University of Groningen Sardinia Radio Telescope observations of Abell 194. The intra-cluster magnetic field power spectrum Govoni, F.; Murgia, M.; Vacca, V.; Loi, F.; Girardi, M.; Gastaldello, F.; Giovannini, G.; Feretti, L.; Paladino, R.; Carretti, E. Published in: Astronomy and astrophysics DOI: 10.1051/0004-6361/201630349 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2017 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Govoni, F., Murgia, M., Vacca, V., Loi, F., Girardi, M., Gastaldello, F., Giovannini, G., Feretti, L., Paladino, R., Carretti, E., Concu, R., Melis, A., Poppi, S., Valente, G., Bernardi, G., Bonafede, A., Boschin, W., Brienza, M., Clarke, T. E., ... Vazza, F. (2017). Sardinia Radio Telescope observations of Abell 194. The intra-cluster magnetic field power spectrum. Astronomy and astrophysics, 603, [A122]. https://doi.org/10.1051/0004-6361/201630349 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license. More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne- amendment. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. A&A 603, A122 (2017) Astronomy DOI: 10.1051/0004-6361/201630349 & c ESO 2017 Astrophysics Sardinia Radio Telescope observations of Abell 194 The intra-cluster magnetic field power spectrum F. Govoni1, M. Murgia1, V. Vacca1, F. Loi1; 2, M. Girardi3; 4, F. Gastaldello5; 6, G. Giovannini7; 8, L. Feretti7, R. Paladino7, E. Carretti1, R. Concu1, A. Melis1, S. Poppi1, G. Valente9; 1, G. Bernardi10; 11, A. Bonafede7; 12, W. Boschin13; 14; 15, M. Brienza16; 17, T. E. Clarke18, S. Colafrancesco19, F. de Gasperin20, D. Eckert21, T. A. Enßlin22, C. Ferrari23, L. Gregorini7, M. Johnston-Hollitt24, H. Junklewitz25, E. Orrù16, P. Parma7, R. Perley26, M. Rossetti5, G. B Taylor27, and F. Vazza7; 12 (Affiliations can be found after the references) Received 23 December 2016 / Accepted 23 March 2017 ABSTRACT Aims. We study the intra-cluster magnetic field in the poor galaxy cluster Abell 194 by complementing radio data, at different frequencies, with data in the optical and X-ray bands. Methods. We analyzed new total intensity and polarization observations of Abell 194 obtained with the Sardinia Radio Telescope (SRT). We used the SRT data in combination with archival Very Large Array observations to derive both the spectral aging and rotation measure (RM) images of the radio galaxies 3C 40A and 3C 40B embedded in Abell 194. To obtain new additional insights into the cluster structure, we investigated the redshifts of 1893 galaxies, resulting in a sample of 143 fiducial cluster members. We analyzed the available ROSAT and Chandra observations to measure the electron density profile of the galaxy cluster. Results. The optical analysis indicates that Abell 194 does not show a major and recent cluster merger, but rather agrees with a scenario of accretion of small groups, mainly along the NE−SW direction. Under the minimum energy assumption, the lifetimes of synchrotron electrons in 3C 40 B measured from the spectral break are found to be 157 ± 11 Myr. The break frequency image and the electron density profile inferred from the X-ray emission are used in combination with the RM data to constrain the intra-cluster magnetic field power spectrum. By assuming a Kolmogorov power-law power spectrum with a minimum scale of fluctuations of Λmin = 1 kpc, we find that the RM data in Abell 194 are well described by a magnetic field with a maximum scale of fluctuations of Λmax = (64 ± 24) kpc. We find a central magnetic field strength of hB0i = (1:5 ± 0:2) µG, which is the lowest ever measured so far in galaxy clusters based on Faraday rotation analysis. Further out, the field decreases with the radius following the gas density to the power of η = 1:1 ± 0:2. Comparing Abell 194 with a small sample of galaxy clusters, there is a hint of a trend between central electron densities and magnetic field strengths. Key words. galaxies: clusters: general – galaxies: clusters: individual: Abell 194 – magnetic fields – large-scale structure of Universe 1. Introduction Highly polarized elongated radio sources named relics are also observed at the periphery of merging systems (e.g., Clarke Galaxy clusters are unique laboratories for the investigation of & Ensslin 2006; Bonafede et al. 2009b; van Weeren et al. 2010). turbulent fluid motions and large-scale magnetic fields (e.g., These radio sources trace the regions where the propagation of Carilli & Taylor 2002; Govoni & Feretti 2004; Murgia 2011). mildly supersonic shock waves compresses the turbulent intra- In the last few years, several efforts have been focused on deter- cluster magnetic field, thereby enhancing the polarized emission mining the effective strength and structure of magnetic fields in and accelerating the relativistic electrons responsible for the syn- galaxy clusters and this topic represents a key project in view of chrotron emission. the Square Kilometre Array (e.g., Johnston-Hollitt et al. 2015). A complementary set of information on galaxy cluster mag- Synchrotron radio halos at the center of galaxy clusters (e.g., netic fields can be obtained from high quality rotation measure Feretti et al. 2012; Ferrari et al. 2008) provide direct evidence (RM) images of powerful and extended radio galaxies. The pres- of the presence of relativistic particles and magnetic fields asso- ence of a magnetized plasma between an observer and a radio ciated with intra-cluster medium. In particular, the detection of source changes the properties of the incoming polarized emis- polarized emission from radio halos is key to investigating the sion. In particular, the position angle of the linearly polarized magnetic field power spectrum in galaxy clusters (Murgia et al. radiation rotates by an amount that is proportional to the line in- 2004; Govoni et al. 2006, 2013, 2015; Vacca et al. 2010). How- tegral of the magnetic field along the line-of-sight times the elec- ever, detecting this polarized signal is a very hard task with cur- tron density of the intervening medium, i.e., the so-called Fara- rent radio facilities and so far only three examples of large-scale day rotation effect. Therefore, information on the intra-cluster filamentary polarized structures have been detected that are pos- magnetic fields can be obtained, in conjunction with X-ray ob- sibly associated with halo emission (A2255; Govoni et al. 2005; servations of the hot gas, through the analysis of the RM of radio Pizzo et al. 2011, MACS J0717.5+3745; Bonafede et al. 2009; galaxies in the background or in the galaxy clusters themselves. A523; Girardi et al. 2016). Rotation measure studies have been performed on statistical Article published by EDP Sciences A122, page 1 of 26 A&A 603, A122 (2017) samples (e.g., Clarke et al. 2001; Johnston-Hollitt & Ekers 2004; (18−26 GHz), a mono-feed C-band receiver (5700−7700 GHz), Govoni et al. 2010) as well as individual clusters (e.g., Perley & and a coaxial dual-feed L=P-band receiver (305−410 MHz, Taylor 1991; Taylor & Perley 1993; Feretti et al. 1995, 1999; 1300−1800 MHz). Taylor et al. 2001; Eilek & Owen 2002; Pratley et al. 2013). The antenna was officially opened on 30 September 2013, These studies reveal that magnetic fields are widespread in the upon completion of the technical commissioning phase (Bolli intra-cluster medium, regardless of the presence of a diffuse ra- et al. 2015). The scientific commissioning of the SRT was car- dio halo emission. ried out in the period 2012–2015 (Prandoni et al. 2017). At the The RM distribution seen over extended radio galaxies is beginning of 2016 the first call for single dish early science pro- generally patchy, indicating that magnetic fields are not regularly grams was issued, and the observations started on February 1st, ordered on cluster scales, but instead they have turbulent struc- 2016. tures down to linear scales as small as a few kpc or less. There- The SMOG project is an SRT early science program (PI fore, RM measurements probe the complex topology of the clus- M. Murgia) focused on a wide-band and wide-field single ter magnetic field and indeed state-of-the-art software tools and dish spectral-polarimetric study of a sample of galaxy clusters. approaches based on a Fourier domain formulation have been By comparing and complementing the SRT observations with developed to constrain the magnetic field power spectrum pa- archival radio data at higher resolution and at different frequen- rameters on the basis of the RM images (Enßlin & Vogt 2003; cies, but also with data in optical and X-ray bands, we want to Murgia et al. 2004; Laing et al. 2008; Kuchar & Enßlin 2011; improve our knowledge of the non-thermal components of the Bonafede et al. 2013). The magnetic field power spectrum has intra-cluster medium on a large scale.