Radio spectral study of the cluster of galaxies Abell 2255

Abstract

Context. The study of the nonthermal components associated with the intra cluster medium (ICM) in galaxy clusters is important in understanding the history and evolution of clusters. Aims. Spectral index studies of halos, relics, and radio galaxies provide useful information on their origin and connection with merger processes. Moreover, they reveal the energy spectrum of the relativistic particles and the magnetic field distribution in galaxy clusters. Methods. We present WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm, 85 cm, and 2 m. The spectral index images allowed us to study the integrated spectrum of halo and relic and to investigate the physical properties of the Beaver head-tail radio galaxy belonging to the cluster. Results. In the radio halo, the spectral index is steeper at the center and flatter at the locations of the radio filaments, clearly detected at 25 cm. In the relics, the spectral index flattens, moving away from the cluster center. For the Beaver radio galaxy, the spectrum severely steepens from the head towards the end of the tail, because of the energy losses suffered by the relativistic particles. In the 2 m map, which is the first high-sensitivity image presented in the literature at such a long wavelength, a new Mpc-size emission region is detected between the known radio halo and the NW relic. Not detecting this feature in the more sensitive 85 cm observations implies that it must have a very steep spectrum (α ≤ −2.6). Conclusions. The observational properties of the radio halo suggest that either we are looking at a superposition of different structures (filaments in the foreground plus real halo in the background) seen in projection across the cluster center or that the halo is intrinsically peculiar. The newly detected extended region to the NW of the halo could be considered as an asymmetric extension of the halo itself. However, since radio halos are known in the literature as structures showing a regular morphology, the new feature could represent the first example of steep Mpc-size diffuse structures (MDS), detected around clusters at very low frequencies. The spectrum of the initial part of the tail of the Beaver, detected at the three wavelengths, is well-fitted by a single injection model.

Spectral index image of the radio halo in the cluster Abell 520, which hosts the famous bow shock

“…Spectral index images have been produced so far for just a few radio halos (e.g. Coma, Giovannini et al 1993;A665 andA2163, Feretti et al 2004;A3562, Giacintucci et al 2005;A2744 and A2219, Orrù et al 2007; A2255, Pizzo & de Bruyn 2009;andA2256, van Weeren et al 2009;and Kale & Dwarakanath 2010), revealing complex distributions with flattening in the regions directly influenced by ongoing mergers. Investigations of the integrated spectrum of radio halos have been performed as well, for instance, in Coma (Thierbach et al 2003), A2319 (Feretti et al 1997), A754 and A1914 (Bacchi et al 2003), A3562 (Giacintucci et al 2005), A2256 (Brentjens 2008;Kale & Dwarakanath 2010), and A697 (van Weeren et al 2011; but see also Venturi et al 2013).…”

A double radio relic in the merging galaxy cluster ZwCl 0008.8+5215

“…Hydrodynamical models of structure formation, including particle acceleration mechanisms (e.g., Hoeft & Brüggen 2007;Hoeft et al 2008;Pfrommer 2008;Battaglia et al 2009) make predictions about the location, orientation and radio power of relics in merging clusters. Amongst the several dozen radio relics known to date (e.g., van Weeren et al 2009b;Rudnick & Lemmerman 2009;Pizzo & de Bruyn 2009;Giacintucci et al 2008;Brentjens 2008;Kempner & Sarazin 2001;Giovannini et al 1999Giovannini et al , 1991, there are a few rare double relic systems, with two relics located symmetrically on opposite sites of the cluster center (e.g., van Weeren et al 2009avan Weeren et al , 2010Brown et al 2010;Bonafede et al 2009;Venturi et al 2007;Bagchi et al 2006;Röttgering et al 1997). These double relics can be used to constrain the merger geometry and timescales involved (Roettiger et al 1999).…”

The correlation between radio power and Mach number for radio relics in galaxy clusters

“…In this way, it would be also possible to obtain a minimum value of the peak spectral index α peak = 0.25 (and also the formulae to obtain MR and MX will change); this would allow to explain, e.g., the peak values α peak ∼ 0.4 for the relics in A3667 observed by Hindson et al (2014), and the average value of αav = 0.8 ± 0.1 found in A2255 by Pizzo & De Bruyn (2009). Therefore, it is necessary to study in detail the spatial properties of the (radio)galaxies close to the relics, and check if both the electrons and the shocks can be produced by radio galaxies jets (we will discuss this case more extensively elsewhere, see Colafrancesco et al in preparation).…”

“…References: (1) Shimwell et al (2015); (2) Ogrean et al (2013b). Notes: a: error bar not present in the cited paper, we assumed 0.1 on the last significant digit; b: error bar assumed to match the value of 0.92 ± 0.02 given by ; c: van Weeren et al (2012) give the value 1.0 ± 0.2, but from the spectrum they publish it is clear that this spectral index does not match well the points at 1.7 and 2.3 GHz and is affected mainly from the point at 147 MHz that has a big error bar, therefore we fixed the value at the maximum value allowed by their error bar; d: error bar assumed to match the value of 0.9 ± 0.1 estimated by Johnston-Hollitt (2003); e: value and error estimated from the map published in Lindner et al (2014); f : value estimated from the full resolution data of Pizzo & De Bruyn (2009) and error fixed to match the value they derive after correcting to have the same angular resolution at the different frequencies (0.8 ± 0.1) and the value 1.4 estimated by Feretti et al (1997); g: tentative estimate derived by us from the jump in the temperature observed between the regions A6 and A15 by Ma et al (2009), that have temperatures of 19 ± 8 and 12 ± 5 keV respectively; h: error bar not present in the cited paper, estimated by us from the errors on the temperature; i: error bar not present in the cited paper, estimated to be of the same order than a previous case discussed in the same paper, before removing one point from the fitting; j : in the cited paper the Mach number is given as M X ∼ > 3.5 − 4. 2.4 ± 0.3 1.7 ± 0.1 (22) 2.0 ± 0.1 A3411 (3.0 ± 0.2) × 10 24 (23) 1.7 ± 0.2 i (24) 0.9 ± 0.1 (24) 2.4 ± 0.3 1.2 ± 0.1 (24) 3.3 ± 0.8 A115 (3.9 ± 0.2) × 10 24 (25) 1.7 ± 0.1 (25) − − 1.1 ± 0.5 (26) 5 ± 11 El Gordo (NW) (3.1 ± 0.9) × 10 25 (27) 4.1 +3.4 −0.9 (28) 0.86 ± 0.15 (27) 2.56 ± 0.45 1.19 ± 0.09 (27) 3.4 ± 0.7 El Gordo (SE) (4.5 ± 1.5) × 10 24 (27) 3.75 ± 0.25 j (28) 1.1 ± 0.1 e (27) 2.1 ± 0.1 1.4 ± 0.1 (27) 2.4 ± 0.3 A2744 (6.5 ± 0.5) × 10 24 (29) 1.7 +0.5 −0.3 (30) 0.9 ± 0.1 (29) 2.4 ± 0.3 1.1 ± 0.1 (29) 5 ± 2 A2255 (4.8 ± 0.2) × 10 23 (31) 1.42 +0.19 −0.15 (32) 0.9 ± 0.1 (31) 2.4 ± 0.3 1.1 ± 0.3 f (31) 5 ± 7 MACS J0717.5+3745 (1.05 ± 0.06) × 10 26 (33)…”