New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations

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 (ΔRM) 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 ΔRM along the line of sight between non-physical pairs. From our analysis, we find no significant difference between the ΔRM distributions of the physical and non-physical pairs, limiting the excess Faraday rotation contribution to <1.9 rad m−2 (${\sim}95{{\ \rm per\ cent}}$ 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 magnetohydrodynamical 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 complex large-scale magnetic fields in the first Galactic quadrant as revealed by the Faraday depth profile disparity

“…The background map shows Hα intensity from WHAMSS (Haffner et al 2003(Haffner et al , 2010. the intrinsic FD of the EGSs (∼ 10-100 rad m −2 ; e.g., Schnitzeler 2010;Oppermann et al 2015;Anderson et al 2019), magnetic fields in the intergalactic medium ( 10 rad m −2 ; e.g., Vernstrom et al 2019;O'Sullivan et al 2020), and the uncertainty of our measurements (≈ 2 rad m −2 ). The use of SEM as our FD uncertainty implicitly assumes that the above sources of FD contaminations are not spatially correlated, which is not strictly the case for small-scale Galactic magnetic field (see Haverkorn et al 2008).…”

The POlarised GLEAM Survey (POGS) II: Results from an all-sky rotation measure synthesis survey at long wavelengths

“…These were the radio galaxies PMN J0351-2744 (e.g., Lenc et al 2017) We identify these four sources plus a further six physical pairs of polarised sources ('polarised doubles') in POGS ExGal, two examples of which are shown in Figure 5. Samples of such physical pairs of polarised sources can be used to provide constraints on the magnetised Cosmic Web (e.g., Vernstrom et al 2019;O'Sullivan et al 2020), but in order to derive meaningful constraints, we would require significantly higher surface density than is achieved with our MWA Phase I polarisation work.…”

“…Indeed, in recent years, the low-frequency polarised sky has been explored in greater detail than ever before, with detections of large-scale diffuse Galactic foreground along many lines of sight (LOSs) (e.g., Jelić et al 2015;Lenc et al 2016;Van Eck et al 2017 as well as many surveys that have begun to study the polarised extragalactic source population (e.g., Bernardi et al 2013;Mulcahy et al 2014;Van Eck et al 2017;Lenc et al 2018;Neld et al 2018;O'Sullivan et al 2019O'Sullivan et al , 2020Stuardi et al 2020;Cantwell et al 2020).…”

LOFAR Deep Fields: probing a broader population of polarized radio galaxies in ELAIS-N1

“…These catalogs are also powerful tools for the study of extragalactic large-scale magnetic fields, as shown by Bonafede et al (2010), who determined the Coma cluster magnetic field strength from RMs. Moreover, due to the high sensitivity and high precision of new-generation radio interferometers, RM catalogs of polarized radio sources will be key components for the investigation of the magnetization of the cosmic web (e.g., Vacca et al 2016;O'Sullivan et al 2020).…”

“…Van Eck et al (2018) reported developments in polarization processing for the 570 deg 2 preliminary data release region from the LOFAR Two-Metre Sky Survey (LoTSS 2 , Shimwell et al 2017Shimwell et al , 2019 and presented a catalog of 92 polarized radio sources at 150 MHz with 4.3arcmin resolution and 1 mJy root mean square (rms) sensitivity. O'Sullivan et al (2019) presented the Faraday RM and depolarization properties of a giant radio galaxy using LOFAR and demonstrated the potential of LOFAR to probe the weak signature of the intergalactic magnetic field (see also O'Sullivan et al 2020).…”