Limiting magnetic fields in the cosmic web with diffuse radio emission

Abstract

We set limits on the presence of the synchrotron cosmic web through the crosscorrelation of the 2.3 GHz S-PASS survey with a model of the local cosmic web derived from constrained magnetohydrodynamic (MHD) simulations. The MHD simulation assumes cosmologically seeded magnetic fields amplified during large-scale structure formation, and a population of relativistic electrons/positrons from proton-proton collisions within the intergalactic medium. We set a model-dependent 3σ upper limit on the synchrotron surface brightness of 0.16 mJy arcmin −2 at 2.3 GHz in filaments. Extrapolating from magnetic field maps created from the simulation, we infer an upper limit (density-weighted) magnetic field of 0.03 (0.13) µG in filaments at the current epoch, and a limit on the primordial magnetic field (PMF) of B P M F <1.0 nG.

Differences in Faraday Rotation between Adjacent Extragalactic Radio Sources as a Probe of Cosmic Magnetic Fields

“…We can still compare this limit to other limits and estimates. Vernstrom et al (2017) and Brown et al (2017) estimated upper limits on the magnetic field of the IGM using cross-correlation analyses and found limits of ∼ 100 to ∼ 500 nG. The recent work by Vacca et al (2018) found an average magnetic field in the IGM from simulations of 20−50 nG, with previous simulations finding similar values of ∼ 10 − 50 nG (Ryu et al 2008;Vazza et al 2014bVazza et al , 2016.…”

S-band Polarization All-Sky Survey (S-PASS): survey description and maps

“…Upper limits have been found so far with correlation techniques between radio emission and cosmic web tracers (e.g. Vernstrom et al 2017;Brown et al 2017) and a possible detection of diffuse synchrotron emission from a filament has recently been reported using the 64-m Sardinia Radio Telescope in combination with interferometric data to subtract blended compact sources (Vacca et al 2018).…”

Science with the Murchison Widefield Array: Phase I results and Phase II opportunities

“…The large-scale structure of the Universe requires the presence of intergalactic shocks, which are in turn expected to accelerate electrons and amplify intergalactic magnetic fields (Keshet et al, 2004b;Brüggen et al, 2005;Hoeft & Brüggen, 2007;Battaglia et al, 2009;Araya-Melo et al, 2012). These shocks should thus produce faint synchrotron emission, which can act as a tracer of large-scale structure, cosmic filaments and primordial magnetic fields (Keshet et al, 2004a;Wilcots, 2004;Donnert et al, 2009;Vazza et al, 2015a,b;Brown et al, 2017b). Detection of this "synchrotron cosmic web" can provide a direct image of the large-scale structure of the Universe, act as a laboratory for studying particle acceleration in low-density shocks, lead to a measurement of the magnetic field strength of the intergalactic medium, and provide a direct discriminant on competing models for the origin of cosmic magnetism.…”

“…The depth of the search reported by Vernstrom et al (2017) was limited by confusion, in that large numbers of unresolved extragalactic radio sources have not been subtracted from the data, and may mimic diffuse radio emission that traces large-scale structure. As for many other continuum science programs, the improved angular resolution and reduced confusion levels offered by MWA Phase II will enable much deeper searches for the synchrotron cosmic web, whether by direct imaging (e.g., Kronberg et al, 2007), statistical cross-correlations (Vernstrom et al, 2017;Brown et al, 2017b) or also in polarimetry Brown et al, 2009).…”