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Register a new userLow Frequency Radio Constraints on the Synchrotron Cosmic Web
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We present a search for the synchrotron emission from the synchrotron cosmic web by cross correlating 180MHz radio images from the Murchison Widefield Array with tracers of large scale structure (LSS). We use t
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Recent improvements in the capabilities of low frequency radio telescopes provide a unique opportunity to study thermal and non-thermal properties of the cosmic web. We argue that the diffuse, polarized emission from giant radio relics traces structure formation shock waves and illuminates the large-scale magnetic field. To show this, we model the population of shock-accelerated relativistic electrons in high-resolution cosmological simulations of galaxy clusters and calculate the resulting radio synchrotron emission. We find that individual shock waves correspond to localized peaks in the radio surface brightness map which enables us to measure Mach numbers for these shocks. We show that the luminosities and number counts of the relics strongly depend on the magnetic field properties, the cluster mass and dynamical state. By suitably combining different cluster data, including Faraday rotation measures, we are able to constrain some macroscopic parameters of the plasma at the structure formation shocks, such as models of turbulence. We also predict upper limits for the properties of the warm-hot intergalactic medium, such as its temperature and density. We predict that the current generation of radio telescopes (LOFAR, GMRT, MWA, LWA) have the potential to discover a substantially larger sample of radio relics, with multiple relics expected for each violently merging cluster. Future experiments (SKA) should enable us to further probe the macroscopic parameters of plasma physics in clusters.
We present the first results of a campaign of ENZO cosmological simulations targeting the shocked and the neutral parts of the cosmic web, obtained with Supercomputing facilities provided by the INAF-CINECA agreement.
It has recently become apparent that the background level of diffuse radio emission on the sky is significantly higher than the level that can result from known extragalactic radio source classes or our Galaxy given our current understanding of its large-scale structure.~ In contrast to the more well-known and well-constrained cosmological and astrophysical backgrounds at microwave, infrared, optical/UV, X-ray, and gamma-ray wavelengths, this ``radio synchrotron background at radio wavelengths provides clear motivation for considering the possibilities of new astrophysical sources and new particle-based emission mechanisms.
Strong accretion shocks are expected to illuminate the warm-hot inter-galactic medium encompassed by the filaments of the cosmic web, through synchrotron radio emission. Given their high sensitivity, low-frequency large radio facilities may already be able to detect signatures of this extended radio emission from the region in between two close and massive galaxy clusters. In this work we exploit the non-detection of such diffuse emission by deep observations of two pairs of relatively close ($simeq 10$ Mpc) and massive ($M_{500}geq 10^{14}M_odot$) galaxy clusters using the LOw-Frequency ARray (LOFAR). By combining the results from the two putative inter-cluster filaments, we derive new independent constraints on the median strength of inter-galactic magnetic fields: $B_{rm 10 Mpc}< 2.5times 10^2,rm nG,(95%, rm CL)$. Based on cosmological simulations and assuming a primordial origin of the B-fields, these estimates can be used to limit the amplitude of primordial seed magnetic fields: $B_0leq10,rm nG$. We advise the observation of similar cluster pairs as a powerful tool to set tight constraints on the amplitude of extragalactic magnetic fields.
Measuring the properties of extragalactic magnetic fields through the effect of Faraday rotation provides a means to understand the origin and evolution of cosmic magnetism. Here we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference ($Delta$RM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to $Delta$RM along the line of sight between non-physical pairs. From our analysis, we find no significant difference between the $Delta$RM distributions of the physical and non-physical pairs, limiting the excess Faraday rotation contribution to $< 1.9$ rad/m$^2$ ($sim$$95%$ confidence). We use this limit with a simple model of an inhomogeneous universe to place an upper limit of 4 nG on the cosmological co-moving magnetic field strength on Mpc scales. We also compare the RM data with a more realistic suite of cosmological MHD simulations, that explore different magnetogenesis scenarios. Both magnetization of the large scale structure by astrophysical processes such as galactic and AGN outflows, and simple primordial scenarios with seed magnetic field strengths $< 0.5$ nG cannot be rejected by the current data; while stronger primordial fields or models with dynamo amplification in filaments are disfavoured.
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