The presence of BIGM can be verified by identifying
faint intergalactic synchrotron radiation at the longer radio
's where the radiative
lifetime is long enough that the synchrotron-emitting electrons can
propagate far enough into the IGM from their acceleration site. This
criterion favors
1m, and requires a
surface brightness sensitivity that exceeds the capability of most
current instruments.
A second method, likewise barely accessible to the best current instruments, is to detect a Faraday RM in an intergalactic context - beyond galaxy clusters (where BIGM is detectable - see above). I illustrate a recent example of each method in the following 2 figures.
Figure 11 shows a 0.4 GHz continuum image of 8° diameter, minus the discrete radio sources, near the center of the Coma supercluster of galaxies [23]. The strong radio source near the center is the radio "halo" that identifies the inner Coma cluster of galaxies.
 |
Figure 11. 0.4 GHz, 10' resolution image of dia. 8° showing extremely faint diffuse radio emission in the Coma supercluster of galaxies [23]. It used a combination of data from the 305m Arecibo Telescope and the NRC-DRAO Synthesis interferometer at Penticton BC. Most of the other diffuse features are not yet identified, and could be of Galactic or extragalactic origin. The rms noise level is ~ 250 mK at 430 MHz, and is set by faint discrete source confusion. |
The extended region to the right of the Coma cluster (center) is ~ 2 Mpc in extent, and has intergalactic field strengths of ~ 10-7 G. This is evidence for an intergalactic magnetic field strength of this order in the IGM beyond galaxy clusters.
Another recent attempt was made to detect Faraday rotation due to BIGM in an intergalactic filament. The well defined filament of the Perseus-Pisces supercluster zone was investigated [24] by combining appropriately smoothed Faraday rotation data with analyses of the 2MASS and CfA surveys of galaxy magnitudes and redshifts. The smoothed 2MASS survey column densities through, the P-P supercluster filament are compared with RM in Figure 12(a). An analogous plot in Fig. 12(b) shows the RM's plotted against weighted pathlength via a 3-D Voronoi tessellation-model of the P-P filament. The results using both methods are consistent with a BIGM of order 10-8 - 10-7 G with a B reversal scale of a few hundred kpc. Further details can be found in [24]. This includes some model assumptions, and is always susceptible to a chance superposition of high latitude Milky Way foreground contribution to RM.
 |
Figure 12. Plots of observed, 7° convolved RM's against calculated magneto-ionic pathlengths using two independent optical galaxy surveys, and methods described briefly in the text, and in Xu et al. [24]. Reproduced from [24]. |
In summary, I have described a recent accumulation of evidence for intergalactic magnetic fields of 10-8 - 10-7 G in galaxy filaments in the low-redshift Universe. The direct observational evidence described in this section is concordant with some magneto-plasma modeling e.g. [21], and direct BIGM from supermassive black holes e.g. [19]. While filament zones over Mpc pathlengths would produce strong UHECR deflections and anisotropies the fraction of such zones over a typical intergalactic propagation path appears to be small. Most of the intergalactic volume consists of voids, where BIGM is much smaller. Although BIGM in the voids is even less explored, it is very small, and can be investigated using analysis of VHECR events - a topic briefly visited below.