Statistical Techniques for Detecting the Intergalactic Magnetic Field From Large Samples of Extragalactic Faraday Rotation Data

Abstract

Rotation measure (RM) grids of extragalactic radio sources have been widely used for studying cosmic magnetism. But their potential for exploring the intergalactic magnetic field (IGMF) in filaments of galaxies is unclear, since other Faraday-rotation media such as the radio source itself, intervening galaxies, and the interstellar medium of our Galaxy are all significant contributors. We study statistical techniques for discriminating the Faraday rotation of filaments from other sources of Faraday rotation in future large-scale surveys of radio polarization. We consider a 30 • × 30 • field-of-view toward the south Galactic pole, while varying the number of sources detected in both present and future observations. We select sources located at high redshifts and toward which depolarization and optical absorption systems are not observed, so as to reduce the RM contributions from the sources and intervening galaxies. It is found that a high-pass filter can satisfactorily reduce the RM contribution from the Galaxy, since the angular scale of this component toward high Galactic latitudes would be much larger than that expected for the IGMF. Present observations do not yet provide a sufficient source density to be able to estimate the RM of filaments. However, from the proposed approach with forthcoming surveys, we predict significant residuals of RM that should be ascribable to filaments. The predicted structure of the IGMF down to scales of 0.1 • should be observable with data from the SKA, if we achieve selections of sources toward which sightlines do not contain intervening galaxies and RM errors are less than a few rad m −2 . Subject headings: intergalactic medium -large-scale structure of universe -magnetic fields -polarization -ISM: magnetic fields

Forecasts for the detection of the magnetised cosmic web from cosmological simulations

“…Govoni et al 2015), Faraday rotation (e.g. Akahori et al 2014;Bonafede et al 2015;Taylor et al 2015), neutral hydrogen emission (e.g. Popping et al 2015), absorption and emission features of highly ionised elements in the WHIM (e.g.…”

Fast Radio Bursts as Probes of Magnetic Fields in the Intergalactic Medium

“…We investigate how † B in Equation (3) compares with the IGMF strength in filaments, and propose a modified formula. We do not consider other contributions to DM and RM, such as those from host galaxies or local environments of FRBs or from the foreground Milky Way (see, e.g., Akahori et al 2014a;Dolag et al 2015;Kulkarni et al 2015;, and also Section 4 below for discussions of those contributions). The rest of this paper is organized as follows: the models and calculation are described in Section 2, the results are shown in Section 3, and the discussion and summary are set out in Section 4.…”

SKA Deep Polarization and Cosmic Magnetism

“…The RM due to the Galactic magnetic field could be substantially reduced with spatial filtering to remove the large scale variance (Akahori et al 2014). Simulations have shown that a RM data set with a sky den- (Mao et al 2010) and lines (Stil.…”

“…et al 2011) are observed ones for ∼ 900 deg 2 fields-of-view toward North (filled, thick) and South (open, thin) Galactic poles. The purple area indicates possible amplitudes for the Milky Way RM toward the poles (Akahori et al 2013). sity of several 100 to 1000 per square degree and with RM precision of ∼1 rad m −2 is required to accurately reconstruction the structure function of RM variance due to the IGMF in the cosmic web (Akahori et al 2014). An SKA1-MID deep polarization survey to below 100 nJy with band 2 & 3 would yield formal errors in RM of 1 rad m −2 for sources with polarized flux density greater than 1.3 µJy.…”