Sardinia Radio Telescope observations of Abell 194

The intra-cluster magnetic field power spectrum

¹ INAF–Osservatorio Astronomico di Cagliari via della Scienza 5, 09047 Selargius (CA), Italy
e-mail: fgovoni@oa-cagliari.inaf.it
² Dip. di Fisica, Università degli Studi di Cagliari, Strada Prov.le Monserrato-Sestu Km 0.700, 09042 Monserrato (CA), Italy
³ Dip. di Fisica, Università degli Studi di Trieste – Sezione di Astronomia, via Tiepolo 11, 34143 Trieste, Italy
⁴ INAF–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF–IASF Milano, via Bassini 15, 20133 Milano, Italy
⁶ Dep. of Physics and Astronomy, University of California at Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697-4575, USA
⁷ INAF–Istituto di Radioastronomia, Bologna via Gobetti 101, 40129 Bologna, Italy
⁸ Dip. di Fisica e Astronomia, Università degli Studi di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
⁹ Agenzia Spaziale Italiana (ASI), 00100 Roma, Italy
¹⁰ SKA SA, 3rd Floor, The Park, Park Road, 7405 Pinelands, The Cape Town, South Africa
¹¹ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
¹² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹³ Fundación G. Galilei – INAF TNG, Rambla J. A. Fernández Pérez 7, 38712 Breña Baja (La Palma), Spain
¹⁴ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Spain
¹⁵ Dep. de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna (Tenerife), Spain
¹⁶ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
¹⁷ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁸ Naval Research Laboratory, Washington, District of Columbia 20375, USA
¹⁹ School of Physics, University of the Witwatersrand, Private Bag 3, 2050 Johannesburg, South Africa
²⁰ University of Leiden, Rapenburg 70, 2311 EZ Leiden, The Netherlands
²¹ Astronomy Department, University of Geneva, 16 Ch. d’Ecogia, 1290 Versoix, Switzerland
²² Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85740 Garching, Germany
²³ Laboratoire Lagrange, UCA, OCA, CNRS, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
²⁴ School of Chemical & Physical Sciences, Victoria University of Wellington, PO Box 600, 6140 Wellington, New Zealand
²⁵ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²⁶ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
²⁷ Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA

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 Λ_min = 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 Λ_max = (64 ± 24) kpc. We find a central magnetic field strength of ⟨ B₀ ⟩ = (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.