2.2 Types of diffuse cluster radio emission
2.2.2 Radio Relics
2.2.2.1 Observed structure
Radio relics can be split into two main categories based on their differing morphological prop- erties, referred to as either elongated or roundish radio relics. Observations have also revealed different combinations of diffuse radio emission involving radio relics. Examples of the different morphologies and configurations are given in Figures 2.4, 2.5 and 2.6.
Elongated relics
The prototypical form of radio relics,elongated relicshave high major- to minor-axis ratios, with the major axis tangential to the edge of the cluster region, and cover large distances (& 1Mpc).
Elongated relics are highly polarized with polarization vectors orientated perpendicular to the major axis. An image of the Coma relic from Giovannini et al. (1991) is given in the left panel of Figure 2.4.
Not all elongated relics have the same morphologies; some elongated relics are exceptionally narrow with a few giant narrow relics such as the “Sausage” relic in CIZA J2242.8+5301 (van Weeren et al., 2011c, Figure 2.5) and the “Toothbrush” relic in 1RXS J0603.3+4214 (van Weeren et al., 2012, Figure 2.6) being extensively researched (van Weeren et al., 2011a; R¨ottgering et al., 2013; Ogrean et al., 2013a; Stroe et al., 2014c,b,a, 2015). The sausage relic is part of a double radio relic system shown in Figure 2.5 and is slightly curved with extremely regular polarization vectors. In certain cases projection effects may play a major role in explaining apparently odd orientations of elongated relics (e.g. A115, Govoni et al., 2001b), however the overall morphol- ogy and location of elongated relics are in line with models of merger shocks, discussed further in§2.2.2.4.
Roundish relics
The second morphological class of radio relics have a more symmetrical, circular structure and are therefore called roundish relics. Although these relics also lie on the periphery of clusters, they are found slightly closer to the cluster centre and tend to be smaller than elongated relics, with typical sizes of∼100 - 350 kpc. To date, most roundish relics have been observed at quite low redshifts,z <0.2, and have very steep curved spectra. An example of a roundish relic is that in A1664 (Govoni et al., 2001b) displayed in the right panel of Figure 2.4.
As they have different shapes and cluster locations compared to elongated relics, roundish relics have different models for their origin. High resolution imaging of roundish relics show a filamentary structure within the relic itself, and they are often located near the central first ranked galaxy (FRG) but not coinciding with it (Govoni et al., 2001b; Slee et al., 2001). Since these sources are often close to AGN they can be explained by the shock-wave re-energisation
of radio lobes from previous AGN activity. The radio lobes were previously unobservable due to synchrotron and IC losses. We note, however, that unlike the shock-driven elongated relics, roundish relics have not as yet been found in pairs. Mathews and Brighenti (2008) suggest a different scenario in which the relics are the result of radio bubbles interacting with the outer re- gions of the ICM. Due to their sometimes small size, the correct identification of roundish radio relics can be complicated by the presence of old radio galaxies with aged spectra (Randall et al., 2010). In some cases, a few roundish relics cannot be readily explained by models involving previous radio activity (Feretti et al., 2006; Solovyeva et al., 2008; Govoni et al., 2001b). More observations are required in order to fully understand the origin of these types of relics.
Double relics
We previously mentioned the link between elongated radio relics and shock fronts caused by cluster mergers, discussed in§2.2.1.3. In mergers with almost equal subcluster mass ratios and a low impact parameter, a pair of shocks, and hence radio relics, are expected on either side of the cluster, aligned with the merger axis. When this configuration is observed the cluster is said to host double radio relics. The first double relic system was observed in A3667 (Rottgering et al., 1997; Johnston-Hollitt et al., 2002; Johnston-Hollitt, 2003) but these configurations have since become quite common with almost 40% of relics belonging to double systems. Figure 2.5 shows examples of three double relic systems of different sizes. Most double relics have elongated structures, in line with the connection to plasma shocks.
Hydrodynamical simulations of CIZA J2242.8+5301 have shown that the observed double relic morphologies result routinely from plane-of-the-sky mergers involving the head-on colli- sion of equal mass clusters (van Weeren et al., 2011c). Thus double relic systems should be found in low-ratio mergers, whereas single relic systems are expected to be in mergers with higher mass ratios or where the merger is occurring to some degree along the line of sight.
Relics and halos
In several cases, radio relics, both double and single, have been observed in clusters hosting a radio halo. These systems are significant indications of there being a link between radio relics,
Figure 2.5: Images of giant radio relics (contours) overlaid on the X-ray emission from the host systems (colors). The three radio relics are reported with the same physical scale. Upper- left and upper-right panels highlight the high-resolution radio images of the northern relics in CIZA2242 and A3667, respectively (credits: van Weeren et al. (2010) and Ogrean et al. (2013a) for CIZA2242; Rottgering et al. (1997) for A3667; and Kale et al. (2012) for A3376). Source:
Brunetti and Jones (2014).
radio halos and mergers, as the relics and halos may stem from the same merger event, although from different physical mechanisms. Figure 2.6 shows some examples of combined radio halo and relic systems. In some cases there is bridge emission between the halo and relic, as is the case for the “Toothbrush” relic and its associated halo in 1RXS J0603.3+4214 (right panel of Figure 2.6.
However the majority of relic systems do not host a radio halo, and in all relic-halo systems, the relic has a surface brightness significantly higher than that of the halo. The lack of radio ha- los in many relic systems may indicate that radio relics can be produced by weak/minor mergers which don’t produce enough turbulent energy to drive an observable radio halo. This may also
Figure 2.6: Examples of galaxy clusters hosting both radio halo and radio relic emission. Left:
GMRT 323 MHz contours showing the radio halo and double relics in PSZ1 G108.18-11.53 at (1,2,4,8,16,32)×3σwithσ = 86µJy beam−1(beam: 1100×800). The contours are overlaid on the smoothed 0.1 - 2.4 keV photon image from the ROSAT all sky survey. Source: de Gasperin et al. (2015). Right: 1.4 GHz WSRT emission of the radio halo and single straight radio relic in 1RXS J0603.3+4214. There is a bridge of radio emission joining the two structures. Source:
van Weeren et al. (2012).
explain the observations of radio relics in cool-core clusters (e.g. A85, Slee et al., 2001): if a mi- nor or off-axis merger were present, a shock wave to induce a relic can occur without disrupting the cluster core.