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

Abstract

Faraday rotation measures (RMs) of extragalactic radio sources provide information on line-of-sight magnetic fields, including contributions from our Galaxy, source environments, and the intergalactic medium (IGM). Looking at differences in RMs, ∆RM, between adjacent sources on the sky can help isolate these different components. In this work, we classify adjacent polarized sources in the NVSS as random or physical pairs. We recompute and correct the uncertainties in the NVSS RM catalog, since these were significantly overestimated. Our sample contains 317 physical and 5111 random pairs, all with Galactic latitudes |b| ≥ 20 • , polarization fractions ≥ 2%, and angular separations between 1.5 and 20 . We find an rms ∆RM of 14.9 ± 0.4 rad m −2 and 4.6 ± 1.1 rad m −2 for random and physical pairs, respectively. This means polarized extragalactic sources that are close on the sky, but at different redshifts, have larger differences in RM than two components of one source. This difference of ∼ 10 rad m −2 is significant at 5σ, and persists in different data subsamples. While there have been other statistical studies of ∆RM between adjacent polarized sources, this is the first unambiguous demonstration that some of this RM difference must be extragalactic, thereby providing a firm upper limit on the RM contribution of the IGM. If the ∆RMs originate local to the sources, then the local magnetic field difference between random sources is a factor of two larger than between components of one source. Alternatively, attributing the difference in ∆RMs to the intervening IGM yields an upper limit on the IGM magnetic field strength of 40 nG.

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).…”

Simulations and observational tests of primordial magnetic fields from Cosmic Microwave Background constraints

“…A few authors have claimed a derivation of upper limits on the average magnetization of the cosmic web, based on the statistical analysis of radio surveys (e.g. Vernstrom et al 2017Vernstrom et al , 2019O'Sullivan et al 2020). Our simulations can already be used for a first tentative comparison with recent work, albeit within the caveat posed by their limited resolution, which are discussed in detail in Section 4 and in the Appendix.…”

Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

“…Gaensler et al 2004) and associated advanced analysis techniques (e.g. Vernstrom et al 2019;O'Sullivan et al 2020;Stuardi et al 2020) might effectively probe such material. The thermal electron density drops rapidly with cluster-centric radius in typical ICM models, and the efficiency of Bremsstrahlung as a tracer of this gas drops more precipitously, since its emissivity is proportional to n 2 e .…”