The Coma cluster magnetic field from Faraday rotation measures

¹ Dip. di Astronomia, Univ. Bologna, via Ranzani 1, 40120 Bologna, Italy e-mail: bonafede@ira.inaf.it
² INAF- Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ INAF- Osservatorio Astronomico di Cagliari, Loc. Poggio, Italy dei Pini, Strada 54, 09012, Capoterra (Ca) Italy
⁴ Max-Plank-Institut für Astrophysik, PO Box 1317, 85741 Garching, Germany
⁵ Department of Physics and Astronomy, University of New Mexico 800 Yale Blvd NE, Albuquerque, NM 87131, USA, and Adjunct Astronomer at the National Radio Astronomy Observatory, USA

Received: 19 November 2009
Accepted: 27 January 2010

Abstract

Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical simulations of the magnetic field.

Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B₀ and radial power-law slope η, where η indicates how the field scales with respect to the gas density profile.

Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ~2 kpc and maximum scale ~34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B₀ = 3.9 μG; η = 0.4) and (B₀ = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B₀ = 4.7 μG; η = 0.5. Values of B₀ > 7 μG and <3 μG as well as η < 0.2 and η > 1.0 are incompatible with RM data at 99% confidence level.