Magnetic field strength in cosmic web filaments

Abstract

ABSTRACT We used the rotation measure (RM) catalogue derived from the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2) at 144 MHz to measure the evolution with redshift of the extragalactic RM (RRM: Residual RM) and the polarization fraction (p) of sources in low-density environments. We also measured the same at 1.4 GHz by cross-matching with the NRAO VLA Sky Survey RM catalogue. We find that RRM versus redshift is flat at 144 MHz, but, once redshift-corrected, it shows evolution at high significance. Also, p evolves with redshift with a decrement by a factor of ∼8 at z ∼ 2. Comparing the 144-MHz and 1.4-GHz data, we find that the observed RRM and p are most likely to have an origin local to the source at 1.4 GHz, while a cosmic web filament origin is favoured at 144 MHz. If we attribute the entire signal to filaments, we infer a mean rest-frame RRM per filament of RRM$_{\rm 0,f} = 0.71 \pm 0.07 \, \, \rm rad\, m^{-2}$ and a magnetic field per filament of Bf = 32 ± 3 nG. This is in agreement with estimates obtained with a complementary method based on synchrotron emission stacking, and with cosmological simulations if primordial magnetic fields are amplified by astrophysical source field seeding. The measurement of an RRM0, f supports the presence of diffuse baryonic gas in filaments. We also estimated a conservative upper limit of the filament magnetic turbulence of $\sigma _{\rm RRM_{\rm 0,f}} =0.039 \pm 0.001 \, \, \rm rad\, m^{-2}$, concluding that the ordered magnetic field component dominates in filaments.