A preliminary image of the nearby Coma cluster as seen by LOFAR at 140 MHz. LOFAR recovers the large-scale radio halo and relic in this cluster, as well as many compact radio sources. The overall extent of the radio emission is around 10 million light years.
The detection and investigation of magnetic fields in the large scale structure of the Universe is crucial to shed light on the origin and evolution of cosmological magnetic fields. Magnetic fields have been detected in galaxy clusters, the knots of the cosmic web, thanks to the observation of diffuse large scale synchrotron sources (radio halos and radio relics) and to the Faraday rotation effect on the signal of background radio sources. Galaxy cluster formation processes completely distort the power spectrum of the seed magnetic field by amplifying its strength and changing its structure. Therefore, galaxy cluster observations alone do not allow us to discriminate between the different scenarios of magnetic field origin and evolution. Magneto-hydro-dynamical simulations indicate the presence of faint (~ 1 nG) magnetic fields also on scales larger than galaxy clusters, along the cosmological filaments, where turbulent gas motions have not yet enhanced the magnetic field and, as a consequence, its strength and structure still reflect the properties of the seed field. Despite being of paramount importance, magnetic fields have not been yet firmly observed along optical filaments connecting galaxy clusters.
LOFAR observations can give us a precious contribution in the detection and investigation of such magnetic fields. Indeed, low frequencies are particularly suitable both for investigating diffuse synchrotron sources and for analysing polarimetric properties of radio sources. Diffuse synchrotron emission represents direct proof of the presence of magnetic fields and it is expected whenever a population of relativistic electrons moves along the flux lines of a magnetic field. When observed in galaxy clusters, diffuse sources appear to be very faint (~ 0.1-1 μJy/arcsec2) and are characterized by a steep radio spectrum (α >1, Sν ∝ ν−α). As a result, observations at low frequencies are ideal for investigating the presence of diffuse emission on scales larger than galaxy clusters and thus inferring information about the associated large-scale magnetic field.
Indirect evidence of the presence of magnetic fields is provided by the rotation of the polarization angle caused by the Faraday effect, in presence of a magneto-ionic medium between the source and the observer. By using this approach, magnetic field strength and structure can be studied both on individual targets (with detailed rotation measure images for extended sources) as well as on statistical samples (with a grid of rotation measure values for a catalog of poin-like sources). Large-bandwidth observations at low frequencies allow us to constraint Faraday depth values with small uncertainties. Faraday rotation observations of extragalactic radio sources are sensitive to magnetic fields anywhere along the line of sight between the source and the observer: the Milky Way, the emitting radio source, galaxy clusters, filaments, sheets, and voids. To detect and investigate magnetic field properties in the large-scale of the Universe, it is necessary to disentangle these contributions. This is a non-trivial task that demands sophisticated statistical approaches.
In the context of the activities of the working group, we are addressing the detection and investigation of large-scale magnetic fields by applying the two appraoches described above and by developing sophisticated techinques of analysis.
A prominent galaxy cluster hosting diffuse radio emission is the Coma cluster, known to host a radio halo and a radio relic connected by a bridge of diffuse emission. The Figure to the right presents a preliminary image produced from LOFAR HBA observations at 140 MHz and 16 MHz of bandwidth (resolution ~ 25 arcsec, rms ~ 0.6 mJy/beam, 1/3 of data processed). The LOFAR data can be used to investigate the properties of the magnetic fields in this region of the sky by analysing the total intensity and polarization properties of diffuse and discrete sources in the field.
By using a reconstruction (Jasche et al. 2010) of the cosmic density field derived on the base on optical counts from the Sloan Digital Sky Survey (SDSS), we are looking for regions in the sky with the highest probability of containing cosmological filaments. The Figure to the left shows the mean fraction of filaments in the region of sky covered by the SDSS and evaluated integrating up to 360Mpc.
The Figure to the right shows the result of a Bayesian analysis of (simulated) extragalactic Faraday rotation data with respect to typical Faraday depth (dispersion) per cosmic distance, to the source luminosity and the sources intrinsic Faraday dispersion. The posterior probability distribution in one and two dimensional parameter cuts is presented. The analysis is based on a LOFAR catalogue built on Tier 1-like observations. The investigation of large-scale magnetic fields through this approach requires the knowledge of the redshift of the sources. In this respect, the planned survey with the WEAVE spectrograph will proceed as soon as the LOFAR Faraday depth catalogue is available.