Effect of Galactic and primordial Magnetic Field on polarization of CMB and 21 cm radiation.

Soma De
(Arizona State University)


I will discuss the effect of both primordial and galactic magnetic field on the polarization of CMB and 21 cm radiation. Faraday Rotation (FR) of CMB polarization, as measured through mode-coupling correlations of E and B modes, can be a promising probe of a stochastic primordial magnetic field (PMF). We use existing estimates of the Milky Way rotation measure (RM) to forecast its detectability with upcoming and future CMB experiments. We find that the galactic RM will not be seen in polarization measurements by Planck, but that it will need to be accounted for by CMB experiments capable of detecting the weak lensing contribution to the B-mode. We find that a realistic future sub-orbital experiment, covering a patch of the sky near the galactic poles, can detect a scale-invariant PMF of 0.1 nano-Gauss at better than 95% confidence level, while a dedicated space-based experiment can detect even smaller fields. Redshifted 21 cm signal from neutral hydrogen is one of the most competitive probes of the epoch of reionization (EoR). Unpolarized 21 cm radiation acquires a certain level of linear polarization during the EoR due to Thompson scattering. This linear polarization, if measured, could probe important information about the EoR. We show that a 99 % accuracy on rotation measure (RM) data is necessary to recover the initial E-mode signal. We conclude that, given the current status of RM observations, it is not possible to retrieve the initial polarization signal of 21 cm from the EoR. However, we are optimistic that it may be possible from the next generation radio surveys.

Finally I will discuss the possibility of any circular polarization that can be picked up by CMB. Circular polarization in the radiation CMB may not be present at the surface of last scattering. However, as the CMB passes through galaxies and galaxy clusters, which could generate a circular polarization by the method of Faraday conversion (FC) (Pacholczyk, 1998, Cooray et al, 2002). Circular polarization (due to Faraday conversion) has no strong local foreground. The unique frequency dependence of FC signal will allow one to eliminate other possible sources of circular polarization enabling to probe the first star explosions.