New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations (2002.06924v2)

Abstract: 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.

• The paper demonstrates that LOFAR's RM analysis reveals no significant excess from intergalactic magnetic fields, thereby constraining them to under 4 nG on Mpc scales.
• It employs a novel method comparing RM differences between physical pairs and random pairs to isolate extragalactic magnetic signals.
• Model comparisons show that astrophysical and uniform primordial scenarios match the data, while dynamo amplification models are inconsistent with observations.

An Essay on "New Constraints on the Magnetization of the Cosmic Web using LOFAR Faraday Rotation Observations"

The research paper by O'Sullivan et al. addresses critical questions related to the understanding of cosmic magnetism, particularly the strength and distribution of intergalactic magnetic fields (IGMF) in the cosmic web. Utilizing data from the LOFAR Two-Metre Sky Survey (LoTSS), this paper capitalizes on the sensitivity and frequency range of LOFAR to examine Faraday rotation measures (RMs) of extragalactic radio sources.

Methodology and Analysis

The researchers implemented a novel approach to differentiate between the Faraday rotation produced by the Milky Way and that contributed by extragalactic magnetic fields. This was achieved by analyzing the RM differences (\dRM) between two categories: physical pairs (PPs), representing two components of the same radio source, and random pairs (RPs), which refer to unrelated sources appearing in proximity in the sky.

By comparing the RM differences between these samples, the paper effectively isolates the signal attributed to large-scale intergalactic fields. Significantly, the analysis reveals no substantial excess Faraday rotation from intergalactic fields, placing a stringent constraint on the cosmological co-moving magnetic field strength at no greater than 4 nG for Mpc scales.

Implications and Model Comparisons

Various models were employed to connect the observational data with theoretical expectations. An inhomogeneous universe model was utilized, where the magnetic field strength scales with local density fluctuations. This modeling strategy represents a step forward in refining the constraints on intergalactic magnetism compared to more rudimentary, homogeneous models.

Additionally, the paper considered different magnetogenesis scenarios within cosmological magneto-hydrodynamical (MHD) simulations. Among these, the astrophysical and uniform primordial field scenarios aligned most congruently with observed data, indicating that these pathways for cosmic magnetic field evolution are plausible under current observation constraints. Notably, scenarios relying on dynamo amplification in cosmic filaments exhibited discrepancies with the data, as the model-predicted RM fluctuations were significantly higher than what LOFAR observations allowed.

Observational Challenges and Future Directions

The research also highlights a crucial aspect of LOFAR's observational capacity: its precision in RM measurements allows for stringent testing of theoretical models by limiting local magnetic contributions from host radio sources. However, advances in understanding and distinguishing between contributions from various cosmic structures require a more diverse and extensive dataset. Impending datasets from the expansion of LoTSS and enhanced redshift measurements could elucidate the evolution and specific contributions of the cosmic web's magnetic components.

Moving forward, the combination of LOFAR's deep surveys with upcoming endeavors at higher frequencies, such as those anticipated from the Square Kilometre Array (SKA), will enhance our capability to probe magnetic field strengths and distributions across cosmic scales, thereby refining theoretical frameworks and advancing our understanding of cosmic magnetism.

In conclusion, this work embodies a significant step in constraining the magnetization of the Universe on large scales. The detailed methodological approach and interpretation of the LOFAR RM data offer robust checkpoints for current cosmological models and suggest pathways for future explorations into the cosmic magnetic frontier.