mm ss.s) 02 56 33.0a Dec. (dd mm ss.s) +00 06 26.3a
redshift 0.363b
LX(1044ergs s−1) 3.01±0.20c Y500(10−4arcmin2) 3.4±1.0d M500,X(1014M) 5.5±1.1c M500,SZ(1014M) 5.0±1.2d