Sardinia Radio Telescope observations of Abell 194

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 analyze new total intensity and polarization observations of Abell 194 obtained with the Sardinia Radio Telescope (SRT). We use 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 in the cluster structure we investigate the redshifts of 1893 galaxies, resulting in a sample of 143 fiducial cluster members. We analyze 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 3C40 B measured from the spectral break are found to be 157±11 Myrs. 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 B 0 = (1.5 ± 0.2) µG, 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.

Characterizing the Uncertainty in Cluster Magnetic Fields Derived from Rotation Measures

“…In general, RM observers would not know η a priori and would, therefore, have to estimate η based on theoretical considerations or simulation results. In cases where there is extensive RM data, one can attempt to estimate η by comparing the radial dependence of σ RM to the radial dependence of n e (e.g., Dolag et al 2001;Guidetti et al 2008;Bonafede et al 2010;Govoni et al 2017). However, the ability to do this depends on the unknown distribution of polarized sources along the line of sight, the assumption of an unchanging coherence scale, and the lack of any local effects around the embedded radio galaxies.…”

“…A study of Abell 194 using RM observations from the Sardinia Radio Telescope and the VLA are presented by Govoni et al (2017). They use a Bayesian type analysis based on three-dimensional simulations assuming a power law magnetic field spectrum, while utilizing both the RM distribution and fractional polarization information.…”

“…RM analyses of ICMs have been applied extensively, using polarized synchrotron sources embedded in the cluster (e.g., Taylor & Perley 1993;Taylor et al 2006;Eilek & Owen 2002;Vacca et al 2012;Feretti et al 1999;Govoni et al 2006;Guidetti et al 2008;Guidetti et al 2010;Govoni et al 2017) as well as multiple unresolved polarized sources behind galaxy clusters. (e.g., Clarke et al 1999;Feretti et al 1999;Bonafede et al 2010) RM measurements have typically suggested maximum magnetic field strengths at cluster centers ∼ 1 − 15µG, with strengths steadily decreasing outwards.…”

“…We note that some recent methods (Vacca et al 2012;Govoni et al 2017) of estimating intracluster magnetic field properties from observed RMs use Monte Carlo simulations and a Bayesian analysis with an assumed power law distribution of magnetic field fluctuations. This approach offers distinct advantages by characterizing some, although not all, of the uncertainties associated with the above issues.…”

The Sardinia Radio Telescope

“…The SRT can also make an important contribution to the investigation of intracluster magnetic fields (see Govoni et al 2017). Determining the origin of these fields and how they evolved over cosmic times, from their genesis in the primordial Universe up to the µG levels observed in nearby galaxy clusters, is one of the major challenges in modern astronomy.…”